Allow constraining opaque types during various unsizing casts

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

@@ -1161,7 +1161,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let InferOk { mut obligations, .. } = self
                     .infcx
                     .at(&obligation.cause, obligation.param_env)
-                    .sup(DefineOpaqueTypes::No, target, source_trait)
+                    .sup(DefineOpaqueTypes::Yes, target, source_trait)
                     .map_err(|_| Unimplemented)?;
 
                 // Register one obligation for 'a: 'b.
@@ -1228,7 +1228,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let InferOk { obligations, .. } = self
                     .infcx
                     .at(&obligation.cause, obligation.param_env)
-                    .eq(DefineOpaqueTypes::No, b, a)
+                    .eq(DefineOpaqueTypes::Yes, b, a)
                     .map_err(|_| Unimplemented)?;
 
                 ImplSource::Builtin(BuiltinImplSource::Misc, obligations)
@@ -1276,7 +1276,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let InferOk { obligations, .. } = self
                     .infcx
                     .at(&obligation.cause, obligation.param_env)
-                    .eq(DefineOpaqueTypes::No, target, new_struct)
+                    .eq(DefineOpaqueTypes::Yes, target, new_struct)
                     .map_err(|_| Unimplemented)?;
                 nested.extend(obligations);
 
@@ -1309,7 +1309,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let InferOk { mut obligations, .. } = self
                     .infcx
                     .at(&obligation.cause, obligation.param_env)
-                    .eq(DefineOpaqueTypes::No, target, new_tuple)
+                    .eq(DefineOpaqueTypes::Yes, target, new_tuple)
                     .map_err(|_| Unimplemented)?;
 
                 // Add a nested `T: Unsize<U>` predicate.

tests/ui/impl-trait/trait_upcasting.rs

@@ -0,0 +1,26 @@
+//! Test that we allow unsizing `Trait<Concrete>` to `Trait<Opaque>` and vice versa
+
+//@ check-pass
+
+trait Trait<T> {}
+
+impl<T, U> Trait<T> for U {}
+
+fn hello() -> &'static (dyn Trait<impl Sized> + Send) {
+    if false {
+        let x = hello();
+        let _: &'static dyn Trait<()> = x;
+    }
+    todo!()
+}
+
+fn bye() -> &'static dyn Trait<impl Sized> {
+    if false {
+        let mut x = bye();
+        let y: &'static (dyn Trait<()> + Send) = &();
+        x = y;
+    }
+    todo!()
+}
+
+fn main() {}

tests/ui/impl-trait/trait_upcasting_reference_mismatch.rs

@@ -0,0 +1,18 @@
+//! Show an uninformative diagnostic that we could possibly improve in the future
+
+trait Trait<T> {}
+
+impl<T, U> Trait<T> for U {}
+
+fn hello() -> &'static (dyn Trait<impl Sized> + Send) {
+    //~^ ERROR: type annotations needed
+    if false {
+        let x = hello();
+        let _: &'static dyn Trait<()> = &x;
+        //^ Note the extra `&`, paired with the blanket impl causing
+        // `impl Sized` to never get a hidden type registered.
+    }
+    todo!()
+}
+
+fn main() {}

tests/ui/impl-trait/trait_upcasting_reference_mismatch.stderr

@@ -0,0 +1,9 @@
+error[E0282]: type annotations needed
+  --> $DIR/trait_upcasting_reference_mismatch.rs:7:35
+   |
+LL | fn hello() -> &'static (dyn Trait<impl Sized> + Send) {
+   |                                   ^^^^^^^^^^ cannot infer type
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0282`.

