Simplify verify_candidate_branch.

Let chains are perfect for this kind of function.

Author: nnethercote

compiler/rustc_mir_transform/src/early_otherwise_branch.rs

@@ -310,42 +310,28 @@ fn verify_candidate_branch<'tcx>(
 ) -> bool{
 // In order for the optimization to be correct, the branch must...
 // ...have exactly one statement
-let[statement] = branch.statements.as_slice()else{
-returnfalse;
-};
-// ...assign the discriminant of `place` in that statement
-letStatementKind::Assign(boxed) = &statement.kindelse{returnfalse};
-let(discr_place,Rvalue::Discriminant(from_place)) = &**boxed else{returnfalse};
-if*from_place != place {
-returnfalse;
-}
-// ...make that assignment to a local
-if discr_place.projection.len() != 0{
-returnfalse;
-}
-// ...terminate on a `SwitchInt` that invalidates that local
-letTerminatorKind::SwitchInt{discr: switch_op, targets, .. } = &branch.terminator().kind
-else{
-returnfalse;
-};
-if*switch_op != Operand::Move(*discr_place){
-returnfalse;
-}
-// ...fall through to `destination` if the switch misses
-if destination != targets.otherwise(){
-returnfalse;
-}
-// ...have a branch for value `value`
-letmut iter = targets.iter();
-letSome((target_value, _)) = iter.next()else{
-returnfalse;
-};
-if target_value != value {
-returnfalse;
-}
-// ...and have no more branches
-ifletSome(_) = iter.next(){
-returnfalse;
+iflet[statement] = branch.statements.as_slice()
+// ...assign the discriminant of `place` in that statement
+ && letStatementKind::Assign(boxed) = &statement.kind
+ && let(discr_place,Rvalue::Discriminant(from_place)) = &**boxed
+ && *from_place == place
+// ...make that assignment to a local
+ && discr_place.projection.is_empty()
+// ...terminate on a `SwitchInt` that invalidates that local
+ && letTerminatorKind::SwitchInt{discr: switch_op, targets, .. } =
+&branch.terminator().kind
+ && *switch_op == Operand::Move(*discr_place)
+// ...fall through to `destination` if the switch misses
+ && destination == targets.otherwise()
+// ...have a branch for value `value`
+ && letmut iter = targets.iter()
+ && letSome((target_value, _)) = iter.next()
+ && target_value == value
+// ...and have no more branches
+ && iter.next().is_none()
+{
+true
+}else{
+false
 }
-true
 }