Make sure we don't lose default struct value when formatting struct

[compiler-errors]

The reason why rust-lang/rustfmt#6424 broke when #129514 landed is because the parser now successfully parses default struct values. That means that where we used to fail in rewrite_macro_inner:

rust/src/tools/rustfmt/src/macros.rs

Lines 263 to 267 in e108481

	let ParsedMacroArgs {
	args: arg_vec,
	vec_with_semi,
	trailing_comma,
	} = match parse_macro_args(context, ts, style, is_forced_bracket) {

... we now succeed, and we now proceed to format the inner struct as a macro arg. Since formatting was never implemented for default struct values, this means that we’ll always rewrite the struct to exclude the default value.

This PR makes it so that we simply don’t rewrite struct fields if they have a default value.

r? @ytmimi

[rustbot]

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

[ytmimi]

RewriteError::SkipFormatting is used to represent #[rustfmt::skip]. I think in this case we could just return Ok(context.snippet(field.span()).to_owned()); (assuming that the span contains the default), or Err(RewriteError::Unknown. At some point I want to add a RewriteError variant for unstable features that don't have defined formatting yet.

[ytmimi]

I think we need to change this to return Err(RewriteError::Unknown); for now. I don't think the field.span() contains the default values. At least I still saw the error reported in rust-lang/rustfmt#6424 when I built rustfmt from this branch and ran it on

frame_support::construct_runtime!(
    pub struct Test {
        System: frame_system = 0,
        SelfDomainId: pallet_domain_id = 1,
    }
);

If possible, could you also include the above snippet in the test case.

[compiler-errors]

You're right, the field span doesn't include the optional const. I just didn't actually re-test my test.

I'm not going to include the snippet because it's identical in behavior to the snippet I included, though. I'll adjust the behavior though, and make sure to re-test it.

[compiler-errors]

This also should go above the contains_skip since that would mess up:

struct Foo2 {
    #[rustfmt::skip]
    default_field:    Spacing =    /* uwu */ 0,
}

which I added a test for.

[ytmimi]

Good catch on the #[rustfmt::skip] case, and thanks for adding the #![feature(default_struct_values)]. I think we're ready to go here.

[ytmimi]

@bors r+

[bors]

@ytmimi: 🔑 Insufficient privileges: Not in reviewers

[compiler-errors]

@bors r=ytmimi

[bors]

📌 Commit 68c46e1 has been approved by ytmimi

It is now in the queue for this repository.