mut_visit.rs

//! A `MutVisitor` represents an AST modification; it accepts an AST piece and
//! mutates it in place. So, for instance, macro expansion is a `MutVisitor`
//! that walks over an AST and modifies it.
//!
//! Note: using a `MutVisitor` (other than the `MacroExpander` `MutVisitor`) on
//! an AST before macro expansion is probably a bad idea. For instance,
//! a `MutVisitor` renaming item names in a module will miss all of those
//! that are created by the expansion of a macro.

use std::ops::DerefMut;
use std::panic;

use rustc_data_structures::flat_map_in_place::FlatMapInPlace;
use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_data_structures::sync::Lrc;
use rustc_span::source_map::Spanned;
use rustc_span::symbol::Ident;
use rustc_span::Span;
use smallvec::{smallvec, Array, SmallVec};
use thin_vec::ThinVec;

use crate::ast::*;
use crate::ptr::P;
use crate::token::{self, Token};
use crate::tokenstream::*;
use crate::visit::{AssocCtxt, BoundKind};

pub trait ExpectOne<A: Array> {
    fn expect_one(self, err: &'static str) -> A::Item;
}

impl<A: Array> ExpectOne<A> for SmallVec<A> {
    fn expect_one(self, err: &'static str) -> A::Item {
        assert!(self.len() == 1, "{}", err);
        self.into_iter().next().unwrap()
    }
}

pub trait WalkItemKind {
    fn walk(&mut self, span: Span, id: NodeId, visitor: &mut impl MutVisitor);
}

pub trait MutVisitor: Sized {
    /// Mutable token visiting only exists for the `macro_rules` token marker and should not be
    /// used otherwise. Token visitor would be entirely separate from the regular visitor if
    /// the marker didn't have to visit AST fragments in nonterminal tokens.
    const VISIT_TOKENS: bool = false;

    // Methods in this trait have one of three forms:
    //
    //   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
    //
    // Any additions to this trait should happen in form of a call to a public
    // `noop_*` function that only calls out to the visitor again, not other
    // `noop_*` functions. This is a necessary API workaround to the problem of
    // not being able to call out to the super default method in an overridden
    // default method.
    //
    // 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_crate(&mut self, c: &mut Crate) {
        walk_crate(self, c)
    }

    fn visit_meta_list_item(&mut self, list_item: &mut NestedMetaItem) {
        walk_meta_list_item(self, list_item);
    }

    fn visit_meta_item(&mut self, meta_item: &mut MetaItem) {
        walk_meta_item(self, meta_item);
    }

    fn visit_use_tree(&mut self, use_tree: &mut UseTree) {
        walk_use_tree(self, use_tree);
    }

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

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

    fn visit_fn_header(&mut self, header: &mut FnHeader) {
        walk_fn_header(self, header);
    }

    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: P<AssocItem>,
        _ctxt: AssocCtxt,
    ) -> SmallVec<[P<AssocItem>; 1]> {
        walk_flat_map_item(self, i)
    }

    fn visit_fn_decl(&mut self, d: &mut P<FnDecl>) {
        walk_fn_decl(self, d);
    }

    /// `Span` and `NodeId` are mutated at the caller site.
    fn visit_fn(&mut self, fk: FnKind<'_>, _: Span, _: NodeId) {
        walk_fn(self, fk)
    }

    fn visit_coroutine_kind(&mut self, a: &mut CoroutineKind) {
        walk_coroutine_kind(self, a);
    }

    fn visit_closure_binder(&mut self, b: &mut ClosureBinder) {
        walk_closure_binder(self, b);
    }

    fn visit_block(&mut self, b: &mut P<Block>) {
        walk_block(self, b);
    }

    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 visit_pat(&mut self, p: &mut P<Pat>) {
        walk_pat(self, p);
    }

    fn visit_anon_const(&mut self, c: &mut AnonConst) {
        walk_anon_const(self, c);
    }

    fn visit_expr(&mut self, e: &mut P<Expr>) {
        walk_expr(self, e);
    }

    /// 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: &mut P<Expr>) {
        self.visit_expr(ex)
    }

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

    fn visit_generic_arg(&mut self, arg: &mut GenericArg) {
        walk_generic_arg(self, arg);
    }

    fn visit_ty(&mut self, t: &mut P<Ty>) {
        walk_ty(self, t);
    }

    fn visit_lifetime(&mut self, l: &mut Lifetime) {
        walk_lifetime(self, l);
    }

    fn visit_assoc_item_constraint(&mut self, c: &mut AssocItemConstraint) {
        walk_assoc_item_constraint(self, c);
    }

    fn visit_foreign_mod(&mut self, nm: &mut ForeignMod) {
        walk_foreign_mod(self, nm);
    }

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

    fn visit_ident(&mut self, i: &mut Ident) {
        walk_ident(self, i);
    }

    fn visit_path(&mut self, p: &mut Path) {
        walk_path(self, p);
    }

    fn visit_path_segment(&mut self, p: &mut PathSegment) {
        walk_path_segment(self, p)
    }

    fn visit_qself(&mut self, qs: &mut Option<P<QSelf>>) {
        walk_qself(self, qs);
    }

    fn visit_generic_args(&mut self, p: &mut GenericArgs) {
        walk_generic_args(self, p);
    }

    fn visit_angle_bracketed_parameter_data(&mut self, p: &mut AngleBracketedArgs) {
        walk_angle_bracketed_parameter_data(self, p);
    }

    fn visit_parenthesized_parameter_data(&mut self, p: &mut ParenthesizedArgs) {
        walk_parenthesized_parameter_data(self, p);
    }

    fn visit_local(&mut self, l: &mut P<Local>) {
        walk_local(self, l);
    }

    fn visit_mac_call(&mut self, mac: &mut MacCall) {
        walk_mac(self, mac);
    }

    fn visit_macro_def(&mut self, def: &mut MacroDef) {
        walk_macro_def(self, def);
    }

    fn visit_label(&mut self, label: &mut Label) {
        walk_label(self, label);
    }

    fn visit_attribute(&mut self, at: &mut Attribute) {
        walk_attribute(self, at);
    }

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

    fn visit_generics(&mut self, generics: &mut Generics) {
        walk_generics(self, generics);
    }

    fn visit_trait_ref(&mut self, tr: &mut TraitRef) {
        walk_trait_ref(self, tr);
    }

    fn visit_poly_trait_ref(&mut self, p: &mut PolyTraitRef) {
        walk_poly_trait_ref(self, p);
    }

    fn visit_variant_data(&mut self, vdata: &mut VariantData) {
        walk_variant_data(self, vdata);
    }

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

    fn visit_param_bound(&mut self, tpb: &mut GenericBound, _ctxt: BoundKind) {
        walk_param_bound(self, tpb);
    }

    fn visit_precise_capturing_arg(&mut self, arg: &mut PreciseCapturingArg) {
        walk_precise_capturing_arg(self, arg);
    }

    fn visit_mt(&mut self, mt: &mut MutTy) {
        walk_mt(self, mt);
    }

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

    fn visit_where_clause(&mut self, where_clause: &mut WhereClause) {
        walk_where_clause(self, where_clause);
    }

    fn visit_where_predicate(&mut self, where_predicate: &mut WherePredicate) {
        walk_where_predicate(self, where_predicate);
    }

    fn visit_vis(&mut self, vis: &mut Visibility) {
        walk_vis(self, vis);
    }

    fn visit_id(&mut self, _id: &mut NodeId) {
        // Do nothing.
    }

    fn visit_span(&mut self, _sp: &mut Span) {
        // Do nothing.
    }

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

    fn visit_inline_asm(&mut self, asm: &mut InlineAsm) {
        walk_inline_asm(self, asm)
    }

    fn visit_inline_asm_sym(&mut self, sym: &mut InlineAsmSym) {
        walk_inline_asm_sym(self, sym)
    }

    fn visit_format_args(&mut self, fmt: &mut FormatArgs) {
        walk_format_args(self, fmt)
    }

    fn visit_capture_by(&mut self, capture_by: &mut CaptureBy) {
        walk_capture_by(self, capture_by)
    }
}

