Add {ast,hir,thir}::PatKind::Missing variants.

"Missing" patterns are possible in bare fn types (`fn f(u32)`) and
similar places. Currently these are represented in the AST with
`ast::PatKind::Ident` with no `by_ref`, no `mut`, an empty ident, and no
sub-pattern. This flows through to `{hir,thir}::PatKind::Binding` for
HIR and THIR.

This is a bit nasty. It's very non-obvious, and easy to forget to check
for the exceptional empty identifier case.

This commit adds a new variant, `PatKind::Missing`, to do it properly.

The process I followed:
- Add a `Missing` variant to `{ast,hir,thir}::PatKind`.
- Chang `parse_param_general` to produce `ast::PatKind::Missing`
  instead of `ast::PatKind::Missing`.
- Look through `kw::Empty` occurrences to find functions where an
  existing empty ident check needs replacing with a `PatKind::Missing`
  check: `print_param`, `check_trait_item`, `is_named_param`.
- Add a `PatKind::Missing => unreachable!(),` arm to every exhaustive
  match identified by the compiler.
- Find which arms are actually reachable by running the test suite,
  changing them to something appropriate, usually by looking at what
  would happen to a `PatKind::Ident`/`PatKind::Binding` with no ref, no
  `mut`, an empty ident, and no subpattern.

Quite a few of the `unreachable!()` arms were never reached. This makes
sense because `PatKind::Missing` can't happen in every pattern, only
in places like bare fn tys and trait fn decls.

I also tried an alternative approach: modifying `ast::Param::pat` to
hold an `Option<P<Pat>>` instead of a `P<Pat>`, but that quickly turned
into a very large and painful change. Adding `PatKind::Missing` is much
easier.

compiler/rustc_lint/src/builtin.rs

@@ -778,21 +778,19 @@ impl EarlyLintPass for AnonymousParameters {
         }
         if let ast::AssocItemKind::Fn(box Fn { ref sig, .. }) = it.kind {
             for arg in sig.decl.inputs.iter() {
-                if let ast::PatKind::Ident(_, ident, None) = arg.pat.kind {
-                    if ident.name == kw::Empty {
-                        let ty_snip = cx.sess().source_map().span_to_snippet(arg.ty.span);
+                if let ast::PatKind::Missing = arg.pat.kind {
+                    let ty_snip = cx.sess().source_map().span_to_snippet(arg.ty.span);
 
-                        let (ty_snip, appl) = if let Ok(ref snip) = ty_snip {
-                            (snip.as_str(), Applicability::MachineApplicable)
-                        } else {
-                            ("<type>", Applicability::HasPlaceholders)
-                        };
-                        cx.emit_span_lint(
-                            ANONYMOUS_PARAMETERS,
-                            arg.pat.span,
-                            BuiltinAnonymousParams { suggestion: (arg.pat.span, appl), ty_snip },
-                        );
-                    }
+                    let (ty_snip, appl) = if let Ok(ref snip) = ty_snip {
+                        (snip.as_str(), Applicability::MachineApplicable)
+                    } else {
+                        ("<type>", Applicability::HasPlaceholders)
+                    };
+                    cx.emit_span_lint(
+                        ANONYMOUS_PARAMETERS,
+                        arg.pat.span,
+                        BuiltinAnonymousParams { suggestion: (arg.pat.span, appl), ty_snip },
+                    );
                 }
             }
         }