Rollup merge of #138685 - nnethercote:use-Option-Ident-for-lowered-param-names, r=compiler-errors

Use `Option<Ident>` for lowered param names.

Parameter patterns are lowered to an `Ident` by `lower_fn_params_to_names`, which is used when lowering bare function types, trait methods, and foreign functions. Currently, there are two exceptional cases where the lowered param can become an empty `Ident`.

- If the incoming pattern is an empty `Ident`. This occurs if the parameter is anonymous, e.g. in a bare function type.

- If the incoming pattern is neither an ident nor an underscore. Any such parameter will have triggered a compile error (hence the `span_delayed_bug`), but lowering still occurs.

This commit replaces these empty `Ident` results with `None`, which eliminates a number of `kw::Empty` uses, and makes it impossible to fail to check for these exceptional cases.

Note: the `FIXME` comment in `is_unwrap_or_empty_symbol` is removed. It actually should have been removed in #138482, the precursor to this PR. That PR changed the lowering of wild patterns to `_` symbols instead of empty symbols, which made the mentioned underscore check load-bearing.

r? ``@compiler-errors``

2 files changed, 2 insertions, 2 deletions

diff --git a/compiler/rustc_metadata/src/rmeta/decoder.rs b/compiler/rustc_metadata/src/rmeta/decoder.rs
index f6cf218db9d..4610a571da0 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder.rs
@@ -1318,7 +1318,7 @@ impl<'a> CrateMetadataRef<'a> {
             .expect("argument names not encoded for a function")
             .decode((self, sess))
             .nth(0)
-            .is_some_and(|ident| ident.name == kw::SelfLower)
+            .is_some_and(|ident| matches!(ident, Some(Ident { name: kw::SelfLower, .. })))
     }
 
     fn get_associated_item_or_field_def_ids(self, id: DefIndex) -> impl Iterator<Item = DefId> {
diff --git a/compiler/rustc_metadata/src/rmeta/mod.rs b/compiler/rustc_metadata/src/rmeta/mod.rs
index 5536c93f84a..dc453b1e747 100644
--- a/compiler/rustc_metadata/src/rmeta/mod.rs
+++ b/compiler/rustc_metadata/src/rmeta/mod.rs
@@ -443,7 +443,7 @@ define_tables! {
     rendered_const: Table<DefIndex, LazyValue<String>>,
     rendered_precise_capturing_args: Table<DefIndex, LazyArray<PreciseCapturingArgKind<Symbol, Symbol>>>,
     asyncness: Table<DefIndex, ty::Asyncness>,
-    fn_arg_names: Table<DefIndex, LazyArray<Ident>>,
+    fn_arg_names: Table<DefIndex, LazyArray<Option<Ident>>>,
     coroutine_kind: Table<DefIndex, hir::CoroutineKind>,
     coroutine_for_closure: Table<DefIndex, RawDefId>,
     coroutine_by_move_body_def_id: Table<DefIndex, RawDefId>,