Eagerly expand bang macros within stability attributes

[dtolnay]

This PR implements support for macro invocations in the values of builtin stability attributes:

- #[stable(feature = "string literal")]
+ #[stable(feature = get_feature!($thing))]

This is analogous to how macro expansion is already supported in the value part of doc attributes since Rust 1.54 (https://blog.rust-lang.org/2021/07/29/Rust-1.54.0.html#attributes-can-invoke-function-like-macros).

#[doc = include_str!("README.md")]

All such macro calls must expand to a literal, like for doc and TokenStream::expand_expr.

I have limited this feature to just the following 4 builtin attributes, none of which are available in stable Rust: #[stable(...)], #[unstable(...)], #[rustc_const_stable(...)], #[rustc_const_unstable(...)]. Other builtin attributes, for example #[deprecated(...)], will continue to reject the above syntax.

Context

I began looking into refactoring the macros which generate our NonZero* integer types (https://github.com/rust-lang/rust/blob/1.74.0/library/core/src/num/nonzero.rs) in preparation for landing the refactor to NonZero<T> (#82363 (comment)).

Today there are 7 separate impl blocks per NonZero integer type. You can see this by counting the impl sections under "Implementations" in https://doc.rust-lang.org/1.74.0/std/num/struct.NonZeroU32.html. There isn't anything wrong with that today, but after the 12 different NonZero* types all get melded into a NonZero<T>, there end up being 67 impl blocks in #100428 on that type.

This is a usability regression compared to today's separate pages per type. With all those blocks expanded, Ctrl+F is obnoxious because you need to skip 12× past every match you don't care about. With all the blocks collapsed, Ctrl+F is useless. Getting to a state in which exactly one type's (e.g. NonZero<u32>) impl blocks are expanded while the rest are collapsed is annoying.

Ideally there would be one impl block per type.

Today's implementation in the standard library is arranged like this:

macro_rules! nonzero_leading_trailing_zeros {
    (...) => {
        $(
            impl $Ty { ... }
        )*
    };
}

nonzero_leading_trailing_zeros!(...);

macro_rules! nonzero_bits {
    (...) => {
        $(
            impl $Ty { ... }
        )*
    };
}

nonzero_bits!(...);
...

The thing that makes it difficult to transpose this into something that produces 1 impl block per type, while remaining maintainable, is that composing stability attributes using a macro is impossible.

rust/library/core/src/num/nonzero.rs

Lines 179 to 192 in c2ec908

	nonzero_integers! {
	#[stable(feature = "nonzero", since = "1.28.0")] #[rustc_const_stable(feature = "nonzero", since = "1.28.0")] NonZeroU8(u8);
	#[stable(feature = "nonzero", since = "1.28.0")] #[rustc_const_stable(feature = "nonzero", since = "1.28.0")] NonZeroU16(u16);
	#[stable(feature = "nonzero", since = "1.28.0")] #[rustc_const_stable(feature = "nonzero", since = "1.28.0")] NonZeroU32(u32);
	#[stable(feature = "nonzero", since = "1.28.0")] #[rustc_const_stable(feature = "nonzero", since = "1.28.0")] NonZeroU64(u64);
	#[stable(feature = "nonzero", since = "1.28.0")] #[rustc_const_stable(feature = "nonzero", since = "1.28.0")] NonZeroU128(u128);
	#[stable(feature = "nonzero", since = "1.28.0")] #[rustc_const_stable(feature = "nonzero", since = "1.28.0")] NonZeroUsize(usize);
	#[stable(feature = "signed_nonzero", since = "1.34.0")] #[rustc_const_stable(feature = "signed_nonzero", since = "1.34.0")] NonZeroI8(i8);
	#[stable(feature = "signed_nonzero", since = "1.34.0")] #[rustc_const_stable(feature = "signed_nonzero", since = "1.34.0")] NonZeroI16(i16);
	#[stable(feature = "signed_nonzero", since = "1.34.0")] #[rustc_const_stable(feature = "signed_nonzero", since = "1.34.0")] NonZeroI32(i32);
	#[stable(feature = "signed_nonzero", since = "1.34.0")] #[rustc_const_stable(feature = "signed_nonzero", since = "1.34.0")] NonZeroI64(i64);
	#[stable(feature = "signed_nonzero", since = "1.34.0")] #[rustc_const_stable(feature = "signed_nonzero", since = "1.34.0")] NonZeroI128(i128);
	#[stable(feature = "signed_nonzero", since = "1.34.0")] #[rustc_const_stable(feature = "signed_nonzero", since = "1.34.0")] NonZeroIsize(isize);
	}

The UI test added in this PR (tests/ui/stability-attribute/eager-expansion.rs) is illustrative of the direction I was headed before realizing this was not supported.

[rustbot]

r? @TaKO8Ki

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[petrochenkov]

I'm quite strongly against doing this.
Assigning to myself to look at this in more detail some time later.

[Mark-Simulacrum]

The thing that makes it difficult to transpose this into something that produces 1 impl block per type, while remaining maintainable, is that composing stability attributes using a macro is impossible.

It seems like an alternative might be some doc(merge) or whatever annotation to teach rustdoc to not preserve the impl block boundaries, if we don't want that at all for these types?

[TaKO8Ki]

r? @petrochenkov

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[petrochenkov]

(I should get to this on new year holidays.)

[bors]

☔ The latest upstream changes (presumably #119146) made this pull request unmergeable. Please resolve the merge conflicts.

[dtolnay]

• Rebased to resolve conflict with Remove DiagCtxt API duplication #119146

[bors]

☔ The latest upstream changes (presumably #119088) made this pull request unmergeable. Please resolve the merge conflicts.

[dtolnay]

• Rebased to resolve conflict with Suggest Upgrading Compiler for Gated Features #119088

[dtolnay]

In #118665 I discovered that tidy requires being able to see #[stable(feature = "…", since = "…")] written out with string literals; #[stable(feature = $feature, since = $since)] or #[stable(feature = feature!(…), since = since!(…))] will never cut it.

tidy error: library/core/src/num/nonzero.rs:67: malformed stability attribute: missing `feature` key
tidy error: library/core/src/num/nonzero.rs:82: malformed stability attribute: missing `feature` key
tidy error: library/core/src/num/nonzero.rs:98: malformed stability attribute: missing the `since` key
tidy error: library/core/src/num/nonzero.rs:112: malformed stability attribute: missing `feature` key
tidy error: library/core/src/num/nonzero.rs:450: malformed stability attribute: missing `feature` key
some tidy checks failed

I am still interesting in seeing something like this PR made to work, but specifically for stability this may not be worth doing now, and #118406 (comment) tells me there isn't a path to extend this to other kinds of inert attrs (like deprecated).