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``

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -1034,7 +1034,13 @@ fn report_trait_method_mismatch<'tcx>(
             let span = tcx
                 .hir_body_param_names(body)
                 .zip(sig.decl.inputs.iter())
-                .map(|(param, ty)| param.span.to(ty.span))
+                .map(|(param_name, ty)| {
+                    if let Some(param_name) = param_name {
+                        param_name.span.to(ty.span)
+                    } else {
+                        ty.span
+                    }
+                })
                 .next()
                 .unwrap_or(impl_err_span);
 

compiler/rustc_hir_analysis/src/hir_ty_lowering/cmse.rs

@@ -49,10 +49,12 @@ pub(crate) fn validate_cmse_abi<'tcx>(
                 Ok(Err(index)) => {
                     // fn(x: u32, u32, u32, u16, y: u16) -> u32,
                     //                           ^^^^^^
-                    let span = bare_fn_ty.param_names[index]
-                        .span
-                        .to(bare_fn_ty.decl.inputs[index].span)
-                        .to(bare_fn_ty.decl.inputs.last().unwrap().span);
+                    let span = if let Some(ident) = bare_fn_ty.param_names[index] {
+                        ident.span.to(bare_fn_ty.decl.inputs[index].span)
+                    } else {
+                        bare_fn_ty.decl.inputs[index].span
+                    }
+                    .to(bare_fn_ty.decl.inputs.last().unwrap().span);
                     let plural = bare_fn_ty.param_names.len() - index != 1;
                     dcx.emit_err(errors::CmseInputsStackSpill { span, plural, abi });
                 }