Run MatchBranchSimplification with opt-level 2

Author: dianqk

compiler/rustc_mir_transform/src/match_branches.rs

@@ -14,7 +14,8 @@ pub(super) struct MatchBranchSimplification;
 
 impl<'tcx> crate::MirPass<'tcx> for MatchBranchSimplification {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
-        sess.mir_opt_level() >= 1
+        // Enable only under -Zmir-opt-level=2 as this can make programs less debuggable.
+        sess.mir_opt_level() >= 2
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {

tests/mir-opt/simplify_locals_fixedpoint.rs

@@ -1,8 +1,12 @@
-// skip-filecheck
 // EMIT_MIR_FOR_EACH_PANIC_STRATEGY
-//@ compile-flags: -Zmir-opt-level=1
+//@ compile-flags: -Zmir-opt-level=1 -Zmir-enable-passes=+MatchBranchSimplification
 
+// EMIT_MIR simplify_locals_fixedpoint.foo.SimplifyLocals-final.diff
 fn foo<T>() {
+    // CHECK-LABEL: fn foo(
+    // CHECK-NOT: let mut {{.*}}: bool;
+    // CHECK-NOT: let mut {{.*}}: u8;
+    // CHECK-NOT: let mut {{.*}}: bool;
     if let (Some(a), None) = (Option::<u8>::None, Option::<T>::None) {
         if a > 42u8 {}
     }
@@ -11,5 +15,3 @@ fn foo<T>() {
 fn main() {
     foo::<()>();
 }
-
-// EMIT_MIR simplify_locals_fixedpoint.foo.SimplifyLocals-final.diff