rustfmt does not format panic! macro under a specific circumstance

[duongdominhchau]

Summary

Somehow my panic!("blah blah blah {}",id) is not formatted, but there is no error.

cargo fmt -- --config=error_on_unformatted=true exits cleanly, cargo fmt -- -v does not print anything unusual. rustup update then cargo +nightly fmt yields the same output.

cargo build produces warnings because I trimmed the code down into silly example, but it is still valid and can be built successfully. In my original project there is no warning, it just runs normally.

I'm unable to pinpoint the exact cause, but I managed to create a quite minimal example here:

pub fn f(root: Container) -> ! {
    let Container { children, .. } = root;
    children.into_iter().for_each(|node| {
            if let Container{ guid, .. } = node {
                if true{
            match guid {
                    Guid::Other(id) => {
                        panic!(
                            "Expecting immediate bookmark root, but found user-created folder GUID={}",id
                        );
                    }
                }
                }
            }
            });
    unreachable!();
}

The children.into_iter() line and the id in panic! is affected, lines before or after are still properly formatted. The children.into_iter() and the inner if let Container lines are required to reproduce the issue, without these two lines it formats just fine. I thought it's about line length, but replacing these lines with lots of nested if true makes the issue disappear.

Please see repro.zip for full code.

Expected behavior

A space should be inserted before id

Actual behavior

The code does not change.

Configuration

None, I use the default.

Reproduction Steps

1. Download the repro.zip and extract it
2. cd into the directory and take note of the existing content of src/main.rs
3. Run cargo fmt and observe that src/main.rs does not change

Meta

• OS: ArchLinux
• Rust toolchain version:

$ cargo --version
cargo 1.98.1 (797e8a9bc 2026-08-05)
$ cargo +nightly --version
cargo 1.100.0-nightly (b2e9d5f9d 2026-09-02)
$ rustup --vesrion
rustup 1.29.0 (2026-03-23)
info: This is the version for the rustup toolchain manager, not the rustc compiler.
info: the currently active `rustc` version is `rustc 1.98.1 (48a229cea 2026-09-01)`
$ cargo fmt -- --version
rustfmt 1.9.0-stable (48a229ceae 2026-09-01)
$ cargo +nightly fmt -- --version
rustfmt 1.10.0-nightly (0ed41eb414 2026-09-04)

[matthewhughes934]

Trying to trim down the reproduction I have:

fn main() {
    {
        {
            {
                ("Expecting immediate bookmark root, but found user-created folder GUID={}aaaaaa",id)
            }
        }
    }
}

• If you remove a level of nesting it formats
• If you remove one character from the string (to bring the line to exactly max_width (100 chars)) it formats

I suspect this is an instance of #3697. If you run the original reproduction above with --config tab_spaces=1 it formats as expected.

EDIT: to clarify this isn't macro specific: if you replace panic! with panic in the original reproduction you get the same result.

[jieyouxu]

I think there's some general all-or-nothing fallback behavior re. max width and string literals 🤔 It feels generally related to:

• Gives up on chains if any line is too long. #3863
• A long string in a match arm breaks formatting for the whole match expression #4797
• [Tracking] Formatting failures with all-or-nothing fallback behavior #6687
• bug: rustfmt silently breaks on this sample #6769

and indeed #3697.