Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1601 insertions, 0 deletions

diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
new file mode 100644
index 000000000..01bd498b3
--- /dev/null
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -0,0 +1,1601 @@
+//! 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 crate::ast::*;
+use crate::ptr::P;
+use crate::token::{self, Token};
+use crate::tokenstream::*;
+
+use rustc_data_structures::map_in_place::MapInPlace;
+use rustc_data_structures::sync::Lrc;
+use rustc_data_structures::thin_vec::ThinVec;
+use rustc_span::source_map::Spanned;
+use rustc_span::symbol::Ident;
+use rustc_span::Span;
+
+use smallvec::{smallvec, Array, SmallVec};
+use std::ops::DerefMut;
+use std::{panic, ptr};
+
+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 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) {
+        noop_visit_crate(c, self)
+    }
+
+    fn visit_meta_list_item(&mut self, list_item: &mut NestedMetaItem) {
+        noop_visit_meta_list_item(list_item, self);
+    }
+
+    fn visit_meta_item(&mut self, meta_item: &mut MetaItem) {
+        noop_visit_meta_item(meta_item, self);
+    }
+
+    fn visit_use_tree(&mut self, use_tree: &mut UseTree) {
+        noop_visit_use_tree(use_tree, self);
+    }
+
+    fn flat_map_foreign_item(&mut self, ni: P<ForeignItem>) -> SmallVec<[P<ForeignItem>; 1]> {
+        noop_flat_map_foreign_item(ni, self)
+    }
+
+    fn flat_map_item(&mut self, i: P<Item>) -> SmallVec<[P<Item>; 1]> {
+        noop_flat_map_item(i, self)
+    }
+
+    fn visit_fn_header(&mut self, header: &mut FnHeader) {
+        noop_visit_fn_header(header, self);
+    }
+
+    fn flat_map_field_def(&mut self, fd: FieldDef) -> SmallVec<[FieldDef; 1]> {
+        noop_flat_map_field_def(fd, self)
+    }
+
+    fn visit_item_kind(&mut self, i: &mut ItemKind) {
+        noop_visit_item_kind(i, self);
+    }
+
+    fn flat_map_trait_item(&mut self, i: P<AssocItem>) -> SmallVec<[P<AssocItem>; 1]> {
+        noop_flat_map_assoc_item(i, self)
+    }
+
+    fn flat_map_impl_item(&mut self, i: P<AssocItem>) -> SmallVec<[P<AssocItem>; 1]> {
+        noop_flat_map_assoc_item(i, self)
+    }
+
+    fn visit_fn_decl(&mut self, d: &mut P<FnDecl>) {
+        noop_visit_fn_decl(d, self);
+    }
+
+    fn visit_asyncness(&mut self, a: &mut Async) {
+        noop_visit_asyncness(a, self);
+    }
+
+    fn visit_closure_binder(&mut self, b: &mut ClosureBinder) {
+        noop_visit_closure_binder(b, self);
+    }
+
+    fn visit_block(&mut self, b: &mut P<Block>) {
+        noop_visit_block(b, self);
+    }
+
+    fn flat_map_stmt(&mut self, s: Stmt) -> SmallVec<[Stmt; 1]> {
+        noop_flat_map_stmt(s, self)
+    }
+
+    fn flat_map_arm(&mut self, arm: Arm) -> SmallVec<[Arm; 1]> {
+        noop_flat_map_arm(arm, self)
+    }
+
+    fn visit_pat(&mut self, p: &mut P<Pat>) {
+        noop_visit_pat(p, self);
+    }
+
+    fn visit_anon_const(&mut self, c: &mut AnonConst) {
+        noop_visit_anon_const(c, self);
+    }
+
+    fn visit_expr(&mut self, e: &mut P<Expr>) {
+        noop_visit_expr(e, self);
+    }
+
+    fn filter_map_expr(&mut self, e: P<Expr>) -> Option<P<Expr>> {
+        noop_filter_map_expr(e, self)
+    }
+
+    fn visit_generic_arg(&mut self, arg: &mut GenericArg) {
+        noop_visit_generic_arg(arg, self);
+    }
+
+    fn visit_ty(&mut self, t: &mut P<Ty>) {
+        noop_visit_ty(t, self);
+    }
+
+    fn visit_lifetime(&mut self, l: &mut Lifetime) {
+        noop_visit_lifetime(l, self);
+    }
+
+    fn visit_constraint(&mut self, t: &mut AssocConstraint) {
+        noop_visit_constraint(t, self);
+    }
+
+    fn visit_foreign_mod(&mut self, nm: &mut ForeignMod) {
+        noop_visit_foreign_mod(nm, self);
+    }
+
+    fn flat_map_variant(&mut self, v: Variant) -> SmallVec<[Variant; 1]> {
+        noop_flat_map_variant(v, self)
+    }
+
+    fn visit_ident(&mut self, i: &mut Ident) {
+        noop_visit_ident(i, self);
+    }
+
+    fn visit_path(&mut self, p: &mut Path) {
+        noop_visit_path(p, self);
+    }
+
+    fn visit_qself(&mut self, qs: &mut Option<QSelf>) {
+        noop_visit_qself(qs, self);
+    }
+
+    fn visit_generic_args(&mut self, p: &mut GenericArgs) {
+        noop_visit_generic_args(p, self);
+    }
+
+    fn visit_angle_bracketed_parameter_data(&mut self, p: &mut AngleBracketedArgs) {
+        noop_visit_angle_bracketed_parameter_data(p, self);
+    }
+
+    fn visit_parenthesized_parameter_data(&mut self, p: &mut ParenthesizedArgs) {
+        noop_visit_parenthesized_parameter_data(p, self);
+    }
+
+    fn visit_local(&mut self, l: &mut P<Local>) {
+        noop_visit_local(l, self);
+    }
+
+    fn visit_mac_call(&mut self, mac: &mut MacCall) {
+        noop_visit_mac(mac, self);
+    }
+
+    fn visit_macro_def(&mut self, def: &mut MacroDef) {
+        noop_visit_macro_def(def, self);
+    }
+
+    fn visit_label(&mut self, label: &mut Label) {
+        noop_visit_label(label, self);
+    }
+
+    fn visit_attribute(&mut self, at: &mut Attribute) {
