Fix trait solver overflow with non_local_definitions lint

Author: Urgau

compiler/rustc_lint/src/non_local_def.rs

@@ -264,7 +264,18 @@ fn impl_trait_ref_has_enough_non_local_candidates<'tcx>(
     binder: EarlyBinder<TraitRef<'tcx>>,
     mut did_has_local_parent: impl FnMut(DefId) -> bool,
 ) -> bool {
-    let infcx = tcx.infer_ctxt().build();
+    let infcx = tcx
+        .infer_ctxt()
+        // We use the new trait solver since the obligation we are trying to
+        // prove here may overflow and those are fatal in the old trait solver.
+        //
+        // Which is unacceptable. Thanksfully the part we use here are very
+        // similar to the new-trait-solver-as-coherence, which is in stabilization.
+        //
+        // https://github.com/rust-lang/rust/issues/123573
+        .with_next_trait_solver(true)
+        .build();
+
     let trait_ref = binder.instantiate(tcx, infcx.fresh_args_for_item(infer_span, trait_def_id));
 
     let trait_ref = trait_ref.fold_with(&mut ReplaceLocalTypesWithInfer {

tests/ui/lint/non_local_definitions.rs

@@ -461,6 +461,17 @@ non_local_macro::non_local_impl!(CargoUpdate);
 non_local_macro::non_local_macro_rules!(my_macro);
 //~^ WARN non-local `macro_rules!` definition
 
+// https://github.com/rust-lang/rust/issues/123573#issue-2229428739
+pub trait StarlarkTypeRepr {}
+
+impl<'a, T: 'a> StarlarkTypeRepr for &[T] where &'a T: StarlarkTypeRepr {}
+
+fn starlark() {
+    struct Local {}
+    impl StarlarkTypeRepr for &Local {}
+    //~^ WARN non-local `impl` definition
+}
+
 fn bitflags() {
     struct Flags;
 

tests/ui/lint/non_local_definitions.stderr

@@ -701,5 +701,16 @@ LL | non_local_macro::non_local_macro_rules!(my_macro);
    = note: the macro `non_local_macro::non_local_macro_rules` may come from an old version of the `non_local_macro` crate, try updating your dependency with `cargo update -p non_local_macro`
    = note: this warning originates in the macro `non_local_macro::non_local_macro_rules` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-warning: 55 warnings emitted
+warning: non-local `impl` definition, they should be avoided as they go against expectation
+  --> $DIR/non_local_definitions.rs:471:5
+   |
+LL |     impl StarlarkTypeRepr for &Local {}
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: move this `impl` block outside the of the current function `starlark`
+   = note: an `impl` definition is non-local if it is nested inside an item and may impact type checking outside of that item. This can be the case if neither the trait or the self type are at the same nesting level as the `impl`
+   = note: one exception to the rule are anon-const (`const _: () = { ... }`) at top-level module and anon-const at the same nesting as the trait or type
+   = note: this lint may become deny-by-default in the edition 2024 and higher, see the tracking issue <https://github.com/rust-lang/rust/issues/120363>
+
+warning: 56 warnings emitted