Rename #[visited_node] attr to #[ast]

[overlookmotel]

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.

[rzvxa]

From day one I was a fan of this more than any other way so that's where I'm.

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

If you remember at first I was more in favor of naming it ast_node but your arguments against it(both here and #3815 (comment)) made me change my decision. #[ast] seems like a good choice👍

[rzvxa]

I had an idea for generated derives which I've never shared publicly. We might want to add an attribute such as #[expand_derive(...)] or #[derived(...)] to easily mark stuff like Hash, PartialEq, and now CloneIn derives.

It is a marker and wouldn't expand the derived macro base on the builtins(or proc_macro_derive), but it is a good way to telegraph that these are being derived for the said type although they are generated and not happening in the compiler.

[overlookmotel]

@Boshen and I discussed on video call just now. He's given his 👍 to this.

[rzvxa]

closed via #4289