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Stabilize #[diagnostic::do_not_recommend] #132056

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Stabilize #[diagnostic::do_not_recommend] #132056

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Commits on Nov 18, 2024

  1. Improve testing coverage for #[diagnostic::do_not_recommend] 

    This PR aims to improve the testing coverage for
`#[diagnostic::do_not_recommend]`. It ensures that all tests are run for
the old and new solver to verify that the behaviour is the same for both
variants. It also adds two new tests:

* A test with 4 traits having wild card impl for each other, with
alternating `#[diagnostic::do_not_recommend]` attributse
* A test with a lifetime dependend wild card impl, which is something
that's not supported yet
    @weiznich
    weiznich committed Nov 18, 2024
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  2. Check #[diagnostic::do_not_recommend] for arguments 

    This commit adds a check that verifies that no arguments are passed to
`#[diagnostic::do_not_recommend]`. If we detect arguments we emit a warning.
    @weiznich
    weiznich committed Nov 18, 2024
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  3. Stabilize #[diagnostic::do_not_recommend] 

    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
    @weiznich
    weiznich committed Nov 18, 2024
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  4. Also warn against #[diagnostic::do_not_recommend] on plain impls

    @weiznich
    weiznich committed Nov 18, 2024
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