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``

1 files changed, 10 insertions, 4 deletions

diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index e24b45c5b19..e08850da4a7 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -1513,16 +1513,22 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }))
     }
 
-    fn lower_fn_params_to_names(&mut self, decl: &FnDecl) -> &'hir [Ident] {
+    fn lower_fn_params_to_names(&mut self, decl: &FnDecl) -> &'hir [Option<Ident>] {
         self.arena.alloc_from_iter(decl.inputs.iter().map(|param| match param.pat.kind {
-            PatKind::Ident(_, ident, _) => self.lower_ident(ident),
-            PatKind::Wild => Ident::new(kw::Underscore, self.lower_span(param.pat.span)),
+            PatKind::Ident(_, ident, _) => {
+                if ident.name != kw::Empty {
+                    Some(self.lower_ident(ident))
+                } else {
+                    None
+                }
+            }
+            PatKind::Wild => Some(Ident::new(kw::Underscore, self.lower_span(param.pat.span))),
             _ => {
                 self.dcx().span_delayed_bug(
                     param.pat.span,
                     "non-ident/wild param pat must trigger an error",
                 );
-                Ident::new(kw::Empty, self.lower_span(param.pat.span))
+                None
             }
         }))
     }