Cap iteration count in ScalarReplacementOfAggregates

compiler/rustc_mir_transform/src/sroa.rs

@@ -28,9 +28,17 @@ impl<'tcx> crate::MirPass<'tcx> for ScalarReplacementOfAggregates {
             return;
         }
 
+        // Cap the number of iterations to avoid non-termination on tainted
+        // input (e.g. when layout cycle errors cause `iter_fields` to keep
+        // producing self-referential field types, cf.
+        // https://github.com/rust-lang/rust/issues/153205). In practice, 2
+        // iterations capture nearly all the value and more than 10 is never
+        // needed for non-contrived code, so 10 is enough.
+        const MAX_ITERATIONS: usize = 10;
+
         let mut excluded = excluded_locals(body);
         let typing_env = body.typing_env(tcx);
-        loop {
+        for _ in 0..MAX_ITERATIONS {
             debug!(?excluded);
             let escaping = escaping_locals(tcx, &excluded, body);
             debug!(?escaping);

tests/ui/layout/hang-sroa-layout-cycle.rs

@@ -0,0 +1,30 @@
+//@ build-fail
+//~^ ERROR cycle detected when computing layout of `Thing<Identity>`
+//@ compile-flags: -Copt-level=3
+
+// Regression test for https://github.com/rust-lang/rust/issues/153205:
+// a struct that contains itself via an associated type used to cause the
+// `ScalarReplacementOfAggregates` MIR pass to loop forever after the
+// layout-cycle error was emitted. The SROA pass now bounds its iteration
+// count, so the compile terminates with the expected layout-cycle error
+// rather than hanging.
+
+trait Apply {
+    type Output<T>;
+}
+
+struct Identity;
+
+impl Apply for Identity {
+    type Output<T> = T;
+}
+
+struct Thing<A: Apply>(A::Output<Self>);
+
+fn foo<A: Apply>() {
+    let _x: Thing<A>;
+}
+
+fn main() {
+    foo::<Identity>();
+}

tests/ui/layout/hang-sroa-layout-cycle.stderr

@@ -0,0 +1,14 @@
+error[E0391]: cycle detected when computing layout of `Thing<Identity>`
+   |
+   = note: ...which requires computing layout of `<Identity as Apply>::Output<Thing<Identity>>`...
+   = note: ...which again requires computing layout of `Thing<Identity>`, completing the cycle
+note: cycle used when optimizing MIR for `main`
+  --> $DIR/hang-sroa-layout-cycle.rs:28:1
+   |
+LL | fn main() {
+   | ^^^^^^^^^
+   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0391`.