Use ExtractIf in evaluate_added_goals_step

Author: compiler-errors

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs

@@ -635,15 +635,24 @@ where
     ///
     /// Goals for the next step get directly added to the nested goals of the `EvalCtxt`.
     fn evaluate_added_goals_step(&mut self) -> Result<Option<Certainty>, NoSolution> {
+        if self.nested_goals.is_empty() {
+            return Ok(Some(Certainty::Yes));
+        }
+
         let cx = self.cx();
         // If this loop did not result in any progress, what's our final certainty.
         let mut unchanged_certainty = Some(Certainty::Yes);
-        for (source, goal, stalled_on) in mem::take(&mut self.nested_goals) {
+
+        let mut nested_goals = mem::take(&mut self.nested_goals);
+        let mut pending_goals = vec![];
+        for (source, goal, stalled_on) in nested_goals.extract_if(.., |(_, _, stalled_on)| {
+            stalled_on.as_ref().is_none_or(|s| !self.delegate.is_still_stalled(s))
+        }) {
             if let Some(certainty) = self.delegate.compute_goal_fast_path(goal, self.origin_span) {
                 match certainty {
                     Certainty::Yes => {}
                     Certainty::Maybe(_) => {
-                        self.nested_goals.push((source, goal, None));
+                        pending_goals.push((source, goal, None));
                         unchanged_certainty = unchanged_certainty.map(|c| c.and(certainty));
                     }
                 }
@@ -680,7 +689,7 @@ where
                 )?;
                 // Add the nested goals from normalization to our own nested goals.
                 trace!(?nested_goals);
-                self.nested_goals.extend(nested_goals.into_iter().map(|(s, g)| (s, g, None)));
+                pending_goals.extend(nested_goals.into_iter().map(|(s, g)| (s, g, None)));
 
                 // Finally, equate the goal's RHS with the unconstrained var.
                 //
@@ -724,7 +733,7 @@ where
                 match certainty {
                     Certainty::Yes => {}
                     Certainty::Maybe(_) => {
-                        self.nested_goals.push((source, with_resolved_vars, stalled_on));
+                        pending_goals.push((source, with_resolved_vars, stalled_on));
                         unchanged_certainty = unchanged_certainty.map(|c| c.and(certainty));
                     }
                 }
@@ -738,13 +747,25 @@ where
                 match certainty {
                     Certainty::Yes => {}
                     Certainty::Maybe(_) => {
-                        self.nested_goals.push((source, goal, stalled_on));
+                        pending_goals.push((source, goal, stalled_on));
                         unchanged_certainty = unchanged_certainty.map(|c| c.and(certainty));
                     }
                 }
             }
         }
 
+        // Nested goals still need to be accounted for in the `unchanged_certainty`.
+        for (_, _, stalled_on) in &nested_goals {
+            if let Some(GoalStalledOn { stalled_cause, .. }) = stalled_on {
+                unchanged_certainty =
+                    unchanged_certainty.map(|c| c.and(Certainty::Maybe(*stalled_cause)));
+            }
+        }
+
+        debug_assert!(self.nested_goals.is_empty());
+        nested_goals.extend(pending_goals);
+        self.nested_goals = nested_goals;
+
         Ok(unchanged_certainty)
     }
 

tests/ui/traits/next-solver/normalization-shadowing/normalizes_to_ignores_unnormalizable_candidate.stderr

@@ -1,10 +1,8 @@
 error[E0284]: type annotations needed: cannot satisfy `<Vec<_> as Trait>::Assoc normalizes-to u8`
-  --> $DIR/normalizes_to_ignores_unnormalizable_candidate.rs:25:9
+  --> $DIR/normalizes_to_ignores_unnormalizable_candidate.rs:25:5
    |
 LL |     foo(unconstrained())
-   |     --- ^^^^^^^^^^^^^^^ cannot satisfy `<Vec<_> as Trait>::Assoc normalizes-to u8`
-   |     |
-   |     required by a bound introduced by this call
+   |     ^^^^^^^^^^^^^^^^^^^^ cannot satisfy `<Vec<_> as Trait>::Assoc normalizes-to u8`
    |
 note: required by a bound in `foo`
   --> $DIR/normalizes_to_ignores_unnormalizable_candidate.rs:14:17