Rollup merge of #139035 - nnethercote:PatKind-Missing, r=oli-obk

Add new `PatKind::Missing` variants

To avoid some ugly uses of `kw::Empty` when handling "missing" patterns, e.g. in bare fn tys. Helps with #137978. Details in the individual commits.

r? ``@oli-obk``

Author: Zalathar

src/items.rs

@@ -2442,11 +2442,7 @@ pub(crate) fn span_hi_for_param(context: &RewriteContext<'_>, param: &ast::Param
 }
 
 pub(crate) fn is_named_param(param: &ast::Param) -> bool {
-    if let ast::PatKind::Ident(_, ident, _) = param.pat.kind {
-        ident.name != symbol::kw::Empty
-    } else {
-        true
-    }
+    !matches!(param.pat.kind, ast::PatKind::Missing)
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]

src/patterns.rs

@@ -42,6 +42,7 @@ pub(crate) fn is_short_pattern(
 
 fn is_short_pattern_inner(context: &RewriteContext<'_>, pat: &ast::Pat) -> bool {
     match &pat.kind {
+        ast::PatKind::Missing => unreachable!(),
         ast::PatKind::Rest | ast::PatKind::Never | ast::PatKind::Wild | ast::PatKind::Err(_) => {
             true
         }
@@ -100,6 +101,7 @@ impl Rewrite for Pat {
 
     fn rewrite_result(&self, context: &RewriteContext<'_>, shape: Shape) -> RewriteResult {
         match self.kind {
+            PatKind::Missing => unreachable!(),
             PatKind::Or(ref pats) => {
                 let pat_strs = pats
                     .iter()