There is code for converting `Attribute` (syntactic) to `MetaItem`
(semantic). There is also code for the reverse direction. The reverse
direction isn't really necessary; it's currently only used when
generating attributes, e.g. in `derive` code.
This commit adds some new functions for creating `Attributes`s directly,
without involving `MetaItem`s: `mk_attr_word`, `mk_attr_name_value_str`,
`mk_attr_nested_word`, and
`ExtCtxt::attr_{word,name_value_str,nested_word}`.
These new methods replace the old functions for creating `Attribute`s:
`mk_attr_inner`, `mk_attr_outer`, and `ExtCtxt::attribute`. Those
functions took `MetaItem`s as input, and relied on many other functions
that created `MetaItems`, which are also removed: `mk_name_value_item`,
`mk_list_item`, `mk_word_item`, `mk_nested_word_item`,
`{MetaItem,MetaItemKind,NestedMetaItem}::token_trees`,
`MetaItemKind::attr_args`, `MetaItemLit::{from_lit_kind,to_token}`,
`ExtCtxt::meta_word`.
Overall this cuts more than 100 lines of code and makes thing simpler.
`rustc_data_structures::thin_vec::ThinVec` looks like this:
```
pub struct ThinVec<T>(Option<Box<Vec<T>>>);
```
It's just a zero word if the vector is empty, but requires two
allocations if it is non-empty. So it's only usable in cases where the
vector is empty most of the time.
This commit removes it in favour of `thin_vec::ThinVec`, which is also
word-sized, but stores the length and capacity in the same allocation as
the elements. It's good in a wider variety of situation, e.g. in enum
variants where the vector is usually/always non-empty.
The commit also:
- Sorts some `Cargo.toml` dependency lists, to make additions easier.
- Sorts some `use` item lists, to make additions easier.
- Changes `clean_trait_ref_with_bindings` to take a
`ThinVec<TypeBinding>` rather than a `&[TypeBinding]`, because this
avoid some unnecessary allocations.
In some places we use `Vec<Attribute>` and some places we use
`ThinVec<Attribute>` (a.k.a. `AttrVec`). This results in various points
where we have to convert between `Vec` and `ThinVec`.
This commit changes the places that use `Vec<Attribute>` to use
`AttrVec`. A lot of this is mechanical and boring, but there are
some interesting parts:
- It adds a few new methods to `ThinVec`.
- It implements `MapInPlace` for `ThinVec`, and introduces a macro to
avoid the repetition of this trait for `Vec`, `SmallVec`, and
`ThinVec`.
Overall, it makes the code a little nicer, and has little effect on
performance. But it is a precursor to removing
`rustc_data_structures::thin_vec::ThinVec` and replacing it with
`thin_vec::ThinVec`, which is implemented more efficiently.
TraitKind -> Trait
TyAliasKind -> TyAlias
ImplKind -> Impl
FnKind -> Fn
All `*Kind`s in AST are supposed to be enums.
Tuple structs are converted to braced structs for the types above, and fields are reordered in syntactic order.
Also, mutable AST visitor now correctly visit spans in defaultness, unsafety, impl polarity and constness.
