rustc_ast/

visit.rs

//! AST walker. Each overridden visit method has full control over what
//! happens with its node, it can do its own traversal of the node's children,
//! call `visit::walk_*` to apply the default traversal algorithm, or prevent
//! deeper traversal by doing nothing.
//!
//! Note: it is an important invariant that the default visitor walks the body
//! of a function in "execution order" (more concretely, reverse post-order
//! with respect to the CFG implied by the AST), meaning that if AST node A may
//! execute before AST node B, then A is visited first. The borrow checker in
//! particular relies on this property.
//!
//! Note: walking an AST before macro expansion is probably a bad idea. For
//! instance, a walker looking for item names in a module will miss all of
//! those that are created by the expansion of a macro.

pub use rustc_ast_ir::visit::VisitorResult;
pub use rustc_ast_ir::{try_visit, visit_opt, walk_list, walk_visitable_list};
use rustc_macros::StableHash;
use rustc_span::{Ident, Span, Spanned, Symbol};
use thin_vec::ThinVec;

use crate::ast::*;
use crate::tokenstream::DelimSpan;

#[derive(Copy, Clone, Debug, PartialEq, Eq, StableHash)]
pub enum AssocCtxt {
    Trait,
    Impl { of_trait: bool },
}

#[derive(Copy, Clone, Debug, PartialEq)]
pub enum FnCtxt {
    Free,
    Foreign,
    Assoc(AssocCtxt),
}

#[derive(Copy, Clone, Debug)]
pub enum BoundKind {
    /// Trait bounds in generics bounds and type/trait alias.
    /// E.g., `<T: Bound>`, `type A: Bound`, or `where T: Bound`.
    Bound,

    /// Trait bounds in `impl` type.
    /// E.g., `type Foo = impl Bound1 + Bound2 + Bound3`.
    Impl,

    /// Trait bounds in trait object type.
    /// E.g., `dyn Bound1 + Bound2 + Bound3`.
    TraitObject,

    /// Super traits of a trait.
    /// E.g., `trait A: B`
    SuperTraits,
}
impl BoundKind {
    pub fn descr(self) -> &'static str {
        match self {
            BoundKind::Bound => "bounds",
            BoundKind::Impl => "`impl Trait`",
            BoundKind::TraitObject => "`dyn` trait object bounds",
            BoundKind::SuperTraits => "supertrait bounds",
        }
    }
}

#[derive(Copy, Clone, Debug)]
pub enum LifetimeCtxt {
    /// Appears in a reference type.
    Ref,
    /// Appears as a bound on a type or another lifetime.
    Bound,
    /// Appears as a generic argument.
    GenericArg,
}

pub(crate) trait Visitable<'a, V: Visitor<'a>> {
    type Extra: Copy;

    #[must_use]
    fn visit(&'a self, visitor: &mut V, extra: Self::Extra) -> V::Result;
}

impl<'a, V: Visitor<'a>, T: ?Sized> Visitable<'a, V> for Box<T>
where
    T: Visitable<'a, V>,
{
    type Extra = T::Extra;
    fn visit(&'a self, visitor: &mut V, extra: Self::Extra) -> V::Result {
        (**self).visit(visitor, extra)
    }
}

impl<'a, V: Visitor<'a>, T> Visitable<'a, V> for Option<T>
where
    T: Visitable<'a, V>,
{
    type Extra = T::Extra;
    fn visit(&'a self, visitor: &mut V, extra: Self::Extra) -> V::Result {
        if let Some(this) = self {
            try_visit!(this.visit(visitor, extra));
        }
        V::Result::output()
    }
}

impl<'a, V: Visitor<'a>, T> Visitable<'a, V> for Spanned<T>
where
    T: Visitable<'a, V>,
{
    type Extra = T::Extra;
    fn visit(&'a self, visitor: &mut V, extra: Self::Extra) -> V::Result {
        let Spanned { span: _, node } = self;
        node.visit(visitor, extra)
    }
}

impl<'a, V: Visitor<'a>, T> Visitable<'a, V> for [T]
where
    T: Visitable<'a, V>,
{
    type Extra = T::Extra;
    fn visit(&'a self, visitor: &mut V, extra: Self::Extra) -> V::Result {
        for item in self {
            try_visit!(item.visit(visitor, extra));
        }
        V::Result::output()
    }
}

impl<'a, V: Visitor<'a>, T> Visitable<'a, V> for Vec<T>
where
    T: Visitable<'a, V>,
{
    type Extra = T::Extra;
    fn visit(&'a self, visitor: &mut V, extra: Self::Extra) -> V::Result {
        for item in self {
            try_visit!(item.visit(visitor, extra));
        }
        V::Result::output()
    }
}

impl<'a, V: Visitor<'a>, T1, T2> Visitable<'a, V> for (T1, T2)
where
    T1: Visitable<'a, V, Extra = ()>,
    T2: Visitable<'a, V, Extra = ()>,
{
    type Extra = ();
    fn visit(&'a self, visitor: &mut V, extra: Self::Extra) -> V::Result {
        try_visit!(self.0.visit(visitor, extra));
        try_visit!(self.1.visit(visitor, extra));
        V::Result::output()
    }
}

impl<'a, V: Visitor<'a>, T1, T2, T3> Visitable<'a, V> for (T1, T2, T3)
where
    T1: Visitable<'a, V, Extra = ()>,
    T2: Visitable<'a, V, Extra = ()>,
    T3: Visitable<'a, V, Extra = ()>,
{
    type Extra = ();
    fn visit(&'a self, visitor: &mut V, extra: Self::Extra) -> V::Result {
        try_visit!(self.0.visit(visitor, extra));
        try_visit!(self.1.visit(visitor, extra));
        try_visit!(self.2.visit(visitor, extra));
        V::Result::output()
    }
}

