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.

1 files changed, 1 insertions, 1 deletions

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 {