Accept "{:}<" in format macro parser

[TaKO8Ki]

I think this should also be compiled successfully.

Fixes #112732

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=5a69e2326988b6150c62087ef60ecf2f

[rustbot]

r? @jackh726

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[compiler-errors]

For the record, this code would now behave differently, right?

fn main() {
    println!("{:}<3}", 0);
}

According to the linked issue, the format spec allows any fill character, so shouldn't we be preserving this old behavior?

[TaKO8Ki]

@compiler-errors Yes. After this change is applied, this case causes an error like the following, In the old parser, it is compiled successfully. I think this might be wrong behavior and new error looks better.

According to the linked issue, the format spec allows any fill character, so shouldn't we be preserving this old behavior?

I guess we don't have detailed specification for formatting and the current behavior is regarded as a specification? Should I avoid changing the behavior of the format macro parse and just add a diagnostic for this case?

Given the following code:

fn main() {
    println!("{:}<3}", 0);
}

Before this change is applied: Compiled successfully

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=53f9ec33ce4fc8293d41b0fb061e0427

After the change is applied:

error: invalid format string: unmatched `}` found
 --> src/main.rs:2:20
  |
2 |     println!("{:}<3}", 0);
  |                    ^ unmatched `}` in format string
  |
  = note: if you intended to print `}`, you can escape it using `}}`

error: could not compile `debug_playground` (bin "debug_playground") due to previous error

[compiler-errors]

Nominating for T-libs-api I think, because this has to do with the specification of the format macro, and it's not strictly a fix.

[joshtriplett]

There's a missing close-brace here.

[TaKO8Ki]

You mean the following brace is not a close-brace?

format!("foo {:}<", 2);
               ↑

But the current rustc can compile this successfully.

format!("{:}", 2)

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=2b950ebb99306f3c8f6d491aab8f84bd

[TaKO8Ki]

They can also be compiled.

fn main() {
    format!("{:} <", 2);
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=ab6c3a65c2ee0de3f856f1f1fb0142e1

fn main() {
    format!("{}<", 2);
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=fc71ece4d69d55ac243e5af4141041fa

[joshtriplett]

There's a missing close-brace here.

[TaKO8Ki]

Same as comments above.

[m-ou-se]

We discussed this in the libs-api meeting today. We're not sure if we can go forward with this, since this would be a breaking change: you could no longer set } as the fill character. ("{:}>}" would stop working.)

I mentioned that {:} is a bit of an anti-pattern. Clippy used to warn about that and suggest replacing it with {}, but I believe I broke that when I refactored the format arguments internals. We should probably add that lint back in rustc, and that logic can then also help with giving a better diagnostic for "{:}>".

[TaKO8Ki]

@m-ou-se

Thank you for discussing it. I see. The example you provided would cause an error like the following after this change is applied. Should } be available as the fill character?

error: invalid format string: unmatched `}` found
 --> src/main.rs:2:18
  |
2 |     format!("{:}>}", 2);
  |                  ^ unmatched `}` in format string
  |
  = note: if you intended to print `}`, you can escape it using `}}`

error: could not compile `debug_playground` (bin "debug_playground") due to previous error

We should probably add that lint back in rustc, and that logic can then also help with giving a better diagnostic for "{:}>".

Ok. I'll create an issue in clippy and work on it when I have time.

[m-ou-se]

Should } be available as the fill character?

Today, every single character is available as fill character. "Every single character except }" doesn't seem like a big improvement. (What about '{' or other potentially confusing ones?)

I agree {:}>} is confusing, but {:} is uncommon enough (and equivalent to {}) that I don't think it's worth coming up with an alternative syntax for specifying } as fill character.

[TaKO8Ki]

Today, every single character is available as fill character. "Every single character except }" doesn't seem like a big improvement. (What about '{' or other potentially confusing ones?)

Understood. You're right. I'll close this pull request.