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, then
assert replaced solutions are equal if Some

4 files changed, 21 insertions, 4 deletions

diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index 5334e767766..56fcfe8e798 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/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)
     }
diff --git a/compiler/rustc_type_ir/src/interner.rs b/compiler/rustc_type_ir/src/interner.rs
index f2492ede4f5..8dec2133a45 100644
--- a/compiler/rustc_type_ir/src/interner.rs
+++ b/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()
+    }
 }
diff --git a/compiler/rustc_type_ir/src/search_graph/global_cache.rs b/compiler/rustc_type_ir/src/search_graph/global_cache.rs
index 47f7cefac6a..0ce927b58bb 100644
--- a/compiler/rustc_type_ir/src/search_graph/global_cache.rs
+++ b/compiler/rustc_type_ir/src/search_graph/global_cache.rs
@@ -44,22 +44,28 @@ impl<X: Cx> GlobalCache<X> {
         cx: X,
         input: X::Input,
 
-        result: X::Result,
+        origin_result: X::Result,
         dep_node: X::DepNodeIndex,
 
         additional_depth: usize,
         encountered_overflow: bool,
         nested_goals: NestedGoals<X>,
     ) {
-        let result = cx.mk_tracked(result, dep_node);
+        let result = cx.mk_tracked(origin_result, dep_node);
         let entry = self.map.entry(input).or_default();
         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 let Some(prev) = &prev {
+                assert!(cx.evaluation_is_concurrent());
+                assert_eq!(cx.get_tracked(&prev.result), origin_result);
+            }
         } else {
             let prev = entry.success.replace(Success { additional_depth, nested_goals, result });
-            assert!(prev.is_none());
+            if let Some(prev) = &prev {
+                assert!(cx.evaluation_is_concurrent());
+                assert_eq!(cx.get_tracked(&prev.result), origin_result);
+            }
         }
     }
 
diff --git a/compiler/rustc_type_ir/src/search_graph/mod.rs b/compiler/rustc_type_ir/src/search_graph/mod.rs
index 418139c3aad..ac4d0795a92 100644
--- a/compiler/rustc_type_ir/src/search_graph/mod.rs
+++ b/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 {