rustc_ast_passes/

ast_validation.rs

//! Validate AST before lowering it to HIR.
//!
//! This pass intends to check that the constructed AST is *syntactically valid* to allow the rest
//! of the compiler to assume that the AST is valid. These checks cannot be performed during parsing
//! because attribute macros are allowed to accept certain pieces of invalid syntax such as a
//! function without body outside of a trait definition:
//!
//! ```ignore (illustrative)
//! #[my_attribute]
//! mod foo {
//!     fn missing_body();
//! }
//! ```
//!
//! These checks are run post-expansion, after AST is frozen, to be able to check for erroneous
//! constructions produced by proc macros. This pass is only intended for simple checks that do not
//! require name resolution or type checking, or other kinds of complex analysis.

use std::mem;
use std::ops::{Deref, DerefMut};
use std::str::FromStr;

use itertools::{Either, Itertools};
use rustc_abi::{CVariadicStatus, CanonAbi, ExternAbi, InterruptKind};
use rustc_ast::visit::{AssocCtxt, BoundKind, FnCtxt, FnKind, Visitor, walk_list};
use rustc_ast::*;
use rustc_ast_pretty::pprust::{self, State};
use rustc_attr_parsing::validate_attr;
use rustc_data_structures::fx::FxIndexMap;
use rustc_errors::{DiagCtxtHandle, LintBuffer};
use rustc_feature::Features;
use rustc_session::Session;
use rustc_session::lint::BuiltinLintDiag;
use rustc_session::lint::builtin::{
    DEPRECATED_WHERE_CLAUSE_LOCATION, MISSING_ABI, MISSING_UNSAFE_ON_EXTERN,
    PATTERNS_IN_FNS_WITHOUT_BODY,
};
use rustc_session::parse::feature_err;
use rustc_span::{Ident, Span, kw, sym};
use rustc_target::spec::{AbiMap, AbiMapping};
use thin_vec::thin_vec;

use crate::errors::{self, TildeConstReason};

/// Is `self` allowed semantically as the first parameter in an `FnDecl`?
enum SelfSemantic {
    Yes,
    No,
}

enum TraitOrImpl {
    Trait { vis: Span, constness: Const },
    TraitImpl { constness: Const, polarity: ImplPolarity, trait_ref_span: Span },
    Impl { constness: Const },
}

impl TraitOrImpl {
    fn constness(&self) -> Option<Span> {
        match self {
            Self::Trait { constness: Const::Yes(span), .. }
            | Self::Impl { constness: Const::Yes(span), .. }
            | Self::TraitImpl { constness: Const::Yes(span), .. } => Some(*span),
            _ => None,
        }
    }
}

struct AstValidator<'a> {
    sess: &'a Session,
    features: &'a Features,

    /// The span of the `extern` in an `extern { ... }` block, if any.
    extern_mod_span: Option<Span>,

    outer_trait_or_trait_impl: Option<TraitOrImpl>,

    has_proc_macro_decls: bool,

    /// Used to ban nested `impl Trait`, e.g., `impl Into<impl Debug>`.
    /// Nested `impl Trait` _is_ allowed in associated type position,
    /// e.g., `impl Iterator<Item = impl Debug>`.
    outer_impl_trait_span: Option<Span>,

    disallow_tilde_const: Option<TildeConstReason>,

    /// Used to ban explicit safety on foreign items when the extern block is not marked as unsafe.
    extern_mod_safety: Option<Safety>,
    extern_mod_abi: Option<ExternAbi>,

    lint_node_id: NodeId,

    is_sdylib_interface: bool,

    lint_buffer: &'a mut LintBuffer,
}

impl<'a> AstValidator<'a> {
    fn with_in_trait_or_impl(
        &mut self,
        in_trait_or_impl: Option<TraitOrImpl>,
        f: impl FnOnce(&mut Self),
    ) {
        let old = mem::replace(&mut self.outer_trait_or_trait_impl, in_trait_or_impl);
        f(self);
        self.outer_trait_or_trait_impl = old;
    }

    fn with_in_trait(&mut self, vis: Span, constness: Const, f: impl FnOnce(&mut Self)) {
        let old = mem::replace(
            &mut self.outer_trait_or_trait_impl,
            Some(TraitOrImpl::Trait { vis, constness }),
        );
        f(self);
        self.outer_trait_or_trait_impl = old;
    }

    fn with_in_extern_mod(
        &mut self,
        extern_mod_safety: Safety,
        abi: Option<ExternAbi>,
        f: impl FnOnce(&mut Self),
    ) {
        let old_safety = mem::replace(&mut self.extern_mod_safety, Some(extern_mod_safety));
        let old_abi = mem::replace(&mut self.extern_mod_abi, abi);
        f(self);
        self.extern_mod_safety = old_safety;
        self.extern_mod_abi = old_abi;
    }

    fn with_tilde_const(
        &mut self,
        disallowed: Option<TildeConstReason>,
        f: impl FnOnce(&mut Self),
    ) {
        let old = mem::replace(&mut self.disallow_tilde_const, disallowed);
        f(self);
        self.disallow_tilde_const = old;
    }

    fn check_type_alias_where_clause_location(
        &mut self,
        ty_alias: &TyAlias,
    ) -> Result<(), errors::WhereClauseBeforeTypeAlias> {
        if ty_alias.ty.is_none() || !ty_alias.generics.where_clause.has_where_token {
            return Ok(());
        }

        let span = ty_alias.generics.where_clause.span;

        let sugg = if !ty_alias.generics.where_clause.predicates.is_empty()
            || !ty_alias.after_where_clause.has_where_token
        {
            let mut state = State::new();

            if !ty_alias.after_where_clause.has_where_token {
                state.space();
                state.word_space("where");
            }

            let mut first = ty_alias.after_where_clause.predicates.is_empty();
            for p in &ty_alias.generics.where_clause.predicates {
                if !first {
                    state.word_space(",");
                }
                first = false;
                state.print_where_predicate(p);
            }

            errors::WhereClauseBeforeTypeAliasSugg::Move {
                left: span,
                snippet: state.s.eof(),
                right: ty_alias.after_where_clause.span.shrink_to_hi(),
            }
        } else {
            errors::WhereClauseBeforeTypeAliasSugg::Remove { span }
        };

        Err(errors::WhereClauseBeforeTypeAlias { span, sugg })
    }

    fn with_impl_trait(&mut self, outer_span: Option<Span>, f: impl FnOnce(&mut Self)) {
        let old = mem::replace(&mut self.outer_impl_trait_span, outer_span);
        f(self);
        self.outer_impl_trait_span = old;
    }

    // Mirrors `visit::walk_ty`, but tracks relevant state.
    fn walk_ty(&mut self, t: &'a Ty) {
        match &t.kind {
            TyKind::ImplTrait(_, bounds) => {
                self.with_impl_trait(Some(t.span), |this| visit::walk_ty(this, t));

                // FIXME(precise_capturing): If we were to allow `use` in other positions
                // (e.g. GATs), then we must validate those as well. However, we don't have
                // a good way of doing this with the current `Visitor` structure.
                let mut use_bounds = bounds
                    .iter()
                    .filter_map(|bound| match bound {
                        GenericBound::Use(_, span) => Some(span),
                        _ => None,
                    })
                    .copied();
                if let Some(bound1) = use_bounds.next()
                    && let Some(bound2) = use_bounds.next()
                {
                    self.dcx().emit_err(errors::DuplicatePreciseCapturing { bound1, bound2 });
                }
            }
            TyKind::TraitObject(..) => self
                .with_tilde_const(Some(TildeConstReason::TraitObject), |this| {
                    visit::walk_ty(this, t)
                }),
            _ => visit::walk_ty(self, t),
        }
    }

    fn dcx(&self) -> DiagCtxtHandle<'a> {
        self.sess.dcx()
    }

    fn visibility_not_permitted(&self, vis: &Visibility, note: errors::VisibilityNotPermittedNote) {
        if let VisibilityKind::Inherited = vis.kind {
            return;
        }

        self.dcx().emit_err(errors::VisibilityNotPermitted {
            span: vis.span,
            note,
            remove_qualifier_sugg: vis.span,
        });
    }

    fn check_decl_no_pat(decl: &FnDecl, mut report_err: impl FnMut(Span, Option<Ident>, bool)) {
        for Param { pat, .. } in &decl.inputs {
            match pat.kind {
                PatKind::Missing | PatKind::Ident(BindingMode::NONE, _, None) | PatKind::Wild => {}
                PatKind::Ident(BindingMode::MUT, ident, None) => {
                    report_err(pat.span, Some(ident), true)
                }
                _ => report_err(pat.span, None, false),
            }
        }
    }

    fn check_impl_fn_not_const(&self, constness: Const, parent_constness: Const) {
        let Const::Yes(span) = constness else {
            return;
        };

        let span = self.sess.source_map().span_extend_while_whitespace(span);

        let Const::Yes(parent_constness) = parent_constness else {
            return;
        };

        self.dcx().emit_err(errors::ImplFnConst { span, parent_constness });
    }

    fn check_trait_fn_not_const(&self, constness: Const, parent: &TraitOrImpl) {
        let Const::Yes(span) = constness else {
            return;
        };