+        noop_visit_attribute(at, self);
+    }
+
+    fn flat_map_param(&mut self, param: Param) -> SmallVec<[Param; 1]> {
+        noop_flat_map_param(param, self)
+    }
+
+    fn visit_generics(&mut self, generics: &mut Generics) {
+        noop_visit_generics(generics, self);
+    }
+
+    fn visit_trait_ref(&mut self, tr: &mut TraitRef) {
+        noop_visit_trait_ref(tr, self);
+    }
+
+    fn visit_poly_trait_ref(&mut self, p: &mut PolyTraitRef) {
+        noop_visit_poly_trait_ref(p, self);
+    }
+
+    fn visit_variant_data(&mut self, vdata: &mut VariantData) {
+        noop_visit_variant_data(vdata, self);
+    }
+
+    fn flat_map_generic_param(&mut self, param: GenericParam) -> SmallVec<[GenericParam; 1]> {
+        noop_flat_map_generic_param(param, self)
+    }
+
+    fn visit_param_bound(&mut self, tpb: &mut GenericBound) {
+        noop_visit_param_bound(tpb, self);
+    }
+
+    fn visit_mt(&mut self, mt: &mut MutTy) {
+        noop_visit_mt(mt, self);
+    }
+
+    fn flat_map_expr_field(&mut self, f: ExprField) -> SmallVec<[ExprField; 1]> {
+        noop_flat_map_expr_field(f, self)
+    }
+
+    fn visit_where_clause(&mut self, where_clause: &mut WhereClause) {
+        noop_visit_where_clause(where_clause, self);
+    }
+
+    fn visit_where_predicate(&mut self, where_predicate: &mut WherePredicate) {
+        noop_visit_where_predicate(where_predicate, self);
+    }
+
+    fn visit_vis(&mut self, vis: &mut Visibility) {
+        noop_visit_vis(vis, self);
+    }
+
+    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]> {
+        noop_flat_map_pat_field(fp, self)
+    }
+
+    fn visit_inline_asm(&mut self, asm: &mut InlineAsm) {
+        noop_visit_inline_asm(asm, self)
+    }
+
+    fn visit_inline_asm_sym(&mut self, sym: &mut InlineAsmSym) {
+        noop_visit_inline_asm_sym(sym, self)
+    }
+}
+
+/// 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) {
+    unsafe {
+        // Safe because `t` is used in a read-only fashion by `read()` before
+        // being overwritten by `write()`.
+        let old_t = ptr::read(t);
+        let new_t =
+            panic::catch_unwind(panic::AssertUnwindSafe(|| f(old_t))).unwrap_or_else(|err| {
+                // Set `t` to some valid but possible meaningless value,
+                // and pass the fatal error further.
+                ptr::write(t, T::dummy());
+                panic::resume_unwind(err);
+            });
+        ptr::write(t, new_t);
+    }
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+#[inline]
+pub 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]
+pub 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`.
+pub fn visit_attrs<T: MutVisitor>(attrs: &mut Vec<Attribute>, vis: &mut T) {
+    visit_vec(attrs, |attr| vis.visit_attribute(attr));
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_thin_attrs<T: MutVisitor>(attrs: &mut AttrVec, vis: &mut T) {
+    for attr in attrs.iter_mut() {
+        vis.visit_attribute(attr);
+    }
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_exprs<T: MutVisitor>(exprs: &mut Vec<P<Expr>>, vis: &mut T) {
+    exprs.flat_map_in_place(|expr| vis.filter_map_expr(expr))
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_bounds<T: MutVisitor>(bounds: &mut GenericBounds, vis: &mut T) {
+    visit_vec(bounds, |bound| vis.visit_param_bound(bound));
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_fn_sig<T: MutVisitor>(FnSig { header, decl, span }: &mut FnSig, vis: &mut T) {
+    vis.visit_fn_header(header);
+    vis.visit_fn_decl(decl);
+    vis.visit_span(span);
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_mac_args<T: MutVisitor>(args: &mut MacArgs, vis: &mut T) {
+    match args {
+        MacArgs::Empty => {}
+        MacArgs::Delimited(dspan, _delim, tokens) => {
+            visit_delim_span(dspan, vis);
+            visit_tts(tokens, vis);
+        }
+        MacArgs::Eq(eq_span, MacArgsEq::Ast(expr)) => {
+            vis.visit_span(eq_span);
+            vis.visit_expr(expr);
+        }
+        MacArgs::Eq(_, MacArgsEq::Hir(lit)) => {
+            unreachable!("in literal form when visiting mac args eq: {:?}", lit)
+        }
+    }
+}
+
+pub fn visit_delim_span<T: MutVisitor>(dspan: &mut DelimSpan, vis: &mut T) {
+    vis.visit_span(&mut dspan.open);
+    vis.visit_span(&mut dspan.close);
+}
+
+pub fn noop_flat_map_pat_field<T: MutVisitor>(
+    mut fp: PatField,
+    vis: &mut T,
+) -> SmallVec<[PatField; 1]> {
+    let PatField { attrs, id, ident, is_placeholder: _, is_shorthand: _, pat, span } = &mut fp;
+    vis.visit_id(id);
+    vis.visit_ident(ident);
+    vis.visit_pat(pat);
+    vis.visit_span(span);
+    visit_thin_attrs(attrs, vis);
+    smallvec![fp]
+}
+
+pub fn noop_visit_use_tree<T: MutVisitor>(use_tree: &mut UseTree, vis: &mut T) {
+    let UseTree { prefix, kind, span } = use_tree;
+    vis.visit_path(prefix);
+    match kind {
+        UseTreeKind::Simple(rename, id1, id2) => {
+            visit_opt(rename, |rename| vis.visit_ident(rename));
