Rollup merge of #149904 - ShoyuVanilla:ns-remove-sg-hack, r=lcnr

`-Znext-solver` Remove the forced ambiguity hack from search graph

As discussed in https://github.com/rust-lang/trait-system-refactor-initiative/issues/257

r? lcnr

compiler/rustc_type_ir/src/search_graph/mod.rs

@@ -916,10 +916,9 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D> {
 /// heads from the stack. This may not necessarily mean that we've actually
 /// reached a fixpoint for that cycle head, which impacts the way we rebase
 /// provisional cache entries.
-#[derive_where(Debug; X: Cx)]
-enum RebaseReason<X: Cx> {
+#[derive(Debug)]
+enum RebaseReason {
     NoCycleUsages,
-    Ambiguity(X::AmbiguityInfo),
     Overflow,
     /// We've actually reached a fixpoint.
     ///
@@ -956,7 +955,7 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D, X> {
         &mut self,
         cx: X,
         stack_entry: &StackEntry<X>,
-        rebase_reason: RebaseReason<X>,
+        rebase_reason: RebaseReason,
     ) {
         let popped_head_index = self.stack.next_index();
         #[allow(rustc::potential_query_instability)]
@@ -1035,9 +1034,6 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D, X> {
                         // is not actually equal to the final provisional result. We
                         // need to discard the provisional cache entry in this case.
                         RebaseReason::NoCycleUsages => return false,
-                        RebaseReason::Ambiguity(info) => {
-                            *result = D::propagate_ambiguity(cx, input, info);
-                        }
                         RebaseReason::Overflow => *result = D::fixpoint_overflow_result(cx, input),
                         RebaseReason::ReachedFixpoint(None) => {}
                         RebaseReason::ReachedFixpoint(Some(path_kind)) => {
@@ -1352,27 +1348,6 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D, X> {
                 return EvaluationResult::finalize(stack_entry, encountered_overflow, result);
             }
 
-            // If computing this goal results in ambiguity with no constraints,
-            // we do not rerun it. It's incredibly difficult to get a different
-            // response in the next iteration in this case. These changes would
-            // likely either be caused by incompleteness or can change the maybe
-            // cause from ambiguity to overflow. Returning ambiguity always
-            // preserves soundness and completeness even if the goal is be known
-            // to succeed or fail.
-            //
-            // This prevents exponential blowup affecting multiple major crates.
-            // As we only get to this branch if we haven't yet reached a fixpoint,
-            // we also taint all provisional cache entries which depend on the
-            // current goal.
-            if let Some(info) = D::is_ambiguous_result(result) {
-                self.rebase_provisional_cache_entries(
-                    cx,
-                    &stack_entry,
-                    RebaseReason::Ambiguity(info),
-                );
-                return EvaluationResult::finalize(stack_entry, encountered_overflow, result);
-            };
-
             // If we've reached the fixpoint step limit, we bail with overflow and taint all
             // provisional cache entries which depend on the current goal.
             i += 1;

tests/ui/traits/next-solver/global-where-bound-normalization.rs

@@ -0,0 +1,45 @@
+//@ check-pass
+//@ compile-flags: -Znext-solver
+
+// Regression test for https://github.com/rust-lang/trait-system-refactor-initiative/issues/257.
+
+#![feature(rustc_attrs)]
+#![expect(internal_features)]
+#![rustc_no_implicit_bounds]
+
+pub trait Bound {}
+impl Bound for u8 {}
+
+pub trait Proj {
+    type Assoc;
+}
+impl<U: Bound> Proj for U {
+    type Assoc = U;
+}
+impl Proj for MyField {
+    type Assoc = u8;
+}
+
+// While wf-checking the global bounds of `fn foo`, elaborating this outlives predicate triggered a
+// cycle in the search graph along a particular probe path, which was not an actual solution.
+// That cycle then resulted in a forced false-positive ambiguity due to a performance hack in the
+// search graph and then ended up floundering the root goal evaluation.
+pub trait Field: Proj<Assoc: Bound + 'static> {}
+
+struct MyField;
+impl Field for MyField {}
+
+trait IdReqField {
+    type This;
+}
+impl<F: Field> IdReqField for F {
+    type This = F;
+}
+
+fn foo()
+where
+    <MyField as IdReqField>::This: Field,
+{
+}
+
+fn main() {}