        let const_trait_impl = self.features.const_trait_impl();
        let make_impl_const_sugg = if const_trait_impl
            && let TraitOrImpl::TraitImpl {
                constness: Const::No,
                polarity: ImplPolarity::Positive,
                trait_ref_span,
                ..
            } = parent
        {
            Some(trait_ref_span.shrink_to_lo())
        } else {
            None
        };

        let map = self.sess.source_map();

        let make_trait_const_sugg = if const_trait_impl
            && let &TraitOrImpl::Trait { vis, constness: ast::Const::No } = parent
        {
            Some(map.span_extend_while_whitespace(vis).shrink_to_hi())
        } else {
            None
        };

        let parent_constness = parent.constness();
        self.dcx().emit_err(errors::TraitFnConst {
            span,
            in_impl: matches!(parent, TraitOrImpl::TraitImpl { .. }),
            const_context_label: parent_constness,
            remove_const_sugg: (
                map.span_extend_while_whitespace(span),
                match parent_constness {
                    Some(_) => rustc_errors::Applicability::MachineApplicable,
                    None => rustc_errors::Applicability::MaybeIncorrect,
                },
            ),
            requires_multiple_changes: make_impl_const_sugg.is_some()
                || make_trait_const_sugg.is_some(),
            make_impl_const_sugg,
            make_trait_const_sugg,
        });
    }

    fn check_async_fn_in_const_trait_or_impl(&self, sig: &FnSig, parent: &TraitOrImpl) {
        let Some(const_keyword) = parent.constness() else { return };

        let Some(CoroutineKind::Async { span: async_keyword, .. }) = sig.header.coroutine_kind
        else {
            return;
        };

        let context = match parent {
            TraitOrImpl::Trait { .. } => "trait",
            TraitOrImpl::TraitImpl { .. } => "trait_impl",
            TraitOrImpl::Impl { .. } => "impl",
        };

        self.dcx().emit_err(errors::AsyncFnInConstTraitOrTraitImpl {
            async_keyword,
            context,
            const_keyword,
        });
    }

    fn check_fn_decl(&self, fn_decl: &FnDecl, self_semantic: SelfSemantic) {
        self.check_decl_num_args(fn_decl);
        self.check_decl_cvariadic_pos(fn_decl);
        self.check_decl_attrs(fn_decl);
        self.check_decl_self_param(fn_decl, self_semantic);
    }

    /// Emits fatal error if function declaration has more than `u16::MAX` arguments
    /// Error is fatal to prevent errors during typechecking
    fn check_decl_num_args(&self, fn_decl: &FnDecl) {
        let max_num_args: usize = u16::MAX.into();
        if fn_decl.inputs.len() > max_num_args {
            let Param { span, .. } = fn_decl.inputs[0];
            self.dcx().emit_fatal(errors::FnParamTooMany { span, max_num_args });
        }
    }

    /// Emits an error if a function declaration has a variadic parameter in the
    /// beginning or middle of parameter list.
    /// Example: `fn foo(..., x: i32)` will emit an error.
    fn check_decl_cvariadic_pos(&self, fn_decl: &FnDecl) {
        match &*fn_decl.inputs {
            [ps @ .., _] => {
                for Param { ty, span, .. } in ps {
                    if let TyKind::CVarArgs = ty.kind {
                        self.dcx().emit_err(errors::FnParamCVarArgsNotLast { span: *span });
                    }
                }
            }
            _ => {}
        }
    }

    fn check_decl_attrs(&self, fn_decl: &FnDecl) {
        fn_decl
            .inputs
            .iter()
            .flat_map(|i| i.attrs.as_ref())
            .filter(|attr| {
                let arr = [
                    sym::allow,
                    sym::cfg_trace,
                    sym::cfg_attr_trace,
                    sym::deny,
                    sym::expect,
                    sym::forbid,
                    sym::warn,
                ];
                !attr.has_any_name(&arr) && rustc_attr_parsing::is_builtin_attr(*attr)
            })
            .for_each(|attr| {
                if attr.is_doc_comment() {
                    self.dcx().emit_err(errors::FnParamDocComment { span: attr.span });
                } else {
                    self.dcx().emit_err(errors::FnParamForbiddenAttr { span: attr.span });
                }
            });
    }

    fn check_decl_self_param(&self, fn_decl: &FnDecl, self_semantic: SelfSemantic) {
        if let (SelfSemantic::No, [param, ..]) = (self_semantic, &*fn_decl.inputs) {
            if param.is_self() {
                self.dcx().emit_err(errors::FnParamForbiddenSelf { span: param.span });
            }
        }
    }

    /// Check that the signature of this function does not violate the constraints of its ABI.
    fn check_extern_fn_signature(&self, abi: ExternAbi, ctxt: FnCtxt, ident: &Ident, sig: &FnSig) {
        match AbiMap::from_target(&self.sess.target).canonize_abi(abi, false) {
            AbiMapping::Direct(canon_abi) | AbiMapping::Deprecated(canon_abi) => {
                match canon_abi {
                    CanonAbi::C
                    | CanonAbi::Rust
                    | CanonAbi::RustCold
                    | CanonAbi::Arm(_)
                    | CanonAbi::GpuKernel
                    | CanonAbi::X86(_) => { /* nothing to check */ }

                    CanonAbi::Custom => {
                        // An `extern "custom"` function must be unsafe.
                        self.reject_safe_fn(abi, ctxt, sig);

                        // An `extern "custom"` function cannot be `async` and/or `gen`.
                        self.reject_coroutine(abi, sig);

                        // An `extern "custom"` function must have type `fn()`.
                        self.reject_params_or_return(abi, ident, sig);
                    }

                    CanonAbi::Interrupt(interrupt_kind) => {
                        // An interrupt handler cannot be `async` and/or `gen`.
                        self.reject_coroutine(abi, sig);

                        if let InterruptKind::X86 = interrupt_kind {
                            // "x86-interrupt" is special because it does have arguments.
                            // FIXME(workingjubilee): properly lint on acceptable input types.
                            let inputs = &sig.decl.inputs;
                            let param_count = inputs.len();
                            if !matches!(param_count, 1 | 2) {
                                let mut spans: Vec<Span> =
                                    inputs.iter().map(|arg| arg.span).collect();
                                if spans.is_empty() {
                                    spans = vec![sig.span];
                                }
                                self.dcx().emit_err(errors::AbiX86Interrupt { spans, param_count });
                            }

                            if let FnRetTy::Ty(ref ret_ty) = sig.decl.output
                                && match &ret_ty.kind {
                                    TyKind::Never => false,
                                    TyKind::Tup(tup) if tup.is_empty() => false,
                                    _ => true,
                                }
                            {
                                self.dcx().emit_err(errors::AbiMustNotHaveReturnType {
                                    span: ret_ty.span,
                                    abi,
                                });
                            }
                        } else {
                            // An `extern "interrupt"` function must have type `fn()`.
                            self.reject_params_or_return(abi, ident, sig);
                        }
                    }
                }
            }
            AbiMapping::Invalid => { /* ignore */ }
        }
    }

    fn reject_safe_fn(&self, abi: ExternAbi, ctxt: FnCtxt, sig: &FnSig) {
        let dcx = self.dcx();

        match sig.header.safety {
            Safety::Unsafe(_) => { /* all good */ }
            Safety::Safe(safe_span) => {
                let source_map = self.sess.psess.source_map();
                let safe_span = source_map.span_until_non_whitespace(safe_span.to(sig.span));
                dcx.emit_err(errors::AbiCustomSafeForeignFunction { span: sig.span, safe_span });
            }
            Safety::Default => match ctxt {
                FnCtxt::Foreign => { /* all good */ }
                FnCtxt::Free | FnCtxt::Assoc(_) => {
                    dcx.emit_err(errors::AbiCustomSafeFunction {
                        span: sig.span,
                        abi,
                        unsafe_span: sig.span.shrink_to_lo(),
                    });
                }
            },
        }
    }

    fn reject_coroutine(&self, abi: ExternAbi, sig: &FnSig) {
        if let Some(coroutine_kind) = sig.header.coroutine_kind {
            let coroutine_kind_span = self
                .sess
                .psess
                .source_map()
                .span_until_non_whitespace(coroutine_kind.span().to(sig.span));

            self.dcx().emit_err(errors::AbiCannotBeCoroutine {
                span: sig.span,
                abi,
                coroutine_kind_span,
                coroutine_kind_str: coroutine_kind.as_str(),
            });
        }
    }

    fn reject_params_or_return(&self, abi: ExternAbi, ident: &Ident, sig: &FnSig) {
        let mut spans: Vec<_> = sig.decl.inputs.iter().map(|p| p.span).collect();
        if let FnRetTy::Ty(ref ret_ty) = sig.decl.output
            && match &ret_ty.kind {
                TyKind::Never => false,
                TyKind::Tup(tup) if tup.is_empty() => false,
                _ => true,
            }
        {
            spans.push(ret_ty.span);
        }

        if !spans.is_empty() {
            let header_span = sig.header_span();
            let suggestion_span = header_span.shrink_to_hi().to(sig.decl.output.span());
            let padding = if header_span.is_empty() { "" } else { " " };

            self.dcx().emit_err(errors::AbiMustNotHaveParametersOrReturnType {
                spans,
                symbol: ident.name,
                suggestion_span,
                padding,
                abi,
            });
        }
    }

