PreferSameLine brace_style with trailing_comma Vertical produces weird results

[djc]

Upgrading rustfmt-nightly from 0.2.15 to 0.3.4 created this change:

@@ -104,8 +122,7 @@ impl FetchCommandMessages {
 
 pub trait FetchBuilderMessages
 where
-    Self: Sized,
-{
+    Self: Sized, {
     fn prepare(self) -> FetchCommandMessages;
 
     fn num(self, num: u32) -> FetchCommandMessages {

This seems like a bug, since trailing_comma = "Vertical" should not trigger if the opening brace is on the same line.

[ytmimi]

Confirming this is reproducible with rustfmt 1.5.1-nightly (0cb294f0 2022-07-12) and configuration options trailing_comma = "Vertical" and brace_style=PreferSameLine.

Input

pub trait FetchBuilderMessages
where
    Self: Sized,
{
     fn prepare(self) -> FetchCommandMessages;
}

output

pub trait FetchBuilderMessages
where
    Self: Sized, { // <--- Note trailing comma is not removed from the bound
    fn prepare(self) -> FetchCommandMessages;
}

If someone is interested in taking this on, I'd start by checking out these lines inrewrite_bounds_on_where_clause:

rustfmt/src/items.rs

Lines 2872 to 2876 in a7bf009

	let comma_tactic = if where_clause_option.suppress_comma || force_single_line {
	SeparatorTactic::Never
	} else {
	context.config.trailing_comma()
	};

[ytmimi]

Linking tracking issues for brace_style(#3376) and trailing_comma (#3379)

[gentoid]

@rustbot claim