Rollup merge of #94869 - jackh726:gats_extended, r=compiler-errors

Add the generic_associated_types_extended feature

Right now, this only ignore obligations that reference new placeholders in `poly_project_and_unify_type`. In the future, this might do other things, like allowing object-safe GATs.

**This feature is *incomplete* and quite likely unsound. This is mostly just for testing out potential future APIs using a "relaxed" set of rules until we figure out *proper* rules.**

Also drive by cleanup of adding a `ProjectAndUnifyResult` enum instead of using a `Result<Result<Option>>`.

r? `@nikomatsakis`

compiler/rustc_middle/src/ty/fold.rs

@@ -1379,3 +1379,50 @@ impl<'tcx> TypeVisitor<'tcx> for LateBoundRegionsCollector {
         ControlFlow::CONTINUE
     }
 }
+
+/// Finds the max universe present
+pub struct MaxUniverse {
+    max_universe: ty::UniverseIndex,
+}
+
+impl MaxUniverse {
+    pub fn new() -> Self {
+        MaxUniverse { max_universe: ty::UniverseIndex::ROOT }
+    }
+
+    pub fn max_universe(self) -> ty::UniverseIndex {
+        self.max_universe
+    }
+}
+
+impl<'tcx> TypeVisitor<'tcx> for MaxUniverse {
+    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+        if let ty::Placeholder(placeholder) = t.kind() {
+            self.max_universe = ty::UniverseIndex::from_u32(
+                self.max_universe.as_u32().max(placeholder.universe.as_u32()),
+            );
+        }
+
+        t.super_visit_with(self)
+    }
+
+    fn visit_const(&mut self, c: ty::consts::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
+        if let ty::ConstKind::Placeholder(placeholder) = c.val() {
+            self.max_universe = ty::UniverseIndex::from_u32(
+                self.max_universe.as_u32().max(placeholder.universe.as_u32()),
+            );
+        }
+
+        c.super_visit_with(self)
+    }
+
+    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
+        if let ty::RePlaceholder(placeholder) = *r {
+            self.max_universe = ty::UniverseIndex::from_u32(
+                self.max_universe.as_u32().max(placeholder.universe.as_u32()),
+            );
+        }
+
+        ControlFlow::CONTINUE
+    }
+}