    /// This ensures that items can only be `unsafe` (or unmarked) outside of extern
    /// blocks.
    ///
    /// This additionally ensures that within extern blocks, items can only be
    /// `safe`/`unsafe` inside of a `unsafe`-adorned extern block.
    fn check_item_safety(&self, span: Span, safety: Safety) {
        match self.extern_mod_safety {
            Some(extern_safety) => {
                if matches!(safety, Safety::Unsafe(_) | Safety::Safe(_))
                    && extern_safety == Safety::Default
                {
                    self.dcx().emit_err(errors::InvalidSafetyOnExtern {
                        item_span: span,
                        block: Some(self.current_extern_span().shrink_to_lo()),
                    });
                }
            }
            None => {
                if matches!(safety, Safety::Safe(_)) {
                    self.dcx().emit_err(errors::InvalidSafetyOnItem { span });
                }
            }
        }
    }

    fn check_fn_ptr_safety(&self, span: Span, safety: Safety) {
        if matches!(safety, Safety::Safe(_)) {
            self.dcx().emit_err(errors::InvalidSafetyOnFnPtr { span });
        }
    }

    fn check_defaultness(&self, span: Span, defaultness: Defaultness) {
        if let Defaultness::Default(def_span) = defaultness {
            let span = self.sess.source_map().guess_head_span(span);
            self.dcx().emit_err(errors::ForbiddenDefault { span, def_span });
        }
    }

    /// If `sp` ends with a semicolon, returns it as a `Span`
    /// Otherwise, returns `sp.shrink_to_hi()`
    fn ending_semi_or_hi(&self, sp: Span) -> Span {
        let source_map = self.sess.source_map();
        let end = source_map.end_point(sp);

        if source_map.span_to_snippet(end).is_ok_and(|s| s == ";") {
            end
        } else {
            sp.shrink_to_hi()
        }
    }

    fn check_type_no_bounds(&self, bounds: &[GenericBound], ctx: &str) {
        let span = match bounds {
            [] => return,
            [b0] => b0.span(),
            [b0, .., bl] => b0.span().to(bl.span()),
        };
        self.dcx().emit_err(errors::BoundInContext { span, ctx });
    }

    fn check_foreign_ty_genericless(&self, generics: &Generics, after_where_clause: &WhereClause) {
        let cannot_have = |span, descr, remove_descr| {
            self.dcx().emit_err(errors::ExternTypesCannotHave {
                span,
                descr,
                remove_descr,
                block_span: self.current_extern_span(),
            });
        };

        if !generics.params.is_empty() {
            cannot_have(generics.span, "generic parameters", "generic parameters");
        }

        let check_where_clause = |where_clause: &WhereClause| {
            if where_clause.has_where_token {
                cannot_have(where_clause.span, "`where` clauses", "`where` clause");
            }
        };

        check_where_clause(&generics.where_clause);
        check_where_clause(&after_where_clause);
    }

    fn check_foreign_kind_bodyless(&self, ident: Ident, kind: &str, body_span: Option<Span>) {
        let Some(body_span) = body_span else {
            return;
        };
        self.dcx().emit_err(errors::BodyInExtern {
            span: ident.span,
            body: body_span,
            block: self.current_extern_span(),
            kind,
        });
    }

    /// An `fn` in `extern { ... }` cannot have a body `{ ... }`.
    fn check_foreign_fn_bodyless(&self, ident: Ident, body: Option<&Block>) {
        let Some(body) = body else {
            return;
        };
        self.dcx().emit_err(errors::FnBodyInExtern {
            span: ident.span,
            body: body.span,
            block: self.current_extern_span(),
        });
    }

    fn current_extern_span(&self) -> Span {
        self.sess.source_map().guess_head_span(self.extern_mod_span.unwrap())
    }

    /// An `fn` in `extern { ... }` cannot have qualifiers, e.g. `async fn`.
    fn check_foreign_fn_headerless(
        &self,
        // Deconstruct to ensure exhaustiveness
        FnHeader { safety: _, coroutine_kind, constness, ext }: FnHeader,
    ) {
        let report_err = |span, kw| {
            self.dcx().emit_err(errors::FnQualifierInExtern {
                span,
                kw,
                block: self.current_extern_span(),
            });
        };
        match coroutine_kind {
            Some(kind) => report_err(kind.span(), kind.as_str()),
            None => (),
        }
        match constness {
            Const::Yes(span) => report_err(span, "const"),
            Const::No => (),
        }
        match ext {
            Extern::None => (),
            Extern::Implicit(span) | Extern::Explicit(_, span) => report_err(span, "extern"),
        }
    }

    /// An item in `extern { ... }` cannot use non-ascii identifier.
    fn check_foreign_item_ascii_only(&self, ident: Ident) {
        if !ident.as_str().is_ascii() {
            self.dcx().emit_err(errors::ExternItemAscii {
                span: ident.span,
                block: self.current_extern_span(),
            });
        }
    }

    /// Reject invalid C-variadic types.
    ///
    /// C-variadics must be:
    /// - Non-const
    /// - Either foreign, or free and `unsafe extern "C"` semantically
    fn check_c_variadic_type(&self, fk: FnKind<'a>, attrs: &'a AttrVec) {
        // `...` is already rejected when it is not the final parameter.
        let variadic_param = match fk.decl().inputs.last() {
            Some(param) if matches!(param.ty.kind, TyKind::CVarArgs) => param,
            _ => return,
        };

        let FnKind::Fn(fn_ctxt, _, Fn { sig, .. }) = fk else {
            // Unreachable because the parser already rejects `...` in closures.
            unreachable!("C variable argument list cannot be used in closures")
        };

        // C-variadics are not yet implemented in const evaluation.
        if let Const::Yes(const_span) = sig.header.constness {
            self.dcx().emit_err(errors::ConstAndCVariadic {
                spans: vec![const_span, variadic_param.span],
                const_span,
                variadic_span: variadic_param.span,
            });
        }

        if let Some(coroutine_kind) = sig.header.coroutine_kind {
            self.dcx().emit_err(errors::CoroutineAndCVariadic {
                spans: vec![coroutine_kind.span(), variadic_param.span],
                coroutine_kind: coroutine_kind.as_str(),
                coroutine_span: coroutine_kind.span(),
                variadic_span: variadic_param.span,
            });
        }

        match fn_ctxt {
            FnCtxt::Foreign => return,
            FnCtxt::Free | FnCtxt::Assoc(_) => {
                if !self.sess.target.arch.supports_c_variadic_definitions() {
                    self.dcx().emit_err(errors::CVariadicNotSupported {
                        variadic_span: variadic_param.span,
                        target: &*self.sess.target.llvm_target,
                    });
                    return;
                }

                match sig.header.ext {
                    Extern::Implicit(_) => {
                        if !matches!(sig.header.safety, Safety::Unsafe(_)) {
                            self.dcx().emit_err(errors::CVariadicMustBeUnsafe {
                                span: variadic_param.span,
                                unsafe_span: sig.safety_span(),
                            });
                        }
                    }
                    Extern::Explicit(StrLit { symbol_unescaped, .. }, _) => {
                        // Just bail if the ABI is not even recognized.
                        let Ok(abi) = ExternAbi::from_str(symbol_unescaped.as_str()) else {
                            return;
                        };

                        self.check_c_variadic_abi(abi, attrs, variadic_param.span, sig);

                        if !matches!(sig.header.safety, Safety::Unsafe(_)) {
                            self.dcx().emit_err(errors::CVariadicMustBeUnsafe {
                                span: variadic_param.span,
                                unsafe_span: sig.safety_span(),
                            });
                        }
                    }
                    Extern::None => {
                        let err = errors::CVariadicNoExtern { span: variadic_param.span };
                        self.dcx().emit_err(err);
                    }
                }
            }
        }
    }

    fn check_c_variadic_abi(
        &self,
        abi: ExternAbi,
        attrs: &'a AttrVec,
        dotdotdot_span: Span,
        sig: &FnSig,
    ) {
        // For naked functions we accept any ABI that is accepted on c-variadic
        // foreign functions, if the c_variadic_naked_functions feature is enabled.
        if attr::contains_name(attrs, sym::naked) {
            match abi.supports_c_variadic() {
                CVariadicStatus::Stable if let ExternAbi::C { .. } = abi => {
                    // With `c_variadic` naked c-variadic `extern "C"` functions are allowed.
                }
                CVariadicStatus::Stable => {
                    // For e.g. aapcs or sysv64 `c_variadic_naked_functions` must also be enabled.
                    if !self.features.enabled(sym::c_variadic_naked_functions) {
                        let msg = format!("Naked c-variadic `extern {abi}` functions are unstable");
                        feature_err(&self.sess, sym::c_variadic_naked_functions, sig.span, msg)
                            .emit();
                    }
                }
                CVariadicStatus::Unstable { feature } => {
                    // Some ABIs need additional features.
                    if !self.features.enabled(sym::c_variadic_naked_functions) {
                        let msg = format!("Naked c-variadic `extern {abi}` functions are unstable");
                        feature_err(&self.sess, sym::c_variadic_naked_functions, sig.span, msg)
                            .emit();
                    }

