Non-idempotency in consecutive block comment

Summary

Reported in https://github.com/rust-lang/rustfmt/issues/6639#issuecomment-5235145262 by @yuxqiu, split out to its own issue.

I tried to format this code:

fn f() -> usize {
    /*A*/ /*B*/
    1 + 2
}

Expected behavior

I expected to see this happen: be idempotent.

Actual behavior

Instead, this happened: formatting is non-idempotent and takes 2 steps to converge:

start

fn f() -> usize {
    /*A*/ /*B*/
    1 + 2
}

start -> form 1

fn f() -> usize {
    /*A*/
 /*B*/
    1 + 2
}

form 1 -> form 2

fn f() -> usize {
    /*A*/
    /*B*/
    1 + 2
}

Configuration

rustfmt cli options used (if applicable):

$ rustfmt +stable --config-path=/dev/null

rustfmt configuration file (e.g. rustfmt.toml, if applicable): none

Reproduction Steps

$ cat foo.rs
fn f() -> usize {
    /*A*/ /*B*/
    1 + 2
}

$ rustfmt +stable --config-path=/dev/null foo.rs # step 1
$ cat foo.rs
fn f() -> usize {
    /*A*/
 /*B*/
    1 + 2
}

$ rustfmt +stable --config-path=/dev/null foo.rs # step 2
$ cat foo.rs
fn f() -> usize {
    /*A*/
    /*B*/
    1 + 2
}

Meta

rustfmt --version:

rustfmt 1.9.0-stable (8bab26f4f6 2026-07-14)