Add -Ztrait-solver=next-coherence

compiler/rustc_middle/src/ty/context.rs

@@ -2337,6 +2337,14 @@ 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
     }

compiler/rustc_session/src/config.rs

@@ -610,6 +610,8 @@ pub enum TraitSolver {
     Chalk,
     /// Experimental trait solver in `rustc_trait_selection::solve`
     Next,
+    /// Use the new trait solver during coherence
+    NextCoherence,
 }
 
 pub enum Input {

compiler/rustc_session/src/options.rs

@@ -986,6 +986,7 @@ pub(crate) fn parse_trait_solver(slot: &mut TraitSolver, v: Option<&str>) -> boo
             Some("classic") => *slot = TraitSolver::Classic,
             Some("chalk") => *slot = TraitSolver::Chalk,
             Some("next") => *slot = TraitSolver::Next,
+            Some("next-coherence") => *slot = TraitSolver::NextCoherence,
             // default trait solver is subject to change..
             Some("default") => *slot = TraitSolver::Classic,
             _ => return false,

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -182,6 +182,7 @@ fn overlap<'tcx>(
         .with_opaque_type_inference(DefiningAnchor::Bubble)
         .skip_leak_check(skip_leak_check.is_yes())
         .intercrate(true)
+        .with_next_trait_solver(tcx.next_trait_solver_in_coherence())
         .build();
     let selcx = &mut SelectionContext::new(&infcx);
     if track_ambiguity_causes.is_yes() {

compiler/rustc_trait_selection/src/traits/engine.rs

@@ -35,6 +35,7 @@ impl<'tcx> TraitEngineExt<'tcx> for dyn TraitEngine<'tcx> {
     fn new(tcx: TyCtxt<'tcx>) -> Box<Self> {
         match tcx.sess.opts.unstable_opts.trait_solver {
             TraitSolver::Classic => Box::new(FulfillmentContext::new()),
+            TraitSolver::NextCoherence => Box::new(FulfillmentContext::new()),
             TraitSolver::Chalk => Box::new(ChalkFulfillmentContext::new()),
             TraitSolver::Next => Box::new(NextFulfillmentCtxt::new()),
         }
@@ -43,6 +44,7 @@ fn new(tcx: TyCtxt<'tcx>) -> Box<Self> {
     fn new_in_snapshot(tcx: TyCtxt<'tcx>) -> Box<Self> {
         match tcx.sess.opts.unstable_opts.trait_solver {
             TraitSolver::Classic => Box::new(FulfillmentContext::new_in_snapshot()),
+            TraitSolver::NextCoherence => Box::new(FulfillmentContext::new_in_snapshot()),
             TraitSolver::Chalk => Box::new(ChalkFulfillmentContext::new_in_snapshot()),
             TraitSolver::Next => Box::new(NextFulfillmentCtxt::new()),
         }