Move hir::Item::ident into hir::ItemKind.

`hir::Item` has an `ident` field.

- It's always non-empty for these item kinds: `ExternCrate`, `Static`,
  `Const`, `Fn`, `Macro`, `Mod`, `TyAlias`, `Enum`, `Struct`, `Union`,
  Trait`, TraitAalis`.

- It's always empty for these item kinds: `ForeignMod`, `GlobalAsm`,
  `Impl`.

- For `Use`, it is non-empty for `UseKind::Single` and empty for
  `UseKind::{Glob,ListStem}`.

All of this is quite non-obvious; the only documentation is a single
comment saying "The name might be a dummy name in case of anonymous
items". Some sites that handle items check for an empty ident, some
don't. This is a very C-like way of doing things, but this is Rust, we
have sum types, we can do this properly and never forget to check for
the exceptional case and never YOLO possibly empty identifiers (or
possibly dummy spans) around and hope that things will work out.

The commit is large but it's mostly obvious plumbing work. Some notable
things.

- A similar transformation makes sense for `ast::Item`, but this is
  already a big change. That can be done later.

- Lots of assertions are added to item lowering to ensure that
  identifiers are empty/non-empty as expected. These will be removable
  when `ast::Item` is done later.

- `ItemKind::Use` doesn't get an `Ident`, but `UseKind::Single` does.

- `lower_use_tree` is significantly simpler. No more confusing `&mut
  Ident` to deal with.

- `ItemKind::ident` is a new method, it returns an `Option<Ident>`. It's
  used with `unwrap` in a few places; sometimes it's hard to tell
  exactly which item kinds might occur. None of these unwraps fail on
  the test suite. It's conceivable that some might fail on alternative
  input. We can deal with those if/when they happen.

- In `trait_path` the `find_map`/`if let` is replaced with a loop, and
  things end up much clearer that way.

- `named_span` no longer checks for an empty name; instead the call site
  now checks for a missing identifier if necessary.

- `maybe_inline_local` doesn't need the `glob` argument, it can be
  computed in-function from the `renamed` argument.

- `arbitrary_source_item_ordering::check_mod` had a big `if` statement
  that was just getting the ident from the item kinds that had one. It
  could be mostly replaced by a single call to the new `ItemKind::ident`
  method.

- `ItemKind` grows from 56 to 64 bytes, but `Item` stays the same size,
  and that's what matters, because `ItemKind` only occurs within `Item`.

Author: nnethercote

compiler/rustc_ast_lowering/src/item.rs

@@ -111,6 +111,7 @@ impl<'a, 'hir> ItemLowerer<'a, 'hir> {
     }
 
     fn lower_foreign_item(&mut self, item: &ForeignItem) {
+        debug_assert_ne!(item.ident.name, kw::Empty);
         self.with_lctx(item.id, |lctx| hir::OwnerNode::ForeignItem(lctx.lower_foreign_item(item)))
     }
 }
@@ -154,14 +155,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
     }
 
     fn lower_item(&mut self, i: &Item) -> &'hir hir::Item<'hir> {
-        let mut ident = i.ident;
         let vis_span = self.lower_span(i.vis.span);
         let hir_id = hir::HirId::make_owner(self.current_hir_id_owner.def_id);
         let attrs = self.lower_attrs(hir_id, &i.attrs, i.span);
-        let kind = self.lower_item_kind(i.span, i.id, hir_id, &mut ident, attrs, vis_span, &i.kind);
+        let kind = self.lower_item_kind(i.span, i.id, hir_id, i.ident, attrs, vis_span, &i.kind);
         let item = hir::Item {
             owner_id: hir_id.expect_owner(),
-            ident: self.lower_ident(ident),
             kind,
             vis_span,
             span: self.lower_span(i.span),
@@ -174,25 +173,34 @@ impl<'hir> LoweringContext<'_, 'hir> {
         span: Span,
         id: NodeId,
         hir_id: hir::HirId,
-        ident: &mut Ident,
+        ident: Ident,
         attrs: &'hir [hir::Attribute],
         vis_span: Span,
         i: &ItemKind,
     ) -> hir::ItemKind<'hir> {
         match i {
-            ItemKind::ExternCrate(orig_name) => hir::ItemKind::ExternCrate(*orig_name),
+            ItemKind::ExternCrate(orig_name) => {
+                debug_assert_ne!(ident.name, kw::Empty);
+                let ident = self.lower_ident(ident);
+                hir::ItemKind::ExternCrate(*orig_name, ident)
+            }
             ItemKind::Use(use_tree) => {
+                debug_assert_eq!(ident.name, kw::Empty);
                 // Start with an empty prefix.
                 let prefix = Path { segments: ThinVec::new(), span: use_tree.span, tokens: None };
 
