Rollup merge of #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 (https://github.com/rust-lang/rust/issues/118155) for details on the experiment.

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

8 files changed, 14 insertions, 4 deletions

diff --git a/src/librustdoc/clean/utils.rs b/src/librustdoc/clean/utils.rs
index c22ad43e049..82f4a38384d 100644
--- a/src/librustdoc/clean/utils.rs
+++ b/src/librustdoc/clean/utils.rs
@@ -303,7 +303,8 @@ pub(crate) fn name_from_pat(p: &hir::Pat<'_>) -> Symbol {
     debug!("trying to get a name from pattern: {p:?}");
 
     Symbol::intern(&match p.kind {
-        PatKind::Wild | PatKind::Struct(..) => return kw::Underscore,
+        // FIXME(never_patterns): does this make sense?
+        PatKind::Wild | PatKind::Never | PatKind::Struct(..) => return kw::Underscore,
         PatKind::Binding(_, _, ident, _) => return ident.name,
         PatKind::TupleStruct(ref p, ..) | PatKind::Path(ref p) => qpath_to_string(p),
         PatKind::Or(pats) => {
diff --git a/src/tools/clippy/clippy_lints/src/equatable_if_let.rs b/src/tools/clippy/clippy_lints/src/equatable_if_let.rs
index 575fead5bf3..630df9a84f5 100644
--- a/src/tools/clippy/clippy_lints/src/equatable_if_let.rs
+++ b/src/tools/clippy/clippy_lints/src/equatable_if_let.rs
@@ -46,7 +46,7 @@ fn unary_pattern(pat: &Pat<'_>) -> bool {
         pats.iter().all(unary_pattern)
     }
     match &pat.kind {
-        PatKind::Slice(_, _, _) | PatKind::Range(_, _, _) | PatKind::Binding(..) | PatKind::Wild | PatKind::Or(_) => {
+        PatKind::Slice(_, _, _) | PatKind::Range(_, _, _) | PatKind::Binding(..) | PatKind::Wild | PatKind::Never | PatKind::Or(_) => {
             false
         },
         PatKind::Struct(_, a, etc) => !etc && a.iter().all(|x| unary_pattern(x.pat)),
diff --git a/src/tools/clippy/clippy_lints/src/matches/match_same_arms.rs b/src/tools/clippy/clippy_lints/src/matches/match_same_arms.rs
index 745be671b86..cbf478620ec 100644
--- a/src/tools/clippy/clippy_lints/src/matches/match_same_arms.rs
+++ b/src/tools/clippy/clippy_lints/src/matches/match_same_arms.rs
@@ -150,6 +150,7 @@ pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, arms: &'tcx [Arm<'_>]) {
 #[derive(Clone, Copy)]
 enum NormalizedPat<'a> {
     Wild,
+    Never,
     Struct(Option<DefId>, &'a [(Symbol, Self)]),
     Tuple(Option<DefId>, &'a [Self]),
     Or(&'a [Self]),
@@ -229,6 +230,7 @@ impl<'a> NormalizedPat<'a> {
             PatKind::Binding(.., Some(pat)) | PatKind::Box(pat) | PatKind::Ref(pat, _) => {
                 Self::from_pat(cx, arena, pat)
             },
+            PatKind::Never => Self::Never,
             PatKind::Struct(ref path, fields, _) => {
                 let fields =
                     arena.alloc_from_iter(fields.iter().map(|f| (f.ident.name, Self::from_pat(cx, arena, f.pat))));
@@ -333,6 +335,7 @@ impl<'a> NormalizedPat<'a> {
     fn has_overlapping_values(&self, other: &Self) -> bool {
         match (*self, *other) {
             (Self::Wild, _) | (_, Self::Wild) => true,
+            (Self::Never, Self::Never) => true,
             (Self::Or(pats), ref other) | (ref other, Self::Or(pats)) => {
                 pats.iter().any(|pat| pat.has_overlapping_values(other))
             },
diff --git a/src/tools/clippy/clippy_lints/src/unnested_or_patterns.rs b/src/tools/clippy/clippy_lints/src/unnested_or_patterns.rs
index 8ff088a208f..952c0dc72b1 100644
--- a/src/tools/clippy/clippy_lints/src/unnested_or_patterns.rs
+++ b/src/tools/clippy/clippy_lints/src/unnested_or_patterns.rs
@@ -226,7 +226,7 @@ fn transform_with_focus_on_idx(alternatives: &mut ThinVec<P<Pat>>, focus_idx: us
         // Therefore they are not some form of constructor `C`,
         // with which a pattern `C(p_0)` may be formed,
         // which we would want to join with other `C(p_j)`s.
-        Ident(.., None) | Lit(_) | Wild | Path(..) | Range(..) | Rest | MacCall(_)
+        Ident(.., None) | Lit(_) | Wild | Never | Path(..) | Range(..) | Rest | MacCall(_)
         // Skip immutable refs, as grouping them saves few characters,
         // and almost always requires adding parens (increasing noisiness).
         // In the case of only two patterns, replacement adds net characters.
diff --git a/src/tools/clippy/clippy_lints/src/utils/author.rs b/src/tools/clippy/clippy_lints/src/utils/author.rs
index e8842ce10f8..e83c04eda20 100644
--- a/src/tools/clippy/clippy_lints/src/utils/author.rs
+++ b/src/tools/clippy/clippy_lints/src/utils/author.rs
@@ -629,6 +629,7 @@ impl<'a, 'tcx> PrintVisitor<'a, 'tcx> {
 
         match pat.value.kind {
             PatKind::Wild => kind!("Wild"),
+            PatKind::Never => kind!("Never"),
             PatKind::Binding(ann, _, name, sub) => {
                 bind!(self, name);
                 opt_bind!(self, sub);
diff --git a/src/tools/clippy/clippy_utils/src/hir_utils.rs b/src/tools/clippy/clippy_utils/src/hir_utils.rs
index 8031e6faa74..34ec83709ff 100644
--- a/src/tools/clippy/clippy_utils/src/hir_utils.rs
+++ b/src/tools/clippy/clippy_utils/src/hir_utils.rs
@@ -1017,6 +1017,7 @@ impl<'a, 'tcx> SpanlessHash<'a, 'tcx> {
                 }
                 e.hash(&mut self.s);
             },
+            PatKind::Never => {},
             PatKind::Wild => {},
         }
     }
diff --git a/src/tools/clippy/clippy_utils/src/lib.rs b/src/tools/clippy/clippy_utils/src/lib.rs
index 172b063df31..fd37713fc60 100644
--- a/src/tools/clippy/clippy_utils/src/lib.rs
+++ b/src/tools/clippy/clippy_utils/src/lib.rs
@@ -1707,6 +1707,7 @@ pub fn is_refutable(cx: &LateContext<'_>, pat: &Pat<'_>) -> bool {
 
     match pat.kind {
         PatKind::Wild => false,
+        PatKind::Never => false, // If `!` typechecked then the type is empty, so not refutable.
         PatKind::Binding(_, _, _, pat) => pat.map_or(false, |pat| is_refutable(cx, pat)),
         PatKind::Box(pat) | PatKind::Ref(pat, _) => is_refutable(cx, pat),
         PatKind::Lit(..) | PatKind::Range(..) => true,
diff --git a/src/tools/rustfmt/src/patterns.rs b/src/tools/rustfmt/src/patterns.rs
index 33f3b4b8a21..8504999b8ff 100644
--- a/src/tools/rustfmt/src/patterns.rs
+++ b/src/tools/rustfmt/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) => "...",