Stabilizable subset of const generics

[withoutboats]

Proposal

We should create a new feature min_const_generics, which contains a subset of the const generics feature suitable for stabilization.

This will impose two restrictions on const generics:

1. They can only have the type of an integral primitive type (integers, bool, and char).
2. Expressions used in const generic position must be either:
   • Just a const generic name in scope (e.g. N)
   • A const expression which contains on no free type or const parameters.

The latter will be implemented simply syntactically: if an expression contains a type or const parameter in its macro-expanded AST, it will error, unless it is just the name of a const generic.

This feature will not depend on lazy normalization of consts.

Mentors or Reviewers

@eddyb @varkor @oli-obk

Process

The main points of the Major Change Process is as follows:

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[oli-obk]

Kicking off the FCP. I personally see no difficulties here

@rfcbot fcp merge

[rfcbot]

Team member @oli-obk has proposed to merge this. The next step is review by the rest of the tagged team members:

• @eddyb
• @estebank
• @matthewjasper
• @nagisa
• @nikomatsakis
• @oli-obk
• @petrochenkov
• @pnkfelix
• @varkor
• @wesleywiser

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[eddyb]

We're not doing seconding for this?

[nikomatsakis]

For public facing changes, FCP is required, but for stabilizations it's usually attached to the PR. It seems like the first step here is going to be carving out a distinct feature gate for this subset and then stabilizing that, and I don't think that necessarily requires FCP. Nonetheless, I approve of the idea.

[spastorino]

Prioritization automation failed because the bot didn't add the proposed-final-comment-period label. I've fixed T-compiler agenda manually. Maybe @rfcbot doesn't have permission to add labels to issues in this repo?.

cc @Mark-Simulacrum

[Mark-Simulacrum]

Added rfcbot with write permissions to this repo.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