/// Use a map-style function (`FnOnce(T) -> T`) to overwrite a `&mut T`. Useful
/// when using a `flat_map_*` or `filter_map_*` method within a `visit_`
/// method.
//
// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
pub fn visit_clobber<T: DummyAstNode>(t: &mut T, f: impl FnOnce(T) -> T) {
    let old_t = std::mem::replace(t, T::dummy());
    *t = f(old_t);
}

// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
#[inline]
fn visit_vec<T, F>(elems: &mut Vec<T>, mut visit_elem: F)
where
    F: FnMut(&mut T),
{
    for elem in elems {
        visit_elem(elem);
    }
}

// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
#[inline]
fn visit_thin_vec<T, F>(elems: &mut ThinVec<T>, mut visit_elem: F)
where
    F: FnMut(&mut T),
{
    for elem in elems {
        visit_elem(elem);
    }
}

// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
#[inline]
fn visit_opt<T, F>(opt: &mut Option<T>, mut visit_elem: F)
where
    F: FnMut(&mut T),
{
    if let Some(elem) = opt {
        visit_elem(elem);
    }
}

// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
fn visit_attrs<T: MutVisitor>(vis: &mut T, attrs: &mut AttrVec) {
    for attr in attrs.iter_mut() {
        vis.visit_attribute(attr);
    }
}

// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
#[allow(unused)]
fn visit_exprs<T: MutVisitor>(vis: &mut T, exprs: &mut Vec<P<Expr>>) {
    exprs.flat_map_in_place(|expr| vis.filter_map_expr(expr))
}

// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
fn visit_thin_exprs<T: MutVisitor>(vis: &mut T, exprs: &mut ThinVec<P<Expr>>) {
    exprs.flat_map_in_place(|expr| vis.filter_map_expr(expr))
}

// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
fn visit_bounds<T: MutVisitor>(vis: &mut T, bounds: &mut GenericBounds, ctxt: BoundKind) {
    visit_vec(bounds, |bound| vis.visit_param_bound(bound, ctxt));
}

// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
fn visit_attr_args<T: MutVisitor>(vis: &mut T, args: &mut AttrArgs) {
    match args {
        AttrArgs::Empty => {}
        AttrArgs::Delimited(args) => visit_delim_args(vis, args),
        AttrArgs::Eq(eq_span, AttrArgsEq::Ast(expr)) => {
            vis.visit_expr(expr);
            vis.visit_span(eq_span);
        }
        AttrArgs::Eq(_eq_span, AttrArgsEq::Hir(lit)) => {
            unreachable!("in literal form when visiting mac args eq: {:?}", lit)
        }
    }
}

// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
fn visit_delim_args<T: MutVisitor>(vis: &mut T, args: &mut DelimArgs) {
    let DelimArgs { dspan, delim: _, tokens } = args;
    visit_tts(vis, tokens);
    visit_delim_span(vis, dspan);
}

pub fn visit_delim_span<T: MutVisitor>(vis: &mut T, DelimSpan { open, close }: &mut DelimSpan) {
    vis.visit_span(open);
    vis.visit_span(close);
}

pub fn walk_flat_map_pat_field<T: MutVisitor>(
    vis: &mut T,
    mut fp: PatField,
) -> SmallVec<[PatField; 1]> {
    let PatField { attrs, id, ident, is_placeholder: _, is_shorthand: _, pat, span } = &mut fp;
    vis.visit_id(id);
    visit_attrs(vis, attrs);
    vis.visit_ident(ident);
    vis.visit_pat(pat);
    vis.visit_span(span);
    smallvec![fp]
}

fn walk_use_tree<T: MutVisitor>(vis: &mut T, use_tree: &mut UseTree) {
    let UseTree { prefix, kind, span } = use_tree;
    vis.visit_path(prefix);
    match kind {
        UseTreeKind::Simple(rename) => visit_opt(rename, |rename| vis.visit_ident(rename)),
        UseTreeKind::Nested { items, span } => {
            for (tree, id) in items {
                vis.visit_id(id);
                vis.visit_use_tree(tree);
            }
            vis.visit_span(span);
        }
        UseTreeKind::Glob => {}
    }
    vis.visit_span(span);
}

pub fn walk_flat_map_arm<T: MutVisitor>(vis: &mut T, mut arm: Arm) -> SmallVec<[Arm; 1]> {
    let Arm { attrs, pat, guard, body, span, id, is_placeholder: _ } = &mut arm;
    vis.visit_id(id);
    visit_attrs(vis, attrs);
    vis.visit_pat(pat);
    visit_opt(guard, |guard| vis.visit_expr(guard));
    visit_opt(body, |body| vis.visit_expr(body));
    vis.visit_span(span);
    smallvec![arm]
}

fn walk_assoc_item_constraint<T: MutVisitor>(
    vis: &mut T,
    AssocItemConstraint { id, ident, gen_args, kind, span }: &mut AssocItemConstraint,
) {
    vis.visit_id(id);
    vis.visit_ident(ident);
    if let Some(gen_args) = gen_args {
        vis.visit_generic_args(gen_args);
    }
    match kind {
        AssocItemConstraintKind::Equality { term } => match term {
            Term::Ty(ty) => vis.visit_ty(ty),
            Term::Const(c) => vis.visit_anon_const(c),
        },
        AssocItemConstraintKind::Bound { bounds } => visit_bounds(vis, bounds, BoundKind::Bound),
    }
    vis.visit_span(span);
}

pub fn walk_ty<T: MutVisitor>(vis: &mut T, ty: &mut P<Ty>) {
    let Ty { id, kind, span, tokens } = ty.deref_mut();
    vis.visit_id(id);
    match kind {
        TyKind::Err(_guar) => {}
        TyKind::Infer | TyKind::ImplicitSelf | TyKind::Dummy | TyKind::Never | TyKind::CVarArgs => {
        }
        TyKind::Slice(ty) => vis.visit_ty(ty),
        TyKind::Ptr(mt) => vis.visit_mt(mt),
        TyKind::Ref(lt, mt) => {
            visit_opt(lt, |lt| vis.visit_lifetime(lt));
            vis.visit_mt(mt);
        }
        TyKind::BareFn(bft) => {
            let BareFnTy { safety, ext: _, generic_params, decl, decl_span } = bft.deref_mut();
            visit_safety(vis, safety);
            generic_params.flat_map_in_place(|param| vis.flat_map_generic_param(param));
            vis.visit_fn_decl(decl);
            vis.visit_span(decl_span);
        }
        TyKind::Tup(tys) => visit_thin_vec(tys, |ty| vis.visit_ty(ty)),
        TyKind::Paren(ty) => vis.visit_ty(ty),
        TyKind::Pat(ty, pat) => {
            vis.visit_ty(ty);
            vis.visit_pat(pat);
        }
        TyKind::Path(qself, path) => {
            vis.visit_qself(qself);
            vis.visit_path(path);
        }
        TyKind::Array(ty, length) => {
            vis.visit_ty(ty);
            vis.visit_anon_const(length);
        }
        TyKind::Typeof(expr) => vis.visit_anon_const(expr),
        TyKind::TraitObject(bounds, _syntax) => {
            visit_vec(bounds, |bound| vis.visit_param_bound(bound, BoundKind::TraitObject))
        }
        TyKind::ImplTrait(id, bounds) => {
            vis.visit_id(id);
            visit_vec(bounds, |bound| vis.visit_param_bound(bound, BoundKind::Impl));
        }
        TyKind::MacCall(mac) => vis.visit_mac_call(mac),
        TyKind::AnonStruct(id, fields) | TyKind::AnonUnion(id, fields) => {
            vis.visit_id(id);
            fields.flat_map_in_place(|field| vis.flat_map_field_def(field));
        }
    }
    visit_lazy_tts(vis, tokens);
    vis.visit_span(span);
}

fn walk_foreign_mod<T: MutVisitor>(vis: &mut T, foreign_mod: &mut ForeignMod) {
    let ForeignMod { safety, abi: _, items } = foreign_mod;
    visit_safety(vis, safety);
    items.flat_map_in_place(|item| vis.flat_map_foreign_item(item));
}

