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

compiler/rustc_error_codes/src/error_codes.rs

@@ -184,6 +184,7 @@ E0373: include_str!("./error_codes/E0373.md"),
 E0374: include_str!("./error_codes/E0374.md"),
 E0375: include_str!("./error_codes/E0375.md"),
 E0376: include_str!("./error_codes/E0376.md"),
+E0377: include_str!("./error_codes/E0377.md"),
 E0378: include_str!("./error_codes/E0378.md"),
 E0379: include_str!("./error_codes/E0379.md"),
 E0380: include_str!("./error_codes/E0380.md"),
@@ -579,8 +580,6 @@ E0791: include_str!("./error_codes/E0791.md"),
 //  E0315, // cannot invoke closure outside of its lifetime
 //  E0319, // trait impls for defaulted traits allowed just for structs/enums
 //  E0372, // coherence not object safe
-    E0377, // the trait `CoerceUnsized` may only be implemented for a coercion
-           // between structures with the same definition
 //  E0385, // {} in an aliasable location
 //  E0402, // cannot use an outer type parameter in this context
 //  E0406, // merged into 420

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).

src/test/ui/error-codes/E0377.rs

@@ -0,0 +1,14 @@
+#![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,
+}
+
+impl<T, U> CoerceUnsized<Bar<U>> for Foo<T> where T: CoerceUnsized<U> {} //~ ERROR E0377
+
+fn main() {}

src/test/ui/error-codes/E0377.stderr

@@ -0,0 +1,9 @@
+error[E0377]: the trait `CoerceUnsized` may only be implemented for a coercion between structures with the same definition; expected `Foo`, found `Bar`
+  --> $DIR/E0377.rs:12:1
+   |
+LL | impl<T, U> CoerceUnsized<Bar<U>> for Foo<T> where T: CoerceUnsized<U> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0377`.

src/tools/tidy/src/error_codes_check.rs

@@ -11,8 +11,8 @@ use regex::Regex;
 
 // A few of those error codes can't be tested but all the others can and *should* be tested!
 const EXEMPTED_FROM_TEST: &[&str] = &[
-    "E0313", "E0377", "E0461", "E0462", "E0465", "E0476", "E0490", "E0514", "E0519", "E0523",
+    "E0313", "E0461", "E0462", "E0465", "E0476", "E0490", "E0514", "E0519", "E0523", "E0554",
-    "E0554", "E0640", "E0717", "E0729", "E0789",
+    "E0640", "E0717", "E0729", "E0789",
 ];
 
 // Some error codes don't have any tests apparently...