Rollup merge of rust-lang#109619 - compiler-errors:new-solver-still-f…

…urther-specializable, r=BoxyUwU

Still-further-specializable projections are ambiguous in new solver

Fixes https://github.com/rust-lang/rust/pull/108896/files#r1148450781

r? `@BoxyUwU` (though feel free to re-roll)

---

This can be used to create an unsound transmute function with the new solver:

```rust
#![feature(specialization)]

trait Default {
   type Id;

   fn intu(&self) -> &Self::Id;
}

impl<T> Default for T {
   default type Id = T;

   fn intu(&self) -> &Self::Id {
        self
   }
}

fn transmute<T: Default<Id = U>, U: Copy>(t: T) -> U {
    *t.intu()
}

use std::num::NonZeroU8;
fn main() {
    let s = transmute::<u8, Option<NonZeroU8>>(0);
    assert_eq!(s, None);
}
```

compiler/rustc_middle/src/ty/mod.rs

@@ -653,8 +653,8 @@ pub enum AliasRelationDirection {
 impl std::fmt::Display for AliasRelationDirection {
  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
  match self {
- AliasRelationDirection::Equate => write!(f, " == "),
- AliasRelationDirection::Subtype => write!(f, " <: "),
+ AliasRelationDirection::Equate => write!(f, "=="),
+ AliasRelationDirection::Subtype => write!(f, "<:"),
  }
  }
 }

compiler/rustc_trait_selection/src/solve/project_goals.rs

@@ -174,7 +174,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
  goal.predicate.def_id(),
  impl_def_id
  )? else {
- return ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes);
+ return ecx.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS);
  };
 
  if !assoc_def.item.defaultness(tcx).has_value() {

tests/ui/traits/new-solver/specialization-transmute.rs

@@ -0,0 +1,30 @@
+// compile-flags: -Ztrait-solver=next
+
+#![feature(specialization)]
+//~^ WARN the feature `specialization` is incomplete
+
+trait Default {
+ type Id;
+
+ fn intu(&self) -> &Self::Id;
+}
+
+impl<T> Default for T {
+ default type Id = T;
+
+ fn intu(&self) -> &Self::Id {
+ self
+ //~^ ERROR cannot satisfy `T <: <T as Default>::Id`
+ }
+}
+
+fn transmute<T: Default<Id = U>, U: Copy>(t: T) -> U {
+ *t.intu()
+}
+
+use std::num::NonZeroU8;
+fn main() {
+ let s = transmute::<u8, Option<NonZeroU8>>(0);
+ //~^ ERROR cannot satisfy `<u8 as Default>::Id == Option<NonZeroU8>
+ assert_eq!(s, None);
+}

tests/ui/traits/new-solver/specialization-transmute.stderr

@@ -0,0 +1,31 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+ --> $DIR/specialization-transmute.rs:3:12
+ |
+LL | #![feature(specialization)]
+ | ^^^^^^^^^^^^^^
+ |
+ = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+ = help: consider using `min_specialization` instead, which is more stable and complete
+ = note: `#[warn(incomplete_features)]` on by default
+
+error[E0284]: type annotations needed: cannot satisfy `T <: <T as Default>::Id`
+ --> $DIR/specialization-transmute.rs:16:9
+ |
+LL | self
+ | ^^^^ cannot satisfy `T <: <T as Default>::Id`
+
+error[E0284]: type annotations needed: cannot satisfy `<u8 as Default>::Id == Option<NonZeroU8>`
+ --> $DIR/specialization-transmute.rs:27:13
+ |
+LL | let s = transmute::<u8, Option<NonZeroU8>>(0);
+ | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot satisfy `<u8 as Default>::Id == Option<NonZeroU8>`
+ |
+note: required by a bound in `transmute`
+ --> $DIR/specialization-transmute.rs:21:25
+ |
+LL | fn transmute<T: Default<Id = U>, U: Copy>(t: T) -> U {
+ | ^^^^^^ required by this bound in `transmute`
+
+error: aborting due to 2 previous errors; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0284`.

tests/ui/traits/new-solver/specialization-unconstrained.rs

@@ -0,0 +1,22 @@
+// compile-flags: -Ztrait-solver=next
+
+#![feature(specialization)]
+//~^ WARN the feature `specialization` is incomplete
+
+// Do not treat the RHS of a projection-goal as an unconstrained `Certainty::Yes` response
+// if the impl is still further specializable.
+
+trait Default {
+ type Id;
+}
+
+impl<T> Default for T {
+ default type Id = T;
+}
+
+fn test<T: Default<Id = U>, U>() {}
+
+fn main() {
+ test::<u32, ()>();
+ //~^ ERROR cannot satisfy `<u32 as Default>::Id == ()`
+}

tests/ui/traits/new-solver/specialization-unconstrained.stderr

@@ -0,0 +1,25 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+ --> $DIR/specialization-unconstrained.rs:3:12
+ |
+LL | #![feature(specialization)]
+ | ^^^^^^^^^^^^^^
+ |
+ = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+ = help: consider using `min_specialization` instead, which is more stable and complete
+ = note: `#[warn(incomplete_features)]` on by default
+
+error[E0284]: type annotations needed: cannot satisfy `<u32 as Default>::Id == ()`
+ --> $DIR/specialization-unconstrained.rs:20:5
+ |
+LL | test::<u32, ()>();
+ | ^^^^^^^^^^^^^^^ cannot satisfy `<u32 as Default>::Id == ()`
+ |
+note: required by a bound in `test`
+ --> $DIR/specialization-unconstrained.rs:17:20
+ |
+LL | fn test<T: Default<Id = U>, U>() {}
+ | ^^^^^^ required by this bound in `test`
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0284`.