impl<'a, V: Visitor<'a>, T1, T2, T3, T4> Visitable<'a, V> for (T1, T2, T3, T4)
where
    T1: Visitable<'a, V, Extra = ()>,
    T2: Visitable<'a, V, Extra = ()>,
    T3: Visitable<'a, V, Extra = ()>,
    T4: Visitable<'a, V, Extra = ()>,
{
    type Extra = ();
    fn visit(&'a self, visitor: &mut V, extra: Self::Extra) -> V::Result {
        try_visit!(self.0.visit(visitor, extra));
        try_visit!(self.1.visit(visitor, extra));
        try_visit!(self.2.visit(visitor, extra));
        try_visit!(self.3.visit(visitor, extra));
        V::Result::output()
    }
}

pub(crate) trait Walkable<'a, V: Visitor<'a>> {
    #[must_use]
    fn walk_ref(&'a self, visitor: &mut V) -> V::Result;
}

macro_rules! visit_visitable {
    ($visitor:expr, $($expr:expr),* $(,)?) => {{
        $(try_visit!(Visitable::visit($expr, $visitor, ()));)*
    }};
}

macro_rules! visit_visitable_with {
    ($visitor:expr, $expr:expr, $extra:expr $(,)?) => {
        try_visit!(Visitable::visit($expr, $visitor, $extra))
    };
}

macro_rules! impl_visitable {
    // The no-extra case.
    (|&$lt:lifetime $self:ident: $SelfTy:ty, $vis:ident: &mut $vis_ty:ident| $block:block) => {
        impl<$lt, $vis_ty: Visitor<$lt>> Visitable<$lt, $vis_ty> for $SelfTy {
            type Extra = ();

            #[inline]
            fn visit(&$lt $self, $vis: &mut $vis_ty, _extra: Self::Extra) -> V::Result {
                $block
            }
        }
    };
    // The with-extra case.
    (|&$lt:lifetime $self:ident: $SelfTy:ty, $vis:ident: &mut $vis_ty:ident,
     $extra:ident: $ExtraTy:ty| $block:block) => {
        impl<$lt, $vis_ty: Visitor<$lt>> Visitable<$lt, $vis_ty> for $SelfTy {
            type Extra = $ExtraTy;

            #[inline]
            fn visit(&$lt $self, $vis: &mut $vis_ty, $extra: Self::Extra) -> V::Result {
                $block
            }
        }
    };
}

macro_rules! impl_walkable {
    (|&$lt:lifetime $self:ident: $SelfTy:ty,
      $vis:ident: &mut $vis_ty:ident| $block:block) => {
        impl<$lt, $vis_ty: Visitor<$lt>> Walkable<$lt, $vis_ty> for $SelfTy {
            fn walk_ref(&$lt $self, $vis: &mut $vis_ty) -> V::Result {
                $block
            }
        }
    };
}

macro_rules! impl_visitable_noop {
    ($($Ty:ty,)*) => {
        $(
            impl_visitable!(|&'a self: $Ty, _vis: &mut V| {
                V::Result::output()
            });
        )*
    };
}

macro_rules! impl_visitable_list {
    ($($Ty:ty,)*) => {
        $(impl<'a, V: Visitor<'a>, T> Visitable<'a, V> for $Ty
        where
            &'a $Ty: IntoIterator<Item = &'a T>,
            T: 'a + Visitable<'a, V>,
        {
            type Extra = <T as Visitable<'a, V>>::Extra;

            fn visit(&'a self, visitor: &mut V, extra: Self::Extra) -> V::Result {
                for i in self {
                    try_visit!(i.visit(visitor, extra));
                }
                V::Result::output()
            }
        })*
    };
}

macro_rules! impl_visitable_direct {
    ($($Ty:ty,)*) => {
        $(impl_visitable!(
            |&'a self: $Ty, visitor: &mut V| {
                Walkable::walk_ref(self, visitor)
            }
        );)*
    };
}

macro_rules! fn_visit {
    ($($visit:ident($Ty:ty $(, $extra:ident: $ExtraTy:ty)?) => $walk:ident;)*) => {
        $(fn $visit(&mut self, node: &'a $Ty $(, $extra: $ExtraTy)?) -> Self::Result {
            Walkable::walk_ref(node, self)
        })*
    };
}

macro_rules! impl_visitable_visit {
    ($($visit:ident($Ty:ty $(, $extra:ident: $ExtraTy:ty)?) => $walk:ident;)*) => {
        $(impl_visitable!(|&'a self: $Ty, visitor: &mut V $(, $extra: $ExtraTy)?| {
            visitor.$visit(self $(, $extra)?)
        });)*
    };
}

macro_rules! fn_walk {
    ($($visit:ident($Ty:ty $(, $extra:ident: $ExtraTy:ty)?) => $walk:ident;)*) => {
        $(pub fn $walk<'a, V: Visitor<'a>>(visitor: &mut V, node: &'a $Ty) -> V::Result {
            Walkable::walk_ref(node, visitor)
        })*
    }
}

