Rollup merge of #95979 - lcnr:coherence-docs, r=compiler-errors

update coherence docs, fix generator + opaque type ICE

the world is confusing, this makes it slightly less so

4 files changed, 82 insertions, 0 deletions

diff --git a/src/test/ui/coherence/coherence-with-closure.rs b/src/test/ui/coherence/coherence-with-closure.rs
new file mode 100644
index 00000000000..6e3281d8508
--- /dev/null
+++ b/src/test/ui/coherence/coherence-with-closure.rs
@@ -0,0 +1,15 @@
+// Test that encountering closures during coherence does not cause issues.
+#![feature(type_alias_impl_trait)]
+type OpaqueClosure = impl Sized;
+fn defining_use() -> OpaqueClosure {
+    || ()
+}
+
+struct Wrapper<T>(T);
+trait Trait {}
+impl Trait for Wrapper<OpaqueClosure> {}
+//~^ ERROR cannot implement trait on type alias impl trait
+impl<T: Sync> Trait for Wrapper<T> {}
+//~^ ERROR conflicting implementations of trait `Trait` for type `Wrapper<OpaqueClosure>`
+
+fn main() {}
diff --git a/src/test/ui/coherence/coherence-with-closure.stderr b/src/test/ui/coherence/coherence-with-closure.stderr
new file mode 100644
index 00000000000..20b986cee69
--- /dev/null
+++ b/src/test/ui/coherence/coherence-with-closure.stderr
@@ -0,0 +1,24 @@
+error: cannot implement trait on type alias impl trait
+  --> $DIR/coherence-with-closure.rs:10:24
+   |
+LL | impl Trait for Wrapper<OpaqueClosure> {}
+   |                        ^^^^^^^^^^^^^
+   |
+note: type alias impl trait defined here
+  --> $DIR/coherence-with-closure.rs:3:22
+   |
+LL | type OpaqueClosure = impl Sized;
+   |                      ^^^^^^^^^^
+
+error[E0119]: conflicting implementations of trait `Trait` for type `Wrapper<OpaqueClosure>`
+  --> $DIR/coherence-with-closure.rs:12:1
+   |
+LL | impl Trait for Wrapper<OpaqueClosure> {}
+   | ------------------------------------- first implementation here
+LL |
+LL | impl<T: Sync> Trait for Wrapper<T> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Wrapper<OpaqueClosure>`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/coherence/coherence-with-generator.rs b/src/test/ui/coherence/coherence-with-generator.rs
new file mode 100644
index 00000000000..d34c391db9f
--- /dev/null
+++ b/src/test/ui/coherence/coherence-with-generator.rs
@@ -0,0 +1,19 @@
+// Test that encountering closures during coherence does not cause issues.
+#![feature(type_alias_impl_trait, generators)]
+type OpaqueGenerator = impl Sized;
+fn defining_use() -> OpaqueGenerator {
+    || {
+        for i in 0..10 {
+            yield i;
+        }
+    }
+}
+
+struct Wrapper<T>(T);
+trait Trait {}
+impl Trait for Wrapper<OpaqueGenerator> {}
+//~^ ERROR cannot implement trait on type alias impl trait
+impl<T: Sync> Trait for Wrapper<T> {}
+//~^ ERROR conflicting implementations of trait `Trait` for type `Wrapper<OpaqueGenerator>`
+
+fn main() {}
diff --git a/src/test/ui/coherence/coherence-with-generator.stderr b/src/test/ui/coherence/coherence-with-generator.stderr
new file mode 100644
index 00000000000..249ad3cb9ec
--- /dev/null
+++ b/src/test/ui/coherence/coherence-with-generator.stderr
@@ -0,0 +1,24 @@
+error: cannot implement trait on type alias impl trait
+  --> $DIR/coherence-with-generator.rs:14:24
+   |
+LL | impl Trait for Wrapper<OpaqueGenerator> {}
+   |                        ^^^^^^^^^^^^^^^
+   |
+note: type alias impl trait defined here
+  --> $DIR/coherence-with-generator.rs:3:24
+   |
+LL | type OpaqueGenerator = impl Sized;
+   |                        ^^^^^^^^^^
+
+error[E0119]: conflicting implementations of trait `Trait` for type `Wrapper<OpaqueGenerator>`
+  --> $DIR/coherence-with-generator.rs:16:1
+   |
+LL | impl Trait for Wrapper<OpaqueGenerator> {}
+   | --------------------------------------- first implementation here
+LL |
+LL | impl<T: Sync> Trait for Wrapper<T> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Wrapper<OpaqueGenerator>`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0119`.