-                self.lower_use_tree(use_tree, &prefix, id, vis_span, ident, attrs)
+                self.lower_use_tree(use_tree, &prefix, id, vis_span, attrs)
             }
             ItemKind::Static(box ast::StaticItem { ty: t, safety: _, mutability: m, expr: e }) => {
+                debug_assert_ne!(ident.name, kw::Empty);
+                let ident = self.lower_ident(ident);
                 let (ty, body_id) =
                     self.lower_const_item(t, span, e.as_deref(), ImplTraitPosition::StaticTy);
-                hir::ItemKind::Static(ty, *m, body_id)
+                hir::ItemKind::Static(ident, ty, *m, body_id)
             }
             ItemKind::Const(box ast::ConstItem { generics, ty, expr, .. }) => {
+                debug_assert_ne!(ident.name, kw::Empty);
+                let ident = self.lower_ident(ident);
                 let (generics, (ty, body_id)) = self.lower_generics(
                     generics,
                     id,
@@ -201,7 +209,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         this.lower_const_item(ty, span, expr.as_deref(), ImplTraitPosition::ConstTy)
                     },
                 );
-                hir::ItemKind::Const(ty, generics, body_id)
+                hir::ItemKind::Const(ident, ty, generics, body_id)
             }
             ItemKind::Fn(box Fn {
                 sig: FnSig { decl, header, span: fn_sig_span },
@@ -211,6 +219,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 define_opaque,
                 ..
             }) => {
+                debug_assert_ne!(ident.name, kw::Empty);
                 self.with_new_scopes(*fn_sig_span, |this| {
                     // Note: we don't need to change the return type from `T` to
                     // `impl Future<Output = T>` here because lower_body
@@ -238,28 +247,44 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         span: this.lower_span(*fn_sig_span),
                     };
                     this.lower_define_opaque(hir_id, &define_opaque);
-                    hir::ItemKind::Fn { sig, generics, body: body_id, has_body: body.is_some() }
+                    let ident = this.lower_ident(ident);
+                    hir::ItemKind::Fn {
+                        ident,
+                        sig,
+                        generics,
+                        body: body_id,
+                        has_body: body.is_some(),
+                    }
                 })
             }
-            ItemKind::Mod(_, mod_kind) => match mod_kind {
-                ModKind::Loaded(items, _, spans, _) => {
-                    hir::ItemKind::Mod(self.lower_mod(items, spans))
+            ItemKind::Mod(_, mod_kind) => {
+                debug_assert_ne!(ident.name, kw::Empty);
+                let ident = self.lower_ident(ident);
+                match mod_kind {
+                    ModKind::Loaded(items, _, spans, _) => {
+                        hir::ItemKind::Mod(ident, self.lower_mod(items, spans))
+                    }
+                    ModKind::Unloaded => panic!("`mod` items should have been loaded by now"),
                 }
-                ModKind::Unloaded => panic!("`mod` items should have been loaded by now"),
-            },
-            ItemKind::ForeignMod(fm) => hir::ItemKind::ForeignMod {
-                abi: fm.abi.map_or(ExternAbi::FALLBACK, |abi| self.lower_abi(abi)),
-                items: self
-                    .arena
-                    .alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item_ref(x))),
-            },
+            }
+            ItemKind::ForeignMod(fm) => {
+                debug_assert_eq!(ident.name, kw::Empty);
+                hir::ItemKind::ForeignMod {
+                    abi: fm.abi.map_or(ExternAbi::FALLBACK, |abi| self.lower_abi(abi)),
+                    items: self
+                        .arena
+                        .alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item_ref(x))),
+                }
+            }
             ItemKind::GlobalAsm(asm) => {
+                debug_assert_eq!(ident.name, kw::Empty);
                 let asm = self.lower_inline_asm(span, asm);
                 let fake_body =
                     self.lower_body(|this| (&[], this.expr(span, hir::ExprKind::InlineAsm(asm))));
                 hir::ItemKind::GlobalAsm { asm, fake_body }
             }
             ItemKind::TyAlias(box TyAlias { generics, where_clauses, ty, .. }) => {
+                debug_assert_ne!(ident.name, kw::Empty);
                 // We lower
                 //
                 // type Foo = impl Trait
@@ -268,6 +293,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 //
                 // type Foo = Foo1
                 // opaque type Foo1: Trait
+                let ident = self.lower_ident(ident);
                 let mut generics = generics.clone();
                 add_ty_alias_where_clause(&mut generics, *where_clauses, true);
                 let (generics, ty) = self.lower_generics(
@@ -293,9 +319,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         ),
                     },
                 );
