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.

compiler/rustc_hir_pretty/src/lib.rs

@@ -2,6 +2,7 @@
 //! the definitions in this file have equivalents in `rustc_ast_pretty`.
 
 // tidy-alphabetical-start
+#![feature(let_chains)]
 #![recursion_limit = "256"]
 // tidy-alphabetical-end
 
@@ -898,7 +899,7 @@ impl<'a> State<'a> {
         ident: Ident,
         m: &hir::FnSig<'_>,
         generics: &hir::Generics<'_>,
-        arg_names: &[Ident],
+        arg_names: &[Option<Ident>],
         body_id: Option<hir::BodyId>,
     ) {
         self.print_fn(m.decl, m.header, Some(ident.name), generics, arg_names, body_id);
@@ -2121,7 +2122,7 @@ impl<'a> State<'a> {
         header: hir::FnHeader,
         name: Option<Symbol>,
         generics: &hir::Generics<'_>,
-        arg_names: &[Ident],
+        arg_names: &[Option<Ident>],
         body_id: Option<hir::BodyId>,
     ) {
         self.print_fn_header_info(header);
@@ -2141,7 +2142,7 @@ impl<'a> State<'a> {
                 s.print_implicit_self(&decl.implicit_self);
             } else {
                 if let Some(arg_name) = arg_names.get(i) {
-                    if arg_name.name != kw::Empty {
+                    if let Some(arg_name) = arg_name {
                         s.word(arg_name.to_string());
                         s.word(":");
                         s.space();
@@ -2451,7 +2452,7 @@ impl<'a> State<'a> {
         decl: &hir::FnDecl<'_>,
         name: Option<Symbol>,
         generic_params: &[hir::GenericParam<'_>],
-        arg_names: &[Ident],
+        arg_names: &[Option<Ident>],
     ) {
         self.ibox(INDENT_UNIT);
         self.print_formal_generic_params(generic_params);