"left behind trailing whitespace" internal error if second consecutive line comment starts with certain characters

Rustfmt fails to format the following file.

fn main() {
    let x = 0; // X
    //.Y
}

• The failure is sensitive to the contents of the second comment. For example // Y or //Y does not hit this bug. //.Y or //~Y or //^Y do.
• The failure does not appear to be sensitive to the contents of the first comment, just that there is a comment.
• The failure is not sensitive to the second comment being the last thing in a block; it occurs even if there are more statements inside the block after the second comment. I left that out above because it is unnecessary for a minimal repro.

Repro against current master (3de1a095e0ed52ade1b88d165d44d80455d3e6c5):

$ echo -e 'fn main() {\n    let x = 0; // X\n    //.Y\n}' | cargo run --bin rustfmt -- --edition=2021

fn main() {
    let x = 0; // X
    
    //.Y
}
error[internal]: left behind trailing whitespace
 --> <stdin>:3:3:1
  |
3 |     
  | ^^^^
  |

warning: rustfmt has failed to format. See previous 1 errors.

I looked through all the open issues matching a "left behind trailing whitespace" search in this repo (of which there are many) and all the closed ones that mention "comment" in the title, and I did not find one that matches this situation. They mostly seemed to involve comments placed in weird places inside an expression or statement, like let x =⏎// comment⏎value; or map(|_|⏎//comment⏎value). In contrast, this issue involves comments in a location where a conscientious person might reasonably put a comment (particularly if there is more stuff in the block after the second comment, as mentioned above). In fact the compiletest_rs crate requires such comments (//~^ ERROR) which is how Miri's test suite ran into this bug.

Reproduce the failure with the cargo run --bin rustfmt -- --edition=2021 command and the Rust snippet in the issue, then trace how rustfmt handles consecutive line comments. Done means the example formats without an internal error or left-behind whitespace, while preserving both comments and the surrounding code.