Tracking issue for RFC 2523, `#[cfg(version(..))]`

[Centril]

This is a tracking issue for #[cfg(version(..))] (rust-lang/rfcs#2523).

Steps:

• Implement the RFC (cc @rust-lang/compiler -- can anyone write up mentoring instructions?)
• Adjust documentation (see instructions on rustc-guide)
• Stabilization PR (see instructions on rustc-guide)

Unresolved questions:

• What is cfg(version(...)) relative to in terms of nightly compilers?

  We could check against what rustc --version says, e.g. nightly being 1.40.0, beta being 1.39.0, and stable being 1.38.0. We could also have cfg(version(...)) at most be relative to the beta compiler. See the RFC for a longer discussion.

  • Resolved in Tracking issue for RFC 2523, #[cfg(version(..))] #64796 (comment)

• Should we also support version = "..." so that crates having a MSRV below when version(...) was stabilized can use the flag?

• Dependency updates cause language changes (src tarballs are vendored without respecting lockfile #79010)

[pickfire]

@csmoe Are you working on this? If not I might want to try out this task first since @petrochenkov mentioned that this task is easier than cfg(accessible = ::path).

[csmoe]

@pickfire I didn't have much bandwidth on this, seems you have already made some progress on cfg(accessible = ::path), so feel free to check this :)
(Don't forget to assign yourself with @rustbot claim)

[pickfire]

@csmoe Nice, thanks a lot for telling me that. I did not know such thing exist.

@rustbot claim

[pickfire]

Can anyone please help to write up the mentoring instructions. Based on what I know, this should be similar to #64797 (comment)

1. Syntax:
   
   1. Add a new `sym::accessible` in https://doc.rust-lang.org/nightly/nightly-rustc/src/syntax_pos/symbol.rs.html#22.
   2. Feature gate `accessible` in [`GATED_CFGS`](https://doc.rust-lang.org/nightly/nightly-rustc/syntax/feature_gate/builtin_attrs/constant.GATED_CFGS.html). Also add `cfg_accessible` to `active.rs`: https://doc.rust-lang.org/nightly/nightly-rustc/src/syntax/feature_gate/active.rs.html#530. This will also require a new `sym::cfg_accessible`.
   3. Introduce a match arm for `sym::accessible` in [`cfg_matches`](https://doc.rust-lang.org/nightly/nightly-rustc/syntax/attr/fn.cfg_matches.html). This should look mostly like [the case for `sym::not`](https://doc.rust-lang.org/nightly/nightly-rustc/src/syntax/attr/builtin.rs.html#591).
      Here you need to extract an [`&ast::Path`](https://doc.rust-lang.org/nightly/nightly-rustc/syntax/ast/struct.Path.html) and delegate the interpretation to a function of the rough shape `fn is_accessible(sess: &ParseSess, path: &Path) -> bool { ... }`

2. Implement `fn is_accessible`.
   
   1. First do some validation. We want to emit errors if [`!path.is_global()`](https://doc.rust-lang.org/nightly/nightly-rustc/syntax/ast/struct.Path.html#method.is_global). Use [`sess.span_diagnostic.struct_span_err`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/struct.Handler.html#method.struct_span_err).

I believed step 1 and 2 should be the same. For step 3, based on what I know requires me to modify somewhere around

rust/src/libsyntax/attr/builtin.rs

Lines 566 to 568 in b56b239

	MetaItemKind::List(..) => {
	error(cfg.span, "unexpected parentheses after `cfg` predicate key")
	}

.

Based on fn is_accessible, the next thing to be done should be to implement fn min_version and then somehow reference version_check on how to do the min_version logic by checking the CFG_VERSION environment variable?

[mibac138]

Hi, @pickfire. Are you still working on this?

[pickfire]

No, I was stuck working on this and don't know how to proceed.

[mibac138]

Ok, thanks for quick response. I'll take my shot at this.
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