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

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)
     }