/// Higher-order macro that puts all the visit/walk hook information in a single place. The
/// passed-in macro should have a left hand side like this:
/// ```ignore (partial)
/// ($($visit:ident($Ty:ty $(, $extra:ident: $ExtraTy:ty)?) => $walk:ident;)*) => { ... }
/// ```
macro_rules! for_each_ast_visit_hook {
    ($macro:ident!) => {
        $macro! {
            // tidy-alphabetical-start
            visit_anon_const(AnonConst) => walk_anon_const;
            visit_arm(Arm) => walk_arm;
            //visit_assoc_item(AssocItem, _ctxt: AssocCtxt) => walk_assoc_item;
            visit_assoc_item_constraint(AssocItemConstraint) => walk_assoc_item_constraint;
            visit_attribute(Attribute) => walk_attribute;
            visit_block(Block) => walk_block;
            visit_capture_by(CaptureBy) => walk_capture_by;
            visit_closure_binder(ClosureBinder) => walk_closure_binder;
            visit_contract(FnContract) => walk_contract;
            visit_coroutine_marker(CoroutineMarker) => walk_coroutine_marker;
            visit_crate(Crate) => walk_crate;
            visit_expr(Expr) => walk_expr;
            visit_expr_field(ExprField) => walk_expr_field;
            visit_field_def(FieldDef) => walk_field_def;
            visit_field_def_extras(FieldDefExtras) => walk_field_def_extras;
            visit_fn_decl(FnDecl) => walk_fn_decl;
            visit_fn_header(FnHeader) => walk_fn_header;
            visit_fn_ret_ty(FnRetTy) => walk_fn_ret_ty;
            //visit_foreign_item(ForeignItem) => walk_foreign_item;
            visit_foreign_mod(ForeignMod) => walk_foreign_mod;
            visit_format_args(FormatArgs) => walk_format_args;
            visit_generic_arg(GenericArg) => walk_generic_arg;
            visit_generic_args(GenericArgs) => walk_generic_args;
            visit_generic_param(GenericParam) => walk_generic_param;
            visit_generics(Generics) => walk_generics;
            visit_impl_restriction(ImplRestriction) => walk_impl_restriction;
            visit_inline_asm(InlineAsm) => walk_inline_asm;
            visit_inline_asm_sym(InlineAsmSym) => walk_inline_asm_sym;
            //visit_item(Item) => walk_item;
            visit_label(Label) => walk_label;
            visit_lifetime(Lifetime, _ctxt: LifetimeCtxt) => walk_lifetime;
            visit_local(Local) => walk_local;
            visit_mac_call(MacCall) => walk_mac;
            visit_macro_def(MacroDef) => walk_macro_def;
            visit_mut_restriction(MutRestriction) => walk_mut_restriction;
            visit_param(Param) => walk_param;
            visit_param_bound(GenericBound, _ctxt: BoundKind) => walk_param_bound;
            visit_pat(Pat) => walk_pat;
            visit_pat_field(PatField) => walk_pat_field;
            visit_path(Path) => walk_path;
            visit_path_segment(PathSegment) => walk_path_segment;
            visit_poly_trait_ref(PolyTraitRef) => walk_poly_trait_ref;
            visit_precise_capturing_arg(PreciseCapturingArg) => walk_precise_capturing_arg;
            visit_qself(QSelf) => walk_qself;
            visit_test_binder_body(TestBinderBody) => walk_test_binder_body;
            visit_test_binder_bound_type_constraint(TestBinderBoundTypeConstraint) =>
                walk_test_binder_bound_type_constraint;
            visit_test_binder_constraint(TestBinderConstraint) => walk_test_binder_constraint;
            visit_test_binder_constraints(TestBinderConstraints) => walk_test_binder_constraints;
            visit_test_binder_exists(TestBinderExists) => walk_test_binder_exists;
            visit_test_binder_forall(TestBinderForall) => walk_test_binder_forall;
            visit_test_binder_item(TestBinderItem) => walk_test_binder_item;
            visit_trait_ref(TraitRef) => walk_trait_ref;
            visit_ty(Ty) => walk_ty;
            visit_ty_pat(TyPat) => walk_ty_pat;
            visit_use_tree(UseTree) => walk_use_tree;
            visit_use_tree_and_id(UseTreeAndId) => walk_use_tree_and_id;
            visit_variant(Variant) => walk_variant;
            visit_variant_data(VariantData) => walk_variant_data;
            visit_vis(Visibility) => walk_vis;
            visit_where_predicate(WherePredicate) => walk_where_predicate;
            visit_where_predicate_kind(WherePredicateKind) => walk_where_predicate_kind;
            // tidy-alphabetical-end
        }
    };
}
pub(crate) use for_each_ast_visit_hook;

macro_rules! common_visitor_and_walkers {
    ($(($mut:ident))? $Visitor:ident$(<$lt:lifetime>)?) => {
        $(${ignore($lt)}
            #[derive(Copy, Clone)]
        )?
        #[derive(Debug)]
        pub enum FnKind<'a> {
            /// E.g., `fn foo()`, `fn foo(&self)`, or `extern "Abi" fn foo()`.
            Fn(FnCtxt, &'a $($mut)? Visibility, &'a $($mut)? Fn),

            /// E.g., `|x, y| body`.
            Closure(
                &'a $($mut)? ClosureBinder,
                &'a $($mut)? Option<CoroutineMarker>,
                &'a $($mut)? Box<FnDecl>,
                &'a $($mut)? Box<Expr>,
            ),
        }

        impl<'a> FnKind<'_> {
            pub fn header(&'a $($mut)? self) -> Option<&'a $($mut)? FnHeader> {
                match *self {
                    FnKind::Fn(_, _, Fn { sig, .. }) => Some(&$($mut)? sig.header),
                    FnKind::Closure(..) => None,
                }
            }

            pub fn ident(&'a $($mut)? self) -> Option<&'a $($mut)? Ident> {
                match self {
                    FnKind::Fn(_, _, Fn { ident, .. }) => Some(ident),
                    _ => None,
                }
            }

            pub fn decl(&'a $($mut)? self) -> &'a $($mut)? FnDecl {
                match self {
                    FnKind::Fn(_, _, Fn { sig, .. }) => &$($mut)? sig.decl,
                    FnKind::Closure(_, _, decl, _) => decl,
                }
            }

            pub fn ctxt(&self) -> Option<FnCtxt> {
                match self {
                    FnKind::Fn(ctxt, ..) => Some(*ctxt),
                    FnKind::Closure(..) => None,
                }
            }
        }