                    if !self.features.enabled(feature) {
                        let msg = format!(
                            "C-variadic functions with the {abi} calling convention are unstable"
                        );
                        feature_err(&self.sess, feature, sig.span, msg).emit();
                    }
                }
                CVariadicStatus::NotSupported => {
                    // Some ABIs, e.g. `extern "Rust"`, never support c-variadic functions.
                    self.dcx().emit_err(errors::CVariadicBadNakedExtern {
                        span: dotdotdot_span,
                        abi: abi.as_str(),
                        extern_span: sig.extern_span(),
                    });
                }
            }
        } else if !matches!(abi, ExternAbi::C { .. }) {
            self.dcx().emit_err(errors::CVariadicBadExtern {
                span: dotdotdot_span,
                abi: abi.as_str(),
                extern_span: sig.extern_span(),
            });
        }
    }

    fn check_item_named(&self, ident: Ident, kind: &str) {
        if ident.name != kw::Underscore {
            return;
        }
        self.dcx().emit_err(errors::ItemUnderscore { span: ident.span, kind });
    }

    fn check_nomangle_item_asciionly(&self, ident: Ident, item_span: Span) {
        if ident.name.as_str().is_ascii() {
            return;
        }
        let span = self.sess.source_map().guess_head_span(item_span);
        self.dcx().emit_err(errors::NoMangleAscii { span });
    }

    fn check_mod_file_item_asciionly(&self, ident: Ident) {
        if ident.name.as_str().is_ascii() {
            return;
        }
        self.dcx().emit_err(errors::ModuleNonAscii { span: ident.span, name: ident.name });
    }

    fn deny_const_auto_traits(&self, constness: Const) {
        if let Const::Yes(span) = constness {
            self.dcx().emit_err(errors::ConstAutoTrait { span });
        }
    }

    fn deny_generic_params(&self, generics: &Generics, ident_span: Span) {
        if !generics.params.is_empty() {
            self.dcx()
                .emit_err(errors::AutoTraitGeneric { span: generics.span, ident: ident_span });
        }
    }

    fn deny_super_traits(&self, bounds: &GenericBounds, ident: Span) {
        if let [.., last] = &bounds[..] {
            let span = bounds.iter().map(|b| b.span()).collect();
            let removal = ident.shrink_to_hi().to(last.span());
            self.dcx().emit_err(errors::AutoTraitBounds { span, removal, ident });
        }
    }

    fn deny_where_clause(&self, where_clause: &WhereClause, ident: Span) {
        if !where_clause.predicates.is_empty() {
            // FIXME: The current diagnostic is misleading since it only talks about
            // super trait and lifetime bounds while we should just say “bounds”.
            self.dcx().emit_err(errors::AutoTraitBounds {
                span: vec![where_clause.span],
                removal: where_clause.span,
                ident,
            });
        }
    }

    fn deny_items(&self, trait_items: &[Box<AssocItem>], ident_span: Span) {
        if !trait_items.is_empty() {
            let spans: Vec<_> = trait_items.iter().map(|i| i.kind.ident().unwrap().span).collect();
            let total = trait_items.first().unwrap().span.to(trait_items.last().unwrap().span);
            self.dcx().emit_err(errors::AutoTraitItems { spans, total, ident: ident_span });
        }
    }

    fn correct_generic_order_suggestion(&self, data: &AngleBracketedArgs) -> String {
        // Lifetimes always come first.
        let lt_sugg = data.args.iter().filter_map(|arg| match arg {
            AngleBracketedArg::Arg(lt @ GenericArg::Lifetime(_)) => {
                Some(pprust::to_string(|s| s.print_generic_arg(lt)))
            }
            _ => None,
        });
        let args_sugg = data.args.iter().filter_map(|a| match a {
            AngleBracketedArg::Arg(GenericArg::Lifetime(_)) | AngleBracketedArg::Constraint(_) => {
                None
            }
            AngleBracketedArg::Arg(arg) => Some(pprust::to_string(|s| s.print_generic_arg(arg))),
        });
        // Constraints always come last.
        let constraint_sugg = data.args.iter().filter_map(|a| match a {
            AngleBracketedArg::Arg(_) => None,
            AngleBracketedArg::Constraint(c) => {
                Some(pprust::to_string(|s| s.print_assoc_item_constraint(c)))
            }
        });
        format!(
            "<{}>",
            lt_sugg.chain(args_sugg).chain(constraint_sugg).collect::<Vec<String>>().join(", ")
        )
    }

    /// Enforce generic args coming before constraints in `<...>` of a path segment.
    fn check_generic_args_before_constraints(&self, data: &AngleBracketedArgs) {
        // Early exit in case it's partitioned as it should be.
        if data.args.iter().is_partitioned(|arg| matches!(arg, AngleBracketedArg::Arg(_))) {
            return;
        }
        // Find all generic argument coming after the first constraint...
        let (constraint_spans, arg_spans): (Vec<Span>, Vec<Span>) =
            data.args.iter().partition_map(|arg| match arg {
                AngleBracketedArg::Constraint(c) => Either::Left(c.span),
                AngleBracketedArg::Arg(a) => Either::Right(a.span()),
            });
        let args_len = arg_spans.len();
        let constraint_len = constraint_spans.len();
        // ...and then error:
        self.dcx().emit_err(errors::ArgsBeforeConstraint {
            arg_spans: arg_spans.clone(),
            constraints: constraint_spans[0],
            args: *arg_spans.iter().last().unwrap(),
            data: data.span,
            constraint_spans: errors::EmptyLabelManySpans(constraint_spans),
            arg_spans2: errors::EmptyLabelManySpans(arg_spans),
            suggestion: self.correct_generic_order_suggestion(data),
            constraint_len,
            args_len,
        });
    }

    fn visit_ty_common(&mut self, ty: &'a Ty) {
        match &ty.kind {
            TyKind::FnPtr(bfty) => {
                self.check_fn_ptr_safety(bfty.decl_span, bfty.safety);
                self.check_fn_decl(&bfty.decl, SelfSemantic::No);
                Self::check_decl_no_pat(&bfty.decl, |span, _, _| {
                    self.dcx().emit_err(errors::PatternFnPointer { span });
                });
                if let Extern::Implicit(extern_span) = bfty.ext {
                    self.handle_missing_abi(extern_span, ty.id);
                }
            }
            TyKind::TraitObject(bounds, ..) => {
                let mut any_lifetime_bounds = false;
                for bound in bounds {
                    if let GenericBound::Outlives(lifetime) = bound {
                        if any_lifetime_bounds {
                            self.dcx()
                                .emit_err(errors::TraitObjectBound { span: lifetime.ident.span });
                            break;
                        }
                        any_lifetime_bounds = true;
                    }
                }
            }
            TyKind::ImplTrait(_, bounds) => {
                if let Some(outer_impl_trait_sp) = self.outer_impl_trait_span {
                    self.dcx().emit_err(errors::NestedImplTrait {
                        span: ty.span,
                        outer: outer_impl_trait_sp,
                        inner: ty.span,
                    });
                }

                if !bounds.iter().any(|b| matches!(b, GenericBound::Trait(..))) {
                    self.dcx().emit_err(errors::AtLeastOneTrait { span: ty.span });
                }
            }
            _ => {}
        }
    }

    fn handle_missing_abi(&mut self, span: Span, id: NodeId) {
        // FIXME(davidtwco): This is a hack to detect macros which produce spans of the
        // call site which do not have a macro backtrace. See #61963.
        if span.edition().at_least_edition_future() && self.features.explicit_extern_abis() {
            self.dcx().emit_err(errors::MissingAbi { span });
        } else if self
            .sess
            .source_map()
            .span_to_snippet(span)
            .is_ok_and(|snippet| !snippet.starts_with("#["))
        {
            self.lint_buffer.buffer_lint(
                MISSING_ABI,
                id,
                span,
                errors::MissingAbiSugg { span, default_abi: ExternAbi::FALLBACK },
            )
        }
    }

    // Used within `visit_item` for item kinds where we don't call `visit::walk_item`.
    fn visit_attrs_vis(&mut self, attrs: &'a AttrVec, vis: &'a Visibility) {
        walk_list!(self, visit_attribute, attrs);
        self.visit_vis(vis);
    }

    // Used within `visit_item` for item kinds where we don't call `visit::walk_item`.
    fn visit_attrs_vis_ident(&mut self, attrs: &'a AttrVec, vis: &'a Visibility, ident: &'a Ident) {
        walk_list!(self, visit_attribute, attrs);
        self.visit_vis(vis);
        self.visit_ident(ident);
    }
}

