Fix pathological performance in trait solver cycles with errors

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -1113,6 +1113,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             debug!("CACHE MISS");
             self.insert_evaluation_cache(param_env, fresh_trait_pred, dep_node, result);
             stack.cache().on_completion(stack.dfn);
+        } else if let Some(_guar) = self.infcx.tainted_by_errors() {
+            // When an error has occurred, we allow global caching of results even if they
+            // appear stack-dependent. This prevents exponential re-evaluation of cycles
+            // in the presence of errors, avoiding compiler hangs like #150907.
+            // This is safe because compilation will fail anyway.
+            debug!("CACHE MISS (tainted by errors)");
+            self.insert_evaluation_cache(param_env, fresh_trait_pred, dep_node, result);
+            stack.cache().on_completion(stack.dfn);
         } else {
             debug!("PROVISIONAL");
             debug!(

tests/ui/traits/cycle-cache-err-150907.rs

@@ -0,0 +1,125 @@
+// Test that we don't hang or take exponential time when evaluating
+// auto-traits for cyclic types containing errors, based on the reproducer
+// provided in issue #150907.
+
+use std::sync::Arc;
+use std::marker::PhantomData;
+
+struct Weak<T>(PhantomData<T>);
+unsafe impl<T: Sync + Send> Send for Weak<T> {}
+unsafe impl<T: Sync + Send> Sync for Weak<T> {}
+
+struct BTreeMap<K, V>(K, V);
+
+trait DeviceOps: Send {}
+
+struct SerialDevice {
+    terminal: Weak<Terminal>,
+}
+
+impl DeviceOps for SerialDevice {}
+
+struct TtyState {
+    terminals: Weak<Terminal>,
+}
+
+struct TerminalMutableState {
+    controller: TerminalController,
+}
+
+struct Terminal {
+    weak_self: Weak<Self>,
+    state: Arc<TtyState>,
+    mutable_state: Weak<TerminalMutableState>,
+}
+
+struct TerminalController {
+    session: Weak<Session>,
+}
+
+struct Kernel {
+    weak_self: Weak<Kernel>,
+    kthreads: KernelThreads,
+    pids: Weak<PidTable>,
+}
+
+struct KernelThreads {
+    system_task: SystemTask,
+    kernel: Weak<Kernel>,
+}
+
+struct SystemTask {
+    system_thread_group: Weak<ThreadGroup>,
+}
+
+enum ProcessEntry {
+    ThreadGroup(Weak<ThreadGroup>),
+}
+
+struct PidEntry {
+    task: Arc<Task>,
+    process: ProcessEntry,
+}
+
+struct PidTable {
+    table: PidEntry,
+    process_groups: Arc<ProcessGroup>,
+}
+
+struct ProcessGroupMutableState {
+    thread_groups: Weak<ThreadGroup>,
+}
+
+struct ProcessGroup {
+    session: Arc<Session>,
+    mutable_state: Arc<ProcessGroupMutableState>,
+}
+
+struct SessionMutableState {
+    process_groups: BTreeMap<(), Weak<ProcessGroup>>,
+    controlling_terminal: ControllingTerminal,
+}
+
+struct Session {
+    mutable_state: Weak<SessionMutableState>,
+}
+
+struct ControllingTerminal {
+    terminal: Arc<Terminal>,
+}
+
+struct TaskPersistentInfoState {
+    thread_group_key: ThreadGroupKey,
+}
+
+struct Task {
+    thread_group_key: ThreadGroupKey,
+    kernel: Arc<Kernel>,
+    thread_group: Arc<ThreadGroup>,
+    persistent_info: Arc<TaskPersistentInfoState>,
+    vfork_event: Arc, //~ ERROR missing generics for struct `Arc`
+}
+
+struct ThreadGroupKey {
+    thread_group: Arc<ThreadGroup>,
+}
+
+struct ThreadGroupMutableState {
+    tasks: BTreeMap<(), TaskContainer>,
+    children: BTreeMap<(), Weak<ThreadGroup>>,
+    process_group: Arc<ProcessGroup>,
+}
+
+struct ThreadGroup {
+    kernel: Arc<Kernel>,
+    mutable_state: Weak<ThreadGroupMutableState>,
+}
+
+struct TaskContainer(Arc<Task>, Arc<TaskPersistentInfoState>);
+
+fn main() {
+    // Trigger auto-trait check for one of the cyclic types
+    is_send::<Kernel>();
+}
+
+fn is_send<T: Send>() {}

tests/ui/traits/cycle-cache-err-150907.stderr

@@ -0,0 +1,14 @@
+error[E0107]: missing generics for struct `Arc`
+  --> $DIR/cycle-cache-err-150907.rs:100:18
+   |
+LL |     vfork_event: Arc,
+   |                  ^^^ expected at least 1 generic argument
+   |
+help: add missing generic argument
+   |
+LL |     vfork_event: Arc<T>,
+   |                     +++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0107`.