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`.

5 files changed, 36 insertions, 34 deletions

diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs
index 6510dbbbe9d..40f8af1f691 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs
@@ -2026,7 +2026,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 }
                 LetVisitor { span }.visit_body(body).break_value()
             }
-            hir::Node::Item(hir::Item { kind: hir::ItemKind::Const(ty, _, _), .. }) => {
+            hir::Node::Item(hir::Item { kind: hir::ItemKind::Const(_, ty, _, _), .. }) => {
                 Some(&ty.peel_refs().kind)
             }
             _ => None,
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/ambiguity.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/ambiguity.rs
index d673e5672a0..59c93db9c8f 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/ambiguity.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/ambiguity.rs
@@ -338,7 +338,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                                 ..
                             },
                             hir::PathSegment {
-                                ident: assoc_item_name,
+                                ident: assoc_item_ident,
                                 res: Res::Def(_, item_id),
                                 ..
                             },
@@ -368,17 +368,16 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
 
                         if let Some(local_def_id) = data.trait_ref.def_id.as_local()
                             && let hir::Node::Item(hir::Item {
-                                ident: trait_name,
-                                kind: hir::ItemKind::Trait(_, _, _, _, trait_item_refs),
+                                kind: hir::ItemKind::Trait(_, _, trait_ident, _, _, trait_item_refs),
                                 ..
                             }) = self.tcx.hir_node_by_def_id(local_def_id)
                             && let Some(method_ref) = trait_item_refs
                                 .iter()
-                                .find(|item_ref| item_ref.ident == *assoc_item_name)
+                                .find(|item_ref| item_ref.ident == *assoc_item_ident)
                         {
                             err.span_label(
                                 method_ref.span,
-                                format!("`{trait_name}::{assoc_item_name}` defined here"),
+                                format!("`{trait_ident}::{assoc_item_ident}` defined here"),
                             );
                         }
 
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs
index 4d87a93be0c..98df09b6f7b 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs
@@ -419,7 +419,12 @@ pub fn report_dyn_incompatibility<'tcx>(
 ) -> Diag<'tcx> {
     let trait_str = tcx.def_path_str(trait_def_id);
     let trait_span = tcx.hir_get_if_local(trait_def_id).and_then(|node| match node {
-        hir::Node::Item(item) => Some(item.ident.span),
+        hir::Node::Item(item) => match item.kind {
+            hir::ItemKind::Trait(_, _, ident, ..) | hir::ItemKind::TraitAlias(ident, _, _) => {
+                Some(ident.span)
+            }
+            _ => unreachable!(),
+        },
         _ => None,
     });
 
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
index 9383b82ff3c..f3ee28fe82f 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
@@ -267,8 +267,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             let node = self.tcx.hir_node_by_def_id(body_id);
             match node {
                 hir::Node::Item(hir::Item {
-                    ident,
-                    kind: hir::ItemKind::Trait(_, _, generics, bounds, _),
+                    kind: hir::ItemKind::Trait(_, _, ident, generics, bounds, _),
                     ..
                 }) if self_ty == self.tcx.types.self_param => {
                     assert!(param_ty);
@@ -282,7 +281,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                         None,
                         projection,
                         trait_pred,
-                        Some((ident, bounds)),
+                        Some((&ident, bounds)),
                     );
                     return;
                 }
@@ -331,7 +330,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 }
                 hir::Node::Item(hir::Item {
                     kind:
-                        hir::ItemKind::Trait(_, _, generics, ..)
+                        hir::ItemKind::Trait(_, _, _, generics, ..)
                         | hir::ItemKind::Impl(hir::Impl { generics, .. }),
                     ..
                 }) if projection.is_some() => {
@@ -352,15 +351,15 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
 
                 hir::Node::Item(hir::Item {
                     kind:
-                        hir::ItemKind::Struct(_, generics)
-                        | hir::ItemKind::Enum(_, generics)
-                        | hir::ItemKind::Union(_, generics)
-                        | hir::ItemKind::Trait(_, _, generics, ..)
+                        hir::ItemKind::Struct(_, _, generics)
+                        | hir::ItemKind::Enum(_, _, generics)
+                        | hir::ItemKind::Union(_, _, generics)
+                        | hir::ItemKind::Trait(_, _, _, generics, ..)
                         | hir::ItemKind::Impl(hir::Impl { generics, .. })
                         | hir::ItemKind::Fn { generics, .. }
-                        | hir::ItemKind::TyAlias(_, generics)
-                        | hir::ItemKind::Const(_, generics, _)
-                        | hir::ItemKind::TraitAlias(generics, _),
+                        | hir::ItemKind::TyAlias(_, _, generics)
+                        | hir::ItemKind::Const(_, _, generics, _)
+                        | hir::ItemKind::TraitAlias(_, generics, _),
                     ..
                 })
                 | hir::Node::TraitItem(hir::TraitItem { generics, .. })
@@ -417,15 +416,15 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
 
