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, ..)

cc #122192 (comment)

Author: bors

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;
@@ -1557,6 +1558,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         if self.is_intercrate() {
             return None;
         }
+
+        // Opaque types are not part of the global cache key, so we'd be caching and loading
+        // values, even though we don't have all the information that went into them.
+        if !self.infcx.defining_opaque_types().is_empty() {
+            return None;
+        }
+
         let tcx = self.tcx();
         let pred = cache_fresh_trait_pred.skip_binder();
 
@@ -1595,6 +1603,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         if self.is_intercrate() {
             return false;
         }
+        // Opaque types are not part of the global cache key, so we'd be caching and loading
+        // values, even though we don't have all the information that went into them.
+        if !self.infcx.defining_opaque_types().is_empty() {
+            return false;
+        }
         match result {
             Ok(Some(SelectionCandidate::ParamCandidate(trait_ref))) => !trait_ref.has_infer(),
             _ => true,

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
     }
 }

tests/ui/impl-trait/equality.rs

@@ -22,7 +22,7 @@ fn sum_to(n: u32) -> impl Foo {
         0
     } else {
         n + sum_to(n - 1)
-        //~^ ERROR cannot satisfy `<u32 as Add<impl Foo>>::Output == i32`
+        //~^ ERROR cannot satisfy `<u32 as Add<impl Foo>>::Output == u32`
     }
 }
 

tests/ui/impl-trait/equality.stderr

@@ -22,11 +22,11 @@ help: change the type of the numeric literal from `u32` to `i32`
 LL |     0_i32
    |       ~~~
 
-error[E0284]: type annotations needed: cannot satisfy `<u32 as Add<impl Foo>>::Output == i32`
+error[E0284]: type annotations needed: cannot satisfy `<u32 as Add<impl Foo>>::Output == u32`
   --> $DIR/equality.rs:24:11
    |
 LL |         n + sum_to(n - 1)
-   |           ^ cannot satisfy `<u32 as Add<impl Foo>>::Output == i32`
+   |           ^ cannot satisfy `<u32 as Add<impl Foo>>::Output == u32`
 
 error: aborting due to 2 previous errors; 1 warning emitted
 

tests/ui/type-alias-impl-trait/escaping-bound-var.rs

@@ -8,6 +8,7 @@ trait Test<'a> {}
 
 pub type Foo = impl for<'a> Trait<'a, Assoc = impl Test<'a>>;
 //~^ ERROR `impl Trait` cannot capture higher-ranked lifetime from outer `impl Trait`
+//~| ERROR unconstrained opaque type
 
 impl Trait<'_> for () {
     type Assoc = ();

tests/ui/type-alias-impl-trait/escaping-bound-var.stderr

@@ -10,6 +10,14 @@ note: lifetime declared here
 LL | pub type Foo = impl for<'a> Trait<'a, Assoc = impl Test<'a>>;
    |                         ^^
 
-error: aborting due to 1 previous error
+error: unconstrained opaque type
+  --> $DIR/escaping-bound-var.rs:9:47
+   |
+LL | pub type Foo = impl for<'a> Trait<'a, Assoc = impl Test<'a>>;
+   |                                               ^^^^^^^^^^^^^
+   |
+   = note: `Foo` must be used in combination with a concrete type within the same module
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0657`.