Match arm pattern formatting exceeds the maximum line length

Formatting the following code

impl Foo {
    fn foo() {
        loop {
            match (pat, self.next_token.kind) {
                (_, TokenKind::Whitespace)
                | (Pat::AnyComment, TokenKind::BlockComment { terminated: true, .. } | TokenKind::LineComment { .. }) => {
                    self.step()
                },
            }
        }
    }
}

Results in:

error[internal]: line formatted, but exceeded maximum width (maximum: 120 (see `max_width` option), found: 122)
   --> \\?\F:\rust-clippy-jj\clippy_dev\src\parse\cursor.rs:104:104:121
    |
104 |                 | (Pat::AnyComment, TokenKind::BlockComment { terminated: true, .. } | TokenKind::LineComment { .. }) => {
    |                                                                                                                         ^^
    |

Config is:

max_width = 120
error_on_line_overflow = true

Making the offending line one character longer causes it to be correctly formatted across multiple lines.

Version:

rustfmt 1.8.0-nightly (4645a79881 2025-09-17)

Start by reproducing the example with max_width = 120 and error_on_line_overflow = true using the reported rustfmt nightly version. Trace the match-arm formatting and line-breaking logic for the pattern shown in clippy_dev/src/parse/cursor.rs:104. Done means the example is split across lines without exceeding 120 columns or reporting an internal overflow error.