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 {
-        return false;
-    };
-    // ...assign the discriminant of `place` in that statement
-    let StatementKind::Assign(boxed) = &statement.kind else { return false };
-    let (discr_place, Rvalue::Discriminant(from_place)) = &**boxed else { return false };
-    if *from_place != place {
-        return false;
-    }
-    // ...make that assignment to a local
-    if discr_place.projection.len() != 0 {
-        return false;
-    }
-    // ...terminate on a `SwitchInt` that invalidates that local
-    let TerminatorKind::SwitchInt { discr: switch_op, targets, .. } = &branch.terminator().kind
-    else {
-        return false;
-    };
-    if *switch_op != Operand::Move(*discr_place) {
-        return false;
-    }
-    // ...fall through to `destination` if the switch misses
-    if destination != targets.otherwise() {
-        return false;
-    }
-    // ...have a branch for value `value`
-    let mut iter = targets.iter();
-    let Some((target_value, _)) = iter.next() else {
-        return false;
-    };
-    if target_value != value {
-        return false;
-    }
-    // ...and have no more branches
-    if let Some(_) = iter.next() {
-        return false;
+    if let [statement] = branch.statements.as_slice()
+        // ...assign the discriminant of `place` in that statement
+        && let StatementKind::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
+        && let TerminatorKind::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`
+        && let mut iter = targets.iter()
+        && let Some((target_value, _)) = iter.next()
+        && target_value == value
+        // ...and have no more branches
+        && iter.next().is_none()
+    {
+        true
+    } else {
+        false
     }
-    true
 }