rustc_lint/

internal.rs

//! Some lints that are only useful in the compiler or crates that use compiler internals, such as
//! Clippy.

use rustc_hir::attrs::AttributeKind;
use rustc_hir::def::Res;
use rustc_hir::def_id::DefId;
use rustc_hir::{Expr, ExprKind, HirId, find_attr};
use rustc_middle::ty::{self, GenericArgsRef, PredicatePolarity};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::hygiene::{ExpnKind, MacroKind};
use rustc_span::{Span, sym};
use {rustc_ast as ast, rustc_hir as hir};

use crate::lints::{
    BadOptAccessDiag, DefaultHashTypesDiag, ImplicitSysrootCrateImportDiag, LintPassByHand,
    NonGlobImportTypeIrInherent, QueryInstability, QueryUntracked, SpanUseEqCtxtDiag,
    SymbolInternStringLiteralDiag, TyQualified, TykindDiag, TykindKind, TypeIrDirectUse,
    TypeIrInherentUsage, TypeIrTraitUsage,
};
use crate::{EarlyContext, EarlyLintPass, LateContext, LateLintPass, LintContext};

declare_tool_lint! {
    /// The `default_hash_type` lint detects use of [`std::collections::HashMap`] and
    /// [`std::collections::HashSet`], suggesting the use of `FxHashMap`/`FxHashSet`.
    ///
    /// This can help as `FxHasher` can perform better than the default hasher. DOS protection is
    /// not required as input is assumed to be trusted.
    pub rustc::DEFAULT_HASH_TYPES,
    Allow,
    "forbid HashMap and HashSet and suggest the FxHash* variants",
    report_in_external_macro: true
}

declare_lint_pass!(DefaultHashTypes => [DEFAULT_HASH_TYPES]);

impl LateLintPass<'_> for DefaultHashTypes {
    fn check_path(&mut self, cx: &LateContext<'_>, path: &hir::Path<'_>, hir_id: HirId) {
        let Res::Def(rustc_hir::def::DefKind::Struct, def_id) = path.res else { return };
        if matches!(
            cx.tcx.hir_node(hir_id),
            hir::Node::Item(hir::Item { kind: hir::ItemKind::Use(..), .. })
        ) {
            // Don't lint imports, only actual usages.
            return;
        }
        let preferred = match cx.tcx.get_diagnostic_name(def_id) {
            Some(sym::HashMap) => "FxHashMap",
            Some(sym::HashSet) => "FxHashSet",
            _ => return,
        };
        cx.emit_span_lint(
            DEFAULT_HASH_TYPES,
            path.span,
            DefaultHashTypesDiag { preferred, used: cx.tcx.item_name(def_id) },
        );
    }
}

declare_tool_lint! {
    /// The `potential_query_instability` lint detects use of methods which can lead to
    /// potential query instability, such as iterating over a `HashMap`.
    ///
    /// Due to the [incremental compilation](https://rustc-dev-guide.rust-lang.org/queries/incremental-compilation.html) model,
    /// queries must return deterministic, stable results. `HashMap` iteration order can change
    /// between compilations, and will introduce instability if query results expose the order.
    pub rustc::POTENTIAL_QUERY_INSTABILITY,
    Allow,
    "require explicit opt-in when using potentially unstable methods or functions",
    report_in_external_macro: true
}

declare_tool_lint! {
    /// The `untracked_query_information` lint detects use of methods which leak information not
    /// tracked by the query system, such as whether a `Steal<T>` value has already been stolen. In
    /// order not to break incremental compilation, such methods must be used very carefully or not
    /// at all.
    pub rustc::UNTRACKED_QUERY_INFORMATION,
    Allow,
    "require explicit opt-in when accessing information not tracked by the query system",
    report_in_external_macro: true
}

declare_lint_pass!(QueryStability => [POTENTIAL_QUERY_INSTABILITY, UNTRACKED_QUERY_INFORMATION]);