pub fn walk_flat_map_variant<T: MutVisitor>(
    visitor: &mut T,
    mut variant: Variant,
) -> SmallVec<[Variant; 1]> {
    let Variant { ident, vis, attrs, id, data, disr_expr, span, is_placeholder: _ } = &mut variant;
    visitor.visit_id(id);
    visit_attrs(visitor, attrs);
    visitor.visit_vis(vis);
    visitor.visit_ident(ident);
    visitor.visit_variant_data(data);
    visit_opt(disr_expr, |disr_expr| visitor.visit_anon_const(disr_expr));
    visitor.visit_span(span);
    smallvec![variant]
}

fn walk_ident<T: MutVisitor>(vis: &mut T, Ident { name: _, span }: &mut Ident) {
    vis.visit_span(span);
}

fn walk_path_segment<T: MutVisitor>(vis: &mut T, segment: &mut PathSegment) {
    let PathSegment { ident, id, args } = segment;
    vis.visit_id(id);
    vis.visit_ident(ident);
    visit_opt(args, |args| vis.visit_generic_args(args));
}

fn walk_path<T: MutVisitor>(vis: &mut T, Path { segments, span, tokens }: &mut Path) {
    for segment in segments {
        vis.visit_path_segment(segment);
    }
    visit_lazy_tts(vis, tokens);
    vis.visit_span(span);
}

fn walk_qself<T: MutVisitor>(vis: &mut T, qself: &mut Option<P<QSelf>>) {
    visit_opt(qself, |qself| {
        let QSelf { ty, path_span, position: _ } = &mut **qself;
        vis.visit_ty(ty);
        vis.visit_span(path_span);
    })
}

fn walk_generic_args<T: MutVisitor>(vis: &mut T, generic_args: &mut GenericArgs) {
    match generic_args {
        GenericArgs::AngleBracketed(data) => vis.visit_angle_bracketed_parameter_data(data),
        GenericArgs::Parenthesized(data) => vis.visit_parenthesized_parameter_data(data),
        GenericArgs::ParenthesizedElided(span) => vis.visit_span(span),
    }
}

fn walk_generic_arg<T: MutVisitor>(vis: &mut T, arg: &mut GenericArg) {
    match arg {
        GenericArg::Lifetime(lt) => vis.visit_lifetime(lt),
        GenericArg::Type(ty) => vis.visit_ty(ty),
        GenericArg::Const(ct) => vis.visit_anon_const(ct),
    }
}

fn walk_angle_bracketed_parameter_data<T: MutVisitor>(vis: &mut T, data: &mut AngleBracketedArgs) {
    let AngleBracketedArgs { args, span } = data;
    visit_thin_vec(args, |arg| match arg {
        AngleBracketedArg::Arg(arg) => vis.visit_generic_arg(arg),
        AngleBracketedArg::Constraint(constraint) => vis.visit_assoc_item_constraint(constraint),
    });
    vis.visit_span(span);
}

fn walk_parenthesized_parameter_data<T: MutVisitor>(vis: &mut T, args: &mut ParenthesizedArgs) {
    let ParenthesizedArgs { inputs, output, span, inputs_span } = args;
    visit_thin_vec(inputs, |input| vis.visit_ty(input));
    walk_fn_ret_ty(vis, output);
    vis.visit_span(span);
    vis.visit_span(inputs_span);
}

fn walk_local<T: MutVisitor>(vis: &mut T, local: &mut P<Local>) {
    let Local { id, pat, ty, kind, span, colon_sp, attrs, tokens } = local.deref_mut();
    vis.visit_id(id);
    visit_attrs(vis, attrs);
    vis.visit_pat(pat);
    visit_opt(ty, |ty| vis.visit_ty(ty));
    match kind {
        LocalKind::Decl => {}
        LocalKind::Init(init) => {
            vis.visit_expr(init);
        }
        LocalKind::InitElse(init, els) => {
            vis.visit_expr(init);
            vis.visit_block(els);
        }
    }
    visit_lazy_tts(vis, tokens);
    visit_opt(colon_sp, |sp| vis.visit_span(sp));
    vis.visit_span(span);
}

fn walk_attribute<T: MutVisitor>(vis: &mut T, attr: &mut Attribute) {
    let Attribute { kind, id: _, style: _, span, allowed_diagnostic_attribute: _ } = attr;
    match kind {
        AttrKind::Normal(normal) => {
            let NormalAttr {
                item: AttrItem { unsafety: _, path, args, tokens },
                tokens: attr_tokens,
            } = &mut **normal;
            vis.visit_path(path);
            visit_attr_args(vis, args);
            visit_lazy_tts(vis, tokens);
            visit_lazy_tts(vis, attr_tokens);
        }
        AttrKind::DocComment(_kind, _sym) => {}
    }
    vis.visit_span(span);
}

fn walk_mac<T: MutVisitor>(vis: &mut T, mac: &mut MacCall) {
    let MacCall { path, args } = mac;
    vis.visit_path(path);
    visit_delim_args(vis, args);
}

fn walk_macro_def<T: MutVisitor>(vis: &mut T, macro_def: &mut MacroDef) {
    let MacroDef { body, macro_rules: _ } = macro_def;
    visit_delim_args(vis, body);
}

fn walk_meta_list_item<T: MutVisitor>(vis: &mut T, li: &mut NestedMetaItem) {
    match li {
        NestedMetaItem::MetaItem(mi) => vis.visit_meta_item(mi),
        NestedMetaItem::Lit(_lit) => {}
    }
}

fn walk_meta_item<T: MutVisitor>(vis: &mut T, mi: &mut MetaItem) {
    let MetaItem { unsafety: _, path: _, kind, span } = mi;
    match kind {
        MetaItemKind::Word => {}
        MetaItemKind::List(mis) => visit_thin_vec(mis, |mi| vis.visit_meta_list_item(mi)),
        MetaItemKind::NameValue(_s) => {}
    }
    vis.visit_span(span);
}

pub fn walk_flat_map_param<T: MutVisitor>(vis: &mut T, mut param: Param) -> SmallVec<[Param; 1]> {
    let Param { attrs, id, pat, span, ty, is_placeholder: _ } = &mut param;
    vis.visit_id(id);
    visit_attrs(vis, attrs);
    vis.visit_pat(pat);
    vis.visit_ty(ty);
    vis.visit_span(span);
    smallvec![param]
}

// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
fn visit_attr_tt<T: MutVisitor>(vis: &mut T, tt: &mut AttrTokenTree) {
    match tt {
        AttrTokenTree::Token(token, _spacing) => {
            visit_token(vis, token);
        }
        AttrTokenTree::Delimited(dspan, _spacing, _delim, tts) => {
            visit_attr_tts(vis, tts);
            visit_delim_span(vis, dspan);
        }
        AttrTokenTree::AttrsTarget(AttrsTarget { attrs, tokens }) => {
            visit_attrs(vis, attrs);
            visit_lazy_tts_opt_mut(vis, Some(tokens));
        }
    }
}

// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
fn visit_tt<T: MutVisitor>(vis: &mut T, tt: &mut TokenTree) {
    match tt {
        TokenTree::Token(token, _spacing) => {
            visit_token(vis, token);
        }
        TokenTree::Delimited(dspan, _spacing, _delim, tts) => {
            visit_tts(vis, tts);
            visit_delim_span(vis, dspan);
        }
    }
}

// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
fn visit_tts<T: MutVisitor>(vis: &mut T, TokenStream(tts): &mut TokenStream) {
    if T::VISIT_TOKENS && !tts.is_empty() {
        let tts = Lrc::make_mut(tts);
        visit_vec(tts, |tree| visit_tt(vis, tree));
    }
}

fn visit_attr_tts<T: MutVisitor>(vis: &mut T, AttrTokenStream(tts): &mut AttrTokenStream) {
    if T::VISIT_TOKENS && !tts.is_empty() {
        let tts = Lrc::make_mut(tts);
        visit_vec(tts, |tree| visit_attr_tt(vis, tree));
    }
}

fn visit_lazy_tts_opt_mut<T: MutVisitor>(vis: &mut T, lazy_tts: Option<&mut LazyAttrTokenStream>) {
    if T::VISIT_TOKENS {
        if let Some(lazy_tts) = lazy_tts {
            let mut tts = lazy_tts.to_attr_token_stream();
            visit_attr_tts(vis, &mut tts);
            *lazy_tts = LazyAttrTokenStream::new(tts);
        }
    }
}

fn visit_lazy_tts<T: MutVisitor>(vis: &mut T, lazy_tts: &mut Option<LazyAttrTokenStream>) {
    visit_lazy_tts_opt_mut(vis, lazy_tts.as_mut());
}