tests/ui/impl-trait/unsize_adt.rs

@@ -1,12 +1,13 @@
-//! Test that we do not allow unsizing `Foo<[Opaque; N]>` to `Foo<[Concrete]>`.
+//! Test that we allow unsizing `Foo<[Opaque; N]>` to `Foo<[Concrete]>`.
+
+//@check-pass
 
 struct Foo<T: ?Sized>(T);
 
 fn hello() -> Foo<[impl Sized; 2]> {
     if false {
         let x = hello();
         let _: &Foo<[i32]> = &x;
-        //~^ ERROR: mismatched types
     }
     todo!()
 }

tests/ui/impl-trait/unsize_slice.rs

@@ -1,10 +1,11 @@
-//! Test that we do not allow unsizing `[Opaque; N]` to `[Concrete]`.
+//! Test that we allow unsizing `[Opaque; N]` to `[Concrete]`.
+
+//@check-pass
 
 fn hello() -> [impl Sized; 2] {
     if false {
         let x = hello();
         let _: &[i32] = &x;
-        //~^ ERROR: mismatched types
     }
     todo!()
 }

tests/ui/impl-trait/unsize_tuple.rs

@@ -1,12 +1,13 @@
-//! Test that we do not allow unsizing `([Opaque; N],)` to `([Concrete],)`.
+//! Test that we allow unsizing `([Opaque; N],)` to `([Concrete],)`.
+
+//@check-pass
 
 #![feature(unsized_tuple_coercion)]
 
 fn hello() -> ([impl Sized; 2],) {
     if false {
         let x = hello();
         let _: &([i32],) = &x;
-        //~^ ERROR: mismatched types
     }
     todo!()
 }

tests/ui/impl-trait/unsized_coercion.next.stderr

@@ -0,0 +1,26 @@
+error[E0271]: type mismatch resolving `impl Trait <: dyn Trait`
+  --> $DIR/unsized_coercion.rs:14:17
+   |
+LL |         let x = hello();
+   |                 ^^^^^^^ types differ
+
+error[E0308]: mismatched types
+  --> $DIR/unsized_coercion.rs:18:14
+   |
+LL | fn hello() -> Box<impl Trait> {
+   |                   ---------- the expected opaque type
+...
+LL |     Box::new(1u32)
+   |     -------- ^^^^ types differ
+   |     |
+   |     arguments to this function are incorrect
+   |
+   = note: expected opaque type `impl Trait`
+                     found type `u32`
+note: associated function defined here
+  --> $SRC_DIR/alloc/src/boxed.rs:LL:COL
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0271, E0308.
+For more information about an error, try `rustc --explain E0271`.

tests/ui/impl-trait/unsized_coercion.rs

@@ -0,0 +1,21 @@
+//! This test checks that opaque types get unsized instead of
+//! constraining their hidden type to a trait object.
+
+//@ revisions: next old
+//@[next] compile-flags: -Znext-solver
+//@[old] check-pass
+
+trait Trait {}
+
+impl Trait for u32 {}
+
+fn hello() -> Box<impl Trait> {
+    if true {
+        let x = hello();
+        //[next]~^ ERROR: type mismatch resolving `impl Trait <: dyn Trait`
+        let y: Box<dyn Trait> = x;
+    }
+    Box::new(1u32) //[next]~ ERROR: mismatched types
+}
+
+fn main() {}

tests/ui/impl-trait/unsized_coercion2.old.stderr

@@ -0,0 +1,12 @@
+error[E0277]: the size for values of type `impl Trait + ?Sized` cannot be known at compilation time
+  --> $DIR/unsized_coercion2.rs:15:33
+   |
+LL |         let y: Box<dyn Trait> = x;
+   |                                 ^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `impl Trait + ?Sized`
+   = note: required for the cast from `Box<impl Trait + ?Sized>` to `Box<dyn Trait>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/impl-trait/unsized_coercion2.rs

@@ -0,0 +1,21 @@
+//! This test checks that opaque types get unsized instead of
+//! constraining their hidden type to a trait object.
+
+//@ revisions: next old
+//@[next] compile-flags: -Znext-solver
+//@[next] check-pass
+
+trait Trait {}
+
+impl Trait for u32 {}
+
+fn hello() -> Box<impl Trait + ?Sized> {
+    if true {
+        let x = hello();
+        let y: Box<dyn Trait> = x;
+        //[old]~^ ERROR: the size for values of type `impl Trait + ?Sized` cannot be know
+    }
+    Box::new(1u32)
+}
+
+fn main() {}