impl<'tcx> LateLintPass<'tcx> for QueryStability {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
        if let Some((callee_def_id, span, generic_args, _recv, _args)) =
            get_callee_span_generic_args_and_args(cx, expr)
            && let Ok(Some(instance)) =
                ty::Instance::try_resolve(cx.tcx, cx.typing_env(), callee_def_id, generic_args)
        {
            let def_id = instance.def_id();
            if find_attr!(cx.tcx.get_all_attrs(def_id), AttributeKind::RustcLintQueryInstability) {
                cx.emit_span_lint(
                    POTENTIAL_QUERY_INSTABILITY,
                    span,
                    QueryInstability { query: cx.tcx.item_name(def_id) },
                );
            } else if has_unstable_into_iter_predicate(cx, callee_def_id, generic_args) {
                let call_span = span.with_hi(expr.span.hi());
                cx.emit_span_lint(
                    POTENTIAL_QUERY_INSTABILITY,
                    call_span,
                    QueryInstability { query: sym::into_iter },
                );
            }

            if find_attr!(
                cx.tcx.get_all_attrs(def_id),
                AttributeKind::RustcLintUntrackedQueryInformation
            ) {
                cx.emit_span_lint(
                    UNTRACKED_QUERY_INFORMATION,
                    span,
                    QueryUntracked { method: cx.tcx.item_name(def_id) },
                );
            }
        }
    }
}

fn has_unstable_into_iter_predicate<'tcx>(
    cx: &LateContext<'tcx>,
    callee_def_id: DefId,
    generic_args: GenericArgsRef<'tcx>,
) -> bool {
    let Some(into_iterator_def_id) = cx.tcx.get_diagnostic_item(sym::IntoIterator) else {
        return false;
    };
    let Some(into_iter_fn_def_id) = cx.tcx.lang_items().into_iter_fn() else {
        return false;
    };
    let predicates = cx.tcx.predicates_of(callee_def_id).instantiate(cx.tcx, generic_args);
    for (predicate, _) in predicates {
        let Some(trait_pred) = predicate.as_trait_clause() else {
            continue;
        };
        if trait_pred.def_id() != into_iterator_def_id
            || trait_pred.polarity() != PredicatePolarity::Positive
        {
            continue;
        }
        // `IntoIterator::into_iter` has no additional method args.
        let into_iter_fn_args =
            cx.tcx.instantiate_bound_regions_with_erased(trait_pred).trait_ref.args;
        let Ok(Some(instance)) = ty::Instance::try_resolve(
            cx.tcx,
            cx.typing_env(),
            into_iter_fn_def_id,
            into_iter_fn_args,
        ) else {
            continue;
        };
        // Does the input type's `IntoIterator` implementation have the
        // `rustc_lint_query_instability` attribute on its `into_iter` method?
        if find_attr!(
            cx.tcx.get_all_attrs(instance.def_id()),
            AttributeKind::RustcLintQueryInstability
        ) {
            return true;
        }
    }
    false
}

/// Checks whether an expression is a function or method call and, if so, returns its `DefId`,
/// `Span`, `GenericArgs`, and arguments. This is a slight augmentation of a similarly named Clippy
/// function, `get_callee_generic_args_and_args`.
fn get_callee_span_generic_args_and_args<'tcx>(
    cx: &LateContext<'tcx>,
    expr: &'tcx Expr<'tcx>,
) -> Option<(DefId, Span, GenericArgsRef<'tcx>, Option<&'tcx Expr<'tcx>>, &'tcx [Expr<'tcx>])> {
    if let ExprKind::Call(callee, args) = expr.kind
        && let callee_ty = cx.typeck_results().expr_ty(callee)
        && let ty::FnDef(callee_def_id, generic_args) = callee_ty.kind()
    {
        return Some((*callee_def_id, callee.span, generic_args, None, args));
    }
    if let ExprKind::MethodCall(segment, recv, args, _) = expr.kind
        && let Some(method_def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id)
    {
        let generic_args = cx.typeck_results().node_args(expr.hir_id);
        return Some((method_def_id, segment.ident.span, generic_args, Some(recv), args));
    }
    None
}

declare_tool_lint! {
    /// The `usage_of_ty_tykind` lint detects usages of `ty::TyKind::<kind>`,
    /// where `ty::<kind>` would suffice.
    pub rustc::USAGE_OF_TY_TYKIND,
    Allow,
    "usage of `ty::TyKind` outside of the `ty::sty` module",
    report_in_external_macro: true
}

declare_tool_lint! {
    /// The `usage_of_qualified_ty` lint detects usages of `ty::TyKind`,
    /// where `Ty` should be used instead.
    pub rustc::USAGE_OF_QUALIFIED_TY,
    Allow,
    "using `ty::{Ty,TyCtxt}` instead of importing it",
    report_in_external_macro: true
}

