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.

2 files changed, 8 insertions, 0 deletions

diff --git a/tests/ui/unpretty/expanded-exhaustive.rs b/tests/ui/unpretty/expanded-exhaustive.rs
index 31af323ecda..4d1f12e3490 100644
--- a/tests/ui/unpretty/expanded-exhaustive.rs
+++ b/tests/ui/unpretty/expanded-exhaustive.rs
@@ -574,6 +574,11 @@ mod items {
 }
 
 mod patterns {
+    /// PatKind::Missing
+    fn pat_missing() {
+        let _: fn(u32, T, &str);
+    }
+
     /// PatKind::Wild
     fn pat_wild() {
         let _;
diff --git a/tests/ui/unpretty/expanded-exhaustive.stdout b/tests/ui/unpretty/expanded-exhaustive.stdout
index 19ae66f7a07..d8da941a340 100644
--- a/tests/ui/unpretty/expanded-exhaustive.stdout
+++ b/tests/ui/unpretty/expanded-exhaustive.stdout
@@ -363,6 +363,7 @@ mod expressions {
 
 
 
+
         { builtin # offset_of(T, field) };
     }
     /// ExprKind::MacCall
@@ -517,6 +518,8 @@ mod items {
     }
 }
 mod patterns {
+    /// PatKind::Missing
+    fn pat_missing() { let _: fn(u32, T, &str); }
     /// PatKind::Wild
     fn pat_wild() { let _; }
     /// PatKind::Ident