/// Applies ident visitor if it's an ident; applies other visits to interpolated nodes.
/// In practice the ident part is not actually used by specific visitors right now,
/// but there's a test below checking that it works.
// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
pub fn visit_token<T: MutVisitor>(vis: &mut T, t: &mut Token) {
    let Token { kind, span } = t;
    match kind {
        token::Ident(name, _ /*raw*/) | token::Lifetime(name) => {
            let mut ident = Ident::new(*name, *span);
            vis.visit_ident(&mut ident);
            *name = ident.name;
            *span = ident.span;
            return; // Avoid visiting the span for the second time.
        }
        token::NtIdent(ident, _is_raw) => {
            vis.visit_ident(ident);
        }
        token::NtLifetime(ident) => {
            vis.visit_ident(ident);
        }
        token::Interpolated(nt) => {
            let nt = Lrc::make_mut(nt);
            visit_nonterminal(vis, nt);
        }
        _ => {}
    }
    vis.visit_span(span);
}

// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
/// Applies the visitor to elements of interpolated nodes.
//
// N.B., this can occur only when applying a visitor to partially expanded
// code, where parsed pieces have gotten implanted ito *other* macro
// invocations. This is relevant for macro hygiene, but possibly not elsewhere.
//
// One problem here occurs because the types for flat_map_item, flat_map_stmt,
// etc., allow the visitor to return *multiple* items; this is a problem for the
// nodes here, because they insist on having exactly one piece. One solution
// would be to mangle the MutVisitor trait to include one-to-many and
// one-to-one versions of these entry points, but that would probably confuse a
// lot of people and help very few. Instead, I'm just going to put in dynamic
// checks. I think the performance impact of this will be pretty much
// nonexistent. The danger is that someone will apply a `MutVisitor` to a
// partially expanded node, and will be confused by the fact that their
// `flat_map_item` or `flat_map_stmt` isn't getting called on `NtItem` or `NtStmt`
// nodes. Hopefully they'll wind up reading this comment, and doing something
// appropriate.
//
// BTW, design choice: I considered just changing the type of, e.g., `NtItem` to
// contain multiple items, but decided against it when I looked at
// `parse_item_or_view_item` and tried to figure out what I would do with
// multiple items there....
fn visit_nonterminal<T: MutVisitor>(vis: &mut T, nt: &mut token::Nonterminal) {
    match nt {
        token::NtItem(item) => visit_clobber(item, |item| {
            // This is probably okay, because the only visitors likely to
            // peek inside interpolated nodes will be renamings/markings,
            // which map single items to single items.
            vis.flat_map_item(item).expect_one("expected visitor to produce exactly one item")
        }),
        token::NtBlock(block) => vis.visit_block(block),
        token::NtStmt(stmt) => visit_clobber(stmt, |stmt| {
            // See reasoning above.
            stmt.map(|stmt| {
                vis.flat_map_stmt(stmt).expect_one("expected visitor to produce exactly one item")
            })
        }),
        token::NtPat(pat) => vis.visit_pat(pat),
        token::NtExpr(expr) => vis.visit_expr(expr),
        token::NtTy(ty) => vis.visit_ty(ty),
        token::NtLiteral(expr) => vis.visit_expr(expr),
        token::NtMeta(item) => {
            let AttrItem { unsafety: _, path, args, tokens } = item.deref_mut();
            vis.visit_path(path);
            visit_attr_args(vis, args);
            visit_lazy_tts(vis, tokens);
        }
        token::NtPath(path) => vis.visit_path(path),
        token::NtVis(visib) => vis.visit_vis(visib),
    }
}

// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
fn visit_defaultness<T: MutVisitor>(vis: &mut T, defaultness: &mut Defaultness) {
    match defaultness {
        Defaultness::Default(span) => vis.visit_span(span),
        Defaultness::Final => {}
    }
}

// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
fn visit_safety<T: MutVisitor>(vis: &mut T, safety: &mut Safety) {
    match safety {
        Safety::Unsafe(span) => vis.visit_span(span),
        Safety::Safe(span) => vis.visit_span(span),
        Safety::Default => {}
    }
}

// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
fn visit_polarity<T: MutVisitor>(vis: &mut T, polarity: &mut ImplPolarity) {
    match polarity {
        ImplPolarity::Positive => {}
        ImplPolarity::Negative(span) => vis.visit_span(span),
    }
}

// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
fn visit_constness<T: MutVisitor>(vis: &mut T, constness: &mut Const) {
    match constness {
        Const::Yes(span) => vis.visit_span(span),
        Const::No => {}
    }
}

fn walk_closure_binder<T: MutVisitor>(vis: &mut T, binder: &mut ClosureBinder) {
    match binder {
        ClosureBinder::NotPresent => {}
        ClosureBinder::For { span: _, generic_params } => {
            generic_params.flat_map_in_place(|param| vis.flat_map_generic_param(param));
        }
    }
}

fn walk_coroutine_kind<T: MutVisitor>(vis: &mut T, coroutine_kind: &mut CoroutineKind) {
    match coroutine_kind {
        CoroutineKind::Async { span, closure_id, return_impl_trait_id }
        | CoroutineKind::Gen { span, closure_id, return_impl_trait_id }
        | CoroutineKind::AsyncGen { span, closure_id, return_impl_trait_id } => {
            vis.visit_id(closure_id);
            vis.visit_id(return_impl_trait_id);
            vis.visit_span(span);
        }
    }
}

fn walk_fn<T: MutVisitor>(vis: &mut T, kind: FnKind<'_>) {
    match kind {
        FnKind::Fn(FnSig { header, decl, span }, generics, body) => {
            // Identifier and visibility are visited as a part of the item.
            vis.visit_fn_header(header);
            vis.visit_generics(generics);
            vis.visit_fn_decl(decl);
            if let Some(body) = body {
                vis.visit_block(body);
            }
            vis.visit_span(span);
        }
        FnKind::Closure(binder, decl, body) => {
            vis.visit_closure_binder(binder);
            vis.visit_fn_decl(decl);
            vis.visit_expr(body);
        }
    }
}

fn walk_fn_decl<T: MutVisitor>(vis: &mut T, decl: &mut P<FnDecl>) {
    let FnDecl { inputs, output } = decl.deref_mut();
    inputs.flat_map_in_place(|param| vis.flat_map_param(param));
    walk_fn_ret_ty(vis, output);
}

fn walk_fn_ret_ty<T: MutVisitor>(vis: &mut T, fn_ret_ty: &mut FnRetTy) {
    match fn_ret_ty {
        FnRetTy::Default(span) => vis.visit_span(span),
        FnRetTy::Ty(ty) => vis.visit_ty(ty),
    }
}

fn walk_param_bound<T: MutVisitor>(vis: &mut T, pb: &mut GenericBound) {
    match pb {
        GenericBound::Trait(ty, _modifier) => vis.visit_poly_trait_ref(ty),
        GenericBound::Outlives(lifetime) => walk_lifetime(vis, lifetime),
        GenericBound::Use(args, span) => {
            for arg in args {
                vis.visit_precise_capturing_arg(arg);
            }
            vis.visit_span(span);
        }
    }
}

fn walk_precise_capturing_arg<T: MutVisitor>(vis: &mut T, arg: &mut PreciseCapturingArg) {
    match arg {
        PreciseCapturingArg::Lifetime(lt) => {
            vis.visit_lifetime(lt);
        }
        PreciseCapturingArg::Arg(path, id) => {
            vis.visit_id(id);
            vis.visit_path(path);
        }
    }
}