declare_lint_pass!(TyTyKind => [
    USAGE_OF_TY_TYKIND,
    USAGE_OF_QUALIFIED_TY,
]);

impl<'tcx> LateLintPass<'tcx> for TyTyKind {
    fn check_path(
        &mut self,
        cx: &LateContext<'tcx>,
        path: &rustc_hir::Path<'tcx>,
        _: rustc_hir::HirId,
    ) {
        if let Some(segment) = path.segments.iter().nth_back(1)
            && lint_ty_kind_usage(cx, &segment.res)
        {
            let span =
                path.span.with_hi(segment.args.map_or(segment.ident.span, |a| a.span_ext).hi());
            cx.emit_span_lint(USAGE_OF_TY_TYKIND, path.span, TykindKind { suggestion: span });
        }
    }

    fn check_ty(&mut self, cx: &LateContext<'_>, ty: &'tcx hir::Ty<'tcx, hir::AmbigArg>) {
        match &ty.kind {
            hir::TyKind::Path(hir::QPath::Resolved(_, path)) => {
                if lint_ty_kind_usage(cx, &path.res) {
                    let span = match cx.tcx.parent_hir_node(ty.hir_id) {
                        hir::Node::PatExpr(hir::PatExpr {
                            kind: hir::PatExprKind::Path(qpath),
                            ..
                        })
                        | hir::Node::Pat(hir::Pat {
                            kind:
                                hir::PatKind::TupleStruct(qpath, ..) | hir::PatKind::Struct(qpath, ..),
                            ..
                        })
                        | hir::Node::Expr(
                            hir::Expr { kind: hir::ExprKind::Path(qpath), .. }
                            | &hir::Expr { kind: hir::ExprKind::Struct(qpath, ..), .. },
                        ) => {
                            if let hir::QPath::TypeRelative(qpath_ty, ..) = qpath
                                && qpath_ty.hir_id == ty.hir_id
                            {
                                Some(path.span)
                            } else {
                                None
                            }
                        }
                        _ => None,
                    };

                    match span {
                        Some(span) => {
                            cx.emit_span_lint(
                                USAGE_OF_TY_TYKIND,
                                path.span,
                                TykindKind { suggestion: span },
                            );
                        }
                        None => cx.emit_span_lint(USAGE_OF_TY_TYKIND, path.span, TykindDiag),
                    }
                } else if !ty.span.from_expansion()
                    && path.segments.len() > 1
                    && let Some(ty) = is_ty_or_ty_ctxt(cx, path)
                {
                    cx.emit_span_lint(
                        USAGE_OF_QUALIFIED_TY,
                        path.span,
                        TyQualified { ty, suggestion: path.span },
                    );
                }
            }
            _ => {}
        }
    }
}

fn lint_ty_kind_usage(cx: &LateContext<'_>, res: &Res) -> bool {
    if let Some(did) = res.opt_def_id() {
        cx.tcx.is_diagnostic_item(sym::TyKind, did) || cx.tcx.is_diagnostic_item(sym::IrTyKind, did)
    } else {
        false
    }
}

fn is_ty_or_ty_ctxt(cx: &LateContext<'_>, path: &hir::Path<'_>) -> Option<String> {
    match &path.res {
        Res::Def(_, def_id) => {
            if let Some(name @ (sym::Ty | sym::TyCtxt)) = cx.tcx.get_diagnostic_name(*def_id) {
                return Some(format!("{}{}", name, gen_args(path.segments.last().unwrap())));
            }
        }
        // Only lint on `&Ty` and `&TyCtxt` if it is used outside of a trait.
        Res::SelfTyAlias { alias_to: did, is_trait_impl: false, .. } => {
            if let ty::Adt(adt, args) = cx.tcx.type_of(did).instantiate_identity().kind()
                && let Some(name @ (sym::Ty | sym::TyCtxt)) = cx.tcx.get_diagnostic_name(adt.did())
            {
                return Some(format!("{}<{}>", name, args[0]));
            }
        }
        _ => (),
    }