+            vis.visit_id(id1);
+            vis.visit_id(id2);
+        }
+        UseTreeKind::Nested(items) => {
+            for (tree, id) in items {
+                vis.visit_use_tree(tree);
+                vis.visit_id(id);
+            }
+        }
+        UseTreeKind::Glob => {}
+    }
+    vis.visit_span(span);
+}
+
+pub fn noop_flat_map_arm<T: MutVisitor>(mut arm: Arm, vis: &mut T) -> SmallVec<[Arm; 1]> {
+    let Arm { attrs, pat, guard, body, span, id, is_placeholder: _ } = &mut arm;
+    visit_thin_attrs(attrs, vis);
+    vis.visit_id(id);
+    vis.visit_pat(pat);
+    visit_opt(guard, |guard| vis.visit_expr(guard));
+    vis.visit_expr(body);
+    vis.visit_span(span);
+    smallvec![arm]
+}
+
+pub fn noop_visit_constraint<T: MutVisitor>(
+    AssocConstraint { id, ident, gen_args, kind, span }: &mut AssocConstraint,
+    vis: &mut T,
+) {
+    vis.visit_id(id);
+    vis.visit_ident(ident);
+    if let Some(ref mut gen_args) = gen_args {
+        vis.visit_generic_args(gen_args);
+    }
+    match kind {
+        AssocConstraintKind::Equality { ref mut term } => match term {
+            Term::Ty(ty) => vis.visit_ty(ty),
+            Term::Const(c) => vis.visit_anon_const(c),
+        },
+        AssocConstraintKind::Bound { ref mut bounds } => visit_bounds(bounds, vis),
+    }
+    vis.visit_span(span);
+}
+
+pub fn noop_visit_ty<T: MutVisitor>(ty: &mut P<Ty>, vis: &mut T) {
+    let Ty { id, kind, span, tokens } = ty.deref_mut();
+    vis.visit_id(id);
+    match kind {
+        TyKind::Infer | TyKind::ImplicitSelf | TyKind::Err | TyKind::Never | TyKind::CVarArgs => {}
+        TyKind::Slice(ty) => vis.visit_ty(ty),
+        TyKind::Ptr(mt) => vis.visit_mt(mt),
+        TyKind::Rptr(lt, mt) => {
+            visit_opt(lt, |lt| noop_visit_lifetime(lt, vis));
+            vis.visit_mt(mt);
+        }
+        TyKind::BareFn(bft) => {
+            let BareFnTy { unsafety, ext: _, generic_params, decl, decl_span } = bft.deref_mut();
+            visit_unsafety(unsafety, vis);
+            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_vec(tys, |ty| vis.visit_ty(ty)),
+        TyKind::Paren(ty) => vis.visit_ty(ty),
+        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))
+        }
+        TyKind::ImplTrait(id, bounds) => {
+            vis.visit_id(id);
+            visit_vec(bounds, |bound| vis.visit_param_bound(bound));
+        }
+        TyKind::MacCall(mac) => vis.visit_mac_call(mac),
+    }
+    vis.visit_span(span);
+    visit_lazy_tts(tokens, vis);
+}
+
+pub fn noop_visit_foreign_mod<T: MutVisitor>(foreign_mod: &mut ForeignMod, vis: &mut T) {
+    let ForeignMod { unsafety, abi: _, items } = foreign_mod;
+    visit_unsafety(unsafety, vis);
+    items.flat_map_in_place(|item| vis.flat_map_foreign_item(item));
+}
+
+pub fn noop_flat_map_variant<T: MutVisitor>(
+    mut variant: Variant,
+    visitor: &mut T,
+) -> SmallVec<[Variant; 1]> {
+    let Variant { ident, vis, attrs, id, data, disr_expr, span, is_placeholder: _ } = &mut variant;
+    visitor.visit_ident(ident);
+    visitor.visit_vis(vis);
+    visit_thin_attrs(attrs, visitor);
+    visitor.visit_id(id);
+    visitor.visit_variant_data(data);
+    visit_opt(disr_expr, |disr_expr| visitor.visit_anon_const(disr_expr));
+    visitor.visit_span(span);
+    smallvec![variant]
+}
+
+pub fn noop_visit_ident<T: MutVisitor>(Ident { name: _, span }: &mut Ident, vis: &mut T) {
+    vis.visit_span(span);
+}
+
+pub fn noop_visit_path<T: MutVisitor>(Path { segments, span, tokens }: &mut Path, vis: &mut T) {
+    vis.visit_span(span);
+    for PathSegment { ident, id, args } in segments {
+        vis.visit_ident(ident);
+        vis.visit_id(id);
+        visit_opt(args, |args| vis.visit_generic_args(args));
+    }
+    visit_lazy_tts(tokens, vis);
+}
+
+pub fn noop_visit_qself<T: MutVisitor>(qself: &mut Option<QSelf>, vis: &mut T) {
+    visit_opt(qself, |QSelf { ty, path_span, position: _ }| {
+        vis.visit_ty(ty);
+        vis.visit_span(path_span);
+    })
+}
+
+pub fn noop_visit_generic_args<T: MutVisitor>(generic_args: &mut GenericArgs, vis: &mut T) {
+    match generic_args {
+        GenericArgs::AngleBracketed(data) => vis.visit_angle_bracketed_parameter_data(data),
+        GenericArgs::Parenthesized(data) => vis.visit_parenthesized_parameter_data(data),
+    }
+}
+
+pub fn noop_visit_generic_arg<T: MutVisitor>(arg: &mut GenericArg, vis: &mut T) {
+    match arg {
+        GenericArg::Lifetime(lt) => vis.visit_lifetime(lt),
+        GenericArg::Type(ty) => vis.visit_ty(ty),
+        GenericArg::Const(ct) => vis.visit_anon_const(ct),
+    }
+}
+
+pub fn noop_visit_angle_bracketed_parameter_data<T: MutVisitor>(
+    data: &mut AngleBracketedArgs,
+    vis: &mut T,
+) {
+    let AngleBracketedArgs { args, span } = data;
+    visit_vec(args, |arg| match arg {
+        AngleBracketedArg::Arg(arg) => vis.visit_generic_arg(arg),
+        AngleBracketedArg::Constraint(constraint) => vis.visit_constraint(constraint),
+    });
+    vis.visit_span(span);
+}
+