        // This macro generates `impl Visitable` and `impl MutVisitable` that do nothing.
        impl_visitable_noop! {
            // tidy-alphabetical-start
            AttrId,
            GeneratedAttr,
            Movability,
            Mutability,
            Pinnedness,
            Result<(), rustc_span::ErrorGuaranteed>,
            Symbol,
            SyntheticAttr,
            bool,
            char,
            crate::token::CommentKind,
            crate::token::Delimiter,
            crate::token::Lit,
            crate::token::LitKind,
            crate::tokenstream::LazyTokenStream,
            crate::tokenstream::TokenStream,
            rustc_data_structures::fx::FxHashMap<Symbol, usize>,
            rustc_span::ByteSymbol,
            rustc_span::ErrorGuaranteed,
            std::borrow::Cow<'_, str>,
            u8,
            usize,
            // tidy-alphabetical-end
        }
        // `Span` is a no-op for the immutable visitor.
        $(${ignore($lt)} impl_visitable_noop! { Span, })?

        // This macro generates `impl Visitable` and `impl MutVisitable` that simply iterate over
        // their contents. We do not use a generic impl for `ThinVec` because we want to allow
        // custom visits for the `MutVisitor`.
        impl_visitable_list! {
            // tidy-alphabetical-start
            ThinVec<(Ident, Option<Ident>)>,
            ThinVec<(NodeId, Path)>,
            ThinVec<AngleBracketedArg>,
            ThinVec<Attribute>,
            ThinVec<Box<Ty>>,
            ThinVec<EiiImpl>,
            ThinVec<GenericBound>,
            ThinVec<Ident>,
            ThinVec<Pat>,
            ThinVec<PathSegment>,
            ThinVec<PreciseCapturingArg>,
            ThinVec<TestBinderConstraint>,
            ThinVec<TestBinderExists>,
            ThinVec<TestBinderForall>,
            ThinVec<TyPat>,
            ThinVec<UseTreeAndId>,
            // tidy-alphabetical-end
        }

        // This macro generates `impl Visitable` and `impl MutVisitable` that forward to `Walkable`
        // or `MutWalkable`. By default, all types that do not have a custom visit method in the
        // visitor should appear here.
        impl_visitable_direct! {
            // tidy-alphabetical-start
            AngleBracketedArg,
            AngleBracketedArgs,
            AsmMacro,
            AssignOpKind,
            AssocItemConstraintKind,
            AttrArgs,
            AttrItem,
            AttrKind,
            AttrStyle,
            BinOpKind,
            BindingMode,
            BlockCheckMode,
            BorrowKind,
            BoundAsyncness,
            BoundConstness,
            BoundPolarity,
            ByRef,
            Closure,
            Const,
            ConstBlockItem,
            ConstItem,
            CoroutineKind,
            Defaultness,
            Delegation,
            DelegationMac,
            DelegationSuffixes,
            DelimArgs,
            DelimSpan,
            EiiDecl,
            EiiImpl,
            EnumDef,
            Extern,
            FnPtrTy,
            ForLoopKind,
            FormatArgPosition,
            FormatArgsPiece,
            FormatArgument,
            FormatArgumentKind,
            FormatArguments,
            FormatPlaceholder,
            GenericParamKind,
            Guard,
            Impl,
            ImplPolarity,
            Inline,
            InlineAsmOperand,
            InlineAsmRegOrRegClass,
            InlineAsmTemplatePiece,
            IsAuto,
            LocalKind,
            MacCallStmt,
            MacStmtStyle,
            MatchKind,
            MethodCall,
            ModKind,
            ModSpans,
            NormalAttr,
            Parens,
            ParenthesizedArgs,
            PatFieldsRest,
            PatKind,
            RangeEnd,
            RangeLimits,
            RangeSyntax,
            Recovered,
            RestrictionKind,
            Safety,
            StaticItem,
            StrLit,
            StrStyle,
            StructExpr,
            StructRest,
            Term,
            TestBinderRegionEquality,
            Trait,
            TraitBoundModifiers,
            TraitObjectSyntax,
            TyAlias,
            TyKind,
            TyPatKind,
            UnOp,
            UnsafeBinderCastKind,
            UnsafeBinderTy,
            UnsafeSource,
            UseTreeKind,
            VisibilityKind,
            WhereBoundPredicate,
            WhereClause,
            WhereEqPredicate,
            WhereRegionPredicate,
            YieldKind,
            // tidy-alphabetical-end
        }

        /// Each method of this trait is a hook to be potentially
        /// overridden. Each method's default implementation recursively visits
        /// the substructure of the input via the corresponding `walk` method;
        #[doc = concat!(
            " e.g., the `visit_item` method by default calls `visit"
            $(, "_", stringify!($mut))?,
            "::walk_item`."
        )]
        ///
        /// If you want to ensure that your code handles every variant
        /// explicitly, you need to override each method. (And you also need
        /// to monitor future changes to this trait in case a new method with a
        /// new default implementation gets introduced.)
        ///
        /// Every `walk_*` method uses deconstruction to access fields of structs and
        /// enums. This will result in a compile error if a field is added, which makes
        /// it more likely the appropriate visit call will be added for it.
        pub trait $Visitor<$($lt)?>: Sized $(${ignore($mut)} + MutVisitorResult<Result = ()>)? {
            $(
                ${ignore($lt)}
                /// The result type of the `visit_*` methods. Can be either `()`,
                /// or `ControlFlow<T>`.
                type Result: VisitorResult = ();
            )?