    None
}

fn gen_args(segment: &hir::PathSegment<'_>) -> String {
    if let Some(args) = &segment.args {
        let lifetimes = args
            .args
            .iter()
            .filter_map(|arg| {
                if let hir::GenericArg::Lifetime(lt) = arg {
                    Some(lt.ident.to_string())
                } else {
                    None
                }
            })
            .collect::<Vec<_>>();

        if !lifetimes.is_empty() {
            return format!("<{}>", lifetimes.join(", "));
        }
    }

    String::new()
}

declare_tool_lint! {
    /// The `non_glob_import_of_type_ir_inherent_item` lint detects
    /// non-glob imports of module `rustc_type_ir::inherent`.
    pub rustc::NON_GLOB_IMPORT_OF_TYPE_IR_INHERENT,
    Allow,
    "non-glob import of `rustc_type_ir::inherent`",
    report_in_external_macro: true
}

declare_tool_lint! {
    /// The `usage_of_type_ir_inherent` lint detects usage of `rustc_type_ir::inherent`.
    ///
    /// This module should only be used within the trait solver.
    pub rustc::USAGE_OF_TYPE_IR_INHERENT,
    Allow,
    "usage `rustc_type_ir::inherent` outside of trait system",
    report_in_external_macro: true
}

declare_tool_lint! {
    /// The `usage_of_type_ir_traits` lint detects usage of `rustc_type_ir::Interner`,
    /// or `rustc_infer::InferCtxtLike`.
    ///
    /// Methods of this trait should only be used within the type system abstraction layer,
    /// and in the generic next trait solver implementation. Look for an analogously named
    /// method on `TyCtxt` or `InferCtxt` (respectively).
    pub rustc::USAGE_OF_TYPE_IR_TRAITS,
    Allow,
    "usage `rustc_type_ir`-specific abstraction traits outside of trait system",
    report_in_external_macro: true
}
declare_tool_lint! {
    /// The `direct_use_of_rustc_type_ir` lint detects usage of `rustc_type_ir`.
    ///
    /// This module should only be used within the trait solver and some desirable
    /// crates like rustc_middle.
    pub rustc::DIRECT_USE_OF_RUSTC_TYPE_IR,
    Allow,
    "usage `rustc_type_ir` abstraction outside of trait system",
    report_in_external_macro: true
}

declare_lint_pass!(TypeIr => [DIRECT_USE_OF_RUSTC_TYPE_IR, NON_GLOB_IMPORT_OF_TYPE_IR_INHERENT, USAGE_OF_TYPE_IR_INHERENT, USAGE_OF_TYPE_IR_TRAITS]);

impl<'tcx> LateLintPass<'tcx> for TypeIr {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
        let res_def_id = match expr.kind {
            hir::ExprKind::Path(hir::QPath::Resolved(_, path)) => path.res.opt_def_id(),
            hir::ExprKind::Path(hir::QPath::TypeRelative(..)) | hir::ExprKind::MethodCall(..) => {
                cx.typeck_results().type_dependent_def_id(expr.hir_id)
            }
            _ => return,
        };
        let Some(res_def_id) = res_def_id else {
            return;
        };
        if let Some(assoc_item) = cx.tcx.opt_associated_item(res_def_id)
            && let Some(trait_def_id) = assoc_item.trait_container(cx.tcx)
            && (cx.tcx.is_diagnostic_item(sym::type_ir_interner, trait_def_id)
                | cx.tcx.is_diagnostic_item(sym::type_ir_infer_ctxt_like, trait_def_id))
        {
            cx.emit_span_lint(USAGE_OF_TYPE_IR_TRAITS, expr.span, TypeIrTraitUsage);
        }
    }

    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'tcx>) {
        let rustc_hir::ItemKind::Use(path, kind) = item.kind else { return };

        let is_mod_inherent = |res: Res| {
            res.opt_def_id()
                .is_some_and(|def_id| cx.tcx.is_diagnostic_item(sym::type_ir_inherent, def_id))
        };

