fix guard patterns interaction with never type

Author: WaffleLapkin

compiler/rustc_hir/src/hir.rs

@@ -1807,8 +1807,9 @@ impl<'hir> Pat<'hir> {
             // Does not constitute a read.
             PatKind::Wild => false,
 
-            // Might not constitute a read, since the condition might be false.
-            PatKind::Guard(_, _) => true,
+            // The guard cannot affect if we make a read or not (it runs after the inner pattern
+            // has matched), therefore it's irrelevant.
+            PatKind::Guard(pat, _) => pat.is_guaranteed_to_constitute_read_for_never(),
 
             // This is unnecessarily restrictive when the pattern that doesn't
             // constitute a read is unreachable.

compiler/rustc_middle/src/hir/mod.rs

@@ -259,7 +259,8 @@ impl<'tcx> TyCtxt<'tcx> {
                         expr.hir_id != lhs.hir_id
                     }
 
-                    // See note on `PatKind::Or` below for why this is `all`.
+                    // See note on `PatKind::Or` in `Pat::is_guaranteed_to_constitute_read_for_never`
+                    // for why this is `all`.
                     ExprKind::Match(scrutinee, arms, _) => {
                         assert_eq!(scrutinee.hir_id, expr.hir_id);
                         arms.iter().all(|arm| arm.pat.is_guaranteed_to_constitute_read_for_never())

tests/ui/reachable/guard_read_for_never.rs

@@ -0,0 +1,16 @@
+// Regression test for <https://github.com/rust-lang/rust/pull/149545#discussion_r2585205872>
+//
+//@ check-pass
+#![feature(guard_patterns, never_type)]
+#![expect(incomplete_features, unused_parens)]
+#![deny(unreachable_code)]
+
+fn main() {
+    unsafe {
+        let x = std::ptr::null::<!>();
+
+        // This should not constitute a read for never, therefore no code here is unreachable
+        let (_ if false): ! = *x;
+        ();
+    }
+}