#[default] without #[derive(Default)] could suggest adding #[derive(Default)]

[clarfonthey]

Given the following code: https://gist.github.com/cbe5d4fb00ed34650d2873725eefbc67

pub enum Test {
    #[default]
    First,
    Second,
}

The current output is:

error: cannot find attribute `default` in this scope
 --> src/lib.rs:2:7
  |
2 |     #[default]
  |       ^^^^^^^

Ideally the output should suggest adding a #[derive(Default)] attribute to the enum. Bonus points if it detects an existing derive attribute and merely adds to the list, rather than adding a new attribute.

Note that this depends on the derive_default_enum feature, which will soon be stabilised as of the FCP in #87517.

[jhpratt]

It would be nice if a solution to this could be applied to user-provided proc macros as well. The metadata should all be there, and the internals as to how the attributes are provided are shared.