        // Path segments except for the final.
        if let Some(seg) = path.segments.iter().find(|seg| is_mod_inherent(seg.res)) {
            cx.emit_span_lint(USAGE_OF_TYPE_IR_INHERENT, seg.ident.span, TypeIrInherentUsage);
        }
        // Final path resolutions, like `use rustc_type_ir::inherent`
        else if let Some(type_ns) = path.res.type_ns
            && is_mod_inherent(type_ns)
        {
            cx.emit_span_lint(
                USAGE_OF_TYPE_IR_INHERENT,
                path.segments.last().unwrap().ident.span,
                TypeIrInherentUsage,
            );
        }

        let (lo, hi, snippet) = match path.segments {
            [.., penultimate, segment] if is_mod_inherent(penultimate.res) => {
                (segment.ident.span, item.kind.ident().unwrap().span, "*")
            }
            [.., segment]
                if let Some(type_ns) = path.res.type_ns
                    && is_mod_inherent(type_ns)
                    && let rustc_hir::UseKind::Single(ident) = kind =>
            {
                let (lo, snippet) =
                    match cx.tcx.sess.source_map().span_to_snippet(path.span).as_deref() {
                        Ok("self") => (path.span, "*"),
                        _ => (segment.ident.span.shrink_to_hi(), "::*"),
                    };
                (lo, if segment.ident == ident { lo } else { ident.span }, snippet)
            }
            _ => return,
        };
        cx.emit_span_lint(
            NON_GLOB_IMPORT_OF_TYPE_IR_INHERENT,
            path.span,
            NonGlobImportTypeIrInherent { suggestion: lo.eq_ctxt(hi).then(|| lo.to(hi)), snippet },
        );
    }

    fn check_path(
        &mut self,
        cx: &LateContext<'tcx>,
        path: &rustc_hir::Path<'tcx>,
        _: rustc_hir::HirId,
    ) {
        if let Some(seg) = path.segments.iter().find(|seg| {
            seg.res
                .opt_def_id()
                .is_some_and(|def_id| cx.tcx.is_diagnostic_item(sym::type_ir, def_id))
        }) {
            cx.emit_span_lint(DIRECT_USE_OF_RUSTC_TYPE_IR, seg.ident.span, TypeIrDirectUse);
        }
    }
}

declare_tool_lint! {
    /// The `lint_pass_impl_without_macro` detects manual implementations of a lint
    /// pass, without using [`declare_lint_pass`] or [`impl_lint_pass`].
    pub rustc::LINT_PASS_IMPL_WITHOUT_MACRO,
    Allow,
    "`impl LintPass` without the `declare_lint_pass!` or `impl_lint_pass!` macros"
}

declare_lint_pass!(LintPassImpl => [LINT_PASS_IMPL_WITHOUT_MACRO]);

impl EarlyLintPass for LintPassImpl {
    fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
        if let ast::ItemKind::Impl(ast::Impl { of_trait: Some(of_trait), .. }) = &item.kind
            && let Some(last) = of_trait.trait_ref.path.segments.last()
            && last.ident.name == sym::LintPass
        {
            let expn_data = of_trait.trait_ref.path.span.ctxt().outer_expn_data();
            let call_site = expn_data.call_site;
            if expn_data.kind != ExpnKind::Macro(MacroKind::Bang, sym::impl_lint_pass)
                && call_site.ctxt().outer_expn_data().kind
                    != ExpnKind::Macro(MacroKind::Bang, sym::declare_lint_pass)
            {
                cx.emit_span_lint(
                    LINT_PASS_IMPL_WITHOUT_MACRO,
                    of_trait.trait_ref.path.span,
                    LintPassByHand,
                );
            }
        }
    }
}

declare_tool_lint! {
    /// The `bad_opt_access` lint detects accessing options by field instead of
    /// the wrapper function.
    pub rustc::BAD_OPT_ACCESS,
    Deny,
    "prevent using options by field access when there is a wrapper function",
    report_in_external_macro: true
}

declare_lint_pass!(BadOptAccess => [BAD_OPT_ACCESS]);

impl LateLintPass<'_> for BadOptAccess {
    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &hir::Expr<'_>) {
        let hir::ExprKind::Field(base, target) = expr.kind else { return };
        let Some(adt_def) = cx.typeck_results().expr_ty(base).ty_adt_def() else { return };
        // Skip types without `#[rustc_lint_opt_ty]` - only so that the rest of the lint can be
        // avoided.
        if !find_attr!(cx.tcx.get_all_attrs(adt_def.did()), AttributeKind::RustcLintOptTy) {
            return;
        }

        for field in adt_def.all_fields() {
            if field.name == target.name
                && let Some(lint_message) = find_attr!(cx.tcx.get_all_attrs(field.did), AttributeKind::RustcLintOptDenyFieldAccess { lint_message, } => lint_message)
            {
                cx.emit_span_lint(
                    BAD_OPT_ACCESS,
                    expr.span,
                    BadOptAccessDiag { msg: lint_message.as_str() },
                );
            }
        }
    }
}

