Formatter adding -- with multi-import statements

[adlerjohn]

Minimal repro:

script;
use std::chain::{panic, panic};

fn main() {
}

Output of forc fmt --check:

script;
-use std::chain::{panic, panic};
+--use std::chain::{panic,panic};

fn main() {
}

[iqdecay]

After spending some time digging around on the formatter, it turns out that this issue probably comes from the fact that a single use std::chain::{panic,log_u8} statement creates 2 root nodes, which from my understanding should not be happening. Tagging this as a compiler issue (because it comes from the parsing).
The way I diagnosed this:

• If you parse the AST as a string you see the lines as they should appear (a leading --- is added to the use statement to signify that they are already formatted).
• The traverse_for_changes is the thing that creates an extra Change element, which make it so that there are 2 sets of change for the same use statement -> this duplication only happens for use statements importing 2 elements from the same crate/module.
• This extra change actually comes from the fact that the AST has 2 root nodes per use statement

In my opinion, this is not right.

[iqdecay]

As explained by @mohammadfawaz:

the compiler treats a use statement with with multiple items:
use a::{b, c, d};
the same as multiple use statements, each importing a single item:
use a::b;
use a::c;
use a::d;

We will have to find a way to fix the formatter without changing the AST generation.