Rollup merge of #112122 - compiler-errors:next-coherence, r=lcnr

Add `-Ztrait-solver=next-coherence`

Flag that conditionally uses the trait solver *only* during coherence, for more testing and/or eventual partial-migration onto the trait solver (in the medium- to long-term).

* This still uses the selection context in some of the coherence methods I think, so it's not "complete". Putting this up for review and/or for further work in-tree.
* I probably need to spend a bit more time making sure that we don't sneakily create any other infcx's during coherence that also need the new solver enabled.

r? `@lcnr`

1 files changed, 9 insertions, 1 deletions

diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index bf6f21968d7..4ee543bbfb6 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -2333,10 +2333,18 @@ impl<'tcx> TyCtxt<'tcx> {
         self.opt_local_def_id_to_hir_id(local_def_id).unwrap()
     }
 
-    pub fn trait_solver_next(self) -> bool {
+    pub fn next_trait_solver_globally(self) -> bool {
         self.sess.opts.unstable_opts.trait_solver == rustc_session::config::TraitSolver::Next
     }
 
+    pub fn next_trait_solver_in_coherence(self) -> bool {
+        matches!(
+            self.sess.opts.unstable_opts.trait_solver,
+            rustc_session::config::TraitSolver::Next
+                | rustc_session::config::TraitSolver::NextCoherence
+        )
+    }
+
     pub fn lower_impl_trait_in_trait_to_assoc_ty(self) -> bool {
         self.sess.opts.unstable_opts.lower_impl_trait_in_trait_to_assoc_ty
     }