Rollup merge of #130866 - compiler-errors:dyn-instantiate-binder, r=lcnr

Allow instantiating object trait binder when upcasting

This PR fixes two bugs (that probably need an FCP).

### We use equality rather than subtyping for upcasting dyn conversions

This code should be valid:

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

trait Foo: for<'h> Bar<'h> {}
trait Bar<'a> {}

fn foo(x: &dyn Foo) {
    let y: &dyn Bar<'static> = x;
}
```
But instead:

```
error[E0308]: mismatched types
 --> src/lib.rs:7:32
  |
7 |     let y: &dyn Bar<'static> = x;
  |                                ^ one type is more general than the other
  |
  = note: expected existential trait ref `for<'h> Bar<'h>`
             found existential trait ref `Bar<'_>`
```

And so should this:

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

fn foo(x: &dyn for<'h> Fn(&'h ())) {
    let y: &dyn FnOnce(&'static ()) = x;
}
```

But instead:

```
error[E0308]: mismatched types
 --> src/lib.rs:4:39
  |
4 |     let y: &dyn FnOnce(&'static ()) = x;
  |                                       ^ one type is more general than the other
  |
  = note: expected existential trait ref `for<'h> FnOnce<(&'h (),)>`
             found existential trait ref `FnOnce<(&(),)>`
```

Specifically, both of these fail because we use *equality* when comparing the supertrait to the *target* of the unsize goal. For the first example, since our supertrait is `for<'h> Bar<'h>` but our target is `Bar<'static>`, there's a higher-ranked type mismatch even though we *should* be able to instantiate that supertrait binder when upcasting. Similarly for the second example.

### New solver uses equality rather than subtyping for no-op (i.e. non-upcasting) dyn conversions

This code should be valid in the new solver, like it is with the old solver:

```rust
// -Znext-solver

fn foo<'a>(x: &mut for<'h> dyn Fn(&'h ())) {
   let _: &mut dyn Fn(&'a ()) = x;
}
```

But instead:

```
error: lifetime may not live long enough
 --> <source>:2:11
  |
1 | fn foo<'a>(x: &mut dyn for<'h> Fn(&'h ())) {
  |        -- lifetime `'a` defined here
2 |    let _: &mut dyn Fn(&'a ()) = x;
  |           ^^^^^^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'static`
  |
  = note: requirement occurs because of a mutable reference to `dyn Fn(&())`
```

Specifically, this fails because we try to coerce `&mut dyn for<'h> Fn(&'h ())` to `&mut dyn Fn(&'a ())`, which registers an `dyn for<'h> Fn(&'h ()): dyn Fn(&'a ())` goal. This fails because the new solver uses *equating* rather than *subtyping* in `Unsize` goals.

This is *mostly* not a problem... You may wonder why the same code passes on the new solver for immutable references:

```
// -Znext-solver

fn foo<'a>(x: &dyn Fn(&())) {
   let _: &dyn Fn(&'a ()) = x; // works
}
```

That's because in this case, we first try to coerce via `Unsize`, but due to the leak check the goal fails. Then, later in coercion, we fall back to a simple subtyping operation, which *does* work.

Since `&T` is covariant over `T`, but `&mut T` is invariant, that's where the discrepancy between these two examples crops up.

