Auto merge of #112001 - saethlin:enable-matchbranchsimplification, r=cjgillot

Enable MatchBranchSimplification

This pass is one of the small number of benefits from `-Zmir-opt-level=3` that has motivated rustc_codegen_cranelift to use it:

19ed0aade6/compiler/rustc_codegen_cranelift/build_system/build_sysroot.rs (L244-L246)

Cranelift's motivation for this is _runtime_ performance improvements in debug builds. Lifting this pass all the way to `-Zmir-opt-level=1` seems to come without significant perf overhead, so that's what I'm suggesting here.

compiler/rustc_mir_transform/src/match_branches.rs

@@ -41,7 +41,7 @@ pub struct MatchBranchSimplification;
 
 impl<'tcx> MirPass<'tcx> for MatchBranchSimplification {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
-        sess.mir_opt_level() >= 3
+        sess.mir_opt_level() >= 1
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
@@ -62,7 +62,12 @@ impl<'tcx> MirPass<'tcx> for MatchBranchSimplification {
                     ..
                 } if targets.iter().len() == 1 => {
                     let (value, target) = targets.iter().next().unwrap();
-                    if target == targets.otherwise() {
+                    // We require that this block and the two possible target blocks all be
+                    // distinct.
+                    if target == targets.otherwise()
+                        || bb_idx == target
+                        || bb_idx == targets.otherwise()
+                    {
                         continue;
                     }
                     (discr, value, target, targets.otherwise())

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -116,6 +116,7 @@ impl<'a, 'tcx> FulfillmentContext<'tcx> {
 }
 
 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
+    #[inline]
     fn register_predicate_obligation(
         &mut self,
         infcx: &InferCtxt<'tcx>,