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

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() {}