Group imports together with imports_granularity results in non-idempotent formatting

[Kobzol]

When group_imports = "StdExternalCrate" is combined with imports_granularity = "Module", rustfmt can format a file in a non-idempotent way, i.e. running rustfmt twice in a row results in a different formatting.

Reduced from a case in the stdlib (rust-lang/rust#126394).

Minimal repro:

# rustfmt.toml
version = "Two"
group_imports = "StdExternalCrate"
imports_granularity = "Module"

$ rustfmt --version
rustfmt 1.7.0-nightly (72fdf91 2024-06-05)

# file.rs
use a::c;
// foo
use a::b;
use a::d;

First run (rustfmt file.rs):

// foo
use a::b;
use a::{c, d};

Second run (rustfmt file.rs):

// foo
use a::{b, c, d};

Removing the group_imports or imports_granularity options removes the issue. The problem seems to be caused by the comment, without it it works fine.

[ytmimi]

Linking the tracking issue for group_imports (#5083) and imports_granularity (#4991)

[nnethercote]

I would argue that the second run shouldn't change anything -- that the comment on use a::b; should prevent it being merged with use a::{c, d};. This is how things behave with an attribute, as in this example:

use a::c;
#[cfg(windows)]
use a::b;
use a::d;

which ends up like this (even after multiple rustfmt invocations):

#[cfg(windows)]
use a::b;
use a::{c, d};

[Kobzol]

Yeah, I also like that better. In any case, any second run shouldn't change anything, that's a bug. Let's hope that it won't be occuring more often in the rustc codebase.