Check projections are well-formed when using projection candidates

src/test/ui/generic-associated-types/projection-bound-cycle-generic.rs

@@ -0,0 +1,62 @@
+// Like `projection-bound-cycle.rs` but this avoids using
+// `feature(trivial_bounds)`.
+
+#![feature(generic_associated_types)]
+//~^ WARNING the feature `generic_associated_types` is incomplete
+
+trait Print {
+    fn print();
+}
+
+trait Foo {
+    type Item: Sized where <Self as Foo>::Item: Sized;
+}
+
+struct Number<T> { t: T }
+
+impl<T> Foo for Number<T> {
+    // Well-formedness checks require that the following
+    // goal is true:
+    // ```
+    // if ([T]: Sized) { # if the where clauses hold
+    //     [T]: Sized # then the bound on the associated type hold
+    // }
+    // ```
+    // which it is :)
+    type Item where [T]: Sized = [T];
+}
+
+struct OnlySized<T> where T: Sized { f: T }
+impl<T> Print for OnlySized<T> {
+    fn print() {
+        println!("{}", std::mem::size_of::<T>());
+    }
+}
+
+trait Bar {
+    type Assoc: Print;
+}
+
+impl<T> Bar for T where T: Foo {
+    // This is not ok, we need to prove `wf(<T as Foo>::Item)`, which requires
+    // knowing that `<T as Foo>::Item: Sized` to satisfy the where clause. We
+    // can use the bound on `Foo::Item` for this, but that requires
+    // `wf(<T as Foo>::Item)`, which is an invalid cycle.
+    type Assoc = OnlySized<<T as Foo>::Item>;
+    //~^ ERROR overflow evaluating the requirement `<T as Foo>::Item: std::marker::Sized`
+}
+
+fn foo<T: Print>() {
+    T::print() // oops, in fact `T = OnlySized<str>` which is ill-formed
+}
+
+fn bar<T: Bar>() {
+    // we have `FromEnv(T: Bar)` hence
+    // `<T as Bar>::Assoc` is well-formed and
+    // `Implemented(<T as Bar>::Assoc: Print)` hold
+    foo::<<T as Bar>::Assoc>()
+}
+
+fn main() {
+    bar::<Number<u8>>()
+}

src/test/ui/generic-associated-types/projection-bound-cycle-generic.stderr

@@ -0,0 +1,21 @@
+warning: the feature `generic_associated_types` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/projection-bound-cycle-generic.rs:4:12
+   |
+LL | #![feature(generic_associated_types)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #44265 <https://github.com/rust-lang/rust/issues/44265> for more information
+
+error[E0275]: overflow evaluating the requirement `<T as Foo>::Item: std::marker::Sized`
+  --> $DIR/projection-bound-cycle-generic.rs:45:5
+   |
+LL | struct OnlySized<T> where T: Sized { f: T }
+   |                  - required by this bound in `OnlySized`
+...
+LL |     type Assoc = OnlySized<<T as Foo>::Item>;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0275`.

src/test/ui/generic-associated-types/projection-bound-cycle.rs

@@ -0,0 +1,64 @@
+// Test case from Chalk.
+// Make sure that we make sure that we don't allow arbitrary bounds to be
+// proven when a bound and a where clause of an associated type are the same.
+
+#![feature(generic_associated_types)]
+//~^ WARNING the feature `generic_associated_types` is incomplete
+#![feature(trivial_bounds)]
+
+trait Print {
+    fn print();
+}
+
+trait Foo {
+    type Item: Sized where <Self as Foo>::Item: Sized;
+}
+
+struct Number { }
+
+impl Foo for Number {
+    // Well-formedness checks require that the following
+    // goal is true:
+    // ```
+    // if (str: Sized) { # if the where clauses hold
+    //     str: Sized # then the bound on the associated type hold
+    // }
+    // ```
+    // which it is :)
+    type Item where str: Sized = str;
+}
+
+struct OnlySized<T> where T: Sized { f: T }
+impl<T> Print for OnlySized<T> {
+    fn print() {
+        println!("{}", std::mem::size_of::<T>());
+    }
+}
+
+trait Bar {
+    type Assoc: Print;
+}
+
+impl<T> Bar for T where T: Foo {
+    // This is not ok, we need to prove `wf(<T as Foo>::Item)`, which requires
+    // knowing that `<T as Foo>::Item: Sized` to satisfy the where clause. We
+    // can use the bound on `Foo::Item` for this, but that requires
+    // `wf(<T as Foo>::Item)`, which is an invalid cycle.
+    type Assoc = OnlySized<<T as Foo>::Item>;
+    //~^ ERROR overflow evaluating the requirement `<T as Foo>::Item: std::marker::Sized`
+}
+
+fn foo<T: Print>() {
+    T::print() // oops, in fact `T = OnlySized<str>` which is ill-formed
+}
+
+fn bar<T: Bar>() {
+    // we have `FromEnv(T: Bar)` hence
+    // `<T as Bar>::Assoc` is well-formed and
+    // `Implemented(<T as Bar>::Assoc: Print)` hold
+    foo::<<T as Bar>::Assoc>()
+}
+
+fn main() {
+    bar::<Number>()
+}