            // Methods in this trait have one of three forms, with the last two forms
            // only occurring on `MutVisitor`:
            //
            //   fn visit_t(&mut self, t: &mut T);                      // common
            //   fn flat_map_t(&mut self, t: T) -> SmallVec<[T; 1]>;    // rare
            //   fn filter_map_t(&mut self, t: T) -> Option<T>;         // rarest
            //
            // When writing these methods, it is better to use destructuring like this:
            //
            //   fn visit_abc(&mut self, ABC { a, b, c: _ }: &mut ABC) {
            //       visit_a(a);
            //       visit_b(b);
            //   }
            //
            // than to use field access like this:
            //
            //   fn visit_abc(&mut self, abc: &mut ABC) {
            //       visit_a(&mut abc.a);
            //       visit_b(&mut abc.b);
            //       // ignore abc.c
            //   }
            //
            // As well as being more concise, the former is explicit about which fields
            // are skipped. Furthermore, if a new field is added, the destructuring
            // version will cause a compile error, which is good. In comparison, the
            // field access version will continue working and it would be easy to
            // forget to add handling for it.
            fn visit_ident(&mut self, Ident { name: _, span }: &$($lt)? $($mut)? Ident)
                -> Self::Result
            {
                visit_visitable!(self, span);
                Self::Result::output()
            }

            crate::visit::for_each_ast_visit_hook! { fn_visit! }

            // We want `Visitor` to take the `NodeId` by value.
            fn visit_id(&mut self, _id: $(&$mut)? NodeId) -> Self::Result {
                Self::Result::output()
            }

            /// This method is a hack to workaround unstable of `stmt_expr_attributes`.
            /// It can be removed once that feature is stabilized.
            fn visit_method_receiver_expr(&mut self, ex: &$($lt)? $($mut)? Expr) -> Self::Result {
                self.visit_expr(ex)
            }

            fn visit_item(&mut self, item: &$($lt)? $($mut)? Item) -> Self::Result {
                walk_item(self, item)
            }

            fn visit_foreign_item(&mut self, item: &$($lt)? $($mut)? ForeignItem) -> Self::Result {
                walk_item(self, item)
            }

            fn visit_assoc_item(&mut self, item: &$($lt)? $($mut)? AssocItem, ctxt: AssocCtxt)
                -> Self::Result
            {
                walk_assoc_item(self, item, ctxt)
            }

            // for `MutVisitor`: `Span` and `NodeId` are mutated at the caller site.
            fn visit_fn(
                &mut self,
                fk: FnKind<$($lt)? $(${ignore($mut)} '_)?>,
                _: &AttrVec,
                _: Span,
                _: NodeId,
            ) -> Self::Result {
                walk_fn(self, fk)
            }

            // (non-mut) `Visitor`-only methods
            $(
                fn visit_stmt(&mut self, s: &$lt Stmt) -> Self::Result {
                    walk_stmt(self, s)
                }
            )?

            // `MutVisitor`-only methods
            $(
                // Span visiting is no longer used, but we keep it for now,
                // in case it's needed for something like #127241.
                #[inline]
                fn visit_span(&mut self, _sp: &$mut Span) {
                    // Do nothing.
                }

                fn flat_map_foreign_item(&mut self, ni: Box<ForeignItem>)
                    -> SmallVec<[Box<ForeignItem>; 1]>
                {
                    walk_flat_map_foreign_item(self, ni)
                }

                fn flat_map_item(&mut self, i: Box<Item>) -> SmallVec<[Box<Item>; 1]> {
                    walk_flat_map_item(self, i)
                }

                fn flat_map_field_def(&mut self, fd: FieldDef) -> SmallVec<[FieldDef; 1]> {
                    walk_flat_map_field_def(self, fd)
                }

                fn flat_map_assoc_item(
                    &mut self,
                    i: Box<AssocItem>,
                    ctxt: AssocCtxt,
                ) -> SmallVec<[Box<AssocItem>; 1]> {
                    walk_flat_map_assoc_item(self, i, ctxt)
                }

                fn flat_map_stmt(&mut self, s: Stmt) -> SmallVec<[Stmt; 1]> {
                    walk_flat_map_stmt(self, s)
                }

                fn flat_map_arm(&mut self, arm: Arm) -> SmallVec<[Arm; 1]> {
                    walk_flat_map_arm(self, arm)
                }

                fn filter_map_expr(&mut self, e: Box<Expr>) -> Option<Box<Expr>> {
                    walk_filter_map_expr(self, e)
                }

                fn flat_map_variant(&mut self, v: Variant) -> SmallVec<[Variant; 1]> {
                    walk_flat_map_variant(self, v)
                }

                fn flat_map_param(&mut self, param: Param) -> SmallVec<[Param; 1]> {
                    walk_flat_map_param(self, param)
                }

                fn flat_map_generic_param(&mut self, param: GenericParam)
                    -> SmallVec<[GenericParam; 1]>
                {
                    walk_flat_map_generic_param(self, param)
                }

                fn flat_map_expr_field(&mut self, f: ExprField) -> SmallVec<[ExprField; 1]> {
                    walk_flat_map_expr_field(self, f)
                }

                fn flat_map_where_predicate(
                    &mut self,
                    where_predicate: WherePredicate,
                ) -> SmallVec<[WherePredicate; 1]> {
                    walk_flat_map_where_predicate(self, where_predicate)
                }

                fn flat_map_pat_field(&mut self, fp: PatField) -> SmallVec<[PatField; 1]> {
                    walk_flat_map_pat_field(self, fp)
                }
            )?
        }

        crate::visit::for_each_ast_visit_hook! { impl_visitable_visit! }

        impl_visitable!(|&$($lt)? $($mut)? self: Ident, visitor: &mut V| {
            visitor.visit_ident(self)
        });