+pub fn noop_visit_parenthesized_parameter_data<T: MutVisitor>(
+    args: &mut ParenthesizedArgs,
+    vis: &mut T,
+) {
+    let ParenthesizedArgs { inputs, output, span, .. } = args;
+    visit_vec(inputs, |input| vis.visit_ty(input));
+    noop_visit_fn_ret_ty(output, vis);
+    vis.visit_span(span);
+}
+
+pub fn noop_visit_local<T: MutVisitor>(local: &mut P<Local>, vis: &mut T) {
+    let Local { id, pat, ty, kind, span, attrs, tokens } = local.deref_mut();
+    vis.visit_id(id);
+    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);
+        }
+    }
+    vis.visit_span(span);
+    visit_thin_attrs(attrs, vis);
+    visit_lazy_tts(tokens, vis);
+}
+
+pub fn noop_visit_attribute<T: MutVisitor>(attr: &mut Attribute, vis: &mut T) {
+    let Attribute { kind, id: _, style: _, span } = attr;
+    match kind {
+        AttrKind::Normal(AttrItem { path, args, tokens }, attr_tokens) => {
+            vis.visit_path(path);
+            visit_mac_args(args, vis);
+            visit_lazy_tts(tokens, vis);
+            visit_lazy_tts(attr_tokens, vis);
+        }
+        AttrKind::DocComment(..) => {}
+    }
+    vis.visit_span(span);
+}
+
+pub fn noop_visit_mac<T: MutVisitor>(mac: &mut MacCall, vis: &mut T) {
+    let MacCall { path, args, prior_type_ascription: _ } = mac;
+    vis.visit_path(path);
+    visit_mac_args(args, vis);
+}
+
+pub fn noop_visit_macro_def<T: MutVisitor>(macro_def: &mut MacroDef, vis: &mut T) {
+    let MacroDef { body, macro_rules: _ } = macro_def;
+    visit_mac_args(body, vis);
+}
+
+pub fn noop_visit_meta_list_item<T: MutVisitor>(li: &mut NestedMetaItem, vis: &mut T) {
+    match li {
+        NestedMetaItem::MetaItem(mi) => vis.visit_meta_item(mi),
+        NestedMetaItem::Literal(_lit) => {}
+    }
+}
+
+pub fn noop_visit_meta_item<T: MutVisitor>(mi: &mut MetaItem, vis: &mut T) {
+    let MetaItem { path: _, kind, span } = mi;
+    match kind {
+        MetaItemKind::Word => {}
+        MetaItemKind::List(mis) => visit_vec(mis, |mi| vis.visit_meta_list_item(mi)),
+        MetaItemKind::NameValue(_s) => {}
+    }
+    vis.visit_span(span);
+}
+
+pub fn noop_flat_map_param<T: MutVisitor>(mut param: Param, vis: &mut T) -> SmallVec<[Param; 1]> {
+    let Param { attrs, id, pat, span, ty, is_placeholder: _ } = &mut param;
+    vis.visit_id(id);
+    visit_thin_attrs(attrs, vis);
+    vis.visit_pat(pat);
+    vis.visit_span(span);
+    vis.visit_ty(ty);
+    smallvec![param]
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_attr_annotated_tt<T: MutVisitor>(tt: &mut AttrAnnotatedTokenTree, vis: &mut T) {
+    match tt {
+        AttrAnnotatedTokenTree::Token(token) => {
+            visit_token(token, vis);
+        }
+        AttrAnnotatedTokenTree::Delimited(DelimSpan { open, close }, _delim, tts) => {
+            vis.visit_span(open);
+            vis.visit_span(close);
+            visit_attr_annotated_tts(tts, vis);
+        }
+        AttrAnnotatedTokenTree::Attributes(data) => {
+            for attr in &mut *data.attrs {
+                match &mut attr.kind {
+                    AttrKind::Normal(_, attr_tokens) => {
+                        visit_lazy_tts(attr_tokens, vis);
+                    }
+                    AttrKind::DocComment(..) => {
+                        vis.visit_span(&mut attr.span);
+                    }
+                }
+            }
+            visit_lazy_tts_opt_mut(Some(&mut data.tokens), vis);
+        }
+    }
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_tt<T: MutVisitor>(tt: &mut TokenTree, vis: &mut T) {
+    match tt {
+        TokenTree::Token(token, _) => {
+            visit_token(token, vis);
+        }
+        TokenTree::Delimited(DelimSpan { open, close }, _delim, tts) => {
+            vis.visit_span(open);
+            vis.visit_span(close);
+            visit_tts(tts, vis);
+        }
+    }
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_tts<T: MutVisitor>(TokenStream(tts): &mut TokenStream, vis: &mut T) {
+    if T::VISIT_TOKENS && !tts.is_empty() {
+        let tts = Lrc::make_mut(tts);
+        visit_vec(tts, |tree| visit_tt(tree, vis));
+    }
+}
+
+pub fn visit_attr_annotated_tts<T: MutVisitor>(
+    AttrAnnotatedTokenStream(tts): &mut AttrAnnotatedTokenStream,
+    vis: &mut T,
+) {
+    if T::VISIT_TOKENS && !tts.is_empty() {
+        let tts = Lrc::make_mut(tts);
+        visit_vec(tts, |(tree, _is_joint)| visit_attr_annotated_tt(tree, vis));
+    }
+}
+
+pub fn visit_lazy_tts_opt_mut<T: MutVisitor>(lazy_tts: Option<&mut LazyTokenStream>, vis: &mut T) {
+    if T::VISIT_TOKENS {
+        if let Some(lazy_tts) = lazy_tts {
+            let mut tts = lazy_tts.create_token_stream();
+            visit_attr_annotated_tts(&mut tts, vis);
+            *lazy_tts = LazyTokenStream::new(tts);
+        }
+    }
+}
+
+pub fn visit_lazy_tts<T: MutVisitor>(lazy_tts: &mut Option<LazyTokenStream>, vis: &mut T) {
+    visit_lazy_tts_opt_mut(lazy_tts.as_mut(), vis);
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+// 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.
