Add some tests

tests/ui/impl-trait/rpit/early_bound.rs

@@ -0,0 +1,13 @@
+use std::convert::identity;
+
+fn test<'a: 'a>(n: bool) -> impl Sized + 'a {
+    //~^ ERROR concrete type differs from previous defining opaque type use
+    let true = n else { loop {} };
+    let _ = || {
+        let _ = identity::<&'a ()>(test(false));
+        //~^ ERROR hidden type for `impl Sized + 'a` captures lifetime that does not appear in bounds
+    };
+    loop {}
+}
+
+fn main() {}

tests/ui/impl-trait/rpit/early_bound.stderr

@@ -0,0 +1,26 @@
+error[E0700]: hidden type for `impl Sized + 'a` captures lifetime that does not appear in bounds
+  --> $DIR/early_bound.rs:7:17
+   |
+LL | fn test<'a: 'a>(n: bool) -> impl Sized + 'a {
+   |         --                  --------------- opaque type defined here
+   |         |
+   |         hidden type `&'a ()` captures the lifetime `'a` as defined here
+...
+LL |         let _ = identity::<&'a ()>(test(false));
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: concrete type differs from previous defining opaque type use
+  --> $DIR/early_bound.rs:3:29
+   |
+LL | fn test<'a: 'a>(n: bool) -> impl Sized + 'a {
+   |                             ^^^^^^^^^^^^^^^ expected `&()`, got `()`
+   |
+note: previous use here
+  --> $DIR/early_bound.rs:7:36
+   |
+LL |         let _ = identity::<&'a ()>(test(false));
+   |                                    ^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0700`.

tests/ui/type-alias-impl-trait/impl_trait_in_trait_defined_outside_trait.rs

@@ -0,0 +1,38 @@
+//! Check that we cannot instantiate a hidden type in the body
+//! of an assoc fn or const unless mentioned in the signature.
+
+#![feature(impl_trait_in_assoc_type)]
+
+trait Trait: Sized {
+    type Assoc;
+    fn foo();
+    fn bar() -> Self::Assoc;
+}
+
+impl Trait for () {
+    type Assoc = impl std::fmt::Debug;
+    fn foo() {
+        let x: Self::Assoc = 42; //~ ERROR: mismatched types
+    }
+    fn bar() -> Self::Assoc {
+        ""
+    }
+}
+
+trait Trait2: Sized {
+    type Assoc;
+    const FOO: ();
+    fn bar() -> Self::Assoc;
+}
+
+impl Trait2 for () {
+    type Assoc = impl std::fmt::Debug;
+    const FOO: () = {
+        let x: Self::Assoc = 42; //~ ERROR: mismatched types
+    };
+    fn bar() -> Self::Assoc {
+        ""
+    }
+}
+
+fn main() {}

tests/ui/type-alias-impl-trait/impl_trait_in_trait_defined_outside_trait.stderr

@@ -0,0 +1,41 @@
+error[E0308]: mismatched types
+  --> $DIR/impl_trait_in_trait_defined_outside_trait.rs:15:30
+   |
+LL |     type Assoc = impl std::fmt::Debug;
+   |                  -------------------- the expected opaque type
+LL |     fn foo() {
+LL |         let x: Self::Assoc = 42;
+   |                -----------   ^^ expected opaque type, found integer
+   |                |
+   |                expected due to this
+   |
+   = note: expected opaque type `<() as Trait>::Assoc`
+                     found type `{integer}`
+note: this item must have the opaque type in its signature in order to be able to register hidden types
+  --> $DIR/impl_trait_in_trait_defined_outside_trait.rs:14:8
+   |
+LL |     fn foo() {
+   |        ^^^
+
+error[E0308]: mismatched types
+  --> $DIR/impl_trait_in_trait_defined_outside_trait.rs:31:30
+   |
+LL |     type Assoc = impl std::fmt::Debug;
+   |                  -------------------- the expected opaque type
+LL |     const FOO: () = {
+LL |         let x: Self::Assoc = 42;
+   |                -----------   ^^ expected opaque type, found integer
+   |                |
+   |                expected due to this
+   |
+   = note: expected opaque type `<() as Trait2>::Assoc`
+                     found type `{integer}`
+note: this item must have the opaque type in its signature in order to be able to register hidden types
+  --> $DIR/impl_trait_in_trait_defined_outside_trait.rs:30:11
+   |
+LL |     const FOO: () = {
+   |           ^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/type-alias-impl-trait/impl_trait_in_trait_defined_outside_trait2.rs

