This commit seeks to stabilize the `#[diagnostic::do_not_recommend]` attribute. This attribute was first proposed as `#[do_not_recommend`] attribute in RFC 2397 (rust-lang/rfcs#2397). It gives the crate authors the ability to not suggest to the compiler to not show certain traits in it's error messages. With the presence of the `#[diagnostic]` tool attribute namespace it was decided to move the attribute there, as that lowers the amount of guarantees the compiler needs to give about the exact way this influences error messages. It turns the attribute into a hint which can be ignored. In addition to the original proposed functionality this attribute now also hides the marked trait in help messages ("This trait is implemented by: "). The attribute does not accept any argument and can only be placed on trait implementations. If it is placed somewhere else a lint warning is emitted and the attribute is otherwise ignored. If an argument is detected a lint warning is emitted and the argument is ignored. This follows the rules outlined by the diagnostic namespace. This attribute allows crates like diesel to improve their error messages drastically. The most common example here is the following error message: ``` error[E0277]: the trait bound `&str: Expression` is not satisfied --> /home/weiznich/Documents/rust/rust/tests/ui/diagnostic_namespace/do_not_recommend.rs:53:15 | LL | SelectInt.check("bar"); | ^^^^^ the trait `Expression` is not implemented for `&str`, which is required by `&str: AsExpression<Integer>` | = help: the following other types implement trait `Expression`: Bound<T> SelectInt note: required for `&str` to implement `AsExpression<Integer>` --> /home/weiznich/Documents/rust/rust/tests/ui/diagnostic_namespace/do_not_recommend.rs:26:13 | LL | impl<T, ST> AsExpression<ST> for T | ^^^^^^^^^^^^^^^^ ^ LL | where LL | T: Expression<SqlType = ST>, | ------------------------ unsatisfied trait bound introduced here ``` By applying the new attribute to the wild card trait implementation of `AsExpression` for `T: Expression` the error message becomes: ``` error[E0277]: the trait bound `&str: AsExpression<Integer>` is not satisfied --> $DIR/as_expression.rs:55:15 | LL | SelectInt.check("bar"); | ^^^^^ the trait `AsExpression<Integer>` is not implemented for `&str` | = help: the trait `AsExpression<Text>` is implemented for `&str` = help: for that trait implementation, expected `Text`, found `Integer` ``` which makes it much easier for users to understand that they are facing a type mismatch. Other explored example usages included * This standard library error message: rust-lang#128008 * That bevy derived example: https://github.com/rust-lang/rust/blob/e1f306899514ea80abc1d1c9f6a57762afb304a3/tests/ui/diagnostic_namespace/do_not_recommend/supress_suggestions_in_help.rs (No more tuple pyramids) Fixes rust-lang#51992

Commits

Commits on Dec 18, 2024

Improve testing coverage for #[diagnostic::do_not_recommend]

Check #[diagnostic::do_not_recommend] for arguments

Stabilize #[diagnostic::do_not_recommend]

Also warn against #[diagnostic::do_not_recommend] on plain impls

Improve testing coverage for `#[diagnostic::do_not_recommend]`

Check `#[diagnostic::do_not_recommend]` for arguments

Stabilize `#[diagnostic::do_not_recommend]`

Also warn against `#[diagnostic::do_not_recommend]` on plain impls