Rollup merge of rust-lang#118157 - Nadrieril:never_pat-feature-gate, r=compiler-errors

Add `never_patterns` feature gate

This PR adds the feature gate and most basic parsing for the experimental `never_patterns` feature. See the tracking issue (rust-lang#118155) for details on the experiment.

`@scottmcm` has agreed to be my lang-team liaison for this experiment.

Author: matthiaskrgr

src/patterns.rs

@@ -40,7 +40,9 @@ pub(crate) fn is_short_pattern(pat: &ast::Pat, pat_str: &str) -> bool {
 
 fn is_short_pattern_inner(pat: &ast::Pat) -> bool {
     match pat.kind {
-        ast::PatKind::Rest | ast::PatKind::Wild | ast::PatKind::Lit(_) => true,
+        ast::PatKind::Rest | ast::PatKind::Never | ast::PatKind::Wild | ast::PatKind::Lit(_) => {
+            true
+        }
         ast::PatKind::Ident(_, _, ref pat) => pat.is_none(),
         ast::PatKind::Struct(..)
         | ast::PatKind::MacCall(..)
@@ -193,6 +195,7 @@ impl Rewrite for Pat {
                     None
                 }
             }
+            PatKind::Never => None,
             PatKind::Range(ref lhs, ref rhs, ref end_kind) => {
                 let infix = match end_kind.node {
                     RangeEnd::Included(RangeSyntax::DotDotDot) => "...",