Comments placed in-between a field and type in a struct get deleted after formatting

System/Tool Info

• Ubuntu 22.04.
• Rust 2024 edition.

rustup toolchain list
stable-x86_64-unknown-linux-gnu (default)
1.85.1-x86_64-unknown-linux-gnu
1.86.0-x86_64-unknown-linux-gnu (active)

cargo fmt --version
rustfmt 1.8.0-stable (05f9846f89 2025-03-31)

cargo --version
cargo 1.86.0 (adf9b6ad1 2025-02-28)

Repro Steps

I wrote the following snippet of code:

#[derive(Debug)]
struct MyCustomStruct {
    my_really_long_name_with_a_long_type:
        std::sync::Arc<std::sync::Mutex<std::collections::HashSet<String>>>,
}

If I try to add a comment here:

#[derive(Debug)]
struct MyCustomStruct {
    my_really_long_name_with_a_long_type:
    // A comment.
        std::sync::Arc<std::sync::Mutex<std::collections::HashSet<String>>>,
}

And then run cargo fmt, the comment gets deleted and the struct looks like the 1st code snippet again. Not sure whether or not it's expected...

Am happy to provide more details, and apologies if something has already been filed for this (skimming the open issues I saw a few similar, but not quite-the-same, items, but I may have just missed it)!

Start by reproducing the example with cargo fmt on the listed Rust toolchain, then trace rustfmt's handling of comments between a struct field name, its type, and the type's indentation. Confirm the expected preservation behavior and add a regression test demonstrating whether the comment remains after formatting.