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.

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -1091,6 +1091,12 @@ enum TestCase<'pat, 'tcx> {
     Or { pats: Box<[FlatPat<'pat, 'tcx>]> },
 }
 
+impl<'pat, 'tcx> TestCase<'pat, 'tcx> {
+    fn as_range(&self) -> Option<&'pat PatRange<'tcx>> {
+        if let Self::Range(v) = self { Some(*v) } else { None }
+    }
+}
+
 #[derive(Debug, Clone)]
 pub(crate) struct MatchPair<'pat, 'tcx> {
     /// This place...

compiler/rustc_mir_build/src/build/matches/test.rs

@@ -510,9 +510,30 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             (TestKind::SwitchInt, &TestCase::Constant { value })
                 if is_switch_ty(match_pair.pattern.ty) =>
             {
-                fully_matched = true;
-                let bits = value.eval_bits(self.tcx, self.param_env);
-                Some(TestBranch::Constant(value, bits))
+                // Beware: there might be some ranges sorted into the failure case; we must not add
+                // a success case that could be matched by one of these ranges.
+                let is_covering_range = |test_case: &TestCase<'_, 'tcx>| {
+                    test_case.as_range().is_some_and(|range| {
+                        matches!(range.contains(value, self.tcx, self.param_env), None | Some(true))
+                    })
+                };
+                let is_conflicting_candidate = |candidate: &&mut Candidate<'_, 'tcx>| {
+                    candidate
+                        .match_pairs
+                        .iter()
+                        .any(|mp| mp.place == *test_place && is_covering_range(&mp.test_case))
+                };
+                if sorted_candidates
+                    .get(&TestBranch::Failure)
+                    .is_some_and(|candidates| candidates.iter().any(is_conflicting_candidate))
+                {
+                    fully_matched = false;
+                    None
+                } else {
+                    fully_matched = true;
+                    let bits = value.eval_bits(self.tcx, self.param_env);
+                    Some(TestBranch::Constant(value, bits))
+                }
             }
             (TestKind::SwitchInt, TestCase::Range(range)) => {
                 fully_matched = false;

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!(),
+    }
+}