        $(
            impl_visitable!(
                |&$lt self: NodeId, visitor: &mut V| {
                    visitor.visit_id(*self)
                }
            );
        )?
        $(
            impl_visitable!(
                |&$mut self: NodeId, visitor: &mut V| {
                    visitor.visit_id(self)
                }
            );

            impl_visitable!(|&mut self: Span, visitor: &mut V| {
                visitor.visit_span(self)
            });
        )?

        impl_visitable!(|&$($lt)? $($mut)? self: Item, vis: &mut V| {
            vis.visit_item(self)
        });
        impl_visitable!(|&$($lt)? $($mut)? self: ForeignItem, vis: &mut V| {
            vis.visit_foreign_item(self)
        });
        impl_visitable!(|&$($lt)? $($mut)? self: AssocItem, vis: &mut V, ctxt: AssocCtxt| {
            vis.visit_assoc_item(self, ctxt)
        });

        pub trait WalkItemKind {
            type Ctxt;
            fn walk<$($lt,)? V: $Visitor$(<$lt>)?>(
                &$($lt)? $($mut)? self,
                attrs: &AttrVec,
                span: Span,
                id: NodeId,
                visibility: &$($lt)? $($mut)? Visibility,
                ctxt: Self::Ctxt,
                vis: &mut V,
            ) -> V::Result;
        }

        fn walk_item_inner<$($lt,)? K: WalkItemKind, V: $Visitor$(<$lt>)?>(
            visitor: &mut V,
            item: &$($lt)? $($mut)? Item<K>,
            ctxt: K::Ctxt,
        ) -> V::Result {
            let Item { attrs, id, kind, vis, span, tokens: _ } = item;
            visit_visitable!(visitor, id, attrs, vis);
            try_visit!(kind.walk(attrs, *span, *id, vis, ctxt, visitor));
            visit_visitable!(visitor, span);
            V::Result::output()
        }

        // Do not implement `Walkable`/`MutWalkable` for *Item to avoid confusion.
        pub fn walk_item<$($lt,)? K: WalkItemKind<Ctxt = ()>, V: $Visitor$(<$lt>)?>(
            visitor: &mut V,
            item: &$($lt)? $($mut)? Item<K>,
        ) -> V::Result {
            walk_item_inner(visitor, item, ())
        }

        // Do not implement `Walkable`/`MutWalkable` for *Item to avoid confusion.
        pub fn walk_assoc_item<$($lt,)? K: WalkItemKind<Ctxt = AssocCtxt>, V: $Visitor$(<$lt>)?>(
            visitor: &mut V,
            item: &$($lt)? $($mut)? Item<K>,
            ctxt: AssocCtxt,
        ) -> V::Result {
            walk_item_inner(visitor, item, ctxt)
        }

        impl WalkItemKind for ItemKind {
            type Ctxt = ();
            fn walk<$($lt,)? V: $Visitor$(<$lt>)?>(
                &$($lt)? $($mut)? self,
                attrs: &AttrVec,
                span: Span,
                id: NodeId,
                visibility: &$($lt)? $($mut)? Visibility,
                _ctxt: Self::Ctxt,
                vis: &mut V,
            ) -> V::Result {
                match self {
                    ItemKind::Fn(func) => {
                        let kind = FnKind::Fn(FnCtxt::Free, visibility, &$($mut)? *func);
                        try_visit!(vis.visit_fn(kind, attrs, span, id));
                    }
                    ItemKind::ExternCrate(orig_name, ident) =>
                        visit_visitable!(vis, orig_name, ident),
                    ItemKind::Use(use_tree) =>
                        visit_visitable!(vis, use_tree),
                    ItemKind::Static(item) =>
                        visit_visitable!(vis, item),
                    ItemKind::ConstBlock(item) =>
                        visit_visitable!(vis, item),
                    ItemKind::Const(item) =>
                        visit_visitable!(vis, item),
                    ItemKind::Mod(safety, ident, mod_kind) =>
                        visit_visitable!(vis, safety, ident, mod_kind),
                    ItemKind::ForeignMod(nm) =>
                        visit_visitable!(vis, nm),
                    ItemKind::GlobalAsm(asm) =>
                        visit_visitable!(vis, asm),
                    ItemKind::TyAlias(ty_alias) =>
                        visit_visitable!(vis, ty_alias),
                    ItemKind::Enum(ident, generics, enum_definition) =>
                        visit_visitable!(vis, ident, generics, enum_definition),
                    ItemKind::Struct(ident, generics, variant_data)
                    | ItemKind::Union(ident, generics, variant_data) =>
                        visit_visitable!(vis, ident, generics, variant_data),
                    ItemKind::Impl(impl_) =>
                        visit_visitable!(vis, impl_),
                    ItemKind::Trait(trait_) =>
                        visit_visitable!(vis, trait_),
                    ItemKind::TraitAlias(TraitAlias { constness, ident, generics, bounds }) => {
                        visit_visitable!(vis, constness, ident, generics);
                        visit_visitable_with!(vis, bounds, BoundKind::Bound)
                    }
                    ItemKind::MacCall(m) =>
                        visit_visitable!(vis, m),
                    ItemKind::MacroDef(ident, def) =>
                        visit_visitable!(vis, ident, def),
                    ItemKind::Delegation(delegation) =>
                        visit_visitable!(vis, delegation),
                    ItemKind::DelegationMac(dm) =>
                        visit_visitable!(vis, dm),
                    ItemKind::TestBinderConstraints(item) =>
                        visit_visitable!(vis, item),
                }
                V::Result::output()
            }
        }

