Trailing comma removed from multiline attribute

I originally had an overly long attribute like this:

#![allow(clippy::needless_pass_by_value, clippy::new_ret_no_self, clippy::new_without_default_derive)]

This got formatted to:

#![allow(
    clippy::needless_pass_by_value,
    clippy::new_ret_no_self,
    clippy::new_without_default_derive
)]

I manually added a comma after the third line, because I expect this list to change in the future.

Expected behavior:

rustfmt keeps the trailing comma.

Actual behavior:

rustfmt removes the trailing comma.

Note that, ideally (IMO), rustfmt would have automatically inserted a trailing comma, or had a setting to specify that behavior. However I don't know if there are situation where the lack of (or presence) of a trailing comma is of semantic significance in (custom) attributes.

If a trailing comma can be significant it should be neither added nor removed automatically, I think. (Unless rustfmt recognizes the specific attribute).

Reproduce the issue in rustfmt using the multiline Rust attribute shown in the report, and compare the formatted output with the manually added trailing comma. Determine whether trailing commas in custom attributes can be changed safely, then verify that the final behavior matches the expected or documented policy.