Auto merge of #141731 - compiler-errors:tweak-fast-path-trait, r=lcnr

Tweak fast path trait handling

(1.) Make it more sound by considering polarity (lol)

(2.) Make it more general, by considering higher-ranked size/copy/clone

(2.) Make it less observable, by only doing copy/clone fast path if there are no regions involved

r? lcnr

2 files changed, 8 insertions, 7 deletions

diff --git a/compiler/rustc_next_trait_solver/src/delegate.rs b/compiler/rustc_next_trait_solver/src/delegate.rs
index 32dc85b3e6a..7b932010d49 100644
--- a/compiler/rustc_next_trait_solver/src/delegate.rs
+++ b/compiler/rustc_next_trait_solver/src/delegate.rs
@@ -3,8 +3,6 @@ use std::ops::Deref;
 use rustc_type_ir::solve::{Certainty, Goal, NoSolution};
 use rustc_type_ir::{self as ty, InferCtxtLike, Interner, TypeFoldable};
 
-use crate::solve::HasChanged;
-
 pub trait SolverDelegate: Deref<Target = Self::Infcx> + Sized {
     type Infcx: InferCtxtLike<Interner = Self::Interner>;
     type Interner: Interner;
@@ -23,7 +21,7 @@ pub trait SolverDelegate: Deref<Target = Self::Infcx> + Sized {
         &self,
         goal: Goal<Self::Interner, <Self::Interner as Interner>::Predicate>,
         span: <Self::Interner as Interner>::Span,
-    ) -> Option<HasChanged>;
+    ) -> Option<Certainty>;
 
     fn fresh_var_for_kind_with_span(
         &self,
diff --git a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs b/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
index b1a842e04af..38d7ff576a5 100644
--- a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
@@ -671,10 +671,13 @@ where
         // If this loop did not result in any progress, what's our final certainty.
         let mut unchanged_certainty = Some(Certainty::Yes);
         for (source, goal, stalled_on) in mem::take(&mut self.nested_goals) {
-            if let Some(has_changed) = self.delegate.compute_goal_fast_path(goal, self.origin_span)
-            {
-                if matches!(has_changed, HasChanged::Yes) {
-                    unchanged_certainty = None;
+            if let Some(certainty) = self.delegate.compute_goal_fast_path(goal, self.origin_span) {
+                match certainty {
+                    Certainty::Yes => {}
+                    Certainty::Maybe(_) => {
+                        self.nested_goals.push((source, goal, None));
+                        unchanged_certainty = unchanged_certainty.map(|c| c.and(certainty));
+                    }
                 }
                 continue;
             }