/// Checks that generic parameters are in the correct order,
/// which is lifetimes, then types and then consts. (`<'a, T, const N: usize>`)
fn validate_generic_param_order(dcx: DiagCtxtHandle<'_>, generics: &[GenericParam], span: Span) {
    let mut max_param: Option<ParamKindOrd> = None;
    let mut out_of_order = FxIndexMap::default();
    let mut param_idents = Vec::with_capacity(generics.len());

    for (idx, param) in generics.iter().enumerate() {
        let ident = param.ident;
        let (kind, bounds, span) = (&param.kind, &param.bounds, ident.span);
        let (ord_kind, ident) = match &param.kind {
            GenericParamKind::Lifetime => (ParamKindOrd::Lifetime, ident.to_string()),
            GenericParamKind::Type { .. } => (ParamKindOrd::TypeOrConst, ident.to_string()),
            GenericParamKind::Const { ty, .. } => {
                let ty = pprust::ty_to_string(ty);
                (ParamKindOrd::TypeOrConst, format!("const {ident}: {ty}"))
            }
        };
        param_idents.push((kind, ord_kind, bounds, idx, ident));
        match max_param {
            Some(max_param) if max_param > ord_kind => {
                let entry = out_of_order.entry(ord_kind).or_insert((max_param, vec![]));
                entry.1.push(span);
            }
            Some(_) | None => max_param = Some(ord_kind),
        };
    }

    if !out_of_order.is_empty() {
        let mut ordered_params = "<".to_string();
        param_idents.sort_by_key(|&(_, po, _, i, _)| (po, i));
        let mut first = true;
        for (kind, _, bounds, _, ident) in param_idents {
            if !first {
                ordered_params += ", ";
            }
            ordered_params += &ident;

            if !bounds.is_empty() {
                ordered_params += ": ";
                ordered_params += &pprust::bounds_to_string(bounds);
            }

            match kind {
                GenericParamKind::Type { default: Some(default) } => {
                    ordered_params += " = ";
                    ordered_params += &pprust::ty_to_string(default);
                }
                GenericParamKind::Type { default: None } => (),
                GenericParamKind::Lifetime => (),
                GenericParamKind::Const { ty: _, span: _, default: Some(default) } => {
                    ordered_params += " = ";
                    ordered_params += &pprust::expr_to_string(&default.value);
                }
                GenericParamKind::Const { ty: _, span: _, default: None } => (),
            }
            first = false;
        }

        ordered_params += ">";

        for (param_ord, (max_param, spans)) in &out_of_order {
            dcx.emit_err(errors::OutOfOrderParams {
                spans: spans.clone(),
                sugg_span: span,
                param_ord,
                max_param,
                ordered_params: &ordered_params,
            });
        }
    }
}

impl<'a> Visitor<'a> for AstValidator<'a> {
    fn visit_attribute(&mut self, attr: &Attribute) {
        validate_attr::check_attr(&self.sess.psess, attr);
    }

    fn visit_ty(&mut self, ty: &'a Ty) {
        self.visit_ty_common(ty);
        self.walk_ty(ty)
    }

    fn visit_item(&mut self, item: &'a Item) {
        if item.attrs.iter().any(|attr| attr.is_proc_macro_attr()) {
            self.has_proc_macro_decls = true;
        }

        let previous_lint_node_id = mem::replace(&mut self.lint_node_id, item.id);

        if let Some(ident) = item.kind.ident()
            && attr::contains_name(&item.attrs, sym::no_mangle)
        {
            self.check_nomangle_item_asciionly(ident, item.span);
        }

        match &item.kind {
            ItemKind::Impl(Impl {
                generics,
                constness,
                of_trait:
                    Some(box TraitImplHeader { safety, polarity, defaultness: _, trait_ref: t }),
                self_ty,
                items,
            }) => {
                self.visit_attrs_vis(&item.attrs, &item.vis);
                self.visibility_not_permitted(
                    &item.vis,
                    errors::VisibilityNotPermittedNote::TraitImpl,
                );
                if let TyKind::Dummy = self_ty.kind {
                    // Abort immediately otherwise the `TyKind::Dummy` will reach HIR lowering,
                    // which isn't allowed. Not a problem for this obscure, obsolete syntax.
                    self.dcx().emit_fatal(errors::ObsoleteAuto { span: item.span });
                }
                if let (&Safety::Unsafe(span), &ImplPolarity::Negative(sp)) = (safety, polarity) {
                    self.dcx().emit_err(errors::UnsafeNegativeImpl {
                        span: sp.to(t.path.span),
                        negative: sp,
                        r#unsafe: span,
                    });
                }

                let disallowed = matches!(constness, Const::No)
                    .then(|| TildeConstReason::TraitImpl { span: item.span });
                self.with_tilde_const(disallowed, |this| this.visit_generics(generics));
                self.visit_trait_ref(t);
                self.visit_ty(self_ty);

                self.with_in_trait_or_impl(
                    Some(TraitOrImpl::TraitImpl {
                        constness: *constness,
                        polarity: *polarity,
                        trait_ref_span: t.path.span,
                    }),
                    |this| {
                        walk_list!(
                            this,
                            visit_assoc_item,
                            items,
                            AssocCtxt::Impl { of_trait: true }
                        );
                    },
                );
            }
            ItemKind::Impl(Impl { generics, of_trait: None, self_ty, items, constness }) => {
                self.visit_attrs_vis(&item.attrs, &item.vis);
                self.visibility_not_permitted(
                    &item.vis,
                    errors::VisibilityNotPermittedNote::IndividualImplItems,
                );

                let disallowed = matches!(constness, ast::Const::No)
                    .then(|| TildeConstReason::Impl { span: item.span });

                self.with_tilde_const(disallowed, |this| this.visit_generics(generics));

                self.visit_ty(self_ty);
                self.with_in_trait_or_impl(
                    Some(TraitOrImpl::Impl { constness: *constness }),
                    |this| {
                        walk_list!(
                            this,
                            visit_assoc_item,
                            items,
                            AssocCtxt::Impl { of_trait: false }
                        );
                    },
                );
            }
            ItemKind::Fn(
                func @ box Fn {
                    defaultness,
                    ident,
                    generics: _,
                    sig,
                    contract: _,
                    body,
                    define_opaque: _,
                },
            ) => {
                self.visit_attrs_vis_ident(&item.attrs, &item.vis, ident);
                self.check_defaultness(item.span, *defaultness);

                let is_intrinsic = item.attrs.iter().any(|a| a.has_name(sym::rustc_intrinsic));
                if body.is_none() && !is_intrinsic && !self.is_sdylib_interface {
                    self.dcx().emit_err(errors::FnWithoutBody {
                        span: item.span,
                        replace_span: self.ending_semi_or_hi(item.span),
                        extern_block_suggestion: match sig.header.ext {
                            Extern::None => None,
                            Extern::Implicit(start_span) => {
                                Some(errors::ExternBlockSuggestion::Implicit {
                                    start_span,
                                    end_span: item.span.shrink_to_hi(),
                                })
                            }
                            Extern::Explicit(abi, start_span) => {
                                Some(errors::ExternBlockSuggestion::Explicit {
                                    start_span,
                                    end_span: item.span.shrink_to_hi(),
                                    abi: abi.symbol_unescaped,
                                })
                            }
                        },
                    });
                }

                let kind = FnKind::Fn(FnCtxt::Free, &item.vis, &*func);
                self.visit_fn(kind, &item.attrs, item.span, item.id);
            }
            ItemKind::ForeignMod(ForeignMod { extern_span, abi, safety, .. }) => {
                let old_item = mem::replace(&mut self.extern_mod_span, Some(item.span));
                self.visibility_not_permitted(
                    &item.vis,
                    errors::VisibilityNotPermittedNote::IndividualForeignItems,
                );

                if &Safety::Default == safety {
                    if item.span.at_least_rust_2024() {
                        self.dcx().emit_err(errors::MissingUnsafeOnExtern { span: item.span });
                    } else {
                        self.lint_buffer.buffer_lint(
                            MISSING_UNSAFE_ON_EXTERN,
                            item.id,
                            item.span,
                            errors::MissingUnsafeOnExternLint {
                                suggestion: item.span.shrink_to_lo(),
                            },
                        );
                    }
                }

                if abi.is_none() {
                    self.handle_missing_abi(*extern_span, item.id);
                }

                let extern_abi = abi.and_then(|abi| ExternAbi::from_str(abi.symbol.as_str()).ok());
                self.with_in_extern_mod(*safety, extern_abi, |this| {
                    visit::walk_item(this, item);
                });
                self.extern_mod_span = old_item;
            }
            ItemKind::Enum(_, _, def) => {
                for variant in &def.variants {
                    self.visibility_not_permitted(
                        &variant.vis,
                        errors::VisibilityNotPermittedNote::EnumVariant,
                    );
                    for field in variant.data.fields() {
                        self.visibility_not_permitted(
                            &field.vis,
                            errors::VisibilityNotPermittedNote::EnumVariant,
                        );
                    }
                }
                self.with_tilde_const(Some(TildeConstReason::Enum { span: item.span }), |this| {
                    visit::walk_item(this, item)
                });
            }
            ItemKind::Trait(box Trait {
                constness,
                is_auto,
                generics,
                ident,
                bounds,
                items,
                ..
            }) => {
                self.visit_attrs_vis_ident(&item.attrs, &item.vis, ident);
                if *is_auto == IsAuto::Yes {
                    // For why we reject `const auto trait`, see rust-lang/rust#149285.
                    self.deny_const_auto_traits(*constness);
                    // Auto traits cannot have generics, super traits nor contain items.
                    self.deny_generic_params(generics, ident.span);
                    self.deny_super_traits(bounds, ident.span);
                    self.deny_where_clause(&generics.where_clause, ident.span);
                    self.deny_items(items, ident.span);
                }

