docs/test: add UI test and long-form error docs for E0377

compiler/rustc_error_codes/src/error_codes/E0377.md

@@ -0,0 +1,29 @@
+The trait `CoerceUnsized` may only be implemented for a coercion between
+structures with the same definition.
+
+Example of erroneous code:
+
+```compile_fail,E0377
+#![feature(coerce_unsized)]
+use std::ops::CoerceUnsized;
+
+pub struct Foo<T: ?Sized> {
+    field_with_unsized_type: T,
+}
+
+pub struct Bar<T: ?Sized> {
+    field_with_unsized_type: T,
+}
+
+// error: the trait `CoerceUnsized` may only be implemented for a coercion
+//        between structures with the same definition
+impl<T, U> CoerceUnsized<Bar<U>> for Foo<T> where T: CoerceUnsized<U> {}
+```
+
+When attempting to implement `CoerceUnsized`, the `impl` signature must look
+like: `impl CoerceUnsized<Type<U>> for Type<T> where T: CoerceUnsized<U>`;
+the *implementer* and *`CoerceUnsized` type parameter* must be the same
+type. In this example, `Bar` and `Foo` (even though structurally identical)
+are *not* the same type and are rejected. Learn more about the `CoerceUnsized`
+trait and DST coercion in
+[the `CoerceUnsized` docs](../std/ops/trait.CoerceUnsized.html).