Rustfmt inserts a made-up `impl Trait` definition into types

[dtolnay]

As of current master, rustfmt applies the following incorrect diff to this valid Rust code:

  #[cfg(any())]
- type Type: Bound;
+ type Type: Bound = impl Bound;

To reproduce: cargo run --bin rustfmt repro.rs

[calebcartwright]

Thanks for the report. Having a major case of déjà vu with this one

[ytmimi]

Summary

Was doing a little digging into this.

TLDR; I think the issue is caused by using the bounds for both the generic_bounds_opt and as the OpaqueType used for the rhs when calling items::rewrite_ty.

Bug Details

When calling items::rewrite_type_alias we hit the first match_arm:

rustfmt/src/items.rs

Lines 1523 to 1527 in eee8f04

	match (visitor_kind, ty_opt) {
	(Item(_), None) => {
	let op_ty = OpaqueType { bounds };
	rewrite_ty(rw_info, Some(bounds), Some(&op_ty), vis)
	}

After we add the generic bounds to the output result (code responsible for that added bellow) we end up with the string type Type: Bound

rustfmt/src/items.rs

Lines 1573 to 1580 in eee8f04

	if let Some(bounds) = generic_bounds_opt {
	if !bounds.is_empty() {
	// 2 = `: `
	let shape = Shape::indented(indent, context.config).offset_left(result.len() + 2)?;
	let type_bounds = bounds.rewrite(context, shape).map(|s| format!(": {}", s))?;
	result.push_str(&type_bounds);
	}
	}

Because we have a rhs we end up adding a = to the end of the result string (code bellow) so now we have
type Type: Bound =

rustfmt/src/items.rs

Lines 1615 to 1636 in eee8f04

	let lhs = match comment_span {
	Some(comment_span)
	if contains_comment(context.snippet_provider.span_to_snippet(comment_span)?) =>
	{
	let comment_shape = if has_where {
	Shape::indented(indent, context.config)
	} else {
	Shape::indented(indent, context.config)
	.block_left(context.config.tab_spaces())?
	};
	combine_strs_with_missing_comments(
	context,
	result.trim_end(),
	"=",
	comment_span,
	comment_shape,
	true,
	)?
	}
	_ => format!("{}=", result),
	};

And lastly because of how OpaqueType::rewrite is implemented (code bellow) we end up adding the impl Bound. before ultimately returning type Type: Bound = impl Bound; from items::rewrite_ty.

rustfmt/src/items.rs

Lines 1874 to 1881 in eee8f04

	impl<'a> Rewrite for OpaqueType<'a> {
	fn rewrite(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<String> {
	let shape = shape.offset_left(5)?; // `impl `
	self.bounds
	.rewrite(context, shape)
	.map(|s| format!("impl {}", s))
	}
	}

Proposed Solution

After going through the code it seems to me that passing None as the rhs argument of items::rewrite_ty, would fix the issue, and locally it seems to have done the trick. I'm going to open a PR, but please let me know if I'm overlooking anything.