                // Equivalent of `visit::walk_item` for `ItemKind::Trait` that inserts a bound
                // context for the supertraits.
                let disallowed = matches!(constness, ast::Const::No)
                    .then(|| TildeConstReason::Trait { span: item.span });
                self.with_tilde_const(disallowed, |this| {
                    this.visit_generics(generics);
                    walk_list!(this, visit_param_bound, bounds, BoundKind::SuperTraits)
                });
                self.with_in_trait(item.span, *constness, |this| {
                    walk_list!(this, visit_assoc_item, items, AssocCtxt::Trait);
                });
            }
            ItemKind::TraitAlias(box TraitAlias { constness, generics, bounds, .. }) => {
                let disallowed = matches!(constness, ast::Const::No)
                    .then(|| TildeConstReason::Trait { span: item.span });
                self.with_tilde_const(disallowed, |this| {
                    this.visit_generics(generics);
                    walk_list!(this, visit_param_bound, bounds, BoundKind::SuperTraits)
                });
            }
            ItemKind::Mod(safety, ident, mod_kind) => {
                if let &Safety::Unsafe(span) = safety {
                    self.dcx().emit_err(errors::UnsafeItem { span, kind: "module" });
                }
                // Ensure that `path` attributes on modules are recorded as used (cf. issue #35584).
                if !matches!(mod_kind, ModKind::Loaded(_, Inline::Yes, _))
                    && !attr::contains_name(&item.attrs, sym::path)
                {
                    self.check_mod_file_item_asciionly(*ident);
                }
                visit::walk_item(self, item)
            }
            ItemKind::Struct(ident, generics, vdata) => {
                self.with_tilde_const(Some(TildeConstReason::Struct { span: item.span }), |this| {
                    match vdata {
                        VariantData::Struct { fields, .. } => {
                            this.visit_attrs_vis_ident(&item.attrs, &item.vis, ident);
                            this.visit_generics(generics);
                            walk_list!(this, visit_field_def, fields);
                        }
                        _ => visit::walk_item(this, item),
                    }
                })
            }
            ItemKind::Union(ident, generics, vdata) => {
                if vdata.fields().is_empty() {
                    self.dcx().emit_err(errors::FieldlessUnion { span: item.span });
                }
                self.with_tilde_const(Some(TildeConstReason::Union { span: item.span }), |this| {
                    match vdata {
                        VariantData::Struct { fields, .. } => {
                            this.visit_attrs_vis_ident(&item.attrs, &item.vis, ident);
                            this.visit_generics(generics);
                            walk_list!(this, visit_field_def, fields);
                        }
                        _ => visit::walk_item(this, item),
                    }
                });
            }
            ItemKind::Const(box ConstItem { defaultness, rhs, .. }) => {
                self.check_defaultness(item.span, *defaultness);
                if rhs.is_none() {
                    self.dcx().emit_err(errors::ConstWithoutBody {
                        span: item.span,
                        replace_span: self.ending_semi_or_hi(item.span),
                    });
                }
                visit::walk_item(self, item);
            }
            ItemKind::Static(box StaticItem { expr, safety, .. }) => {
                self.check_item_safety(item.span, *safety);
                if matches!(safety, Safety::Unsafe(_)) {
                    self.dcx().emit_err(errors::UnsafeStatic { span: item.span });
                }

                if expr.is_none() {
                    self.dcx().emit_err(errors::StaticWithoutBody {
                        span: item.span,
                        replace_span: self.ending_semi_or_hi(item.span),
                    });
                }
                visit::walk_item(self, item);
            }
            ItemKind::TyAlias(
                ty_alias @ box TyAlias { defaultness, bounds, after_where_clause, ty, .. },
            ) => {
                self.check_defaultness(item.span, *defaultness);
                if ty.is_none() {
                    self.dcx().emit_err(errors::TyAliasWithoutBody {
                        span: item.span,
                        replace_span: self.ending_semi_or_hi(item.span),
                    });
                }
                self.check_type_no_bounds(bounds, "this context");

                if self.features.lazy_type_alias() {
                    if let Err(err) = self.check_type_alias_where_clause_location(ty_alias) {
                        self.dcx().emit_err(err);
                    }
                } else if after_where_clause.has_where_token {
                    self.dcx().emit_err(errors::WhereClauseAfterTypeAlias {
                        span: after_where_clause.span,
                        help: self.sess.is_nightly_build(),
                    });
                }
                visit::walk_item(self, item);
            }
            _ => visit::walk_item(self, item),
        }

        self.lint_node_id = previous_lint_node_id;
    }

    fn visit_foreign_item(&mut self, fi: &'a ForeignItem) {
        match &fi.kind {
            ForeignItemKind::Fn(box Fn { defaultness, ident, sig, body, .. }) => {
                self.check_defaultness(fi.span, *defaultness);
                self.check_foreign_fn_bodyless(*ident, body.as_deref());
                self.check_foreign_fn_headerless(sig.header);
                self.check_foreign_item_ascii_only(*ident);
                self.check_extern_fn_signature(
                    self.extern_mod_abi.unwrap_or(ExternAbi::FALLBACK),
                    FnCtxt::Foreign,
                    ident,
                    sig,
                );
            }
            ForeignItemKind::TyAlias(box TyAlias {
                defaultness,
                ident,
                generics,
                after_where_clause,
                bounds,
                ty,
                ..
            }) => {
                self.check_defaultness(fi.span, *defaultness);
                self.check_foreign_kind_bodyless(*ident, "type", ty.as_ref().map(|b| b.span));
                self.check_type_no_bounds(bounds, "`extern` blocks");
                self.check_foreign_ty_genericless(generics, after_where_clause);
                self.check_foreign_item_ascii_only(*ident);
            }
            ForeignItemKind::Static(box StaticItem { ident, safety, expr, .. }) => {
                self.check_item_safety(fi.span, *safety);
                self.check_foreign_kind_bodyless(*ident, "static", expr.as_ref().map(|b| b.span));
                self.check_foreign_item_ascii_only(*ident);
            }
            ForeignItemKind::MacCall(..) => {}
        }

        visit::walk_item(self, fi)
    }

    // Mirrors `visit::walk_generic_args`, but tracks relevant state.
    fn visit_generic_args(&mut self, generic_args: &'a GenericArgs) {
        match generic_args {
            GenericArgs::AngleBracketed(data) => {
                self.check_generic_args_before_constraints(data);

                for arg in &data.args {
                    match arg {
                        AngleBracketedArg::Arg(arg) => self.visit_generic_arg(arg),
                        // Associated type bindings such as `Item = impl Debug` in
                        // `Iterator<Item = Debug>` are allowed to contain nested `impl Trait`.
                        AngleBracketedArg::Constraint(constraint) => {
                            self.with_impl_trait(None, |this| {
                                this.visit_assoc_item_constraint(constraint);
                            });
                        }
                    }
                }
            }
            GenericArgs::Parenthesized(data) => {
                walk_list!(self, visit_ty, &data.inputs);
                if let FnRetTy::Ty(ty) = &data.output {
                    // `-> Foo` syntax is essentially an associated type binding,
                    // so it is also allowed to contain nested `impl Trait`.
                    self.with_impl_trait(None, |this| this.visit_ty(ty));
                }
            }
            GenericArgs::ParenthesizedElided(_span) => {}
        }
    }

    fn visit_generics(&mut self, generics: &'a Generics) {
        let mut prev_param_default = None;
        for param in &generics.params {
            match param.kind {
                GenericParamKind::Lifetime => (),
                GenericParamKind::Type { default: Some(_), .. }
                | GenericParamKind::Const { default: Some(_), .. } => {
                    prev_param_default = Some(param.ident.span);
                }
                GenericParamKind::Type { .. } | GenericParamKind::Const { .. } => {
                    if let Some(span) = prev_param_default {
                        self.dcx().emit_err(errors::GenericDefaultTrailing { span });
                        break;
                    }
                }
            }
        }

        validate_generic_param_order(self.dcx(), &generics.params, generics.span);

        for predicate in &generics.where_clause.predicates {
            let span = predicate.span;
            if let WherePredicateKind::EqPredicate(predicate) = &predicate.kind {
                deny_equality_constraints(self, predicate, span, generics);
            }
        }
        walk_list!(self, visit_generic_param, &generics.params);
        for predicate in &generics.where_clause.predicates {
            match &predicate.kind {
                WherePredicateKind::BoundPredicate(bound_pred) => {
                    // This is slightly complicated. Our representation for poly-trait-refs contains a single
                    // binder and thus we only allow a single level of quantification. However,
                    // the syntax of Rust permits quantification in two places in where clauses,
                    // e.g., `T: for <'a> Foo<'a>` and `for <'a, 'b> &'b T: Foo<'a>`. If both are
                    // defined, then error.
                    if !bound_pred.bound_generic_params.is_empty() {
                        for bound in &bound_pred.bounds {
                            match bound {
                                GenericBound::Trait(t) => {
                                    if !t.bound_generic_params.is_empty() {
                                        self.dcx()
                                            .emit_err(errors::NestedLifetimes { span: t.span });
                                    }
                                }
                                GenericBound::Outlives(_) => {}
                                GenericBound::Use(..) => {}
                            }
                        }
                    }
                }
                _ => {}
            }
            self.visit_where_predicate(predicate);
        }
    }

