Generate default impl when converting #[derive(Debug)] to manual impl

[yoshuawuyts]

This patch makes it so when you convert #[derive(Debug)] to a manual impl, a default body is provided that's equivalent to the original output of #[derive(Debug)]. This should make it drastically easier to write custom Debug impls, especially when all you want to do is quickly omit a single field which is !Debug.

This is implemented for enums, record structs, tuple structs, empty structs - and it sets us up to implement variations on this in the future for other traits (like PartialEq and Hash).

Thanks!

Codegen diff

This is the difference in codegen for record structs with this patch:

struct Foo {
    bar: String,
}

impl fmt::Debug for Foo {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        todo!();
+        f.debug_struct("Foo").field("bar", &self.bar).finish()
    }
}

[yoshuawuyts]

CI is currently failing on:

thread 'tidy::files_are_tidy' panicked at '
TODO markers or todo! macros should not be committed to the master branch,
use FIXME instead
/home/runner/work/rust-analyzer/rust-analyzer/crates/ide_assists/src/tests/generated.rs
', crates/rust-analyzer/tests/slow-tests/tidy.rs:292:9

But looking at the code diff, the only change we've made to the file is removing a todo! statement. This makes it seem like the error might be originating outside this PR? What would the right fix for this be?

cc/ @Veykril.

[Veykril]

Heh, thats a fun one. So the generated tests still contain a todo!() invocation, but your changes removed the only ${0:todo!()} snippet. So now the tidy script is still picking up a todo in the generated.rs but there is no snippet so its not tidy according to it.

I think you can just add the path of the generated.rs file to the tidy exclusions for now.
That is here https://github.com/rust-analyzer/rust-analyzer/blob/f0d32591a68ecacd61a59aaa9df924890d0212d7/crates/rust-analyzer/tests/slow-tests/tidy.rs#L270-L282

[yoshuawuyts]

Updated with feedback from review. Waiting on CI to pass now ✨

edit: yay, it passes! 🎉 - think it's good to merge now!

[bors]

✌️ yoshuawuyts can now approve this pull request. To approve and merge a pull request, simply reply with bors r+. More detailed instructions are available here.

[lnicola]

Can we avoid some of the unwraps in there, even if by doing nothing when something is missing? I'm sure that at least some of them should be safe, but that can change, and in the past assists were the part of RA that was most prone to random panics.

[Veykril]

I agree, minimizing unwraps where possible is a good choice.

[yoshuawuyts]

Implemented all remaining feedback!

bors r+

[bors]

Build succeeded:

• Rust (macos-latest)
• Rust (ubuntu-latest)
• Rust (windows-latest)
• TypeScript (ubuntu-latest)
• TypeScript (windows-latest)

[lnicola]