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.

2 files changed, 5 insertions, 5 deletions

diff --git a/compiler/rustc_middle/src/hir/map.rs b/compiler/rustc_middle/src/hir/map.rs
index 2e589150d3e..f17efab81ec 100644
--- a/compiler/rustc_middle/src/hir/map.rs
+++ b/compiler/rustc_middle/src/hir/map.rs
@@ -280,11 +280,11 @@ impl<'tcx> TyCtxt<'tcx> {
         })
     }
 
-    pub fn hir_body_param_names(self, id: BodyId) -> impl Iterator<Item = Ident> {
+    pub fn hir_body_param_names(self, id: BodyId) -> impl Iterator<Item = Option<Ident>> {
         self.hir_body(id).params.iter().map(|param| match param.pat.kind {
-            PatKind::Binding(_, _, ident, _) => ident,
-            PatKind::Wild => Ident::new(kw::Underscore, param.pat.span),
-            _ => Ident::empty(),
+            PatKind::Binding(_, _, ident, _) => Some(ident),
+            PatKind::Wild => Some(Ident::new(kw::Underscore, param.pat.span)),
+            _ => None,
         })
     }
 
diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index 94a5a3769a3..527c18addbe 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -1410,7 +1410,7 @@ rustc_queries! {
         desc { |tcx| "computing target features for inline asm of `{}`", tcx.def_path_str(def_id) }
     }
 
-    query fn_arg_names(def_id: DefId) -> &'tcx [rustc_span::Ident] {
+    query fn_arg_names(def_id: DefId) -> &'tcx [Option<rustc_span::Ident>] {
         desc { |tcx| "looking up function parameter names for `{}`", tcx.def_path_str(def_id) }
         separate_provide_extern
     }