`ERROR synthetic syntax` warning on any interaction with the editor

[Paladynee]

rust-analyzer version: rust-analyzer version: 0.3.2353-standalone (37acea8 2025-03-23)

rustc version: rustc 1.87.0-nightly (f8c27dfe1 2025-03-24)

editor or extension: VSCode

relevant settings: i dont know

code snippet to reproduce:
minimal:

fn minimal_case() {
    let anything_display = 1;
    format_args!("{}", anything_display);
}

other examples:

fn higher_macros() {
    panic!("panic uses format args{}", "");
}

fn real_code() {
    let vec: Vec<u8> = vec![];
    let formatted = format!("real {} code that can happen", vec.len());
    println!("very {} in real life ", formatted);
}

fn main() {
    minimal_case();
    higher_macros();
    real_code();
}

any macro that calls format_args with an argument that can be formatted, will cause this "warning". the warning is in the OUTPUT section of the editor, under Rust Analyzer Language Server from the dropdown:

...
2025-03-26T18:36:02.853090122+03:00 ERROR synthetic syntax
2025-03-26T18:36:02.853171069+03:00 ERROR synthetic syntax
2025-03-26T18:36:02.853212834+03:00 ERROR synthetic syntax
2025-03-26T18:36:02.853218002+03:00 ERROR synthetic syntax
2025-03-26T18:36:16.090820077+03:00 ERROR synthetic syntax
2025-03-26T18:36:16.09091122+03:00 ERROR synthetic syntax
2025-03-26T18:36:16.090965557+03:00 ERROR synthetic syntax
2025-03-26T18:36:16.090972821+03:00 ERROR synthetic syntax
2025-03-26T18:38:08.535456768+03:00 ERROR synthetic syntax
2025-03-26T18:38:08.535573823+03:00 ERROR synthetic syntax
2025-03-26T18:38:08.53561922+03:00 ERROR synthetic syntax
2025-03-26T18:38:08.535625855+03:00 ERROR synthetic syntax
...

It will produce as many ERROR synthetic syntax messages as there are calls to format_args! in the source code (whether direct, or indirect, as shown above).

I am pretty much sure this can't be reproduced, otherwise it would be recognized immediately. This doesn't cause any issue per se, but it is still annoying to not be able to see Rust Analyzer Language Server output because of this error.

It is also a very cryptic error message with no payload attached. Why is it like this?

[lnicola]

I'm not able to reproduce this, but I think those error come up from time to time. Just to be sure, do you get the docs when hovering on panic_args!?

[lnicola]

Ah, never mind, I can reproduce it on nightly.

[Veykril]

That message means we tried to emit a diagnostic for some desugared thing, something we shouldn't be doing. I can confirm that my logs are also apparently spammed with this message so we either changed some desugaring recently or when a specific diagnostic gets reported. Though I don't see any such relevant changes

[lnicola]

nightly-2025-03-22 works fine, nightly-2025-03-23 fails.

[Veykril]

Ah right, rust-lang/rust#136932 got merged so we need to adjust our lowering

[Veykril]

Still seems to be an issue

[maia-s]

I still get this issue with rustc 1.88.0-nightly (10fa3c449 2025-04-26)