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

3 files changed, 25 insertions, 2 deletions

diff --git a/src/test/ui/error-codes/E0377.rs b/src/test/ui/error-codes/E0377.rs
new file mode 100644
index 00000000000..6da2c20956a
--- /dev/null
+++ b/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() {}
diff --git a/src/test/ui/error-codes/E0377.stderr b/src/test/ui/error-codes/E0377.stderr
new file mode 100644
index 00000000000..bf7d8c8d39d
--- /dev/null
+++ b/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`.
diff --git a/src/tools/tidy/src/error_codes_check.rs b/src/tools/tidy/src/error_codes_check.rs
index 610e322e129..1b119e4113e 100644
--- a/src/tools/tidy/src/error_codes_check.rs
+++ b/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",
-    "E0554", "E0640", "E0717", "E0729", "E0789",
+    "E0313", "E0461", "E0462", "E0465", "E0476", "E0490", "E0514", "E0519", "E0523", "E0554",
+    "E0640", "E0717", "E0729", "E0789",
 ];
 
 // Some error codes don't have any tests apparently...