Rollup merge of rust-lang#110618 - compiler-errors:eval-ctxt-tainted, r=BoxyUwU

Track if EvalCtxt has been tainted, make sure it can't be used to make query responses after

Just some additional protection against missing probes or strange candidate assembly behavior in the new solver.

For background, we don't ever want to call `evaluate_added_goals_and_make_canonical_response` if a previous call to `try_evaluate_added_goals` has bailed with `NoSolution`, since our nested goals are left in an undefined state at that point. This most commonly suggests a missing `EvalCtxt::probe`, but could also signify some other shenanigans like dropping a `QueryResult` on the floor without properly `?`'ing it.

r? `@lcnr`

Author: matthiaskrgr

compiler/rustc_trait_selection/src/solve/eval_ctxt.rs

@@ -57,6 +57,14 @@ pub struct EvalCtxt<'a, 'tcx> {
     pub(super) search_graph: &'a mut SearchGraph<'tcx>,
 
     pub(super) nested_goals: NestedGoals<'tcx>,
+
+    // Has this `EvalCtxt` errored out with `NoSolution` in `try_evaluate_added_goals`?
+    //
+    // If so, then it can no longer be used to make a canonical query response,
+    // since subsequent calls to `try_evaluate_added_goals` have possibly dropped
+    // ambiguous goals. Instead, a probe needs to be introduced somewhere in the
+    // evaluation code.
+    tainted: Result<(), NoSolution>,
 }
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
@@ -121,6 +129,7 @@ impl<'tcx> InferCtxtEvalExt<'tcx> for InferCtxt<'tcx> {
             max_input_universe: ty::UniverseIndex::ROOT,
             var_values: CanonicalVarValues::dummy(),
             nested_goals: NestedGoals::new(),
+            tainted: Ok(()),
         };
         let result = ecx.evaluate_goal(IsNormalizesToHack::No, goal);
 
@@ -172,6 +181,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
                 max_input_universe: canonical_goal.max_universe,
                 search_graph,
                 nested_goals: NestedGoals::new(),
+                tainted: Ok(()),
             };
             ecx.compute_goal(goal)
         })
@@ -391,6 +401,10 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
             },
         );
 
+        if response.is_err() {
+            self.tainted = Err(NoSolution);
+        }
+
         self.nested_goals = goals;
         response
     }
@@ -404,6 +418,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             max_input_universe: self.max_input_universe,
             search_graph: self.search_graph,
             nested_goals: self.nested_goals.clone(),
+            tainted: self.tainted,
         };
         self.infcx.probe(|_| f(&mut ecx))
     }

compiler/rustc_trait_selection/src/solve/eval_ctxt/canonical.rs

@@ -51,6 +51,13 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         certainty: Certainty,
     ) -> QueryResult<'tcx> {
         let goals_certainty = self.try_evaluate_added_goals()?;
+        assert_eq!(
+            self.tainted,
+            Ok(()),
+            "EvalCtxt is tainted -- nested goals may have been dropped in a \
+            previous call to `try_evaluate_added_goals!`"
+        );
+
         let certainty = certainty.unify_with(goals_certainty);
 
         let external_constraints = self.compute_external_query_constraints()?;