    fn visit_param_bound(&mut self, bound: &'a GenericBound, ctxt: BoundKind) {
        match bound {
            GenericBound::Trait(trait_ref) => {
                match (ctxt, trait_ref.modifiers.constness, trait_ref.modifiers.polarity) {
                    (
                        BoundKind::TraitObject,
                        BoundConstness::Always(_),
                        BoundPolarity::Positive,
                    ) => {
                        self.dcx().emit_err(errors::ConstBoundTraitObject { span: trait_ref.span });
                    }
                    (_, BoundConstness::Maybe(span), BoundPolarity::Positive)
                        if let Some(reason) = self.disallow_tilde_const =>
                    {
                        self.dcx().emit_err(errors::TildeConstDisallowed { span, reason });
                    }
                    _ => {}
                }

                // Negative trait bounds are not allowed to have associated constraints
                if let BoundPolarity::Negative(_) = trait_ref.modifiers.polarity
                    && let Some(segment) = trait_ref.trait_ref.path.segments.last()
                {
                    match segment.args.as_deref() {
                        Some(ast::GenericArgs::AngleBracketed(args)) => {
                            for arg in &args.args {
                                if let ast::AngleBracketedArg::Constraint(constraint) = arg {
                                    self.dcx().emit_err(errors::ConstraintOnNegativeBound {
                                        span: constraint.span,
                                    });
                                }
                            }
                        }
                        // The lowered form of parenthesized generic args contains an associated type binding.
                        Some(ast::GenericArgs::Parenthesized(args)) => {
                            self.dcx().emit_err(errors::NegativeBoundWithParentheticalNotation {
                                span: args.span,
                            });
                        }
                        Some(ast::GenericArgs::ParenthesizedElided(_)) | None => {}
                    }
                }
            }
            GenericBound::Outlives(_) => {}
            GenericBound::Use(_, span) => match ctxt {
                BoundKind::Impl => {}
                BoundKind::Bound | BoundKind::TraitObject | BoundKind::SuperTraits => {
                    self.dcx().emit_err(errors::PreciseCapturingNotAllowedHere {
                        loc: ctxt.descr(),
                        span: *span,
                    });
                }
            },
        }

        visit::walk_param_bound(self, bound)
    }

    fn visit_fn(&mut self, fk: FnKind<'a>, attrs: &AttrVec, span: Span, id: NodeId) {
        // Only associated `fn`s can have `self` parameters.
        let self_semantic = match fk.ctxt() {
            Some(FnCtxt::Assoc(_)) => SelfSemantic::Yes,
            _ => SelfSemantic::No,
        };
        self.check_fn_decl(fk.decl(), self_semantic);

        if let Some(&FnHeader { safety, .. }) = fk.header() {
            self.check_item_safety(span, safety);
        }

        if let FnKind::Fn(ctxt, _, fun) = fk
            && let Extern::Explicit(str_lit, _) = fun.sig.header.ext
            && let Ok(abi) = ExternAbi::from_str(str_lit.symbol.as_str())
        {
            self.check_extern_fn_signature(abi, ctxt, &fun.ident, &fun.sig);
        }

        self.check_c_variadic_type(fk, attrs);

        // Functions cannot both be `const async` or `const gen`
        if let Some(&FnHeader {
            constness: Const::Yes(const_span),
            coroutine_kind: Some(coroutine_kind),
            ..
        }) = fk.header()
        {
            self.dcx().emit_err(errors::ConstAndCoroutine {
                spans: vec![coroutine_kind.span(), const_span],
                const_span,
                coroutine_span: coroutine_kind.span(),
                coroutine_kind: coroutine_kind.as_str(),
                span,
            });
        }

        if let FnKind::Fn(
            _,
            _,
            Fn {
                sig: FnSig { header: FnHeader { ext: Extern::Implicit(extern_span), .. }, .. },
                ..
            },
        ) = fk
        {
            self.handle_missing_abi(*extern_span, id);
        }

        // Functions without bodies cannot have patterns.
        if let FnKind::Fn(ctxt, _, Fn { body: None, sig, .. }) = fk {
            Self::check_decl_no_pat(&sig.decl, |span, ident, mut_ident| {
                if mut_ident && matches!(ctxt, FnCtxt::Assoc(_)) {
                    if let Some(ident) = ident {
                        self.lint_buffer.buffer_lint(
                            PATTERNS_IN_FNS_WITHOUT_BODY,
                            id,
                            span,
                            BuiltinLintDiag::PatternsInFnsWithoutBody {
                                span,
                                ident,
                                is_foreign: matches!(ctxt, FnCtxt::Foreign),
                            },
                        )
                    }
                } else {
                    match ctxt {
                        FnCtxt::Foreign => self.dcx().emit_err(errors::PatternInForeign { span }),
                        _ => self.dcx().emit_err(errors::PatternInBodiless { span }),
                    };
                }
            });
        }

        let tilde_const_allowed =
            matches!(fk.header(), Some(FnHeader { constness: ast::Const::Yes(_), .. }))
                || matches!(fk.ctxt(), Some(FnCtxt::Assoc(_)))
                    && self
                        .outer_trait_or_trait_impl
                        .as_ref()
                        .and_then(TraitOrImpl::constness)
                        .is_some();

        let disallowed = (!tilde_const_allowed).then(|| match fk {
            FnKind::Fn(_, _, f) => TildeConstReason::Function { ident: f.ident.span },
            FnKind::Closure(..) => TildeConstReason::Closure,
        });
        self.with_tilde_const(disallowed, |this| visit::walk_fn(this, fk));
    }

    fn visit_assoc_item(&mut self, item: &'a AssocItem, ctxt: AssocCtxt) {
        if let Some(ident) = item.kind.ident()
            && attr::contains_name(&item.attrs, sym::no_mangle)
        {
            self.check_nomangle_item_asciionly(ident, item.span);
        }

        if ctxt == AssocCtxt::Trait || self.outer_trait_or_trait_impl.is_none() {
            self.check_defaultness(item.span, item.kind.defaultness());
        }

        if let AssocCtxt::Impl { .. } = ctxt {
            match &item.kind {
                AssocItemKind::Const(box ConstItem { rhs: None, .. }) => {
                    self.dcx().emit_err(errors::AssocConstWithoutBody {
                        span: item.span,
                        replace_span: self.ending_semi_or_hi(item.span),
                    });
                }
                AssocItemKind::Fn(box Fn { body, .. }) => {
                    if body.is_none() && !self.is_sdylib_interface {
                        self.dcx().emit_err(errors::AssocFnWithoutBody {
                            span: item.span,
                            replace_span: self.ending_semi_or_hi(item.span),
                        });
                    }
                }
                AssocItemKind::Type(box TyAlias { bounds, ty, .. }) => {
                    if ty.is_none() {
                        self.dcx().emit_err(errors::AssocTypeWithoutBody {
                            span: item.span,
                            replace_span: self.ending_semi_or_hi(item.span),
                        });
                    }
                    self.check_type_no_bounds(bounds, "`impl`s");
                }
                _ => {}
            }
        }

        if let AssocItemKind::Type(ty_alias) = &item.kind
            && let Err(err) = self.check_type_alias_where_clause_location(ty_alias)
        {
            let sugg = match err.sugg {
                errors::WhereClauseBeforeTypeAliasSugg::Remove { .. } => None,
                errors::WhereClauseBeforeTypeAliasSugg::Move { snippet, right, .. } => {
                    Some((right, snippet))
                }
            };
            self.lint_buffer.buffer_lint(
                DEPRECATED_WHERE_CLAUSE_LOCATION,
                item.id,
                err.span,
                BuiltinLintDiag::DeprecatedWhereclauseLocation(err.span, sugg),
            );
        }

        match &self.outer_trait_or_trait_impl {
            Some(parent @ (TraitOrImpl::Trait { .. } | TraitOrImpl::TraitImpl { .. })) => {
                self.visibility_not_permitted(
                    &item.vis,
                    errors::VisibilityNotPermittedNote::TraitImpl,
                );
                if let AssocItemKind::Fn(box Fn { sig, .. }) = &item.kind {
                    self.check_trait_fn_not_const(sig.header.constness, parent);
                    self.check_async_fn_in_const_trait_or_impl(sig, parent);
                }
            }
            Some(parent @ TraitOrImpl::Impl { constness }) => {
                if let AssocItemKind::Fn(box Fn { sig, .. }) = &item.kind {
                    self.check_impl_fn_not_const(sig.header.constness, *constness);
                    self.check_async_fn_in_const_trait_or_impl(sig, parent);
                }
            }
            None => {}
        }

