From e798fdf7befc40255ce46caa37a0e6c1e8756b6c Mon Sep 17 00:00:00 2001
From: Ezra Shaw <ezrasure@outlook.com>
Date: Tue, 20 Dec 2022 18:31:15 +1300
Subject: [PATCH] docs/test: add UI test and long-form error docs for `E0377`

---
 compiler/rustc_error_codes/src/error_codes.rs |  3 +-
 .../src/error_codes/E0377.md                  | 29 +++++++++++++++++++
 src/test/ui/error-codes/E0377.rs              | 14 +++++++++
 src/test/ui/error-codes/E0377.stderr          |  9 ++++++
 src/tools/tidy/src/error_codes_check.rs       |  4 +--
 5 files changed, 55 insertions(+), 4 deletions(-)
 create mode 100644 compiler/rustc_error_codes/src/error_codes/E0377.md
 create mode 100644 src/test/ui/error-codes/E0377.rs
 create mode 100644 src/test/ui/error-codes/E0377.stderr

diff --git a/compiler/rustc_error_codes/src/error_codes.rs b/compiler/rustc_error_codes/src/error_codes.rs
index 883a4bbe8e8..4e149fc2b99 100644
--- a/compiler/rustc_error_codes/src/error_codes.rs
+++ b/compiler/rustc_error_codes/src/error_codes.rs
@@ -184,6 +184,7 @@
 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 @@
 //  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
diff --git a/compiler/rustc_error_codes/src/error_codes/E0377.md b/compiler/rustc_error_codes/src/error_codes/E0377.md
new file mode 100644
index 00000000000..b1d36406332
--- /dev/null
+++ b/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).
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 @@
 
 // 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...