add Fold/VisitMut methods for Vec<T>, Option<T>, Punctuated, etc.

[programmerjake]

I'm trying to build a Fold visitor that removes all Attributes that match a pattern, using Fold::fold_attribute doesn't work because I want to replace the attribute with nothing, not another attribute.

I think adding Fold::fold_attributes is a good solution.

I also ran into this problem when trying to transform Stmt into nothing, I had to construct a no-op Stmt.

[programmerjake]

as mentioned above, this is also a problem for Stmt and other things where you have a list of them, the PR just fixed the problem for attributes.

[dtolnay]

Vec<Stmt> is exclusively present in Block so it can easily be handled in fold_block already.

[programmerjake]

that takes care of Vec<Stmt> but not the many instances of Punctuated, e.g. there's several places where there's Punctuated<Expr, _> and Punctuated<Pat, _>

[dtolnay]

What use case did you have for a method dedicated to those?

[programmerjake]

well, for fields I've written proc macros that transform type definitions by removing fields that have a #[my_macro(skip)] annotation. If I implemented it slightly differently, I would also be transforming field types based on annotations on the fields, e.g.:

#[my_io]
struct S {
    #[my_io(in)]
    a: u32,
    #[my_io(out)]
    b: i16,
}

would covert to something like

struct S {
    a: Input<u32>,
    b: Output<i16>,
}