-                hir::ItemKind::TyAlias(ty, generics)
+                hir::ItemKind::TyAlias(ident, ty, generics)
             }
             ItemKind::Enum(enum_definition, generics) => {
+                debug_assert_ne!(ident.name, kw::Empty);
+                let ident = self.lower_ident(ident);
                 let (generics, variants) = self.lower_generics(
                     generics,
                     id,
@@ -306,25 +334,29 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         )
                     },
                 );
-                hir::ItemKind::Enum(hir::EnumDef { variants }, generics)
+                hir::ItemKind::Enum(ident, hir::EnumDef { variants }, generics)
             }
             ItemKind::Struct(struct_def, generics) => {
+                debug_assert_ne!(ident.name, kw::Empty);
+                let ident = self.lower_ident(ident);
                 let (generics, struct_def) = self.lower_generics(
                     generics,
                     id,
                     ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| this.lower_variant_data(hir_id, struct_def),
                 );
-                hir::ItemKind::Struct(struct_def, generics)
+                hir::ItemKind::Struct(ident, struct_def, generics)
             }
             ItemKind::Union(vdata, generics) => {
+                debug_assert_ne!(ident.name, kw::Empty);
+                let ident = self.lower_ident(ident);
                 let (generics, vdata) = self.lower_generics(
                     generics,
                     id,
                     ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| this.lower_variant_data(hir_id, vdata),
                 );
-                hir::ItemKind::Union(vdata, generics)
+                hir::ItemKind::Union(ident, vdata, generics)
             }
             ItemKind::Impl(box Impl {
                 safety,
@@ -336,6 +368,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 self_ty: ty,
                 items: impl_items,
             }) => {
+                debug_assert_eq!(ident.name, kw::Empty);
                 // Lower the "impl header" first. This ordering is important
                 // for in-band lifetimes! Consider `'a` here:
                 //
@@ -401,6 +434,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 }))
             }
             ItemKind::Trait(box Trait { is_auto, safety, generics, bounds, items }) => {
+                debug_assert_ne!(ident.name, kw::Empty);
+                let ident = self.lower_ident(ident);
                 let (generics, (safety, items, bounds)) = self.lower_generics(
                     generics,
                     id,
@@ -417,9 +452,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         (safety, items, bounds)
                     },
                 );
-                hir::ItemKind::Trait(*is_auto, safety, generics, bounds, items)
+                hir::ItemKind::Trait(*is_auto, safety, ident, generics, bounds, items)
             }
             ItemKind::TraitAlias(generics, bounds) => {
+                debug_assert_ne!(ident.name, kw::Empty);
+                let ident = self.lower_ident(ident);
                 let (generics, bounds) = self.lower_generics(
                     generics,
                     id,
@@ -431,9 +468,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         )
                     },
                 );
-                hir::ItemKind::TraitAlias(generics, bounds)
+                hir::ItemKind::TraitAlias(ident, generics, bounds)
             }
             ItemKind::MacroDef(MacroDef { body, macro_rules }) => {
+                debug_assert_ne!(ident.name, kw::Empty);
+                let ident = self.lower_ident(ident);
                 let body = P(self.lower_delim_args(body));
                 let def_id = self.local_def_id(id);
                 let def_kind = self.tcx.def_kind(def_id);
@@ -444,11 +483,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     );
                 };
                 let macro_def = self.arena.alloc(ast::MacroDef { body, macro_rules: *macro_rules });
-                hir::ItemKind::Macro(macro_def, macro_kind)
+                hir::ItemKind::Macro(ident, macro_def, macro_kind)
             }
             ItemKind::Delegation(box delegation) => {
+                debug_assert_ne!(ident.name, kw::Empty);
+                let ident = self.lower_ident(ident);
                 let delegation_results = self.lower_delegation(delegation, id);
                 hir::ItemKind::Fn {
+                    ident,
                     sig: delegation_results.sig,
                     generics: delegation_results.generics,
                     body: delegation_results.body_id,
@@ -479,15 +521,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
         prefix: &Path,
         id: NodeId,
         vis_span: Span,
-        ident: &mut Ident,
         attrs: &'hir [hir::Attribute],
     ) -> hir::ItemKind<'hir> {
         let path = &tree.prefix;
         let segments = prefix.segments.iter().chain(path.segments.iter()).cloned().collect();
 
