mir-opt: Do not transform non-int type in match_branches

compiler/rustc_mir_transform/src/match_branches.rs

@@ -284,12 +284,14 @@ fn can_cast(
     let v = match src_layout.ty.kind() {
         ty::Uint(_) => from_scalar.to_uint(src_layout.size),
         ty::Int(_) => from_scalar.to_int(src_layout.size) as u128,
-        _ => unreachable!("invalid int"),
+        // We can also transform the values of other integer representations (such as char),
+        // although this may not be practical in real-world scenarios.
+        _ => return false,
     };
     let size = match *cast_ty.kind() {
         ty::Int(t) => Integer::from_int_ty(&tcx, t).size(),
         ty::Uint(t) => Integer::from_uint_ty(&tcx, t).size(),
-        _ => unreachable!("invalid int"),
+        _ => return false,
     };
     let v = size.truncate(v);
     let cast_scalar = ScalarInt::try_from_uint(v, size).unwrap();

tests/mir-opt/matches_reduce_branches.match_non_int_failed.MatchBranchSimplification.diff

@@ -0,0 +1,29 @@
+- // MIR for `match_non_int_failed` before MatchBranchSimplification
++ // MIR for `match_non_int_failed` after MatchBranchSimplification
+
+  fn match_non_int_failed(_1: char) -> u8 {
+      let mut _0: u8;
+
+      bb0: {
+          switchInt(copy _1) -> [97: bb1, 98: bb2, otherwise: bb3];
+      }
+
+      bb1: {
+          _0 = const 97_u8;
+          goto -> bb4;
+      }
+
+      bb2: {
+          _0 = const 98_u8;
+          goto -> bb4;
+      }
+
+      bb3: {
+          unreachable;
+      }
+
+      bb4: {
+          return;
+      }
+  }
+

tests/mir-opt/matches_reduce_branches.rs

@@ -628,6 +628,37 @@ fn match_i128_u128(i: EnumAi128) -> u128 {
     }
 }
 
+// EMIT_MIR matches_reduce_branches.match_non_int_failed.MatchBranchSimplification.diff
+#[custom_mir(dialect = "runtime")]
+fn match_non_int_failed(i: char) -> u8 {
+    // CHECK-LABEL: fn match_non_int_failed(
+    // CHECK: switchInt
+    // CHECK: return
+    mir! {
+        {
+            match i {
+                'a' => bb1,
+                'b' => bb2,
+                _ => unreachable_bb,
+            }
+        }
+        bb1 = {
+            RET = 97;
+            Goto(ret)
+        }
+        bb2 = {
+            RET = 98;
+            Goto(ret)
+        }
+        unreachable_bb = {
+            Unreachable()
+        }
+        ret = {
+            Return()
+        }
+    }
+}
+
 fn main() {
     let _ = foo(None);
     let _ = foo(Some(()));
@@ -665,4 +696,5 @@ fn main() {
     let _ = match_i128_u128(EnumAi128::A);
 
     let _ = my_is_some(None);
+    let _ = match_non_int_failed('a');
 }