At present we only have #[visited_node] attrs on types which are visited.

We will also need to add an attribute to all types which are part of AST but are not visited, for:

• CloneIn trait for AST nodes #4284
• Make AST #[repr(C)] #4296
• AST transfer

Currently:

#[visited_node]
enum Expression<'a> { /* ... */ }

I propose:

#[ast(visit)]
enum Expression<'a> { /* ... */ }

or:

#[ast]
#[visit]
enum Expression<'a> { /* ... */ }

Types which are part of AST, but which are not visited would omit the visit part e.g.:

#[ast]
pub enum FunctionType { /* ... */ }

Bikeshedding

We could call the attr something other than #[ast]. I considered #[ast_node] but:

1. That name will be familiar to some from SWC, and I don't think we should use it because these attrs do something very different - they're no-op markers for codegen, rather than macros generating a huge amount of trait impls and other code.
2. Some of these types (e.g. FunctionType) are not AST nodes per se.

The common factor between all these types is that they're part of the AST, so #[ast] seems descriptive.

But I'm not at all set on that name, if someone has a better suggestion.

cc @Boshen @rzvxa.