pub fn walk_flat_map_generic_param<T: MutVisitor>(
    vis: &mut T,
    mut param: GenericParam,
) -> SmallVec<[GenericParam; 1]> {
    let GenericParam { id, ident, attrs, bounds, kind, colon_span, is_placeholder: _ } = &mut param;
    vis.visit_id(id);
    visit_attrs(vis, attrs);
    vis.visit_ident(ident);
    visit_vec(bounds, |bound| vis.visit_param_bound(bound, BoundKind::Bound));
    match kind {
        GenericParamKind::Lifetime => {}
        GenericParamKind::Type { default } => {
            visit_opt(default, |default| vis.visit_ty(default));
        }
        GenericParamKind::Const { ty, kw_span: _, default } => {
            vis.visit_ty(ty);
            visit_opt(default, |default| vis.visit_anon_const(default));
        }
    }
    if let Some(colon_span) = colon_span {
        vis.visit_span(colon_span);
    }
    smallvec![param]
}

fn walk_label<T: MutVisitor>(vis: &mut T, Label { ident }: &mut Label) {
    vis.visit_ident(ident);
}

fn walk_lifetime<T: MutVisitor>(vis: &mut T, Lifetime { id, ident }: &mut Lifetime) {
    vis.visit_id(id);
    vis.visit_ident(ident);
}

fn walk_generics<T: MutVisitor>(vis: &mut T, generics: &mut Generics) {
    let Generics { params, where_clause, span } = generics;
    params.flat_map_in_place(|param| vis.flat_map_generic_param(param));
    vis.visit_where_clause(where_clause);
    vis.visit_span(span);
}

fn walk_ty_alias_where_clauses<T: MutVisitor>(vis: &mut T, tawcs: &mut TyAliasWhereClauses) {
    let TyAliasWhereClauses { before, after, split: _ } = tawcs;
    let TyAliasWhereClause { has_where_token: _, span: span_before } = before;
    let TyAliasWhereClause { has_where_token: _, span: span_after } = after;
    vis.visit_span(span_before);
    vis.visit_span(span_after);
}

fn walk_where_clause<T: MutVisitor>(vis: &mut T, wc: &mut WhereClause) {
    let WhereClause { has_where_token: _, predicates, span } = wc;
    visit_thin_vec(predicates, |predicate| vis.visit_where_predicate(predicate));
    vis.visit_span(span);
}

fn walk_where_predicate<T: MutVisitor>(vis: &mut T, pred: &mut WherePredicate) {
    match pred {
        WherePredicate::BoundPredicate(bp) => {
            let WhereBoundPredicate { span, bound_generic_params, bounded_ty, bounds } = bp;
            bound_generic_params.flat_map_in_place(|param| vis.flat_map_generic_param(param));
            vis.visit_ty(bounded_ty);
            visit_vec(bounds, |bound| vis.visit_param_bound(bound, BoundKind::Bound));
            vis.visit_span(span);
        }
        WherePredicate::RegionPredicate(rp) => {
            let WhereRegionPredicate { span, lifetime, bounds } = rp;
            vis.visit_lifetime(lifetime);
            visit_vec(bounds, |bound| vis.visit_param_bound(bound, BoundKind::Bound));
            vis.visit_span(span);
        }
        WherePredicate::EqPredicate(ep) => {
            let WhereEqPredicate { span, lhs_ty, rhs_ty } = ep;
            vis.visit_ty(lhs_ty);
            vis.visit_ty(rhs_ty);
            vis.visit_span(span);
        }
    }
}

fn walk_variant_data<T: MutVisitor>(vis: &mut T, vdata: &mut VariantData) {
    match vdata {
        VariantData::Struct { fields, recovered: _ } => {
            fields.flat_map_in_place(|field| vis.flat_map_field_def(field));
        }
        VariantData::Tuple(fields, id) => {
            vis.visit_id(id);
            fields.flat_map_in_place(|field| vis.flat_map_field_def(field));
        }
        VariantData::Unit(id) => vis.visit_id(id),
    }
}

fn walk_trait_ref<T: MutVisitor>(vis: &mut T, TraitRef { path, ref_id }: &mut TraitRef) {
    vis.visit_id(ref_id);
    vis.visit_path(path);
}

fn walk_poly_trait_ref<T: MutVisitor>(vis: &mut T, p: &mut PolyTraitRef) {
    let PolyTraitRef { bound_generic_params, trait_ref, span } = p;
    bound_generic_params.flat_map_in_place(|param| vis.flat_map_generic_param(param));
    vis.visit_trait_ref(trait_ref);
    vis.visit_span(span);
}

pub fn walk_flat_map_field_def<T: MutVisitor>(
    visitor: &mut T,
    mut fd: FieldDef,
) -> SmallVec<[FieldDef; 1]> {
    let FieldDef { span, ident, vis, id, ty, attrs, is_placeholder: _ } = &mut fd;
    visitor.visit_id(id);
    visit_attrs(visitor, attrs);
    visitor.visit_vis(vis);
    visit_opt(ident, |ident| visitor.visit_ident(ident));
    visitor.visit_ty(ty);
    visitor.visit_span(span);
    smallvec![fd]
}

pub fn walk_flat_map_expr_field<T: MutVisitor>(
    vis: &mut T,
    mut f: ExprField,
) -> SmallVec<[ExprField; 1]> {
    let ExprField { ident, expr, span, is_shorthand: _, attrs, id, is_placeholder: _ } = &mut f;
    vis.visit_id(id);
    visit_attrs(vis, attrs);
    vis.visit_ident(ident);
    vis.visit_expr(expr);
    vis.visit_span(span);
    smallvec![f]
}

fn walk_mt<T: MutVisitor>(vis: &mut T, MutTy { ty, mutbl: _ }: &mut MutTy) {
    vis.visit_ty(ty);
}

pub fn walk_block<T: MutVisitor>(vis: &mut T, block: &mut P<Block>) {
    let Block { id, stmts, rules: _, span, tokens, could_be_bare_literal: _ } = block.deref_mut();
    vis.visit_id(id);
    stmts.flat_map_in_place(|stmt| vis.flat_map_stmt(stmt));
    visit_lazy_tts(vis, tokens);
    vis.visit_span(span);
}

pub fn walk_item_kind(
    kind: &mut impl WalkItemKind,
    span: Span,
    id: NodeId,
    vis: &mut impl MutVisitor,
) {
    kind.walk(span, id, vis)
}

