Auto merge of #128034 - Nadrieril:explain-unreachable, r=compiler-errors

exhaustiveness: Explain why a given pattern is considered unreachable

This PR tells the user why a given pattern is considered unreachable. I reused the intersection information we were already computing; even though it's incomplete I convinced myself that it is sufficient to always get a set of patterns that cover the unreachable one.

I'm not a fan of the diagnostic messages I came up with, I'm open to suggestions.

Fixes https://github.com/rust-lang/rust/issues/127870. This is also the other one of the two diagnostic improvements I wanted to do before https://github.com/rust-lang/rust/pull/122792.

Note: the first commit is an unrelated drive-by tweak.

r? `@compiler-errors`

1 files changed, 20 insertions, 0 deletions

diff --git a/compiler/rustc_pattern_analysis/tests/intersection.rs b/compiler/rustc_pattern_analysis/tests/intersection.rs
index 4a96b7248da..8c8cb3c796d 100644
--- a/compiler/rustc_pattern_analysis/tests/intersection.rs
+++ b/compiler/rustc_pattern_analysis/tests/intersection.rs
@@ -67,4 +67,24 @@ fn test_nested() {
         ),
         &[&[], &[]],
     );
+    let ty = Ty::Tuple(&[Ty::Bool; 3]);
+    assert_intersects(
+        pats!(ty;
+            (true, true, _),
+            (true, _, true),
+            (false, _, _),
+        ),
+        &[&[], &[], &[]],
+    );
+    let ty = Ty::Tuple(&[Ty::Bool, Ty::Bool, Ty::U8]);
+    assert_intersects(
+        pats!(ty;
+            (true, _, _),
+            (_, true, 0..10),
+            (_, true, 10..),
+            (_, true, 3),
+            _,
+        ),
+        &[&[], &[], &[], &[1], &[0, 1, 2, 3]],
+    );
 }