+pub fn visit_token<T: MutVisitor>(t: &mut Token, vis: &mut T) {
+    let Token { kind, span } = t;
+    match kind {
+        token::Ident(name, _) | 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::Interpolated(nt) => {
+            let mut nt = Lrc::make_mut(nt);
+            visit_nonterminal(&mut nt, vis);
+        }
+        _ => {}
+    }
+    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....
+pub fn visit_nonterminal<T: MutVisitor>(nt: &mut token::Nonterminal, vis: &mut T) {
+    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::NtIdent(ident, _is_raw) => vis.visit_ident(ident),
+        token::NtLifetime(ident) => vis.visit_ident(ident),
+        token::NtLiteral(expr) => vis.visit_expr(expr),
+        token::NtMeta(item) => {
+            let AttrItem { path, args, tokens } = item.deref_mut();
+            vis.visit_path(path);
+            visit_mac_args(args, vis);
+            visit_lazy_tts(tokens, vis);
+        }
+        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`.
+pub fn visit_defaultness<T: MutVisitor>(defaultness: &mut Defaultness, vis: &mut T) {
+    match defaultness {
+        Defaultness::Default(span) => vis.visit_span(span),
+        Defaultness::Final => {}
+    }
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_unsafety<T: MutVisitor>(unsafety: &mut Unsafe, vis: &mut T) {
+    match unsafety {
+        Unsafe::Yes(span) => vis.visit_span(span),
+        Unsafe::No => {}
+    }
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_polarity<T: MutVisitor>(polarity: &mut ImplPolarity, vis: &mut T) {
+    match polarity {
+        ImplPolarity::Positive => {}
+        ImplPolarity::Negative(span) => vis.visit_span(span),
+    }
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_constness<T: MutVisitor>(constness: &mut Const, vis: &mut T) {
+    match constness {
+        Const::Yes(span) => vis.visit_span(span),
+        Const::No => {}
+    }
+}
+
+pub fn noop_visit_closure_binder<T: MutVisitor>(binder: &mut ClosureBinder, vis: &mut T) {
+    match binder {
+        ClosureBinder::NotPresent => {}
+        ClosureBinder::For { span: _, generic_params } => {
+            let mut vec = std::mem::take(generic_params).into_vec();
+            vec.flat_map_in_place(|param| vis.flat_map_generic_param(param));
+            *generic_params = P::from_vec(vec);
+        }
+    }
+}
+
+pub fn noop_visit_asyncness<T: MutVisitor>(asyncness: &mut Async, vis: &mut T) {
+    match asyncness {
+        Async::Yes { span: _, closure_id, return_impl_trait_id } => {
+            vis.visit_id(closure_id);
+            vis.visit_id(return_impl_trait_id);
+        }
+        Async::No => {}
+    }
+}
+
+pub fn noop_visit_fn_decl<T: MutVisitor>(decl: &mut P<FnDecl>, vis: &mut T) {
+    let FnDecl { inputs, output } = decl.deref_mut();
+    inputs.flat_map_in_place(|param| vis.flat_map_param(param));
+    noop_visit_fn_ret_ty(output, vis);
+}
+
+pub fn noop_visit_fn_ret_ty<T: MutVisitor>(fn_ret_ty: &mut FnRetTy, vis: &mut T) {
+    match fn_ret_ty {
+        FnRetTy::Default(span) => vis.visit_span(span),
+        FnRetTy::Ty(ty) => vis.visit_ty(ty),
+    }
+}
+
+pub fn noop_visit_param_bound<T: MutVisitor>(pb: &mut GenericBound, vis: &mut T) {
+    match pb {
+        GenericBound::Trait(ty, _modifier) => vis.visit_poly_trait_ref(ty),
+        GenericBound::Outlives(lifetime) => noop_visit_lifetime(lifetime, vis),
+    }
+}
+
+pub fn noop_flat_map_generic_param<T: MutVisitor>(
+    mut param: GenericParam,
+    vis: &mut T,
+) -> SmallVec<[GenericParam; 1]> {
+    let GenericParam { id, ident, attrs, bounds, kind, colon_span, is_placeholder: _ } = &mut param;
+    vis.visit_id(id);
+    vis.visit_ident(ident);
+    if let Some(ref mut colon_span) = colon_span {
+        vis.visit_span(colon_span);
+    }
+    visit_thin_attrs(attrs, vis);
+    visit_vec(bounds, |bound| noop_visit_param_bound(bound, vis));
+    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));
+        }
+    }
+    smallvec![param]
+}
+
+pub fn noop_visit_label<T: MutVisitor>(Label { ident }: &mut Label, vis: &mut T) {
+    vis.visit_ident(ident);
+}
+
+fn noop_visit_lifetime<T: MutVisitor>(Lifetime { id, ident }: &mut Lifetime, vis: &mut T) {
+    vis.visit_id(id);
+    vis.visit_ident(ident);
+}
+
+pub fn noop_visit_generics<T: MutVisitor>(generics: &mut Generics, vis: &mut T) {
+    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);
+}
+
+pub fn noop_visit_where_clause<T: MutVisitor>(wc: &mut WhereClause, vis: &mut T) {
+    let WhereClause { has_where_token: _, predicates, span } = wc;
+    visit_vec(predicates, |predicate| vis.visit_where_predicate(predicate));
+    vis.visit_span(span);
+}
+
+pub fn noop_visit_where_predicate<T: MutVisitor>(pred: &mut WherePredicate, vis: &mut T) {
+    match pred {
+        WherePredicate::BoundPredicate(bp) => {
+            let WhereBoundPredicate { span, bound_generic_params, bounded_ty, bounds } = bp;
+            vis.visit_span(span);
+            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));
+        }
+        WherePredicate::RegionPredicate(rp) => {
+            let WhereRegionPredicate { span, lifetime, bounds } = rp;
+            vis.visit_span(span);
+            noop_visit_lifetime(lifetime, vis);
+            visit_vec(bounds, |bound| noop_visit_param_bound(bound, vis));
+        }
+        WherePredicate::EqPredicate(ep) => {
+            let WhereEqPredicate { id, span, lhs_ty, rhs_ty } = ep;
+            vis.visit_id(id);
+            vis.visit_span(span);
+            vis.visit_ty(lhs_ty);
+            vis.visit_ty(rhs_ty);
+        }
+    }
+}
+
+pub fn noop_visit_variant_data<T: MutVisitor>(vdata: &mut VariantData, vis: &mut T) {
+    match vdata {
+        VariantData::Struct(fields, ..) => {
+            fields.flat_map_in_place(|field| vis.flat_map_field_def(field));