tests/ui/impl-trait/unsized_coercion3.next.stderr

@@ -0,0 +1,38 @@
+error[E0271]: type mismatch resolving `impl Trait + ?Sized <: dyn Send`
+  --> $DIR/unsized_coercion3.rs:13:17
+   |
+LL |         let x = hello();
+   |                 ^^^^^^^ types differ
+
+error[E0308]: mismatched types
+  --> $DIR/unsized_coercion3.rs:19:14
+   |
+LL | fn hello() -> Box<impl Trait + ?Sized> {
+   |                   ------------------- the expected opaque type
+...
+LL |     Box::new(1u32)
+   |     -------- ^^^^ types differ
+   |     |
+   |     arguments to this function are incorrect
+   |
+   = note: expected opaque type `impl Trait + ?Sized`
+                     found type `u32`
+note: associated function defined here
+  --> $SRC_DIR/alloc/src/boxed.rs:LL:COL
+
+error[E0277]: the size for values of type `impl Trait + ?Sized` cannot be known at compilation time
+  --> $DIR/unsized_coercion3.rs:19:14
+   |
+LL |     Box::new(1u32)
+   |     -------- ^^^^ doesn't have a size known at compile-time
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Sized` is not implemented for `impl Trait + ?Sized`
+note: required by a bound in `Box::<T>::new`
+  --> $SRC_DIR/alloc/src/boxed.rs:LL:COL
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0271, E0277, E0308.
+For more information about an error, try `rustc --explain E0271`.

tests/ui/impl-trait/unsized_coercion3.old.stderr

@@ -0,0 +1,26 @@
+error: cannot check whether the hidden type of opaque type satisfies auto traits
+  --> $DIR/unsized_coercion3.rs:15:32
+   |
+LL |         let y: Box<dyn Send> = x;
+   |                                ^
+   |
+   = note: fetching the hidden types of an opaque inside of the defining scope is not supported. You can try moving the opaque type and the item that actually registers a hidden type into a new submodule
+note: opaque type is declared here
+  --> $DIR/unsized_coercion3.rs:11:19
+   |
+LL | fn hello() -> Box<impl Trait + ?Sized> {
+   |                   ^^^^^^^^^^^^^^^^^^^
+   = note: required for the cast from `Box<impl Trait + ?Sized>` to `Box<dyn Send>`
+
+error[E0277]: the size for values of type `impl Trait + ?Sized` cannot be known at compilation time
+  --> $DIR/unsized_coercion3.rs:15:32
+   |
+LL |         let y: Box<dyn Send> = x;
+   |                                ^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `impl Trait + ?Sized`
+   = note: required for the cast from `Box<impl Trait + ?Sized>` to `Box<dyn Send>`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/impl-trait/unsized_coercion3.rs

@@ -0,0 +1,24 @@
+//! This test checks that opaque types get unsized instead of
+//! constraining their hidden type to a trait object.
+
+//@ revisions: next old
+//@[next] compile-flags: -Znext-solver
+
+trait Trait {}
+
+impl Trait for u32 {}
+
+fn hello() -> Box<impl Trait + ?Sized> {
+    if true {
+        let x = hello();
+        //[next]~^ ERROR: type mismatch resolving `impl Trait + ?Sized <: dyn Send`
+        let y: Box<dyn Send> = x;
+        //[old]~^ ERROR: the size for values of type `impl Trait + ?Sized` cannot be know
+        //[old]~| ERROR: cannot check whether the hidden type of opaque type satisfies auto traits
+    }
+    Box::new(1u32)
+    //[next]~^ ERROR: mismatched types
+    //[next]~| ERROR: the size for values of type `impl Trait + ?Sized` cannot be know
+}
+
+fn main() {}

tests/ui/impl-trait/unsized_coercion4.rs

@@ -0,0 +1,20 @@
+//! This test checks that opaque types get unsized instead of
+//! constraining their hidden type to a trait object.
+
+//@ revisions: next old
+//@[next] compile-flags: -Znext-solver
+//@check-pass
+
+trait Trait {}
+
+impl Trait for u32 {}
+
+fn hello() -> Box<impl Trait + ?Sized> {
+    if true {
+        let x = hello() as Box<u32>;
+        let y: Box<dyn Send> = x;
+    }
+    Box::new(1u32)
+}
+
+fn main() {}