Tracking Issue for Rust 2024: Reserve `gen` keyword

[traviscross]

This is a tracking issue for reserving the gen keyword in Rust 2024 according to:

• Add gen blocks and reserve keyword in Rust 2024 rfcs#3513

The feature gate for the issue is #![feature(gen_blocks)].

About tracking issues

Tracking issues are used to record the overall progress of implementation. They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions. A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature. Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Reserve the gen keyword in nightly Rust 2024.
  • Implement gen blocks in the 2024 edition #116447
• Implement migration lints.
  • Deny gen keyword in edition_2024_compat lints #123680
• N/A: Add documentation to the dev guide.
  • See the instructions.
• Add documentation to the reference.
  • Document gen keyword as reserved in Rust 2024 reference#1501
• Add documentation to the edition guide.
  • 2024: Document reserving gen keyword edition-guide#300
• Ensure ready for Rust 2024 stabilization.

Unresolved Questions

• Whether we should use some keyword other than gen.
  • Bikeshedding the 'gen' keyword & syntax #123731
• Whether we should reserve gen as a full keyword or as a contextual one.

Related

• Tracking Issue for gen blocks and functions (gen_blocks) #117078
• keyword-idents-2024 unable to migrate loop labels or lifetimes #125986
• Implement raw lifetimes and labels ('r#ident) #126452
• Deny keyword lifetimes pre-expansion #126762

Implementation history

• Implement gen blocks in the 2024 edition #116447
• False positive in keyword_idents_2024 #124862

cc @oli-obk @compiler-errors

[oli-obk]

Does the gen_blocks feature need to be stabilized for the 2024? or is it fine to just stabilize the syntax in the parser?

nope, just syntax is fine, all that is needed to fix this issue is writing documentation

[jhpratt]

Minor bug in the lint: #124862

[zachgrayio]

what will it take to support this in rust-analyzer?

[ehuss]

Results from the crater run showed a few limitations of the migration lint, which I don't think there is much we can do about. The following is a summary:

attribute macro issues:

use thiserror::Error;
#[derive(Error, Debug)]
pub enum Error {
    #[error("it is {gen}")]
    A{gen: i32}
}

sn_consensus@3.5.2
reductionml-core@0.1.0 (via schemars macro)
hermit-toolkit-hmip20@0.1.0 (via schemars macro)
hermit-toolkit-hmip721@0.1.1 (via schemars macro)
hermit-toolkit-permit@0.1.0 (via schemars macro)
hermit-toolkit-utils@0.1.0 (via schemars macro)
hermit_toolkit_hmip21@0.1.5 (via schemars macro)
https://github.com/mahamzaidi/secret-marketplace (via schemars macro)
https://github.com/kubecfg/kubit (via schemars macro)
https://github.com/hatuank97lhp/ptrans2 (via schemars macro)
https://github.com/chinhnotional/custom-snip721 (via schemars macro)

proc-macro got confused via quote!:

let gen = 123;
quote!(let a = #gen);

trait-gen-0.3.2
llml_derive_crate@0.1.0

build.rs generated code:
boring-sys@4.6.0
bssl-cmake-sys@0.1.2405130
switchtec-user-sys
wolfssl-sys@1.0.0

Pulled via include! which isn't fixed:
zcash_script@0.1.16

[traviscross]

We reviewed this in the edition call today. This item is ready for Rust 2024. It's implemented; the automatic migration lints have landed; we've tested those with crater, and the item is documented in the reference and in the edition guide.

Thanks to @oli-obk and to @compiler-errors for pushing this forward, and thanks to @ehuss for writing up the content for the edition guide and for running crater and analyzing its results.