`format_strings` breaks escaped character when string too long

[t-webber]

Summary

Formatting this code:

fn main() {
    panic!("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\nbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb")
}

Produces this code:

fn main() {
    panic!(
        "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\\
         nbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"
    )
}

Which adds some spaces after the newline and an extra n character.

Configuration

My rustfmt.toml:

format_strings = true

(unstable, tracking issue: #3353)

Reproduction Steps

1. Copy the code from above in your main.rs and rustfmt.toml
2. Run cargo fmt

Meta

$ rustfmt --version
rustfmt 1.9.0-nightly (fb0a5a5a9c 2026-05-08)
$ rustc --version
rustc 1.97.0-nightly (fb0a5a5a9 2026-05-08)

@rustbot label +S-has-mcve +UO-format_strings

[rustbot]

Error: Label UO-format_strings can only be set by Rust team members

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