+        }
+        VariantData::Tuple(fields, id) => {
+            fields.flat_map_in_place(|field| vis.flat_map_field_def(field));
+            vis.visit_id(id);
+        }
+        VariantData::Unit(id) => vis.visit_id(id),
+    }
+}
+
+pub fn noop_visit_trait_ref<T: MutVisitor>(TraitRef { path, ref_id }: &mut TraitRef, vis: &mut T) {
+    vis.visit_path(path);
+    vis.visit_id(ref_id);
+}
+
+pub fn noop_visit_poly_trait_ref<T: MutVisitor>(p: &mut PolyTraitRef, vis: &mut T) {
+    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 noop_flat_map_field_def<T: MutVisitor>(
+    mut fd: FieldDef,
+    visitor: &mut T,
+) -> SmallVec<[FieldDef; 1]> {
+    let FieldDef { span, ident, vis, id, ty, attrs, is_placeholder: _ } = &mut fd;
+    visitor.visit_span(span);
+    visit_opt(ident, |ident| visitor.visit_ident(ident));
+    visitor.visit_vis(vis);
+    visitor.visit_id(id);
+    visitor.visit_ty(ty);
+    visit_thin_attrs(attrs, visitor);
+    smallvec![fd]
+}
+
+pub fn noop_flat_map_expr_field<T: MutVisitor>(
+    mut f: ExprField,
+    vis: &mut T,
+) -> SmallVec<[ExprField; 1]> {
+    let ExprField { ident, expr, span, is_shorthand: _, attrs, id, is_placeholder: _ } = &mut f;
+    vis.visit_ident(ident);
+    vis.visit_expr(expr);
+    vis.visit_id(id);
+    vis.visit_span(span);
+    visit_thin_attrs(attrs, vis);
+    smallvec![f]
+}
+
+pub fn noop_visit_mt<T: MutVisitor>(MutTy { ty, mutbl: _ }: &mut MutTy, vis: &mut T) {
+    vis.visit_ty(ty);
+}
+
+pub fn noop_visit_block<T: MutVisitor>(block: &mut P<Block>, vis: &mut T) {
+    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));
+    vis.visit_span(span);
+    visit_lazy_tts(tokens, vis);
+}
+
+pub fn noop_visit_item_kind<T: MutVisitor>(kind: &mut ItemKind, vis: &mut T) {
+    match kind {
+        ItemKind::ExternCrate(_orig_name) => {}
+        ItemKind::Use(use_tree) => vis.visit_use_tree(use_tree),
+        ItemKind::Static(ty, _, expr) => {
+            vis.visit_ty(ty);
+            visit_opt(expr, |expr| vis.visit_expr(expr));
+        }
+        ItemKind::Const(defaultness, ty, expr) => {
+            visit_defaultness(defaultness, vis);
+            vis.visit_ty(ty);
+            visit_opt(expr, |expr| vis.visit_expr(expr));
+        }
+        ItemKind::Fn(box Fn { defaultness, generics, sig, body }) => {
+            visit_defaultness(defaultness, vis);
+            visit_fn_sig(sig, vis);
+            vis.visit_generics(generics);
+            visit_opt(body, |body| vis.visit_block(body));
+        }
+        ItemKind::Mod(unsafety, mod_kind) => {
+            visit_unsafety(unsafety, vis);
+            match mod_kind {
+                ModKind::Loaded(items, _inline, ModSpans { inner_span, inject_use_span }) => {
+                    vis.visit_span(inner_span);
+                    vis.visit_span(inject_use_span);
+                    items.flat_map_in_place(|item| vis.flat_map_item(item));
+                }
+                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(defaultness, vis);
+            vis.visit_generics(generics);
+            vis.visit_span(&mut where_clauses.0.1);
+            vis.visit_span(&mut where_clauses.1.1);
+            visit_bounds(bounds, vis);
+            visit_opt(ty, |ty| vis.visit_ty(ty));
+        }
+        ItemKind::Enum(EnumDef { variants }, generics) => {
+            variants.flat_map_in_place(|variant| vis.flat_map_variant(variant));
+            vis.visit_generics(generics);
+        }
+        ItemKind::Struct(variant_data, generics) | ItemKind::Union(variant_data, generics) => {
+            vis.visit_variant_data(variant_data);
+            vis.visit_generics(generics);
+        }
+        ItemKind::Impl(box Impl {
+            defaultness,
+            unsafety,
+            generics,
+            constness,
+            polarity,
+            of_trait,
+            self_ty,
+            items,
+        }) => {
+            visit_defaultness(defaultness, vis);
+            visit_unsafety(unsafety, vis);
+            vis.visit_generics(generics);
+            visit_constness(constness, vis);
+            visit_polarity(polarity, vis);
+            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_impl_item(item));
+        }
+        ItemKind::Trait(box Trait { unsafety, is_auto: _, generics, bounds, items }) => {
+            visit_unsafety(unsafety, vis);
+            vis.visit_generics(generics);
+            visit_bounds(bounds, vis);
+            items.flat_map_in_place(|item| vis.flat_map_trait_item(item));
+        }
+        ItemKind::TraitAlias(generics, bounds) => {
+            vis.visit_generics(generics);
+            visit_bounds(bounds, vis);
+        }
+        ItemKind::MacCall(m) => vis.visit_mac_call(m),
+        ItemKind::MacroDef(def) => vis.visit_macro_def(def),
+    }
+}
+
+pub fn noop_flat_map_assoc_item<T: MutVisitor>(
+    mut item: P<AssocItem>,
+    visitor: &mut T,
+) -> SmallVec<[P<AssocItem>; 1]> {
+    let Item { id, ident, vis, attrs, kind, span, tokens } = item.deref_mut();
+    visitor.visit_id(id);
+    visitor.visit_ident(ident);
+    visitor.visit_vis(vis);
+    visit_attrs(attrs, visitor);
+    match kind {
+        AssocItemKind::Const(defaultness, ty, expr) => {
+            visit_defaultness(defaultness, visitor);
+            visitor.visit_ty(ty);
+            visit_opt(expr, |expr| visitor.visit_expr(expr));
+        }
+        AssocItemKind::Fn(box Fn { defaultness, generics, sig, body }) => {
+            visit_defaultness(defaultness, visitor);
+            visitor.visit_generics(generics);
+            visit_fn_sig(sig, visitor);
+            visit_opt(body, |body| visitor.visit_block(body));
+        }
+        AssocItemKind::TyAlias(box TyAlias {
+            defaultness,
+            generics,
+            where_clauses,
+            bounds,
+            ty,
+            ..
