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.

3 files changed, 3 insertions, 0 deletions

diff --git a/src/tools/clippy/clippy_utils/src/ast_utils/mod.rs b/src/tools/clippy/clippy_utils/src/ast_utils/mod.rs
index 6023ae9cc7b..ed99f966d2b 100644
--- a/src/tools/clippy/clippy_utils/src/ast_utils/mod.rs
+++ b/src/tools/clippy/clippy_utils/src/ast_utils/mod.rs
@@ -33,6 +33,7 @@ pub fn eq_id(l: Ident, r: Ident) -> bool {
 pub fn eq_pat(l: &Pat, r: &Pat) -> bool {
     use PatKind::*;
     match (&l.kind, &r.kind) {
+        (Missing, _) | (_, Missing) => unreachable!(),
         (Paren(l), _) => eq_pat(l, r),
         (_, Paren(r)) => eq_pat(l, r),
         (Wild, Wild) | (Rest, Rest) => true,
diff --git a/src/tools/clippy/clippy_utils/src/hir_utils.rs b/src/tools/clippy/clippy_utils/src/hir_utils.rs
index 9938e64d242..b813cd361ed 100644
--- a/src/tools/clippy/clippy_utils/src/hir_utils.rs
+++ b/src/tools/clippy/clippy_utils/src/hir_utils.rs
@@ -1124,6 +1124,7 @@ impl<'a, 'tcx> SpanlessHash<'a, 'tcx> {
     pub fn hash_pat(&mut self, pat: &Pat<'_>) {
         std::mem::discriminant(&pat.kind).hash(&mut self.s);
         match pat.kind {
+            PatKind::Missing => unreachable!(),
             PatKind::Binding(BindingMode(by_ref, mutability), _, _, pat) => {
                 std::mem::discriminant(&by_ref).hash(&mut self.s);
                 std::mem::discriminant(&mutability).hash(&mut self.s);
diff --git a/src/tools/clippy/clippy_utils/src/lib.rs b/src/tools/clippy/clippy_utils/src/lib.rs
index 1307ff79bc5..bdcbe1437d0 100644
--- a/src/tools/clippy/clippy_utils/src/lib.rs
+++ b/src/tools/clippy/clippy_utils/src/lib.rs
@@ -1857,6 +1857,7 @@ pub fn is_refutable(cx: &LateContext<'_>, pat: &Pat<'_>) -> bool {
     }
 
     match pat.kind {
+        PatKind::Missing => unreachable!(),
         PatKind::Wild | PatKind::Never => false, // If `!` typechecked then the type is empty, so not refutable.
         PatKind::Binding(_, _, _, pat) => pat.is_some_and(|pat| is_refutable(cx, pat)),
         PatKind::Box(pat) | PatKind::Ref(pat, _) => is_refutable(cx, pat),