src/test/ui/generic-associated-types/projection-bound-cycle.stderr

@@ -0,0 +1,21 @@
+warning: the feature `generic_associated_types` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/projection-bound-cycle.rs:5:12
+   |
+LL | #![feature(generic_associated_types)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #44265 <https://github.com/rust-lang/rust/issues/44265> for more information
+
+error[E0275]: overflow evaluating the requirement `<T as Foo>::Item: std::marker::Sized`
+  --> $DIR/projection-bound-cycle.rs:47:5
+   |
+LL | struct OnlySized<T> where T: Sized { f: T }
+   |                  - required by this bound in `OnlySized`
+...
+LL |     type Assoc = OnlySized<<T as Foo>::Item>;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0275`.

src/test/ui/specialization/issue-38091-2.rs

@@ -0,0 +1,23 @@
+#![feature(specialization)]
+//~^ WARN the feature `specialization` is incomplete
+
+trait Iterate<'a> {
+    type Ty: Valid;
+    fn iterate(self);
+}
+impl<'a, T> Iterate<'a> for T where T: Check {
+    default type Ty = ();
+    default fn iterate(self) {}
+}
+
+trait Check {}
+impl<'a, T> Check for T where <T as Iterate<'a>>::Ty: Valid {}
+
+trait Valid {}
+
+impl Valid for () {}
+
+fn main() {
+    Iterate::iterate(0);
+    //~^ ERROR overflow evaluating the requirement `{integer}: Check`
+}

src/test/ui/specialization/issue-38091-2.stderr

@@ -0,0 +1,23 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/issue-38091-2.rs:1:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+error[E0275]: overflow evaluating the requirement `{integer}: Check`
+  --> $DIR/issue-38091-2.rs:21:5
+   |
+LL |     fn iterate(self);
+   |     ----------------- required by `Iterate::iterate`
+...
+LL |     Iterate::iterate(0);
+   |     ^^^^^^^^^^^^^^^^
+   |
+   = note: required because of the requirements on the impl of `Iterate<'_>` for `{integer}`
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0275`.

src/test/ui/specialization/issue-38091.rs

@@ -18,4 +18,5 @@ trait Valid {}
 
 fn main() {
     Iterate::iterate(0);
+    //~^ ERROR overflow evaluating the requirement `{integer}: Check`
 }

src/test/ui/specialization/issue-38091.stderr

@@ -16,6 +16,18 @@ LL |     type Ty: Valid;
 LL |     default type Ty = ();
    |     ^^^^^^^^^^^^^^^^^^^^^ the trait `Valid` is not implemented for `()`
 
-error: aborting due to previous error; 1 warning emitted
+error[E0275]: overflow evaluating the requirement `{integer}: Check`
+  --> $DIR/issue-38091.rs:20:5
+   |
+LL |     fn iterate(self);
+   |     ----------------- required by `Iterate::iterate`
+...
+LL |     Iterate::iterate(0);
+   |     ^^^^^^^^^^^^^^^^
+   |
+   = note: required because of the requirements on the impl of `Iterate<'_>` for `{integer}`
 
-For more information about this error, try `rustc --explain E0277`.
+error: aborting due to 2 previous errors; 1 warning emitted
+
+Some errors have detailed explanations: E0275, E0277.
+For more information about an error, try `rustc --explain E0275`.