Cargo fails to format a line comment followed by a block comment

Problem

cargo fmt fails to format the following code (playground link):

fn main() {
    todo!() // line comment
    /*
        block comment
    */
}

with this error:

error[internal]: left behind trailing whitespace
 --> /playground/src/main.rs:2:2:1
  |
2 |     
  | ^^^^
  |

The code is formatted like is this (Note the 4 spaces on the added line between the line and the block comments):

fn main() {
    todo!() // line comment
    
    /*
        block comment
    */
}

The actual code before the line comment doesn't seem to matter, only that there is some code on the same line as the comment.

Steps

1. Write any function that has some code, then a line comment on the same line, then a block comment on the line right afterwards
2. Run cargo fmt

Possible Solution(s)

No response

Notes

Affects at least stable 1.93.0 and nightly 1.95.0, and affects both Windows and WSL (Ubuntu).

Version

cargo 1.95.0-nightly (fe2f314ae 2026-01-30)
release: 1.95.0-nightly
commit-hash: fe2f314aef06e688a9517da1ac0577bb1854d01f
commit-date: 2026-01-30
host: x86_64-unknown-linux-gnu
libgit2: 1.9.2 (sys:0.20.3 vendored)
libcurl: 8.15.0-DEV (sys:0.4.83+curl-8.15.0 vendored ssl:OpenSSL/3.5.4)
ssl: OpenSSL 3.5.4 30 Sep 2025
os: Ubuntu 24.4.0 (noble) [64-bit]

Start by reproducing the issue with the Rust snippet in the report and running cargo fmt. Trace the formatter's handling of a line comment followed by a block comment; done means cargo fmt completes without the trailing-whitespace error and preserves the comments without inserting invalid whitespace.