Tracking Issue for const mem::discriminant

[lcnr]

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

History

• Implementation: make mem::discriminant const #69825
• Stabilization PR: Const stabilize mem::discriminant #116240

Unresolved issues

• Interpreter calls discriminant_for_variant on zero variant enum #89765 ICE when used on zero variant enums

[RalfJung]

The intrinsic is already implemented in Miri, you can copy that to rustc if you want.

If you do, please also submit a PR to Miri to remove the implementation there.

[stephanemagnenat]

It seems that mem::discriminant is now const, as the doc shows. Shouldn't this issue be closed?

[memoryruins]

@stephanemagnenat the const part of it is still unstable.

rust/library/core/src/mem/mod.rs

Lines 1014 to 1018 in d95745e

	#[stable(feature = "discriminant_value", since = "1.21.0")]
	#[rustc_const_unstable(feature = "const_discriminant", issue = "69821")]
	pub const fn discriminant<T>(v: &T) -> Discriminant<T> {
	Discriminant(intrinsics::discriminant_value(v))
	}

If you view nightly docs, it is much clearer, showing (const: unstable) next to the function. It will take a couple of releases before that rustdoc feature is part of stable docs.

[stephanemagnenat]

Thank you for the clarification!

[stephanemagnenat]

Btw, I saw on the PR (#69825) that people were asking for use cases to decide whether to stabilise this feature. I believe I have such a use case: I am making a parser for a DSL that must match existing data structures, and these can have members of various types. I thus have a custom Value type, and would like these data structures to return a description of themselves that contains which variant of Value they accept. I was planning to use the Discriminant<Value> type, but as I cannot have const values of this type in stable, I'll probably use the enum-kinds crate for now.

[joshtriplett]

We reviewed this in today's @rust-lang/lang meeting. It looks like there are mentions of wanting a use case to justify making this available in const. Leaving aside that there appear to have been mentions of use cases, in general we feel that if a function exists as non-const and it can be written as const then we should make it available in const.

[stephanemagnenat]

I have another use case (currently I ended up using the strum and num_traits packages), in addition to the one I mentioned on June 25 last year. I have a block-based visual programming language (see https://cand.li for a screenshot). Each block has possibly parameters (e.g. speed vector), and all blocks together are joined in a union. Sometimes, we want configurations where only a subset of blocks can be used (e.g. simple mode). It would be convenient to have such configurations const, so that they can be used in const contexts (as a global configuration for example). Ideally that feature would use the macro evoked by @aleokdev but even without it, there could be a const prototype for each block type (e.g. the one we show on the block palette) out of which the discriminant can be extracted in a const way.

[dtolnay]

@rfcbot fcp merge

Adds const on the following function, which has been stable since Rust 1.21:

// core::mem

pub const fn discriminant<T>(v: &T) -> Discriminant<T>

Example use cases benefiting from const: #69821 (comment), #69821 (comment)

[rfcbot]

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

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se
• @nikomatsakis
• @pnkfelix
• @scottmcm
• @tmandry

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!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[traviscross]

@rustbot labels +I-lang-nominated

[nikomatsakis]

@rfcbot reviewed

[rfcbot]

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

[traviscross]

@rustbot labels -I-lang-nominated

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[Dylan-DPC]

Closing this as it has been stabilised.