@@ -0,0 +1,22 @@
+//! Check that we cannot instantiate a hidden type from another assoc type.
+
+#![feature(impl_trait_in_assoc_type)]
+
+trait Trait: Sized {
+    type Assoc;
+    type Foo;
+    fn foo() -> Self::Assoc;
+}
+
+impl Trait for () {
+    type Assoc = impl std::fmt::Debug;
+    type Foo = [(); {
+        let x: Self::Assoc = 42; //~ ERROR: mismatched types
+        3
+    }];
+    fn foo() -> Self::Assoc {
+        ""
+    }
+}
+
+fn main() {}

tests/ui/type-alias-impl-trait/impl_trait_in_trait_defined_outside_trait2.stderr

@@ -0,0 +1,17 @@
+error[E0308]: mismatched types
+  --> $DIR/impl_trait_in_trait_defined_outside_trait2.rs:14:30
+   |
+LL |     type Assoc = impl std::fmt::Debug;
+   |                  -------------------- the expected opaque type
+LL |     type Foo = [(); {
+LL |         let x: Self::Assoc = 42;
+   |                -----------   ^^ expected opaque type, found integer
+   |                |
+   |                expected due to this
+   |
+   = note: expected opaque type `<() as Trait>::Assoc`
+                     found type `{integer}`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/type-alias-impl-trait/impl_trait_in_trait_defined_outside_trait3.rs

@@ -0,0 +1,38 @@
+//! Check that non-defining assoc items can use the opaque type
+//! opaquely.
+
+// check-pass
+
+#![feature(impl_trait_in_assoc_type)]
+
+trait Trait: Sized {
+    type Assoc;
+    fn foo();
+    fn bar() -> Self::Assoc;
+}
+
+impl Trait for () {
+    type Assoc = impl std::fmt::Debug;
+    fn foo() {
+        let x: Self::Assoc = Self::bar();
+    }
+    fn bar() -> Self::Assoc {
+        ""
+    }
+}
+
+trait Trait2: Sized {
+    type Assoc;
+    const FOO: ();
+    const BAR: Self::Assoc;
+}
+
+impl Trait2 for () {
+    type Assoc = impl Copy;
+    const FOO: () = {
+        let x: Self::Assoc = Self::BAR;
+    };
+    const BAR: Self::Assoc = "";
+}
+
+fn main() {}

tests/ui/type-alias-impl-trait/in-assoc-ty-early-bound.rs

@@ -0,0 +1,17 @@
+#![feature(impl_trait_in_assoc_type)]
+
+trait Foo {
+    type Assoc<'a, 'b>;
+    fn bar<'a: 'a, 'b: 'b>(_: &'a ()) -> Self::Assoc<'a, 'b>;
+}
+
+impl Foo for () {
+    type Assoc<'a, 'b> = impl Sized;
+    fn bar<'a: 'a, 'b: 'b>(x: &'a ()) -> Self::Assoc<'a, 'b> {
+        let closure = |x: &'a ()| -> Self::Assoc<'b, 'a> { x };
+        //~^ ERROR `<() as Foo>::Assoc<'b, 'a>` captures lifetime that does not appear in bounds
+        x
+    }
+}
+
+fn main() {}

tests/ui/type-alias-impl-trait/in-assoc-ty-early-bound.stderr

@@ -0,0 +1,13 @@
+error[E0700]: hidden type for `<() as Foo>::Assoc<'b, 'a>` captures lifetime that does not appear in bounds
+  --> $DIR/in-assoc-ty-early-bound.rs:11:60
+   |
+LL |     type Assoc<'a, 'b> = impl Sized;
+   |                          ---------- opaque type defined here
+LL |     fn bar<'a: 'a, 'b: 'b>(x: &'a ()) -> Self::Assoc<'a, 'b> {
+   |            -- hidden type `&'a ()` captures the lifetime `'a` as defined here
+LL |         let closure = |x: &'a ()| -> Self::Assoc<'b, 'a> { x };
+   |                                                            ^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0700`.

tests/ui/type-alias-impl-trait/in-assoc-ty-early-bound2.rs

@@ -0,0 +1,21 @@
+#![feature(impl_trait_in_assoc_type)]
+
+trait Foo {
+    type Assoc<'a>;
+    fn bar<'a: 'a>();
+}
+
+impl Foo for () {
+    type Assoc<'a> = impl Sized; //~ ERROR unconstrained opaque type
+    fn bar<'a: 'a>()
+    where
+        Self::Assoc<'a>:,
+    {
+        let _ = |x: &'a ()| {
+            let _: Self::Assoc<'a> = x;
+            //~^ ERROR `<() as Foo>::Assoc<'a>` captures lifetime that does not appear in bound
+        };
+    }
+}
+
+fn main() {}

tests/ui/type-alias-impl-trait/in-assoc-ty-early-bound2.stderr

@@ -0,0 +1,22 @@
+error[E0700]: hidden type for `<() as Foo>::Assoc<'a>` captures lifetime that does not appear in bounds
+  --> $DIR/in-assoc-ty-early-bound2.rs:15:20
+   |
+LL |     type Assoc<'a> = impl Sized;
+   |                      ---------- opaque type defined here
+LL |     fn bar<'a: 'a>()
+   |            -- hidden type `&'a ()` captures the lifetime `'a` as defined here
+...
+LL |             let _: Self::Assoc<'a> = x;
+   |                    ^^^^^^^^^^^^^^^
+
+error: unconstrained opaque type
+  --> $DIR/in-assoc-ty-early-bound2.rs:9:22
+   |
+LL |     type Assoc<'a> = impl Sized;
+   |                      ^^^^^^^^^^
+   |
+   = note: `Assoc` must be used in combination with a concrete type within the same impl
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0700`.