declare_tool_lint! {
    pub rustc::SPAN_USE_EQ_CTXT,
    Allow,
    "forbid uses of `==` with `Span::ctxt`, suggest `Span::eq_ctxt` instead",
    report_in_external_macro: true
}

declare_lint_pass!(SpanUseEqCtxt => [SPAN_USE_EQ_CTXT]);

impl<'tcx> LateLintPass<'tcx> for SpanUseEqCtxt {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &hir::Expr<'_>) {
        if let hir::ExprKind::Binary(
            hir::BinOp { node: hir::BinOpKind::Eq | hir::BinOpKind::Ne, .. },
            lhs,
            rhs,
        ) = expr.kind
        {
            if is_span_ctxt_call(cx, lhs) && is_span_ctxt_call(cx, rhs) {
                cx.emit_span_lint(SPAN_USE_EQ_CTXT, expr.span, SpanUseEqCtxtDiag);
            }
        }
    }
}

fn is_span_ctxt_call(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> bool {
    match &expr.kind {
        hir::ExprKind::MethodCall(..) => cx
            .typeck_results()
            .type_dependent_def_id(expr.hir_id)
            .is_some_and(|call_did| cx.tcx.is_diagnostic_item(sym::SpanCtxt, call_did)),

        _ => false,
    }
}

declare_tool_lint! {
    /// The `symbol_intern_string_literal` detects `Symbol::intern` being called on a string literal
    pub rustc::SYMBOL_INTERN_STRING_LITERAL,
    // rustc_driver crates out of the compiler can't/shouldn't add preinterned symbols;
    // bootstrap will deny this manually
    Allow,
    "Forbid uses of string literals in `Symbol::intern`, suggesting preinterning instead",
    report_in_external_macro: true
}

declare_lint_pass!(SymbolInternStringLiteral => [SYMBOL_INTERN_STRING_LITERAL]);

impl<'tcx> LateLintPass<'tcx> for SymbolInternStringLiteral {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx rustc_hir::Expr<'tcx>) {
        if let hir::ExprKind::Call(path, [arg]) = expr.kind
            && let hir::ExprKind::Path(ref qpath) = path.kind
            && let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id()
            && cx.tcx.is_diagnostic_item(sym::SymbolIntern, def_id)
            && let hir::ExprKind::Lit(kind) = arg.kind
            && let rustc_ast::LitKind::Str(_, _) = kind.node
        {
            cx.emit_span_lint(
                SYMBOL_INTERN_STRING_LITERAL,
                kind.span,
                SymbolInternStringLiteralDiag,
            );
        }
    }
}

declare_tool_lint! {
    /// The `implicit_sysroot_crate_import` detects use of `extern crate` to import non-sysroot crates
    /// (e.g. crates.io deps) from the sysroot, which is dangerous because these crates are not guaranteed
    /// to exist exactly once, and so may be missing entirely or appear multiple times resulting in ambiguity.
    pub rustc::IMPLICIT_SYSROOT_CRATE_IMPORT,
    Allow,
    "Forbid uses of non-sysroot crates in `extern crate`",
    report_in_external_macro: true
}

declare_lint_pass!(ImplicitSysrootCrateImport => [IMPLICIT_SYSROOT_CRATE_IMPORT]);

impl EarlyLintPass for ImplicitSysrootCrateImport {
    fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
        fn is_whitelisted(crate_name: &str) -> bool {
            // Whitelist of allowed crates.
            crate_name.starts_with("rustc_")
                || matches!(
                    crate_name,
                    "test" | "self" | "core" | "alloc" | "std" | "proc_macro" | "tikv_jemalloc_sys"
                )
        }

        if let ast::ItemKind::ExternCrate(original_name, imported_name) = &item.kind {
            let name = original_name.as_ref().unwrap_or(&imported_name.name).as_str();
            let externs = &cx.builder.sess().opts.externs;
            if externs.get(name).is_none() && !is_whitelisted(name) {
                cx.emit_span_lint(
                    IMPLICIT_SYSROOT_CRATE_IMPORT,
                    item.span,
                    ImplicitSysrootCrateImportDiag { name },
                );
            }
        }
    }
}