compiler: Inform the solver of concurrency

Parallel compilation of a program can cause unexpected event sequencing.
Inform the solver when this is true so it can skip invalid asserts.

Author: workingjubilee

compiler/rustc_middle/src/ty/context.rs

@@ -181,6 +181,10 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
         }
     }
 
+    fn evaluation_is_concurrent(&self) -> bool {
+        self.sess.threads() > 1
+    }
+
     fn expand_abstract_consts<T: TypeFoldable<TyCtxt<'tcx>>>(self, t: T) -> T {
         self.expand_abstract_consts(t)
     }

compiler/rustc_type_ir/src/interner.rs

@@ -137,6 +137,8 @@ pub trait Interner:
         f: impl FnOnce(&mut search_graph::GlobalCache<Self>) -> R,
     ) -> R;
 
+    fn evaluation_is_concurrent(&self) -> bool;
+
     fn expand_abstract_consts<T: TypeFoldable<Self>>(self, t: T) -> T;
 
     type GenericsOf: GenericsOf<Self>;
@@ -404,4 +406,7 @@ impl<I: Interner> search_graph::Cx for I {
     ) -> R {
         I::with_global_cache(self, mode, f)
     }
+    fn evaluation_is_concurrent(&self) -> bool {
+        self.evaluation_is_concurrent()
+    }
 }

compiler/rustc_type_ir/src/search_graph/global_cache.rs

@@ -56,10 +56,14 @@ impl<X: Cx> GlobalCache<X> {
         if encountered_overflow {
             let with_overflow = WithOverflow { nested_goals, result };
             let prev = entry.with_overflow.insert(additional_depth, with_overflow);
-            assert!(prev.is_none());
+            if !cx.evaluation_is_concurrent() {
+                assert!(prev.is_none());
+            };
         } else {
             let prev = entry.success.replace(Success { additional_depth, nested_goals, result });
-            assert!(prev.is_none());
+            if !cx.evaluation_is_concurrent() {
+                assert!(prev.is_none());
+            }
         }
     }
 

compiler/rustc_type_ir/src/search_graph/mod.rs

@@ -53,6 +53,8 @@ pub trait Cx: Copy {
         mode: SolverMode,
         f: impl FnOnce(&mut GlobalCache<Self>) -> R,
     ) -> R;
+
+    fn evaluation_is_concurrent(&self) -> bool;
 }
 
 pub trait Delegate {

tests/ui/traits/next-solver/global-cache-and-parallel-frontend.rs

@@ -0,0 +1,15 @@
+//@ compile-flags: -Zthreads=16
+
+#[derive(Clone, Eq)] //~ ERROR [E0277]
+pub struct Struct<T>(T);
+
+impl<T: Clone, U> PartialEq<U> for Struct<T>
+where
+    U: Into<Struct<T>> + Clone
+{
+    fn eq(&self, _other: &U) -> bool {
+        todo!()
+    }
+}
+
+fn main() {}

tests/ui/traits/next-solver/global-cache-and-parallel-frontend.stderr

@@ -0,0 +1,24 @@
+error[E0277]: the trait bound `T: Clone` is not satisfied
+  --> $DIR/global-cache-and-parallel-frontend.rs:3:17
+   |
+LL | #[derive(Clone, Eq)]
+   |                 ^^ the trait `Clone` is not implemented for `T`, which is required by `Struct<T>: PartialEq`
+   |
+note: required for `Struct<T>` to implement `PartialEq`
+  --> $DIR/global-cache-and-parallel-frontend.rs:6:19
+   |
+LL | impl<T: Clone, U> PartialEq<U> for Struct<T>
+   |         -----     ^^^^^^^^^^^^     ^^^^^^^^^
+   |         |
+   |         unsatisfied trait bound introduced here
+note: required by a bound in `Eq`
+  --> $SRC_DIR/core/src/cmp.rs:LL:COL
+   = note: this error originates in the derive macro `Eq` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider restricting type parameter `T`
+   |
+LL | pub struct Struct<T: std::clone::Clone>(T);
+   |                    +++++++++++++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.