Unify all attributes into one; support every item; add default

[wackbyte]

Adds support for all items and the default qualifier, all with one attribute.

Implementation notes

Behavior change

My implementation of the attribute has a slightly different behavior:

// In the original implementation:
#[fn_qualifiers()] // removes the `const` qualifier!
const fn const_fn_or_is_it() {}

// In this new implementation:
#[qualifiers()] // doesn't touch the item's qualifiers at all
const fn definitely_a_const_fn() {}

Parsing logic

In order to support const, fn, static, and type items in all positions, I had to implement custom item types to parse them specially.

// Example:
#[qualifiers(pub)]
static ITEM_STATIC: u8 = 100;

extern "C" {
    #[qualifiers(pub)]
    static FOREIGN_ITEM_STATIC: u8;
    // If this was parsed as a plain `Item`/`ItemStatic`, it would require the `= <expr>` at the end!
}

// (custom `const`, `fn`, and `type` items are required for `default` support)

Questions

Modules

Modules can't be supported due to hygiene issues.

Is this true? In my testing they seemed to work fine (except for the rust-analyzer issue I noted).

Fields

I did not implement support for named fields. I have a concept in mind, though:

#[field_qualifiers(x(pub), y(pub))]
struct InHeader {
    x: String,
    y: u32,
    // TODO: how does this work for tuple structs? `#[field_qualifiers(_0(pub))]`?
}

[wackbyte]

Note: This file is named the way it is so that cargo test ignores it (so you don't need nightly installed to compile)

[wackbyte]

I've implemented #[field_qualifiers] as I described it above.

[JohnScience]

@wackbyte You were given the contributor rights. You can merge the pull request if you want.

[wackbyte]

Thanks!

[ryoqun]

@JohnScience @wackbyte hey, this pr looks quite impressive. :) when will this be published on crates.io as new ver?

this crate is so useful for our usecase like this: solana-labs/solana#32822 (comment) thanks for the great work.

seems the latest published version v0.1.6 works for now. but I'm planning to heavily use this crate across the codebase.

[wackbyte]

@JohnScience would have the ability to do that for you.

[JohnScience]

@wackbyte You have been given access on crates.io as well

[JohnScience]

@ryoqun Restored the legacy API to avoid breakage and published the 0.2 version on crates.io. Please take a look at the note on legacy attributes.

[ryoqun]

@ryoqun Restored the legacy API to avoid breakage and published the 0.2 version on crates.io. Please take a look at the note on legacy attributes.

really thanks so much (even with polishes with bunch of v0.2.x rels); just created update pr at our side: solana-labs/solana#32838