---

r? lcnr or reassign :D

1 files changed, 73 insertions, 20 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 20b540831aa..fba1d1025ca 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -16,6 +16,7 @@ use rustc_hir::LangItem;
 use rustc_hir::def_id::DefId;
 use rustc_infer::infer::BoundRegionConversionTime::{self, HigherRankedType};
 use rustc_infer::infer::DefineOpaqueTypes;
+use rustc_infer::infer::at::ToTrace;
 use rustc_infer::infer::relate::TypeRelation;
 use rustc_infer::traits::TraitObligation;
 use rustc_middle::bug;
@@ -44,7 +45,7 @@ use super::{
     TraitQueryMode, const_evaluatable, project, util, wf,
 };
 use crate::error_reporting::InferCtxtErrorExt;
-use crate::infer::{InferCtxt, InferCtxtExt, InferOk, TypeFreshener};
+use crate::infer::{InferCtxt, InferOk, TypeFreshener};
 use crate::solve::InferCtxtSelectExt as _;
 use crate::traits::normalize::{normalize_with_depth, normalize_with_depth_to};
 use crate::traits::project::{ProjectAndUnifyResult, ProjectionCacheKeyExt};
@@ -2579,16 +2580,31 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                 // Check that a_ty's supertrait (upcast_principal) is compatible
                 // with the target (b_ty).
                 ty::ExistentialPredicate::Trait(target_principal) => {
+                    let hr_source_principal = upcast_principal.map_bound(|trait_ref| {
+                        ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)
+                    });
+                    let hr_target_principal = bound.rebind(target_principal);
+
                     nested.extend(
                         self.infcx
-                            .at(&obligation.cause, obligation.param_env)
-                            .eq(
-                                DefineOpaqueTypes::Yes,
-                                upcast_principal.map_bound(|trait_ref| {
-                                    ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)
-                                }),
-                                bound.rebind(target_principal),
-                            )
+                            .enter_forall(hr_target_principal, |target_principal| {
+                                let source_principal =
+                                    self.infcx.instantiate_binder_with_fresh_vars(
+                                        obligation.cause.span,
+                                        HigherRankedType,
+                                        hr_source_principal,
+                                    );
+                                self.infcx.at(&obligation.cause, obligation.param_env).eq_trace(
+                                    DefineOpaqueTypes::Yes,
+                                    ToTrace::to_trace(
+                                        &obligation.cause,
+                                        hr_target_principal,
+                                        hr_source_principal,
+                                    ),
+                                    target_principal,
+                                    source_principal,
+                                )
+                            })
                             .map_err(|_| SelectionError::Unimplemented)?
                             .into_obligations(),
                     );
@@ -2599,19 +2615,40 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                 // return ambiguity. Otherwise, if exactly one matches, equate
                 // it with b_ty's projection.
                 ty::ExistentialPredicate::Projection(target_projection) => {
-                    let target_projection = bound.rebind(target_projection);
+                    let hr_target_projection = bound.rebind(target_projection);
+
                     let mut matching_projections =
-                        a_data.projection_bounds().filter(|source_projection| {
+                        a_data.projection_bounds().filter(|&hr_source_projection| {
                             // Eager normalization means that we can just use can_eq
                             // here instead of equating and processing obligations.
-                            source_projection.item_def_id() == target_projection.item_def_id()
-                                && self.infcx.can_eq(
-                                    obligation.param_env,
-                                    *source_projection,
-                                    target_projection,
-                                )
+                            hr_source_projection.item_def_id() == hr_target_projection.item_def_id()
+                                && self.infcx.probe(|_| {
+                                    self.infcx
+                                        .enter_forall(hr_target_projection, |target_projection| {
+                                            let source_projection =
+                                                self.infcx.instantiate_binder_with_fresh_vars(
+                                                    obligation.cause.span,
+                                                    HigherRankedType,
+                                                    hr_source_projection,
+                                                );
+                                            self.infcx
+                                                .at(&obligation.cause, obligation.param_env)
+                                                .eq_trace(
+                                                    DefineOpaqueTypes::Yes,
+                                                    ToTrace::to_trace(
+                                                        &obligation.cause,
+                                                        hr_target_projection,
+                                                        hr_source_projection,
+                                                    ),
+                                                    target_projection,
+                                                    source_projection,
+                                                )
+                                        })
+                                        .is_ok()
+                                })
                         });
-                    let Some(source_projection) = matching_projections.next() else {
+
+                    let Some(hr_source_projection) = matching_projections.next() else {
                         return Err(SelectionError::Unimplemented);
                     };
                     if matching_projections.next().is_some() {
@@ -2619,8 +2656,24 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                     }
                     nested.extend(
                         self.infcx
-                            .at(&obligation.cause, obligation.param_env)
-                            .eq(DefineOpaqueTypes::Yes, source_projection, target_projection)
+                            .enter_forall(hr_target_projection, |target_projection| {
+                                let source_projection =
+                                    self.infcx.instantiate_binder_with_fresh_vars(
+                                        obligation.cause.span,
+                                        HigherRankedType,
+                                        hr_source_projection,
+                                    );
+                                self.infcx.at(&obligation.cause, obligation.param_env).eq_trace(
+                                    DefineOpaqueTypes::Yes,
+                                    ToTrace::to_trace(
+                                        &obligation.cause,
+                                        hr_target_projection,
+                                        hr_source_projection,
+                                    ),
+                                    target_projection,
+                                    source_projection,
+                                )
+                            })
                             .map_err(|_| SelectionError::Unimplemented)?
                             .into_obligations(),
                     );