Duplicate comma with `trailing_comma = "Always"` in struct pattern

[dnbln]

// src/main.rs
fn main() {
    let Foo { a, .. } = b;
}

# rustfmt.toml
edition = "2021"

unstable_features = true
trailing_comma = "Always"

Reformat with rustfmt --config-path rustfmt.toml src/main.rs

// src/main.rs
fn main() {
    let Foo { a,, .. } = b;
}

rustfmt --version: rustfmt 1.4.38-nightly (4961b10 2021-11-04)

[calebcartwright]

Thanks for the report.

@ytmimi could you take a look at this one? Issue should be in/around some of the same struct-related code you were looking at not too long ago

[ytmimi]

I'd be happy to take a look! I'll let you know what I can find.

[ytmimi]

@calebcartwright I took a look at this, and the reason that we're adding the comma twice is because when we try to rewrite the struct pattern in patterns::rewrite_struct_pat, there's an assumption that a trailing comma is always needed when the tactic is not DefinitiveListTactic::Vertical.

rustfmt/src/patterns.rs

Lines 330 to 339 in eee8f04

	if !fields_str.is_empty() {
	// there are preceding struct fields being matched on
	if tactic == DefinitiveListTactic::Vertical {
	// if the tactic is Vertical, write_list already added a trailing ,
	fields_str.push(' ');
	} else {
	fields_str.push_str(", ");
	}
	}
	}

In this particular case the tactic is actually DefinitiveListTactic::Horizontal so we end up appending ", " to the end of the fields_str.

would it make more sense to check if !fmt.needs_trailing_separator() instead of !tactic == DefinitiveListTactic::Horizontal to decide whether or not we append
" " or ", "?