        impl WalkItemKind for AssocItemKind {
            type Ctxt = AssocCtxt;
            fn walk<$($lt,)? V: $Visitor$(<$lt>)?>(
                &$($lt)? $($mut)? self,
                attrs: &AttrVec,
                span: Span,
                id: NodeId,
                visibility: &$($lt)? $($mut)? Visibility,
                ctxt: Self::Ctxt,
                vis: &mut V,
            ) -> V::Result {
                match self {
                    AssocItemKind::Const(item) =>
                        visit_visitable!(vis, item),
                    AssocItemKind::Fn(func) => {
                        let kind = FnKind::Fn(FnCtxt::Assoc(ctxt), visibility, &$($mut)? *func);
                        try_visit!(vis.visit_fn(kind, attrs, span, id))
                    }
                    AssocItemKind::Type(alias) =>
                        visit_visitable!(vis, alias),
                    AssocItemKind::MacCall(mac) =>
                        visit_visitable!(vis, mac),
                    AssocItemKind::Delegation(delegation) =>
                        visit_visitable!(vis, delegation),
                    AssocItemKind::DelegationMac(dm) =>
                        visit_visitable!(vis, dm),
                }
                V::Result::output()
            }
        }

        impl WalkItemKind for ForeignItemKind {
            type Ctxt = ();
            fn walk<$($lt,)? V: $Visitor$(<$lt>)?>(
                &$($lt)? $($mut)? self,
                attrs: &AttrVec,
                span: Span,
                id: NodeId,
                visibility: &$($lt)? $($mut)? Visibility,
                _ctxt: Self::Ctxt,
                vis: &mut V,
            ) -> V::Result {
                match self {
                    ForeignItemKind::Static(item) =>
                        visit_visitable!(vis, item),
                    ForeignItemKind::Fn(func) => {
                        let kind = FnKind::Fn(FnCtxt::Foreign, visibility, &$($mut)? *func);
                        try_visit!(vis.visit_fn(kind, attrs, span, id))
                    }
                    ForeignItemKind::TyAlias(alias) =>
                        visit_visitable!(vis, alias),
                    ForeignItemKind::MacCall(mac) =>
                        visit_visitable!(vis, mac),
                }
                V::Result::output()
            }
        }

        pub fn walk_fn<$($lt,)? V: $Visitor$(<$lt>)?>(
            vis: &mut V,
            kind: FnKind<$($lt)? $(${ignore($mut)} '_)?>,
        ) -> V::Result {
            match kind {
                FnKind::Fn(
                    _ctxt,
                    // Visibility is visited as a part of the item.
                    _vis,
                    Fn {
                        defaultness,
                        ident,
                        sig,
                        generics,
                        contract,
                        body,
                        define_opaque,
                        eii_impl,
                    },
                ) => {
                    let FnSig { header, decl, span } = sig;
                    visit_visitable!(vis,
                        defaultness, ident, header, generics, decl,
                        contract, body, span, define_opaque, eii_impl
                    );
                }
                FnKind::Closure(binder, coroutine_marker, decl, body) =>
                    visit_visitable!(vis, binder, coroutine_marker, decl, body),
            }
            V::Result::output()
        }

        impl_walkable!(|&$($lt)? $($mut)? self: Impl, vis: &mut V| {
            let Impl { generics, of_trait, self_ty, items, constness: _ } = self;
            try_visit!(vis.visit_generics(generics));
            if let Some(of_trait) = of_trait {
                let TraitImplHeader { defaultness, safety, polarity, trait_ref } = of_trait;
                visit_visitable!(vis, defaultness, safety, polarity, trait_ref);
            }
            try_visit!(vis.visit_ty(self_ty));
            visit_visitable_with!(vis, items, AssocCtxt::Impl { of_trait: of_trait.is_some() });
            V::Result::output()
        });

        // Special case to call `visit_method_receiver_expr`.
        impl_walkable!(|&$($lt)? $($mut)? self: MethodCall, vis: &mut V| {
            let MethodCall { seg, receiver, args, span } = self;
            try_visit!(vis.visit_method_receiver_expr(receiver));
            visit_visitable!(vis, seg, args, span);
            V::Result::output()
        });