impl WalkItemKind for ItemKind {
    fn walk(&mut self, span: Span, id: NodeId, vis: &mut impl MutVisitor) {
        match self {
            ItemKind::ExternCrate(_orig_name) => {}
            ItemKind::Use(use_tree) => vis.visit_use_tree(use_tree),
            ItemKind::Static(box StaticItem { ty, safety: _, mutability: _, expr }) => {
                vis.visit_ty(ty);
                visit_opt(expr, |expr| vis.visit_expr(expr));
            }
            ItemKind::Const(item) => {
                visit_const_item(item, vis);
            }
            ItemKind::Fn(box Fn { defaultness, generics, sig, body }) => {
                visit_defaultness(vis, defaultness);
                vis.visit_fn(FnKind::Fn(sig, generics, body), span, id);
            }
            ItemKind::Mod(safety, mod_kind) => {
                visit_safety(vis, safety);
                match mod_kind {
                    ModKind::Loaded(items, _inline, ModSpans { inner_span, inject_use_span }) => {
                        items.flat_map_in_place(|item| vis.flat_map_item(item));
                        vis.visit_span(inner_span);
                        vis.visit_span(inject_use_span);
                    }
                    ModKind::Unloaded => {}
                }
            }
            ItemKind::ForeignMod(nm) => vis.visit_foreign_mod(nm),
            ItemKind::GlobalAsm(asm) => vis.visit_inline_asm(asm),
            ItemKind::TyAlias(box TyAlias { defaultness, generics, where_clauses, bounds, ty }) => {
                visit_defaultness(vis, defaultness);
                vis.visit_generics(generics);
                visit_bounds(vis, bounds, BoundKind::Bound);
                visit_opt(ty, |ty| vis.visit_ty(ty));
                walk_ty_alias_where_clauses(vis, where_clauses);
            }
            ItemKind::Enum(EnumDef { variants }, generics) => {
                vis.visit_generics(generics);
                variants.flat_map_in_place(|variant| vis.flat_map_variant(variant));
            }
            ItemKind::Struct(variant_data, generics) | ItemKind::Union(variant_data, generics) => {
                vis.visit_generics(generics);
                vis.visit_variant_data(variant_data);
            }
            ItemKind::Impl(box Impl {
                defaultness,
                safety,
                generics,
                constness,
                polarity,
                of_trait,
                self_ty,
                items,
            }) => {
                visit_defaultness(vis, defaultness);
                visit_safety(vis, safety);
                vis.visit_generics(generics);
                visit_constness(vis, constness);
                visit_polarity(vis, polarity);
                visit_opt(of_trait, |trait_ref| vis.visit_trait_ref(trait_ref));
                vis.visit_ty(self_ty);
                items.flat_map_in_place(|item| vis.flat_map_assoc_item(item, AssocCtxt::Impl));
            }
            ItemKind::Trait(box Trait { safety, is_auto: _, generics, bounds, items }) => {
                visit_safety(vis, safety);
                vis.visit_generics(generics);
                visit_bounds(vis, bounds, BoundKind::Bound);
                items.flat_map_in_place(|item| vis.flat_map_assoc_item(item, AssocCtxt::Trait));
            }
            ItemKind::TraitAlias(generics, bounds) => {
                vis.visit_generics(generics);
                visit_bounds(vis, bounds, BoundKind::Bound);
            }
            ItemKind::MacCall(m) => vis.visit_mac_call(m),
            ItemKind::MacroDef(def) => vis.visit_macro_def(def),
            ItemKind::Delegation(box Delegation {
                id,
                qself,
                path,
                rename,
                body,
                from_glob: _,
            }) => {
                vis.visit_id(id);
                vis.visit_qself(qself);
                vis.visit_path(path);
                if let Some(rename) = rename {
                    vis.visit_ident(rename);
                }
                if let Some(body) = body {
                    vis.visit_block(body);
                }
            }
            ItemKind::DelegationMac(box DelegationMac { qself, prefix, suffixes, body }) => {
                vis.visit_qself(qself);
                vis.visit_path(prefix);
                if let Some(suffixes) = suffixes {
                    for (ident, rename) in suffixes {
                        vis.visit_ident(ident);
                        if let Some(rename) = rename {
                            vis.visit_ident(rename);
                        }
                    }
                }
                if let Some(body) = body {
                    vis.visit_block(body);
                }
            }
        }
    }
}

impl WalkItemKind for AssocItemKind {
    fn walk(&mut self, span: Span, id: NodeId, visitor: &mut impl MutVisitor) {
        match self {
            AssocItemKind::Const(item) => {
                visit_const_item(item, visitor);
            }
            AssocItemKind::Fn(box Fn { defaultness, generics, sig, body }) => {
                visit_defaultness(visitor, defaultness);
                visitor.visit_fn(FnKind::Fn(sig, generics, body), span, id);
            }
            AssocItemKind::Type(box TyAlias {
                defaultness,
                generics,
                where_clauses,
                bounds,
                ty,
            }) => {
                visit_defaultness(visitor, defaultness);
                visitor.visit_generics(generics);
                visit_bounds(visitor, bounds, BoundKind::Bound);
                visit_opt(ty, |ty| visitor.visit_ty(ty));
                walk_ty_alias_where_clauses(visitor, where_clauses);
            }
            AssocItemKind::MacCall(mac) => visitor.visit_mac_call(mac),
            AssocItemKind::Delegation(box Delegation {
                id,
                qself,
                path,
                rename,
                body,
                from_glob: _,
            }) => {
                visitor.visit_id(id);
                visitor.visit_qself(qself);
                visitor.visit_path(path);
                if let Some(rename) = rename {
                    visitor.visit_ident(rename);
                }
                if let Some(body) = body {
                    visitor.visit_block(body);
                }
            }
            AssocItemKind::DelegationMac(box DelegationMac { qself, prefix, suffixes, body }) => {
                visitor.visit_qself(qself);
                visitor.visit_path(prefix);
                if let Some(suffixes) = suffixes {
                    for (ident, rename) in suffixes {
                        visitor.visit_ident(ident);
                        if let Some(rename) = rename {
                            visitor.visit_ident(rename);
                        }
                    }
                }
                if let Some(body) = body {
                    visitor.visit_block(body);
                }
            }
        }
    }
}

fn visit_const_item<T: MutVisitor>(
    ConstItem { defaultness, generics, ty, expr }: &mut ConstItem,
    visitor: &mut T,
) {
    visit_defaultness(visitor, defaultness);
    visitor.visit_generics(generics);
    visitor.visit_ty(ty);
    visit_opt(expr, |expr| visitor.visit_expr(expr));
}

fn walk_fn_header<T: MutVisitor>(vis: &mut T, header: &mut FnHeader) {
    let FnHeader { safety, coroutine_kind, constness, ext: _ } = header;
    visit_constness(vis, constness);
    coroutine_kind.as_mut().map(|coroutine_kind| vis.visit_coroutine_kind(coroutine_kind));
    visit_safety(vis, safety);
}

pub fn walk_crate<T: MutVisitor>(vis: &mut T, krate: &mut Crate) {
    let Crate { attrs, items, spans, id, is_placeholder: _ } = krate;
    vis.visit_id(id);
    visit_attrs(vis, attrs);
    items.flat_map_in_place(|item| vis.flat_map_item(item));
    let ModSpans { inner_span, inject_use_span } = spans;
    vis.visit_span(inner_span);
    vis.visit_span(inject_use_span);
}

/// Mutates one item, returning the item again.
pub fn walk_flat_map_item<K: WalkItemKind>(
    visitor: &mut impl MutVisitor,
    mut item: P<Item<K>>,
) -> SmallVec<[P<Item<K>>; 1]> {
    let Item { ident, attrs, id, kind, vis, span, tokens } = item.deref_mut();
    visitor.visit_id(id);
    visit_attrs(visitor, attrs);
    visitor.visit_vis(vis);
    visitor.visit_ident(ident);
    kind.walk(*span, *id, visitor);
    visit_lazy_tts(visitor, tokens);
    visitor.visit_span(span);
    smallvec![item]
}

impl WalkItemKind for ForeignItemKind {
    fn walk(&mut self, span: Span, id: NodeId, visitor: &mut impl MutVisitor) {
        match self {
            ForeignItemKind::Static(box StaticItem { ty, mutability: _, expr, safety: _ }) => {
                visitor.visit_ty(ty);
                visit_opt(expr, |expr| visitor.visit_expr(expr));
            }
            ForeignItemKind::Fn(box Fn { defaultness, generics, sig, body }) => {
                visit_defaultness(visitor, defaultness);
                visitor.visit_fn(FnKind::Fn(sig, generics, body), span, id);
            }
            ForeignItemKind::TyAlias(box TyAlias {
                defaultness,
                generics,
                where_clauses,
                bounds,
                ty,
            }) => {
                visit_defaultness(visitor, defaultness);
                visitor.visit_generics(generics);
                visit_bounds(visitor, bounds, BoundKind::Bound);
                visit_opt(ty, |ty| visitor.visit_ty(ty));
                walk_ty_alias_where_clauses(visitor, where_clauses);
            }
            ForeignItemKind::MacCall(mac) => visitor.visit_mac_call(mac),
        }
    }
}