+        }) => {
+            visit_defaultness(defaultness, visitor);
+            visitor.visit_generics(generics);
+            visitor.visit_span(&mut where_clauses.0.1);
+            visitor.visit_span(&mut where_clauses.1.1);
+            visit_bounds(bounds, visitor);
+            visit_opt(ty, |ty| visitor.visit_ty(ty));
+        }
+        AssocItemKind::MacCall(mac) => visitor.visit_mac_call(mac),
+    }
+    visitor.visit_span(span);
+    visit_lazy_tts(tokens, visitor);
+    smallvec![item]
+}
+
+pub fn noop_visit_fn_header<T: MutVisitor>(header: &mut FnHeader, vis: &mut T) {
+    let FnHeader { unsafety, asyncness, constness, ext: _ } = header;
+    visit_constness(constness, vis);
+    vis.visit_asyncness(asyncness);
+    visit_unsafety(unsafety, vis);
+}
+
+pub fn noop_visit_crate<T: MutVisitor>(krate: &mut Crate, vis: &mut T) {
+    let Crate { attrs, items, spans, id, is_placeholder: _ } = krate;
+    vis.visit_id(id);
+    visit_attrs(attrs, vis);
+    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 into possibly many items.
+pub fn noop_flat_map_item<T: MutVisitor>(
+    mut item: P<Item>,
+    visitor: &mut T,
+) -> SmallVec<[P<Item>; 1]> {
+    let Item { ident, attrs, id, kind, vis, span, tokens } = item.deref_mut();
+    visitor.visit_ident(ident);
+    visit_attrs(attrs, visitor);
+    visitor.visit_id(id);
+    visitor.visit_item_kind(kind);
+    visitor.visit_vis(vis);
+    visitor.visit_span(span);
+    visit_lazy_tts(tokens, visitor);
+
+    smallvec![item]
+}
+
+pub fn noop_flat_map_foreign_item<T: MutVisitor>(
+    mut item: P<ForeignItem>,
+    visitor: &mut T,
+) -> SmallVec<[P<ForeignItem>; 1]> {
+    let Item { ident, attrs, id, kind, vis, span, tokens } = item.deref_mut();
+    visitor.visit_id(id);
+    visitor.visit_ident(ident);
+    visitor.visit_vis(vis);
+    visit_attrs(attrs, visitor);
+    match kind {
+        ForeignItemKind::Static(ty, _, expr) => {
+            visitor.visit_ty(ty);
+            visit_opt(expr, |expr| visitor.visit_expr(expr));
+        }
+        ForeignItemKind::Fn(box Fn { defaultness, generics, sig, body }) => {
+            visit_defaultness(defaultness, visitor);
+            visitor.visit_generics(generics);
+            visit_fn_sig(sig, visitor);
+            visit_opt(body, |body| visitor.visit_block(body));
+        }
+        ForeignItemKind::TyAlias(box TyAlias {
+            defaultness,
+            generics,
+            where_clauses,
+            bounds,
+            ty,
+            ..
+        }) => {
+            visit_defaultness(defaultness, visitor);
+            visitor.visit_generics(generics);
+            visitor.visit_span(&mut where_clauses.0.1);
+            visitor.visit_span(&mut where_clauses.1.1);
+            visit_bounds(bounds, visitor);
+            visit_opt(ty, |ty| visitor.visit_ty(ty));
+        }
+        ForeignItemKind::MacCall(mac) => visitor.visit_mac_call(mac),
+    }
+    visitor.visit_span(span);
+    visit_lazy_tts(tokens, visitor);
+    smallvec![item]
+}
+
+pub fn noop_visit_pat<T: MutVisitor>(pat: &mut P<Pat>, vis: &mut T) {
+    let Pat { id, kind, span, tokens } = pat.deref_mut();
+    vis.visit_id(id);
+    match kind {
+        PatKind::Wild | PatKind::Rest => {}
+        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_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::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_vec(elems, |elem| vis.visit_pat(elem))
+        }
+        PatKind::Paren(inner) => vis.visit_pat(inner),
+        PatKind::MacCall(mac) => vis.visit_mac_call(mac),
+    }
+    vis.visit_span(span);
+    visit_lazy_tts(tokens, vis);
+}
+
+pub fn noop_visit_anon_const<T: MutVisitor>(AnonConst { id, value }: &mut AnonConst, vis: &mut T) {
+    vis.visit_id(id);
+    vis.visit_expr(value);
+}
+
+pub fn noop_visit_inline_asm<T: MutVisitor>(asm: &mut InlineAsm, vis: &mut T) {
+    for (op, _) in &mut asm.operands {
+        match op {
+            InlineAsmOperand::In { expr, .. }
+            | InlineAsmOperand::Out { expr: Some(expr), .. }
+            | InlineAsmOperand::InOut { expr, .. } => vis.visit_expr(expr),
+            InlineAsmOperand::Out { expr: None, .. } => {}
+            InlineAsmOperand::SplitInOut { in_expr, out_expr, .. } => {
+                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),
+        }
+    }
+}
+
+pub fn noop_visit_inline_asm_sym<T: MutVisitor>(
+    InlineAsmSym { id, qself, path }: &mut InlineAsmSym,
+    vis: &mut T,
+) {
+    vis.visit_id(id);
+    vis.visit_qself(qself);
+    vis.visit_path(path);
+}
+
+pub fn noop_visit_expr<T: MutVisitor>(
+    Expr { kind, id, span, attrs, tokens }: &mut Expr,
+    vis: &mut T,
+) {
+    match kind {
+        ExprKind::Box(expr) => vis.visit_expr(expr),
+        ExprKind::Array(exprs) => visit_exprs(exprs, vis),
+        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_exprs(exprs, vis),
+        ExprKind::Call(f, args) => {
+            vis.visit_expr(f);
+            visit_exprs(args, vis);