         match tree.kind {
             UseTreeKind::Simple(rename) => {
-                *ident = tree.ident();
+                let mut ident = tree.ident();
 
                 // First, apply the prefix to the path.
                 let mut path = Path { segments, span: path.span, tokens: None };
@@ -498,13 +539,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 {
                     let _ = path.segments.pop();
                     if rename.is_none() {
-                        *ident = path.segments.last().unwrap().ident;
+                        ident = path.segments.last().unwrap().ident;
                     }
                 }
 
                 let res = self.lower_import_res(id, path.span);
                 let path = self.lower_use_path(res, &path, ParamMode::Explicit);
-                hir::ItemKind::Use(path, hir::UseKind::Single)
+                let ident = self.lower_ident(ident);
+                hir::ItemKind::Use(path, hir::UseKind::Single(ident))
             }
             UseTreeKind::Glob => {
                 let res = self.expect_full_res(id);
@@ -551,20 +593,16 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     // own its own names, we have to adjust the owner before
                     // lowering the rest of the import.
                     self.with_hir_id_owner(id, |this| {
-                        let mut ident = *ident;
-
                         // `prefix` is lowered multiple times, but in different HIR owners.
                         // So each segment gets renewed `HirId` with the same
                         // `ItemLocalId` and the new owner. (See `lower_node_id`)
-                        let kind =
-                            this.lower_use_tree(use_tree, &prefix, id, vis_span, &mut ident, attrs);
+                        let kind = this.lower_use_tree(use_tree, &prefix, id, vis_span, attrs);
                         if !attrs.is_empty() {
                             this.attrs.insert(hir::ItemLocalId::ZERO, attrs);
                         }
 
                         let item = hir::Item {
                             owner_id: hir::OwnerId { def_id: new_hir_id },
-                            ident: this.lower_ident(ident),
                             kind,
                             vis_span,
                             span: this.lower_span(use_tree.span),
@@ -604,7 +642,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             hir::ItemKind::Impl(impl_) => {
                 self.is_in_trait_impl = impl_.of_trait.is_some();
             }
-            hir::ItemKind::Trait(_, _, _, _, _) => {}
+            hir::ItemKind::Trait(..) => {}
             kind => {
                 span_bug!(item.span, "assoc item has unexpected kind of parent: {}", kind.descr())
             }
@@ -760,6 +798,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     }
 
     fn lower_trait_item(&mut self, i: &AssocItem) -> &'hir hir::TraitItem<'hir> {
+        debug_assert_ne!(i.ident.name, kw::Empty);
         let hir_id = hir::HirId::make_owner(self.current_hir_id_owner.def_id);
         let attrs = self.lower_attrs(hir_id, &i.attrs, i.span);
         let trait_item_def_id = hir_id.expect_owner();
@@ -907,6 +946,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     }
 
     fn lower_impl_item(&mut self, i: &AssocItem) -> &'hir hir::ImplItem<'hir> {
+        debug_assert_ne!(i.ident.name, kw::Empty);
         // Since `default impl` is not yet implemented, this is always true in impls.
         let has_value = true;
         let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value);

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -691,7 +691,9 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
             true,
             td.is_local(),
             td.as_local().and_then(|tld| match self.infcx.tcx.hir_node_by_def_id(tld) {
-                Node::Item(hir::Item { kind: hir::ItemKind::Trait(_, _, _, _, items), .. }) => {
+                Node::Item(hir::Item {
+                    kind: hir::ItemKind::Trait(_, _, _, _, _, items), ..
+                }) => {
                     let mut f_in_trait_opt = None;
                     for hir::TraitItemRef { id: fi, kind: k, .. } in *items {
                         let hi = fi.hir_id();
@@ -980,7 +982,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
             let arg = match tcx.hir_get_if_local(callee_def_id) {
                 Some(
                     hir::Node::Item(hir::Item {
-                        ident, kind: hir::ItemKind::Fn { sig, .. }, ..
+                        kind: hir::ItemKind::Fn { ident, sig, .. }, ..
                     })
                     | hir::Node::TraitItem(hir::TraitItem {
                         ident,
@@ -1021,7 +1023,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
             // return type.
             match tcx.hir_node_by_def_id(tcx.hir_get_parent_item(fn_call_id).def_id) {
                 hir::Node::Item(hir::Item {
-                    ident, kind: hir::ItemKind::Fn { sig, .. }, ..
+                    kind: hir::ItemKind::Fn { ident, sig, .. }, ..
                 })
                 | hir::Node::TraitItem(hir::TraitItem {
                     ident,