pub fn walk_pat<T: MutVisitor>(vis: &mut T, pat: &mut P<Pat>) {
    let Pat { id, kind, span, tokens } = pat.deref_mut();
    vis.visit_id(id);
    match kind {
        PatKind::Err(_guar) => {}
        PatKind::Wild | PatKind::Rest | PatKind::Never => {}
        PatKind::Ident(_binding_mode, ident, sub) => {
            vis.visit_ident(ident);
            visit_opt(sub, |sub| vis.visit_pat(sub));
        }
        PatKind::Lit(e) => vis.visit_expr(e),
        PatKind::TupleStruct(qself, path, elems) => {
            vis.visit_qself(qself);
            vis.visit_path(path);
            visit_thin_vec(elems, |elem| vis.visit_pat(elem));
        }
        PatKind::Path(qself, path) => {
            vis.visit_qself(qself);
            vis.visit_path(path);
        }
        PatKind::Struct(qself, path, fields, _etc) => {
            vis.visit_qself(qself);
            vis.visit_path(path);
            fields.flat_map_in_place(|field| vis.flat_map_pat_field(field));
        }
        PatKind::Box(inner) => vis.visit_pat(inner),
        PatKind::Deref(inner) => vis.visit_pat(inner),
        PatKind::Ref(inner, _mutbl) => vis.visit_pat(inner),
        PatKind::Range(e1, e2, Spanned { span: _, node: _ }) => {
            visit_opt(e1, |e| vis.visit_expr(e));
            visit_opt(e2, |e| vis.visit_expr(e));
            vis.visit_span(span);
        }
        PatKind::Tuple(elems) | PatKind::Slice(elems) | PatKind::Or(elems) => {
            visit_thin_vec(elems, |elem| vis.visit_pat(elem))
        }
        PatKind::Paren(inner) => vis.visit_pat(inner),
        PatKind::MacCall(mac) => vis.visit_mac_call(mac),
    }
    visit_lazy_tts(vis, tokens);
    vis.visit_span(span);
}

fn walk_anon_const<T: MutVisitor>(vis: &mut T, AnonConst { id, value }: &mut AnonConst) {
    vis.visit_id(id);
    vis.visit_expr(value);
}

fn walk_inline_asm<T: MutVisitor>(vis: &mut T, asm: &mut InlineAsm) {
    // FIXME: Visit spans inside all this currently ignored stuff.
    let InlineAsm {
        template: _,
        template_strs: _,
        operands,
        clobber_abis: _,
        options: _,
        line_spans: _,
    } = asm;
    for (op, span) in operands {
        match op {
            InlineAsmOperand::In { expr, reg: _ }
            | InlineAsmOperand::Out { expr: Some(expr), reg: _, late: _ }
            | InlineAsmOperand::InOut { expr, reg: _, late: _ } => vis.visit_expr(expr),
            InlineAsmOperand::Out { expr: None, reg: _, late: _ } => {}
            InlineAsmOperand::SplitInOut { in_expr, out_expr, reg: _, late: _ } => {
                vis.visit_expr(in_expr);
                if let Some(out_expr) = out_expr {
                    vis.visit_expr(out_expr);
                }
            }
            InlineAsmOperand::Const { anon_const } => vis.visit_anon_const(anon_const),
            InlineAsmOperand::Sym { sym } => vis.visit_inline_asm_sym(sym),
            InlineAsmOperand::Label { block } => vis.visit_block(block),
        }
        vis.visit_span(span);
    }
}

fn walk_inline_asm_sym<T: MutVisitor>(
    vis: &mut T,
    InlineAsmSym { id, qself, path }: &mut InlineAsmSym,
) {
    vis.visit_id(id);
    vis.visit_qself(qself);
    vis.visit_path(path);
}

fn walk_format_args<T: MutVisitor>(vis: &mut T, fmt: &mut FormatArgs) {
    // FIXME: visit the template exhaustively.
    let FormatArgs { span, template: _, arguments } = fmt;
    for FormatArgument { kind, expr } in arguments.all_args_mut() {
        match kind {
            FormatArgumentKind::Named(ident) | FormatArgumentKind::Captured(ident) => {
                vis.visit_ident(ident)
            }
            FormatArgumentKind::Normal => {}
        }
        vis.visit_expr(expr);
    }
    vis.visit_span(span);
}

pub fn walk_expr<T: MutVisitor>(vis: &mut T, Expr { kind, id, span, attrs, tokens }: &mut Expr) {
    vis.visit_id(id);
    visit_attrs(vis, attrs);
    match kind {
        ExprKind::Array(exprs) => visit_thin_exprs(vis, exprs),
        ExprKind::ConstBlock(anon_const) => {
            vis.visit_anon_const(anon_const);
        }
        ExprKind::Repeat(expr, count) => {
            vis.visit_expr(expr);
            vis.visit_anon_const(count);
        }
        ExprKind::Tup(exprs) => visit_thin_exprs(vis, exprs),
        ExprKind::Call(f, args) => {
            vis.visit_expr(f);
            visit_thin_exprs(vis, args);
        }
        ExprKind::MethodCall(box MethodCall {
            seg: PathSegment { ident, id, args: seg_args },
            receiver,
            args: call_args,
            span,
        }) => {
            vis.visit_method_receiver_expr(receiver);
            vis.visit_id(id);
            vis.visit_ident(ident);
            visit_opt(seg_args, |args| vis.visit_generic_args(args));
            visit_thin_exprs(vis, call_args);
            vis.visit_span(span);
        }
        ExprKind::Binary(_binop, lhs, rhs) => {
            vis.visit_expr(lhs);
            vis.visit_expr(rhs);
        }
        ExprKind::Unary(_unop, ohs) => vis.visit_expr(ohs),
        ExprKind::Cast(expr, ty) => {
            vis.visit_expr(expr);
            vis.visit_ty(ty);
        }
        ExprKind::Type(expr, ty) => {
            vis.visit_expr(expr);
            vis.visit_ty(ty);
        }
        ExprKind::AddrOf(_kind, _mut, ohs) => vis.visit_expr(ohs),
        ExprKind::Let(pat, scrutinee, span, _recovered) => {
            vis.visit_pat(pat);
            vis.visit_expr(scrutinee);
            vis.visit_span(span);
        }
        ExprKind::If(cond, tr, fl) => {
            vis.visit_expr(cond);
            vis.visit_block(tr);
            visit_opt(fl, |fl| ensure_sufficient_stack(|| vis.visit_expr(fl)));
        }
        ExprKind::While(cond, body, label) => {
            visit_opt(label, |label| vis.visit_label(label));
            vis.visit_expr(cond);
            vis.visit_block(body);
        }
        ExprKind::ForLoop { pat, iter, body, label, kind: _ } => {
            visit_opt(label, |label| vis.visit_label(label));
            vis.visit_pat(pat);
            vis.visit_expr(iter);
            vis.visit_block(body);
        }
        ExprKind::Loop(body, label, span) => {
            visit_opt(label, |label| vis.visit_label(label));
            vis.visit_block(body);
            vis.visit_span(span);
        }
        ExprKind::Match(expr, arms, _kind) => {
            vis.visit_expr(expr);
            arms.flat_map_in_place(|arm| vis.flat_map_arm(arm));
        }
        ExprKind::Closure(box Closure {
            binder,
            capture_clause,
            constness,
            coroutine_kind,
            movability: _,
            fn_decl,
            body,
            fn_decl_span,
            fn_arg_span,
        }) => {
            visit_constness(vis, constness);
            coroutine_kind.as_mut().map(|coroutine_kind| vis.visit_coroutine_kind(coroutine_kind));
            vis.visit_capture_by(capture_clause);
            vis.visit_fn(FnKind::Closure(binder, fn_decl, body), *span, *id);
            vis.visit_span(fn_decl_span);
            vis.visit_span(fn_arg_span);
        }
        ExprKind::Block(blk, label) => {
            visit_opt(label, |label| vis.visit_label(label));
            vis.visit_block(blk);
        }
        ExprKind::Gen(_capture_by, body, _kind, decl_span) => {
            vis.visit_block(body);
            vis.visit_span(decl_span);
        }
        ExprKind::Await(expr, await_kw_span) => {
            vis.visit_expr(expr);
            vis.visit_span(await_kw_span);
        }
        ExprKind::Assign(el, er, span) => {
            vis.visit_expr(el);
            vis.visit_expr(er);
            vis.visit_span(span);
        }
        ExprKind::AssignOp(_op, el, er) => {
            vis.visit_expr(el);
            vis.visit_expr(er);
        }
        ExprKind::Field(el, ident) => {
            vis.visit_expr(el);
            vis.visit_ident(ident);
        }
        ExprKind::Index(el, er, brackets_span) => {
            vis.visit_expr(el);
            vis.visit_expr(er);
            vis.visit_span(brackets_span);
        }
        ExprKind::Range(e1, e2, _lim) => {
            visit_opt(e1, |e1| vis.visit_expr(e1));
            visit_opt(e2, |e2| vis.visit_expr(e2));
        }
        ExprKind::Underscore => {}
        ExprKind::Path(qself, path) => {
            vis.visit_qself(qself);
            vis.visit_path(path);
        }
        ExprKind::Break(label, expr) => {
            visit_opt(label, |label| vis.visit_label(label));
            visit_opt(expr, |expr| vis.visit_expr(expr));
        }
        ExprKind::Continue(label) => {
            visit_opt(label, |label| vis.visit_label(label));
        }
        ExprKind::Ret(expr) => {
            visit_opt(expr, |expr| vis.visit_expr(expr));
        }
        ExprKind::Yeet(expr) => {
            visit_opt(expr, |expr| vis.visit_expr(expr));
        }
        ExprKind::Become(expr) => vis.visit_expr(expr),
        ExprKind::InlineAsm(asm) => vis.visit_inline_asm(asm),
        ExprKind::FormatArgs(fmt) => vis.visit_format_args(fmt),
        ExprKind::OffsetOf(container, fields) => {
            vis.visit_ty(container);
            for field in fields.iter_mut() {
                vis.visit_ident(field);
            }
        }
        ExprKind::MacCall(mac) => vis.visit_mac_call(mac),
        ExprKind::Struct(se) => {
            let StructExpr { qself, path, fields, rest } = se.deref_mut();
            vis.visit_qself(qself);
            vis.visit_path(path);
            fields.flat_map_in_place(|field| vis.flat_map_expr_field(field));
            match rest {
                StructRest::Base(expr) => vis.visit_expr(expr),
                StructRest::Rest(_span) => {}
                StructRest::None => {}
            }
        }
        ExprKind::Paren(expr) => {
            vis.visit_expr(expr);
        }
        ExprKind::Yield(expr) => {
            visit_opt(expr, |expr| vis.visit_expr(expr));
        }
        ExprKind::Try(expr) => vis.visit_expr(expr),
        ExprKind::TryBlock(body) => vis.visit_block(body),
        ExprKind::Lit(_token) => {}
        ExprKind::IncludedBytes(_bytes) => {}
        ExprKind::Err(_guar) => {}
        ExprKind::Dummy => {}
    }
    visit_lazy_tts(vis, tokens);
    vis.visit_span(span);
}

