Description
I've been working on a PR to clippy that involved looking at the different AST node types in rustc_ast::ast. I noticed that while most of the subset of those types that can contain attributes use AttrVec, (an alias of a ThinVec of Attributes) some node types use a plain Vec<Attribute>.
In particular, as of this writing, Variant, StructField, Item, Crate, and Arm all use Vec<Attribute>. (I found these via a search in rustc_ast for "attr" and checking each field in the results, so hopefully that list is complete.)
ThinVec's documentation says:
A vector type optimized for cases where this size is usually 0 (cf. SmallVector). The Option<Box<..>> wrapping allows us to represent a zero sized vector with None, which uses only a single (null) pointer.
I would suspect that for most of the types that use a plain Vec<Attribute> the Vec is empty almost all of the time, with the possible exception of Items, since #[derive(Debug)] etc. is pretty common.
So should more of the types that use a plain Vec<Attribute> be using an AttrVec instead? Are these fields plain Vecs just because no one changed them, or was it somehow determined that those types aren't copied enough for this optimization to matter?