Auto merge of #126024 - oli-obk:candidate_key_caching_is_unsound_yay, r=<try>

Do not use global cache for selection candidates if opaque types can be constrained

fixes #105787

r? `@ghost`

This is certainly the crudest way to make the cache sound wrt opaque types, but if perf lets us get away with this, let's do it in the old solver and let the new solver fix this correctly once and for all.

If perf is prohibitively bad, I'll look into alternatives (using canonical queries, checking whether any opaque types got constrained or whether decisions based on the availability of opaque types were made, still using the cache for things that can't possibly constrain opaque types (probably sound, famous last words), ..)

cc #122192 (comment)

* [ ] check if this actually fixes #105787 or if it just fixes the opaque type reproducer

Author: bors

compiler/rustc_middle/src/traits/select.rs

@@ -9,15 +9,15 @@ use rustc_errors::ErrorGuaranteed;
 
 use crate::ty;
 
-use rustc_hir::def_id::DefId;
+use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_macros::{HashStable, TypeVisitable};
 use rustc_query_system::cache::Cache;
 
 pub type SelectionCache<'tcx> = Cache<
     // This cache does not use `ParamEnvAnd` in its keys because `ParamEnv::and` can replace
     // caller bounds with an empty list if the `TraitPredicate` looks global, which may happen
     // after erasing lifetimes from the predicate.
-    (ty::ParamEnv<'tcx>, ty::TraitPredicate<'tcx>),
+    (&'tcx ty::List<LocalDefId>, ty::ParamEnv<'tcx>, ty::TraitPredicate<'tcx>),
     SelectionResult<'tcx, SelectionCandidate<'tcx>>,
 >;
 

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -49,6 +49,7 @@ use rustc_middle::ty::{self, PolyProjectionPredicate, Upcast};
 use rustc_middle::ty::{Ty, TyCtxt, TypeFoldable, TypeVisitableExt};
 use rustc_span::symbol::sym;
 use rustc_span::Symbol;
+use rustc_type_ir::InferCtxtLike as _;
 
 use std::cell::{Cell, RefCell};
 use std::cmp;
@@ -1559,13 +1560,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
         let tcx = self.tcx();
         let pred = cache_fresh_trait_pred.skip_binder();
+        let opaques = self.infcx.defining_opaque_types();
 
         if self.can_use_global_caches(param_env) {
-            if let Some(res) = tcx.selection_cache.get(&(param_env, pred), tcx) {
+            if let Some(res) = tcx.selection_cache.get(&(opaques, param_env, pred), tcx) {
                 return Some(res);
             }
         }
-        self.infcx.selection_cache.get(&(param_env, pred), tcx)
+        self.infcx.selection_cache.get(&(opaques, param_env, pred), tcx)
     }
 
     /// Determines whether can we safely cache the result
@@ -1611,6 +1613,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ) {
         let tcx = self.tcx();
         let pred = cache_fresh_trait_pred.skip_binder();
+        let opaques = self.infcx.defining_opaque_types();
 
         if !self.can_cache_candidate(&candidate) {
             debug!(?pred, ?candidate, "insert_candidate_cache - candidate is not cacheable");
@@ -1624,14 +1627,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 if !candidate.has_infer() {
                     debug!(?pred, ?candidate, "insert_candidate_cache global");
                     // This may overwrite the cache with the same value.
-                    tcx.selection_cache.insert((param_env, pred), dep_node, candidate);
+                    tcx.selection_cache.insert((opaques, param_env, pred), dep_node, candidate);
                     return;
                 }
             }
         }
 
         debug!(?pred, ?candidate, "insert_candidate_cache local");
-        self.infcx.selection_cache.insert((param_env, pred), dep_node, candidate);
+        self.infcx.selection_cache.insert((opaques, param_env, pred), dep_node, candidate);
     }
 
     /// Looks at the item bounds of the projection or opaque type.

tests/crashes/119272.rs renamed to tests/ui/auto-traits/opaque_type_candidate_selection.rs

@@ -1,10 +1,11 @@
-//@ known-bug: #119272
+//! used to ICE: #119272
 #![feature(type_alias_impl_trait)]
 mod defining_scope {
     use super::*;
     pub type Alias<T> = impl Sized;
 
     pub fn cast<T>(x: Container<Alias<T>, T>) -> Container<T, T> {
+        //~^ ERROR: type annotations needed
         x
     }
 }

tests/ui/auto-traits/opaque_type_candidate_selection.stderr

@@ -0,0 +1,21 @@
+error[E0284]: type annotations needed
+  --> $DIR/opaque_type_candidate_selection.rs:7:20
+   |
+LL |     pub fn cast<T>(x: Container<Alias<T>, T>) -> Container<T, T> {
+   |                    ^ cannot infer type
+   |
+   = note: cannot satisfy `<Alias<T> as Trait<T>>::Assoc == _`
+note: required because it appears within the type `Container<Alias<T>, T>`
+  --> $DIR/opaque_type_candidate_selection.rs:13:8
+   |
+LL | struct Container<T: Trait<U>, U> {
+   |        ^^^^^^^^^
+   = help: unsized fn params are gated as an unstable feature
+help: function arguments must have a statically known size, borrowed types always have a known size
+   |
+LL |     pub fn cast<T>(x: &Container<Alias<T>, T>) -> Container<T, T> {
+   |                       +
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0284`.

tests/ui/coherence/occurs-check/opaques.next.stderr

@@ -1,5 +1,5 @@
 error[E0119]: conflicting implementations of trait `Trait<_>`
-  --> $DIR/opaques.rs:30:1
+  --> $DIR/opaques.rs:28:1
    |
 LL | impl<T> Trait<T> for T {
    | ---------------------- first implementation here
@@ -8,7 +8,7 @@ LL | impl<T> Trait<T> for defining_scope::Alias<T> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation
 
 error[E0282]: type annotations needed
-  --> $DIR/opaques.rs:13:20
+  --> $DIR/opaques.rs:11:20
    |
 LL |     pub fn cast<T>(x: Container<Alias<T>, T>) -> Container<T, T> {
    |                    ^ cannot infer type

tests/ui/coherence/occurs-check/opaques.old.stderr

@@ -0,0 +1,21 @@
+error[E0284]: type annotations needed
+  --> $DIR/opaques.rs:11:20
+   |
+LL |     pub fn cast<T>(x: Container<Alias<T>, T>) -> Container<T, T> {
+   |                    ^ cannot infer type
+   |
+   = note: cannot satisfy `<Alias<T> as Trait<T>>::Assoc == _`
+note: required because it appears within the type `Container<Alias<T>, T>`
+  --> $DIR/opaques.rs:17:8
+   |
+LL | struct Container<T: Trait<U>, U> {
+   |        ^^^^^^^^^
+   = help: unsized fn params are gated as an unstable feature
+help: function arguments must have a statically known size, borrowed types always have a known size
+   |
+LL |     pub fn cast<T>(x: &Container<Alias<T>, T>) -> Container<T, T> {
+   |                       +
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0284`.

tests/ui/coherence/occurs-check/opaques.rs

@@ -3,15 +3,13 @@
 
 // A regression test for #105787
 
-//@[old] known-bug: #105787
-//@[old] check-pass
 #![feature(type_alias_impl_trait)]
 mod defining_scope {
     use super::*;
     pub type Alias<T> = impl Sized;
 
     pub fn cast<T>(x: Container<Alias<T>, T>) -> Container<T, T> {
-        //[next]~^ ERROR type annotations needed
+        //~^ ERROR type annotations needed
         x
     }
 }