Auto merge of #95031 - compiler-errors:param-env-cache, r=Aaron1011

Do not use `ParamEnv::and` when building a cache key from a param-env and trait eval candidate

Do not use `ParamEnv::and` to cache a param-env with a selection/evaluation candidate.

This is because if the param-env is `RevealAll` mode, and the candidate looks global (i.e. it has erased regions, which can show up when we normalize a projection type under a binder<sup>1</sup>), then when we use `ParamEnv::and` to pair the candidate and the param-env for use as a cache key, we will throw away the param-env's caller bounds, and we'll end up caching a candidate that we inferred from the param-env with a empty param-env, which may cause cache-hit later when we have an empty param-env, and possibly mess with normalization like we see in the referenced issue during codegen.

Not sure how to trigger this with a more structured test, but changing `check-pass` to `build-pass` triggers the case that https://github.com/rust-lang/rust/issues/94903 detected.

<sup>1.</sup> That is, we will replace the late-bound region with a placeholder, which gets canonicalized and turned into an infererence variable, which gets erased during region freshening right before we cache the result. Sorry, it's quite a few steps.

Fixes #94903
r? `@Aaron1011` (or reassign as you see fit)

1 files changed, 2 insertions, 1 deletions

diff --git a/src/test/ui/higher-rank-trait-bounds/normalize-under-binder/issue-80706.rs b/src/test/ui/higher-rank-trait-bounds/normalize-under-binder/issue-80706.rs
index 112227c85c4..00a866f220b 100644
--- a/src/test/ui/higher-rank-trait-bounds/normalize-under-binder/issue-80706.rs
+++ b/src/test/ui/higher-rank-trait-bounds/normalize-under-binder/issue-80706.rs
@@ -1,4 +1,4 @@
-// check-pass
+// build-pass
 // edition:2018
 
 type BoxFuture<T> = std::pin::Pin<Box<dyn std::future::Future<Output=T>>>;
@@ -65,6 +65,7 @@ async fn run<S>(dep: &str)
 where
     S: Storage,
     for<'a> SaveUser<'a>: StorageRequest<S>,
+    for<'a> SaveUser<'a>: StorageRequestReturnType,
 {
     User { dep }.save().await;
 }