Document assoc-lang-items.rs

Author: jieyouxu

tests/ui/assoc-lang-items.rs

@@ -1,3 +1,17 @@
+//! Check that associated items can be marked as lang items, so that they don't have to be looked up
+//! by name or by definition order indirectly.
+//!
+//! This test is not *quite* high-fidelity: it checks that you can use lang items on associated
+//! items by looking at the error message *as a proxy*. That is, the error message is about
+//! undefined lang items and not invalid attribute target, indicating that it has reached lang item
+//! machinery (which is relying on knowing the implementation detail). However, it's annoying to
+//! write a full-fidelity test for this, so I think this is acceptable even though it's not *great*.
+//!
+//! This was implemented in <https://github.com/rust-lang/rust/pull/72559> to help with
+//! <https://github.com/rust-lang/rust/issues/70718>, which is itself relevant for e.g. `Fn::Output`
+//! or `Future::Output` or specific use cases like [Use `T`'s discriminant type in
+//! `mem::Discriminant<T>` instead of `u64`](https://github.com/rust-lang/rust/pull/70705).
+
 #![feature(lang_items)]
 
 trait Foo {

tests/ui/assoc-lang-items.stderr

@@ -1,23 +1,23 @@
 error[E0522]: definition of an unknown lang item: `dummy_lang_item_1`
-  --> $DIR/assoc-lang-items.rs:4:5
+  --> $DIR/assoc-lang-items.rs:18:5
    |
 LL |     #[lang = "dummy_lang_item_1"]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ definition of unknown lang item `dummy_lang_item_1`
 
 error[E0522]: definition of an unknown lang item: `dummy_lang_item_2`
-  --> $DIR/assoc-lang-items.rs:7:5
+  --> $DIR/assoc-lang-items.rs:21:5
    |
 LL |     #[lang = "dummy_lang_item_2"]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ definition of unknown lang item `dummy_lang_item_2`
 
 error[E0522]: definition of an unknown lang item: `dummy_lang_item_3`
-  --> $DIR/assoc-lang-items.rs:10:5
+  --> $DIR/assoc-lang-items.rs:24:5
    |
 LL |     #[lang = "dummy_lang_item_3"]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ definition of unknown lang item `dummy_lang_item_3`
 
 error[E0522]: definition of an unknown lang item: `dummy_lang_item_4`
-  --> $DIR/assoc-lang-items.rs:17:5
+  --> $DIR/assoc-lang-items.rs:31:5
    |
 LL |     #[lang = "dummy_lang_item_4"]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ definition of unknown lang item `dummy_lang_item_4`