Internal error "left behind trailing whitespace" for assignment with RHS comment

Describe the bug

error[internal]: left behind trailing whitespace
 --> /playground/src/main.rs:4:4:17
  |
4 |     let new_len = 
  |                  ^
  |

To Reproduce

Repros on the playground, https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=8d26c934db762f2165ab9b0cc4d2ef61

    pub unsafe fn as_chunks_mut_unchecked<const N: usize>(&mut self) -> &mut [[T; N]] {
        debug_assert_ne!(N, 0);
        debug_assert_eq!(self.len() % N, 0);
        let new_len = 
            // SAFETY: Our precondition is exactly what's needed to call this
            unsafe { crate::intrinsics::exact_div(self.len(), N) };
        // SAFETY: We cast a slice of `new_len * N` elements into
        // a slice of `new_len` many `N` elements chunks.
        unsafe { from_raw_parts_mut(self.as_mut_ptr().cast(), new_len) }
    }

Start with the playground reproduction and the Rust snippet in the issue, then run rustfmt to confirm the internal “left behind trailing whitespace” error. Trace the formatter path responsible for the assignment and comment layout, and use the reproduction as the regression case; done means this input formats without the internal error.