        impl_walkable!(|&$($lt)? $($mut)? self: Expr, vis: &mut V| {
            let Expr { id, kind, span, attrs, tokens: _ } = self;
            visit_visitable!(vis, id, attrs);
            match kind {
                ExprKind::Array(exprs) =>
                    visit_visitable!(vis, exprs),
                ExprKind::ConstBlock(anon_const) =>
                    visit_visitable!(vis, anon_const),
                ExprKind::Repeat(element, count) =>
                    visit_visitable!(vis, element, count),
                ExprKind::Struct(se) =>
                    visit_visitable!(vis, se),
                ExprKind::Tup(exprs) =>
                    visit_visitable!(vis, exprs),
                ExprKind::Call(callee_expression, arguments) =>
                    visit_visitable!(vis, callee_expression, arguments),
                ExprKind::MethodCall(mc) =>
                    visit_visitable!(vis, mc),
                ExprKind::Binary(op, lhs, rhs) =>
                    visit_visitable!(vis, op, lhs, rhs),
                ExprKind::AddrOf(kind, mutbl, subexpression) =>
                    visit_visitable!(vis, kind, mutbl, subexpression),
                ExprKind::Unary(op, subexpression) =>
                    visit_visitable!(vis, op, subexpression),
                ExprKind::Cast(subexpression, typ) | ExprKind::Type(subexpression, typ) =>
                    visit_visitable!(vis, subexpression, typ),
                ExprKind::Let(pat, expr, span, _recovered) =>
                    visit_visitable!(vis, pat, expr, span),
                ExprKind::If(head_expression, if_block, optional_else) =>
                    visit_visitable!(vis, head_expression, if_block, optional_else),
                ExprKind::While(subexpression, block, opt_label) =>
                    visit_visitable!(vis, subexpression, block, opt_label),
                ExprKind::ForLoop(ForLoop { pat, iter, body, label, kind }) =>
                    visit_visitable!(vis, pat, iter, body, label, kind),
                ExprKind::Loop(block, opt_label, span) =>
                    visit_visitable!(vis, block, opt_label, span),
                ExprKind::Match(subexpression, arms, kind) =>
                    visit_visitable!(vis, subexpression, arms, kind),
                ExprKind::Closure(Closure {
                    binder,
                    capture_clause,
                    coroutine_marker,
                    constness,
                    movability,
                    fn_decl,
                    body,
                    fn_decl_span,
                    fn_arg_span,
                }) => {
                    visit_visitable!(vis, constness, movability, capture_clause);
                    let kind = FnKind::Closure(binder, coroutine_marker, fn_decl, body);
                    try_visit!(vis.visit_fn(kind, attrs, *span, *id));
                    visit_visitable!(vis, fn_decl_span, fn_arg_span);
                }
                ExprKind::Block(block, opt_label) =>
                    visit_visitable!(vis, block, opt_label),
                ExprKind::Gen(capt, body, kind, decl_span) =>
                    visit_visitable!(vis, capt, body, kind, decl_span),
                ExprKind::Await(expr, span)
                | ExprKind::Move(expr, span)
                | ExprKind::Use(expr, span) =>
                    visit_visitable!(vis, expr, span),
                ExprKind::Assign(lhs, rhs, span) =>
                    visit_visitable!(vis, lhs, rhs, span),
                ExprKind::AssignOp(op, lhs, rhs) =>
                    visit_visitable!(vis, op, lhs, rhs),
                ExprKind::Field(subexpression, ident) =>
                    visit_visitable!(vis, subexpression, ident),
                ExprKind::Index(main_expression, index_expression, span) =>
                    visit_visitable!(vis, main_expression, index_expression, span),
                ExprKind::Range(start, end, limit) =>
                    visit_visitable!(vis, start, end, limit),
                ExprKind::Underscore => {}
                ExprKind::Path(maybe_qself, path) =>
                    visit_visitable!(vis, maybe_qself, path),
                ExprKind::Break(opt_label, opt_expr) =>
                    visit_visitable!(vis, opt_label, opt_expr),
                ExprKind::Continue(opt_label) =>
                    visit_visitable!(vis, opt_label),
                ExprKind::Ret(optional_expression) | ExprKind::Yeet(optional_expression) =>
                    visit_visitable!(vis, optional_expression),
                ExprKind::Become(expr) =>
                    visit_visitable!(vis, expr),
                ExprKind::MacCall(mac) =>
                    visit_visitable!(vis, mac),
                ExprKind::Paren(subexpression) =>
                    visit_visitable!(vis, subexpression),
                ExprKind::InlineAsm(asm) =>
                    visit_visitable!(vis, asm),
                ExprKind::FormatArgs(f) =>
                    visit_visitable!(vis, f),
                ExprKind::OffsetOf(container, fields) =>
                    visit_visitable!(vis, container, fields),
                ExprKind::Yield(kind) =>
                    visit_visitable!(vis, kind),
                ExprKind::Try(subexpression) =>
                    visit_visitable!(vis, subexpression),
                ExprKind::TryBlock(body, optional_type) =>
                    visit_visitable!(vis, body, optional_type),
                ExprKind::Lit(token) =>
                    visit_visitable!(vis, token),
                ExprKind::IncludedBytes(bytes) =>
                    visit_visitable!(vis, bytes),
                ExprKind::UnsafeBinderCast(kind, expr, ty) =>
                    visit_visitable!(vis, kind, expr, ty),
                ExprKind::GcaMacro(expr) =>
                    visit_visitable!(vis, expr),
                ExprKind::Err(_guar) => {}
                ExprKind::Dummy => {}
            }

            visit_visitable!(vis, span);
            V::Result::output()
        });

        crate::visit::for_each_ast_visit_hook! { fn_walk! }
    };
}
pub(crate) use common_visitor_and_walkers;

common_visitor_and_walkers!(Visitor<'a>);

macro_rules! generate_list_visit_fns {
    ($($visit_fn:ident, $Ty:ty $(, $extra:ident: $ExtraTy:ty)?;)+) => {
        $(impl_visitable!(|&'a self: ThinVec<$Ty>, visitor: &mut V $(, $extra: $ExtraTy)?| {
            walk_list!(visitor, $visit_fn, self $(, $extra)?);
            V::Result::output()
        });)+
    }
}

generate_list_visit_fns! {
    // tidy-alphabetical-start
    visit_arm, Arm;
    visit_assoc_item, Box<AssocItem>, ctxt: AssocCtxt;
    visit_expr, Box<Expr>;
    visit_expr_field, ExprField;
    visit_field_def, FieldDef;
    visit_foreign_item, Box<ForeignItem>;
    visit_generic_param, GenericParam;
    visit_item, Box<Item>;
    visit_param, Param;
    visit_pat_field, PatField;
    visit_stmt, Stmt;
    visit_variant, Variant;
    visit_where_predicate, WherePredicate;
    // tidy-alphabetical-end
}

pub fn walk_stmt<'a, V: Visitor<'a>>(visitor: &mut V, statement: &'a Stmt) -> V::Result {
    let Stmt { id, kind, span: _ } = statement;
    try_visit!(visitor.visit_id(*id));
    match kind {
        StmtKind::Let(local) => try_visit!(visitor.visit_local(local)),
        StmtKind::Item(item) => try_visit!(visitor.visit_item(item)),
        StmtKind::Expr(expr) | StmtKind::Semi(expr) => try_visit!(visitor.visit_expr(expr)),
        StmtKind::Empty => {}
        StmtKind::MacCall(mac) => {
            let MacCallStmt { mac, attrs, style: _, tokens: _ } = &**mac;
            walk_list!(visitor, visit_attribute, attrs);
            try_visit!(visitor.visit_mac_call(mac));
        }
    }
    V::Result::output()
}