Attributes on struct rest patterns are ignored

[Skepfyr]

Attributes on struct field rest patterns are entirely ignored by the compiler, in fact they're dropped by the parser, so they can't generate any errors (beyond parse errors) but are accepted by the parser.

Specifically, this bit of the AST doesn't store the attributes:

rust/compiler/rustc_ast/src/ast.rs

Lines 845 to 854 in 01a26c0

	/// Whether the `..` is present in a struct fields pattern.
	#[derive(Clone, Copy, Encodable, Decodable, Debug, PartialEq)]
	pub enum PatFieldsRest {
	/// `module::StructName { field, ..}`
	Rest,
	/// `module::StructName { field, syntax error }`
	Recovered(ErrorGuaranteed),
	/// `module::StructName { field }`
	None,
	}

The parser drops them on the floor here.

This means that the following code doesn't produce any errors:

let Foo { #[bad] .. } = Foo {};

And entertainingly the attribute is deleted by rustfmt...I assume that's because it's not represented in the AST.

Now, as far as I know there aren't any attributes that would be reasonable to use in this position, which is probably why the bug still exists.

[ehuss]

Now, as far as I know there aren't any attributes that would be reasonable to use in this position, which is probably why the bug still exists.

I would expect #[cfg] to work there, but it is also ignored.

[petrochenkov]

Duplicate of #81282.
I think my answer in #81282 (comment) is still relevant.

[ehuss]

Ah, I knew there was an issue somewhere, but I was searching for the word "rest" and wasn't finding that. Thanks!