pub fn noop_filter_map_expr<T: MutVisitor>(vis: &mut T, mut e: P<Expr>) -> Option<P<Expr>> {
    Some({
        vis.visit_expr(&mut e);
        e
    })
}

pub fn walk_flat_map_stmt<T: MutVisitor>(
    vis: &mut T,
    Stmt { kind, mut span, mut id }: Stmt,
) -> SmallVec<[Stmt; 1]> {
    vis.visit_id(&mut id);
    let stmts: SmallVec<_> = walk_flat_map_stmt_kind(vis, kind)
        .into_iter()
        .map(|kind| Stmt { id, kind, span })
        .collect();
    if stmts.len() > 1 {
        panic!(
            "cloning statement `NodeId`s is prohibited by default, \
             the visitor should implement custom statement visiting"
        );
    }
    vis.visit_span(&mut span);
    stmts
}

fn walk_flat_map_stmt_kind<T: MutVisitor>(vis: &mut T, kind: StmtKind) -> SmallVec<[StmtKind; 1]> {
    match kind {
        StmtKind::Let(mut local) => smallvec![StmtKind::Let({
            vis.visit_local(&mut local);
            local
        })],
        StmtKind::Item(item) => vis.flat_map_item(item).into_iter().map(StmtKind::Item).collect(),
        StmtKind::Expr(expr) => vis.filter_map_expr(expr).into_iter().map(StmtKind::Expr).collect(),
        StmtKind::Semi(expr) => vis.filter_map_expr(expr).into_iter().map(StmtKind::Semi).collect(),
        StmtKind::Empty => smallvec![StmtKind::Empty],
        StmtKind::MacCall(mut mac) => {
            let MacCallStmt { mac: mac_, style: _, attrs, tokens } = mac.deref_mut();
            visit_attrs(vis, attrs);
            vis.visit_mac_call(mac_);
            visit_lazy_tts(vis, tokens);
            smallvec![StmtKind::MacCall(mac)]
        }
    }
}

fn walk_vis<T: MutVisitor>(vis: &mut T, visibility: &mut Visibility) {
    let Visibility { kind, span, tokens } = visibility;
    match kind {
        VisibilityKind::Public | VisibilityKind::Inherited => {}
        VisibilityKind::Restricted { path, id, shorthand: _ } => {
            vis.visit_id(id);
            vis.visit_path(path);
        }
    }
    visit_lazy_tts(vis, tokens);
    vis.visit_span(span);
}

fn walk_capture_by<T: MutVisitor>(vis: &mut T, capture_by: &mut CaptureBy) {
    match capture_by {
        CaptureBy::Ref => {}
        CaptureBy::Value { move_kw } => {
            vis.visit_span(move_kw);
        }
    }
}

/// Some value for the AST node that is valid but possibly meaningless. Similar
/// to `Default` but not intended for wide use. The value will never be used
/// meaningfully, it exists just to support unwinding in `visit_clobber` in the
/// case where its closure panics.
pub trait DummyAstNode {
    fn dummy() -> Self;
}

impl<T> DummyAstNode for Option<T> {
    fn dummy() -> Self {
        Default::default()
    }
}

impl<T: DummyAstNode + 'static> DummyAstNode for P<T> {
    fn dummy() -> Self {
        P(DummyAstNode::dummy())
    }
}

impl DummyAstNode for Item {
    fn dummy() -> Self {
        Item {
            attrs: Default::default(),
            id: DUMMY_NODE_ID,
            span: Default::default(),
            vis: Visibility {
                kind: VisibilityKind::Public,
                span: Default::default(),
                tokens: Default::default(),
            },
            ident: Ident::empty(),
            kind: ItemKind::ExternCrate(None),
            tokens: Default::default(),
        }
    }
}

impl DummyAstNode for Expr {
    fn dummy() -> Self {
        Expr {
            id: DUMMY_NODE_ID,
            kind: ExprKind::Dummy,
            span: Default::default(),
            attrs: Default::default(),
            tokens: Default::default(),
        }
    }
}

impl DummyAstNode for Ty {
    fn dummy() -> Self {
        Ty {
            id: DUMMY_NODE_ID,
            kind: TyKind::Dummy,
            span: Default::default(),
            tokens: Default::default(),
        }
    }
}

impl DummyAstNode for Pat {
    fn dummy() -> Self {
        Pat {
            id: DUMMY_NODE_ID,
            kind: PatKind::Wild,
            span: Default::default(),
            tokens: Default::default(),
        }
    }
}

impl DummyAstNode for Stmt {
    fn dummy() -> Self {
        Stmt { id: DUMMY_NODE_ID, kind: StmtKind::Empty, span: Default::default() }
    }
}

impl DummyAstNode for Crate {
    fn dummy() -> Self {
        Crate {
            attrs: Default::default(),
            items: Default::default(),
            spans: Default::default(),
            id: DUMMY_NODE_ID,
            is_placeholder: Default::default(),
        }
    }
}

impl<N: DummyAstNode, T: DummyAstNode> DummyAstNode for crate::ast_traits::AstNodeWrapper<N, T> {
    fn dummy() -> Self {
        crate::ast_traits::AstNodeWrapper::new(N::dummy(), T::dummy())
    }
}

#[derive(Debug)]
pub enum FnKind<'a> {
    /// E.g., `fn foo()`, `fn foo(&self)`, or `extern "Abi" fn foo()`.
    Fn(&'a mut FnSig, &'a mut Generics, &'a mut Option<P<Block>>),

    /// E.g., `|x, y| body`.
    Closure(&'a mut ClosureBinder, &'a mut P<FnDecl>, &'a mut P<Expr>),
}