Fix a subtle regression

Before, the SwitchInt cases were computed in two passes: if the first
pass accepted e.g. 0..=5 and then 1, the second pass would not accept
0..=5 anymore because 1 would be listed in the SwitchInt options.

Now there's a single pass, so if we sort 0..=5 we must take care to not
sort a subsequent 1.

1 files changed, 14 insertions, 0 deletions

diff --git a/tests/ui/pattern/usefulness/integer-ranges/regression-switchint-sorting-with-ranges.rs b/tests/ui/pattern/usefulness/integer-ranges/regression-switchint-sorting-with-ranges.rs
new file mode 100644
index 00000000000..bacb60a108b
--- /dev/null
+++ b/tests/ui/pattern/usefulness/integer-ranges/regression-switchint-sorting-with-ranges.rs
@@ -0,0 +1,14 @@
+//@ run-pass
+//
+// Regression test for match lowering to MIR: when gathering candidates, by the time we get to the
+// range we know the range will only match on the failure case of the switchint. Hence we mustn't
+// add the `1` to the switchint or the range would be incorrectly sorted.
+#![allow(unreachable_patterns)]
+fn main() {
+    match 1 {
+        10 => unreachable!(),
+        0..=5 => {}
+        1 => unreachable!(),
+        _ => unreachable!(),
+    }
+}