+        }
+        ExprKind::MethodCall(PathSegment { ident, id, args }, exprs, span) => {
+            vis.visit_ident(ident);
+            vis.visit_id(id);
+            visit_opt(args, |args| vis.visit_generic_args(args));
+            visit_exprs(exprs, vis);
+            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(_, _, ohs) => vis.visit_expr(ohs),
+        ExprKind::Let(pat, scrutinee, _) => {
+            vis.visit_pat(pat);
+            vis.visit_expr(scrutinee);
+        }
+        ExprKind::If(cond, tr, fl) => {
+            vis.visit_expr(cond);
+            vis.visit_block(tr);
+            visit_opt(fl, |fl| vis.visit_expr(fl));
+        }
+        ExprKind::While(cond, body, label) => {
+            vis.visit_expr(cond);
+            vis.visit_block(body);
+            visit_opt(label, |label| vis.visit_label(label));
+        }
+        ExprKind::ForLoop(pat, iter, body, label) => {
+            vis.visit_pat(pat);
+            vis.visit_expr(iter);
+            vis.visit_block(body);
+            visit_opt(label, |label| vis.visit_label(label));
+        }
+        ExprKind::Loop(body, label) => {
+            vis.visit_block(body);
+            visit_opt(label, |label| vis.visit_label(label));
+        }
+        ExprKind::Match(expr, arms) => {
+            vis.visit_expr(expr);
+            arms.flat_map_in_place(|arm| vis.flat_map_arm(arm));
+        }
+        ExprKind::Closure(binder, _capture_by, asyncness, _movability, decl, body, span) => {
+            vis.visit_closure_binder(binder);
+            vis.visit_asyncness(asyncness);
+            vis.visit_fn_decl(decl);
+            vis.visit_expr(body);
+            vis.visit_span(span);
+        }
+        ExprKind::Block(blk, label) => {
+            vis.visit_block(blk);
+            visit_opt(label, |label| vis.visit_label(label));
+        }
+        ExprKind::Async(_capture_by, node_id, body) => {
+            vis.visit_id(node_id);
+            vis.visit_block(body);
+        }
+        ExprKind::Await(expr) => vis.visit_expr(expr),
+        ExprKind::Assign(el, er, _) => {
+            vis.visit_expr(el);
+            vis.visit_expr(er);
+        }
+        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) => {
+            vis.visit_expr(el);
+            vis.visit_expr(er);
+        }
+        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::InlineAsm(asm) => vis.visit_inline_asm(asm),
+        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(_) | ExprKind::Err => {}
+    }
+    vis.visit_id(id);
+    vis.visit_span(span);
+    visit_thin_attrs(attrs, vis);
+    visit_lazy_tts(tokens, vis);
+}
+
+pub fn noop_filter_map_expr<T: MutVisitor>(mut e: P<Expr>, vis: &mut T) -> Option<P<Expr>> {
+    Some({
+        vis.visit_expr(&mut e);
+        e
+    })
+}
+
+pub fn noop_flat_map_stmt<T: MutVisitor>(
+    Stmt { kind, mut span, mut id }: Stmt,
+    vis: &mut T,
+) -> SmallVec<[Stmt; 1]> {
+    vis.visit_id(&mut id);
+    vis.visit_span(&mut span);
+    let stmts: SmallVec<_> = noop_flat_map_stmt_kind(kind, vis)
+        .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"
+        );
+    }
+    stmts
+}
+
+pub fn noop_flat_map_stmt_kind<T: MutVisitor>(
+    kind: StmtKind,
+    vis: &mut T,
+) -> SmallVec<[StmtKind; 1]> {
+    match kind {
+        StmtKind::Local(mut local) => smallvec![StmtKind::Local({
+            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();
+            vis.visit_mac_call(mac_);
+            visit_thin_attrs(attrs, vis);
+            visit_lazy_tts(tokens, vis);
+            smallvec![StmtKind::MacCall(mac)]
+        }
+    }
+}
+
+pub fn noop_visit_vis<T: MutVisitor>(visibility: &mut Visibility, vis: &mut T) {
+    match &mut visibility.kind {
+        VisibilityKind::Public | VisibilityKind::Inherited => {}
+        VisibilityKind::Restricted { path, id } => {
+            vis.visit_path(path);
+            vis.visit_id(id);
+        }
+    }
+    vis.visit_span(&mut visibility.span);
+}
+
+/// Some value for the AST node that is valid but possibly meaningless.
+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<T> DummyAstNode for ThinVec<T> {
+    fn dummy() -> Self {
+        Default::default()
+    }
+}
+
+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::Err,
+            span: Default::default(),
+            attrs: Default::default(),
+            tokens: Default::default(),
+        }
+    }
+}
+
+impl DummyAstNode for Ty {
+    fn dummy() -> Self {
+        Ty {
+            id: DUMMY_NODE_ID,
+            kind: TyKind::Err,
+            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 Block {
+    fn dummy() -> Self {
+        Block {
+            stmts: Default::default(),
+            id: DUMMY_NODE_ID,
+            rules: BlockCheckMode::Default,
+            span: Default::default(),
+            tokens: Default::default(),
+            could_be_bare_literal: 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(),
+        }
+    }
+}