        if let AssocItemKind::Const(ci) = &item.kind {
            self.check_item_named(ci.ident, "const");
        }

        let parent_is_const =
            self.outer_trait_or_trait_impl.as_ref().and_then(TraitOrImpl::constness).is_some();

        match &item.kind {
            AssocItemKind::Fn(func)
                if parent_is_const
                    || ctxt == AssocCtxt::Trait
                    || matches!(func.sig.header.constness, Const::Yes(_)) =>
            {
                self.visit_attrs_vis_ident(&item.attrs, &item.vis, &func.ident);
                let kind = FnKind::Fn(FnCtxt::Assoc(ctxt), &item.vis, &*func);
                self.visit_fn(kind, &item.attrs, item.span, item.id);
            }
            AssocItemKind::Type(_) => {
                let disallowed = (!parent_is_const).then(|| match self.outer_trait_or_trait_impl {
                    Some(TraitOrImpl::Trait { .. }) => {
                        TildeConstReason::TraitAssocTy { span: item.span }
                    }
                    Some(TraitOrImpl::TraitImpl { .. }) => {
                        TildeConstReason::TraitImplAssocTy { span: item.span }
                    }
                    Some(TraitOrImpl::Impl { .. }) | None => {
                        TildeConstReason::InherentAssocTy { span: item.span }
                    }
                });
                self.with_tilde_const(disallowed, |this| {
                    this.with_in_trait_or_impl(None, |this| {
                        visit::walk_assoc_item(this, item, ctxt)
                    })
                })
            }
            _ => self.with_in_trait_or_impl(None, |this| visit::walk_assoc_item(this, item, ctxt)),
        }
    }

    fn visit_anon_const(&mut self, anon_const: &'a AnonConst) {
        self.with_tilde_const(
            Some(TildeConstReason::AnonConst { span: anon_const.value.span }),
            |this| visit::walk_anon_const(this, anon_const),
        )
    }
}

/// When encountering an equality constraint in a `where` clause, emit an error. If the code seems
/// like it's setting an associated type, provide an appropriate suggestion.
fn deny_equality_constraints(
    this: &AstValidator<'_>,
    predicate: &WhereEqPredicate,
    predicate_span: Span,
    generics: &Generics,
) {
    let mut err = errors::EqualityInWhere { span: predicate_span, assoc: None, assoc2: None };

    // Given `<A as Foo>::Bar = RhsTy`, suggest `A: Foo<Bar = RhsTy>`.
    if let TyKind::Path(Some(qself), full_path) = &predicate.lhs_ty.kind
        && let TyKind::Path(None, path) = &qself.ty.kind
        && let [PathSegment { ident, args: None, .. }] = &path.segments[..]
    {
        for param in &generics.params {
            if param.ident == *ident
                && let [PathSegment { ident, args, .. }] = &full_path.segments[qself.position..]
            {
                // Make a new `Path` from `foo::Bar` to `Foo<Bar = RhsTy>`.
                let mut assoc_path = full_path.clone();
                // Remove `Bar` from `Foo::Bar`.
                assoc_path.segments.pop();
                let len = assoc_path.segments.len() - 1;
                let gen_args = args.as_deref().cloned();
                // Build `<Bar = RhsTy>`.
                let arg = AngleBracketedArg::Constraint(AssocItemConstraint {
                    id: rustc_ast::node_id::DUMMY_NODE_ID,
                    ident: *ident,
                    gen_args,
                    kind: AssocItemConstraintKind::Equality {
                        term: predicate.rhs_ty.clone().into(),
                    },
                    span: ident.span,
                });
                // Add `<Bar = RhsTy>` to `Foo`.
                match &mut assoc_path.segments[len].args {
                    Some(args) => match args.deref_mut() {
                        GenericArgs::Parenthesized(_) | GenericArgs::ParenthesizedElided(..) => {
                            continue;
                        }
                        GenericArgs::AngleBracketed(args) => {
                            args.args.push(arg);
                        }
                    },
                    empty_args => {
                        *empty_args = Some(
                            AngleBracketedArgs { span: ident.span, args: thin_vec![arg] }.into(),
                        );
                    }
                }
                err.assoc = Some(errors::AssociatedSuggestion {
                    span: predicate_span,
                    ident: *ident,
                    param: param.ident,
                    path: pprust::path_to_string(&assoc_path),
                })
            }
        }
    }

    let mut suggest =
        |poly: &PolyTraitRef, potential_assoc: &PathSegment, predicate: &WhereEqPredicate| {
            if let [trait_segment] = &poly.trait_ref.path.segments[..] {
                let assoc = pprust::path_to_string(&ast::Path::from_ident(potential_assoc.ident));
                let ty = pprust::ty_to_string(&predicate.rhs_ty);
                let (args, span) = match &trait_segment.args {
                    Some(args) => match args.deref() {
                        ast::GenericArgs::AngleBracketed(args) => {
                            let Some(arg) = args.args.last() else {
                                return;
                            };
                            (format!(", {assoc} = {ty}"), arg.span().shrink_to_hi())
                        }
                        _ => return,
                    },
                    None => (format!("<{assoc} = {ty}>"), trait_segment.span().shrink_to_hi()),
                };
                let removal_span = if generics.where_clause.predicates.len() == 1 {
                    // We're removing th eonly where bound left, remove the whole thing.
                    generics.where_clause.span
                } else {
                    let mut span = predicate_span;
                    let mut prev_span: Option<Span> = None;
                    let mut preds = generics.where_clause.predicates.iter().peekable();
                    // Find the predicate that shouldn't have been in the where bound list.
                    while let Some(pred) = preds.next() {
                        if let WherePredicateKind::EqPredicate(_) = pred.kind
                            && pred.span == predicate_span
                        {
                            if let Some(next) = preds.peek() {
                                // This is the first predicate, remove the trailing comma as well.
                                span = span.with_hi(next.span.lo());
                            } else if let Some(prev_span) = prev_span {
                                // Remove the previous comma as well.
                                span = span.with_lo(prev_span.hi());
                            }
                        }
                        prev_span = Some(pred.span);
                    }
                    span
                };
                err.assoc2 = Some(errors::AssociatedSuggestion2 {
                    span,
                    args,
                    predicate: removal_span,
                    trait_segment: trait_segment.ident,
                    potential_assoc: potential_assoc.ident,
                });
            }
        };

    if let TyKind::Path(None, full_path) = &predicate.lhs_ty.kind {
        // Given `A: Foo, Foo::Bar = RhsTy`, suggest `A: Foo<Bar = RhsTy>`.
        for bounds in generics.params.iter().map(|p| &p.bounds).chain(
            generics.where_clause.predicates.iter().filter_map(|pred| match &pred.kind {
                WherePredicateKind::BoundPredicate(p) => Some(&p.bounds),
                _ => None,
            }),
        ) {
            for bound in bounds {
                if let GenericBound::Trait(poly) = bound
                    && poly.modifiers == TraitBoundModifiers::NONE
                {
                    if full_path.segments[..full_path.segments.len() - 1]
                        .iter()
                        .map(|segment| segment.ident.name)
                        .zip(poly.trait_ref.path.segments.iter().map(|segment| segment.ident.name))
                        .all(|(a, b)| a == b)
                        && let Some(potential_assoc) = full_path.segments.last()
                    {
                        suggest(poly, potential_assoc, predicate);
                    }
                }
            }
        }
        // Given `A: Foo, A::Bar = RhsTy`, suggest `A: Foo<Bar = RhsTy>`.
        if let [potential_param, potential_assoc] = &full_path.segments[..] {
            for (ident, bounds) in generics.params.iter().map(|p| (p.ident, &p.bounds)).chain(
                generics.where_clause.predicates.iter().filter_map(|pred| match &pred.kind {
                    WherePredicateKind::BoundPredicate(p)
                        if let ast::TyKind::Path(None, path) = &p.bounded_ty.kind
                            && let [segment] = &path.segments[..] =>
                    {
                        Some((segment.ident, &p.bounds))
                    }
                    _ => None,
                }),
            ) {
                if ident == potential_param.ident {
                    for bound in bounds {
                        if let ast::GenericBound::Trait(poly) = bound
                            && poly.modifiers == TraitBoundModifiers::NONE
                        {
                            suggest(poly, potential_assoc, predicate);
                        }
                    }
                }
            }
        }
    }
    this.dcx().emit_err(err);
}

pub fn check_crate(
    sess: &Session,
    features: &Features,
    krate: &Crate,
    is_sdylib_interface: bool,
    lints: &mut LintBuffer,
) -> bool {
    let mut validator = AstValidator {
        sess,
        features,
        extern_mod_span: None,
        outer_trait_or_trait_impl: None,
        has_proc_macro_decls: false,
        outer_impl_trait_span: None,
        disallow_tilde_const: Some(TildeConstReason::Item),
        extern_mod_safety: None,
        extern_mod_abi: None,
        lint_node_id: CRATE_NODE_ID,
        is_sdylib_interface,
        lint_buffer: lints,
    };
    visit::walk_crate(&mut validator, krate);

    validator.has_proc_macro_decls
}