                 hir::Node::Item(hir::Item {
                     kind:
-                        hir::ItemKind::Struct(_, generics)
-                        | hir::ItemKind::Enum(_, generics)
-                        | hir::ItemKind::Union(_, generics)
-                        | hir::ItemKind::Trait(_, _, generics, ..)
+                        hir::ItemKind::Struct(_, _, generics)
+                        | hir::ItemKind::Enum(_, _, generics)
+                        | hir::ItemKind::Union(_, _, generics)
+                        | hir::ItemKind::Trait(_, _, _, generics, ..)
                         | hir::ItemKind::Impl(hir::Impl { generics, .. })
                         | hir::ItemKind::Fn { generics, .. }
-                        | hir::ItemKind::TyAlias(_, generics)
-                        | hir::ItemKind::Const(_, generics, _)
-                        | hir::ItemKind::TraitAlias(generics, _),
+                        | hir::ItemKind::TyAlias(_, _, generics)
+                        | hir::ItemKind::Const(_, _, generics, _)
+                        | hir::ItemKind::TraitAlias(_, generics, _),
                     ..
                 }) if !param_ty => {
                     // Missing generic type parameter bound.
@@ -847,7 +846,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     name.to_string()
                 }
                 Some(hir::Node::Item(hir::Item {
-                    ident, kind: hir::ItemKind::Fn { .. }, ..
+                    kind: hir::ItemKind::Fn { ident, .. }, ..
                 })) => {
                     err.span_label(ident.span, "consider calling this function");
                     ident.to_string()
@@ -1593,20 +1592,20 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 if let Some(typeck_results) = &self.typeck_results
                     && let ty = typeck_results.expr_ty_adjusted(base)
                     && let ty::FnDef(def_id, _args) = ty.kind()
-                    && let Some(hir::Node::Item(hir::Item { ident, span, vis_span, .. })) =
-                        self.tcx.hir_get_if_local(*def_id)
+                    && let Some(hir::Node::Item(item)) = self.tcx.hir_get_if_local(*def_id)
                 {
+                    let (ident, _, _, _) = item.expect_fn();
                     let msg = format!("alternatively, consider making `fn {ident}` asynchronous");
-                    if vis_span.is_empty() {
+                    if item.vis_span.is_empty() {
                         err.span_suggestion_verbose(
-                            span.shrink_to_lo(),
+                            item.span.shrink_to_lo(),
                             msg,
                             "async ",
                             Applicability::MaybeIncorrect,
                         );
                     } else {
                         err.span_suggestion_verbose(
-                            vis_span.shrink_to_hi(),
+                            item.vis_span.shrink_to_hi(),
                             msg,
                             " async",
                             Applicability::MaybeIncorrect,
@@ -3308,8 +3307,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 let mut is_auto_trait = false;
                 match tcx.hir_get_if_local(data.impl_or_alias_def_id) {
                     Some(Node::Item(hir::Item {
-                        kind: hir::ItemKind::Trait(is_auto, ..),
-                        ident,
+                        kind: hir::ItemKind::Trait(is_auto, _, ident, ..),
                         ..
                     })) => {
                         // FIXME: we should do something else so that it works even on crate foreign
diff --git a/compiler/rustc_trait_selection/src/errors.rs b/compiler/rustc_trait_selection/src/errors.rs
index fe859eb53cd..7159397c4b1 100644
--- a/compiler/rustc_trait_selection/src/errors.rs
+++ b/compiler/rustc_trait_selection/src/errors.rs
@@ -516,7 +516,7 @@ impl Subdiagnostic for AddLifetimeParamsSuggestion<'_> {
                     match self.tcx.parent_hir_node(self.tcx.local_def_id_to_hir_id(anon_reg.scope))
                     {
                         hir::Node::Item(hir::Item {
-                            kind: hir::ItemKind::Trait(_, _, generics, ..),
+                            kind: hir::ItemKind::Trait(_, _, _, generics, ..),
                             ..
                         })
                         | hir::Node::Item(hir::Item {