Add some tests around where bounds on associated items and their lack of effect on impls

tests/ui/traits/auxiliary/trivial3.rs

@@ -0,0 +1 @@
+pub trait Trait {}

tests/ui/traits/auxiliary/trivial4.rs

@@ -0,0 +1,3 @@
+pub trait Trait {}
+
+impl Trait for () {}

tests/ui/traits/trivial_impl.rs

@@ -0,0 +1,18 @@
+//! This test checks that we do need to implement
+//! all members, even if their where bounds only hold
+//! due to other impls.
+
+trait Foo<T> {
+    fn foo()
+    where
+        Self: Foo<()>;
+}
+
+impl Foo<()> for () {
+    fn foo() {}
+}
+
+impl Foo<u32> for () {}
+//~^ ERROR: not all trait items implemented, missing: `foo`
+
+fn main() {}

tests/ui/traits/trivial_impl.stderr

@@ -0,0 +1,14 @@
+error[E0046]: not all trait items implemented, missing: `foo`
+  --> $DIR/trivial_impl.rs:15:1
+   |
+LL | /     fn foo()
+LL | |     where
+LL | |         Self: Foo<()>;
+   | |______________________- `foo` from trait
+...
+LL |   impl Foo<u32> for () {}
+   |   ^^^^^^^^^^^^^^^^^^^^ missing `foo` in implementation
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0046`.

tests/ui/traits/trivial_impl2.rs

@@ -0,0 +1,13 @@
+//! This test checks that we currently need to implement
+//! members, even if their where bounds don't hold for the impl type.
+
+trait Foo<T> {
+    fn foo()
+    where
+        Self: Foo<()>;
+}
+
+impl Foo<u32> for () {}
+//~^ ERROR: not all trait items implemented, missing: `foo`
+
+fn main() {}

tests/ui/traits/trivial_impl2.stderr

@@ -0,0 +1,14 @@
+error[E0046]: not all trait items implemented, missing: `foo`
+  --> $DIR/trivial_impl2.rs:10:1
+   |
+LL | /     fn foo()
+LL | |     where
+LL | |         Self: Foo<()>;
+   | |______________________- `foo` from trait
+...
+LL |   impl Foo<u32> for () {}
+   |   ^^^^^^^^^^^^^^^^^^^^ missing `foo` in implementation
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0046`.

tests/ui/traits/trivial_impl3.rs

@@ -0,0 +1,19 @@
+//! Check that we don't break orphan rules.
+//! The dependency may add an impl for `u8` later,
+//! which would break this crate. We want to avoid adding
+//! more ways in which adding an impl can be a breaking change.
+
+// aux-build:trivial3.rs
+
+extern crate trivial3;
+
+pub trait Foo {
+    fn foo()
+    where
+        Self: trivial3::Trait;
+}
+
+impl Foo for u8 {}
+//~^ ERROR not all trait items implemented, missing: `foo`
+
+fn main() {}

tests/ui/traits/trivial_impl3.stderr

@@ -0,0 +1,14 @@
+error[E0046]: not all trait items implemented, missing: `foo`
+  --> $DIR/trivial_impl3.rs:16:1
+   |
+LL | /     fn foo()
+LL | |     where
+LL | |         Self: trivial3::Trait;
+   | |______________________________- `foo` from trait
+...
+LL |   impl Foo for u8 {}
+   |   ^^^^^^^^^^^^^^^ missing `foo` in implementation
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0046`.

tests/ui/traits/trivial_impl4.rs

@@ -0,0 +1,21 @@
+//! Check that we don't break orphan rules.
+//! The dependency may add an impl for `u8` later,
+//! which would break this crate. We want to avoid adding
+//! more ways in which adding an impl can be a breaking change.
+//! This test differs from `trivial_impl3` because there actually
+//! exists any impl for `Trait`, which has an effect on coherence.
+
+// aux-build:trivial4.rs
+
+extern crate trivial4;
+
+pub trait Foo {
+    fn foo()
+    where
+        Self: trivial4::Trait;
+}
+
+impl Foo for u8 {}
+//~^ ERROR not all trait items implemented, missing: `foo`
+
+fn main() {}

tests/ui/traits/trivial_impl4.stderr

@@ -0,0 +1,14 @@
+error[E0046]: not all trait items implemented, missing: `foo`
+  --> $DIR/trivial_impl4.rs:18:1
+   |
+LL | /     fn foo()
+LL | |     where
+LL | |         Self: trivial4::Trait;
+   | |______________________________- `foo` from trait
+...
+LL |   impl Foo for u8 {}
+   |   ^^^^^^^^^^^^^^^ missing `foo` in implementation
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0046`.

tests/ui/traits/trivial_impl_sized.rs

@@ -0,0 +1,26 @@
+//! This test checks that we currently need to implement
+//! members, even if their where bounds don't hold for the impl type.
+
+trait Foo {
+    fn foo()
+    where
+        Self: Sized;
+}
+
+impl Foo for () {
+    fn foo() {}
+}
+
+// Must not be allowed
+impl Foo for i32 {}
+//~^ ERROR: not all trait items implemented, missing: `foo`
+
+// Should be allowed
+impl Foo for dyn std::fmt::Debug {}
+//~^ ERROR: not all trait items implemented, missing: `foo`
+
+impl Foo for dyn std::fmt::Display {
+    fn foo() {}
+}
+
+fn main() {}

tests/ui/traits/trivial_impl_sized.stderr

@@ -0,0 +1,25 @@
+error[E0046]: not all trait items implemented, missing: `foo`
+  --> $DIR/trivial_impl_sized.rs:15:1
+   |
+LL | /     fn foo()
+LL | |     where
+LL | |         Self: Sized;
+   | |____________________- `foo` from trait
+...
+LL |   impl Foo for i32 {}
+   |   ^^^^^^^^^^^^^^^^ missing `foo` in implementation
+
+error[E0046]: not all trait items implemented, missing: `foo`
+  --> $DIR/trivial_impl_sized.rs:19:1
+   |
+LL | /     fn foo()
+LL | |     where
+LL | |         Self: Sized;
+   | |____________________- `foo` from trait
+...
+LL |   impl Foo for dyn std::fmt::Debug {}
+   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `foo` in implementation
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0046`.