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, 7 insertions, 7 deletions

diff --git a/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs b/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
index 2694a1eda78..978186f76a1 100644
--- a/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
@@ -2514,12 +2514,12 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
         let ty::Tuple(params) = tupled_params.kind() else { return };
 
         // Find the first argument with a matching type, get its name
-        let Some((_, this_name)) =
-            params.iter().zip(tcx.hir_body_param_names(closure.body)).find(|(param_ty, name)| {
+        let Some(this_name) = params.iter().zip(tcx.hir_body_param_names(closure.body)).find_map(
+            |(param_ty, name)| {
                 // FIXME: also support deref for stuff like `Rc` arguments
-                param_ty.peel_refs() == local_ty && name != &Ident::empty()
-            })
-        else {
+                if param_ty.peel_refs() == local_ty { name } else { None }
+            },
+        ) else {
             return;
         };
 
@@ -3787,7 +3787,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                 method_args,
                 *fn_span,
                 call_source.from_hir_call(),
-                Some(self.infcx.tcx.fn_arg_names(method_did)[0]),
+                self.infcx.tcx.fn_arg_names(method_did)[0],
             )
         {
             err.note(format!("borrow occurs due to deref coercion to `{deref_target_ty}`"));
diff --git a/compiler/rustc_borrowck/src/diagnostics/mod.rs b/compiler/rustc_borrowck/src/diagnostics/mod.rs
index 208d510db2e..899e145c2c0 100644
--- a/compiler/rustc_borrowck/src/diagnostics/mod.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/mod.rs
@@ -1029,7 +1029,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                 method_args,
                 *fn_span,
                 call_source.from_hir_call(),
-                Some(self.infcx.tcx.fn_arg_names(method_did)[0]),
+                self.infcx.tcx.fn_arg_names(method_did)[0],
             );
 
             return FnSelfUse {