Recognize discriminant reads as no-ops in RemoveNoopLandingPads

tmiasko · tmiasko · commit ecd7862dfb56 · 2020-10-10T00:00:00.000Z
The cleanup blocks often contain read of discriminants. Teach
RemoveNoopLandingPads to recognize them as no-ops to remove
additional no-op landing pads.
diff --git a/compiler/rustc_mir/src/transform/remove_noop_landing_pads.rs b/compiler/rustc_mir/src/transform/remove_noop_landing_pads.rs
@@ -43,7 +43,7 @@ impl RemoveNoopLandingPads {
                     // These are all nops in a landing pad
                 }
 
-                StatementKind::Assign(box (place, Rvalue::Use(_))) => {
+                StatementKind::Assign(box (place, Rvalue::Use(_) | Rvalue::Discriminant(_))) => {
                     if place.as_local().is_some() {
                         // Writing to a local (e.g., a drop flag) does not
                         // turn a landing pad to a non-nop

Original file line number	Diff line number	Diff line change
`@@ -43,7 +43,7 @@ impl RemoveNoopLandingPads {`
`43`	`43`	`// These are all nops in a landing pad`
`44`	`44`	`}`
`45`	`45`
`46`		`- StatementKind::Assign(box (place, Rvalue::Use(_))) => {`
	`46`	`+ StatementKind::Assign(box (place, Rvalue::Use(_) \| Rvalue::Discriminant(_))) => {`
`47`	`47`	`if place.as_local().is_some() {`
`48`	`48`	`// Writing to a local (e.g., a drop flag) does not`
`49`	`49`	`// turn a landing pad to a non-nop`