Tracking Issue for const_unicode_case_lookup

[mx00s]

Feature gate: #![feature(const_unicode_case_lookup)]

This is a tracking issue for making char::is_lowercase and char::is_uppercase const.

Public API

• https://doc.rust-lang.org/nightly/core/primitive.char.html#method.is_uppercase
• https://doc.rust-lang.org/nightly/core/primitive.char.html#method.is_lowercase

Example:

const CAPITAL_DELTA_IS_UPPERCASE: bool = 'Δ'.is_uppercase();

Steps / History

• Implementation: Make char::is_lowercase and char::is_uppercase const #101401
• Final comment period (FCP)¹
• Stabilization PR

Unresolved Questions

• None yet.

Footnotes

1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩

[DaniPopes]

This has been unstable for more than a year, and since there are no reported issues or blockers I think this can be stabilized.

The full API is making char::{is_lowercase,is_uppercase} const-stable.

[dtolnay]

From skimming the implementation PR, I am apprehensive about seeing it convert many Unicode tables from static to const. Consts are defined to behave as if their definition is copy-pasted at every use. Statics are not; they appear at most once, at a single address. See also #82676.

I would not want to regret being forced to define Unicode tables as const when we later find that static's semantics are more appropriate for them.

It would be good to get reassurance from a compiler perspective about whether committing to representing Unicode tables as const is going to be problematic.

[RalfJung]

From skimming the implementation PR, I am apprehensive about seeing it convert many Unicode tables from static to const. Consts are defined to behave as if their definition is copy-pasted at every use. Statics are not; they appear at most once, at a single address.

#131641 resolves that. So I think this is ready for const-stabilization?

[dtolnay]

• https://doc.rust-lang.org/nightly/core/primitive.char.html#method.is_uppercase
• https://doc.rust-lang.org/nightly/core/primitive.char.html#method.is_lowercase

@rfcbot fcp merge

[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

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.

[joshtriplett]

This looks great. Could we do the same for the other char methods, like is_digit and is_alphabetic?

[RalfJung]

@Amanieu @BurntSushi @m-ou-se friendly FCP checkbox reminder. :) This function is long stable, so making it const-stable should be uncontroversial.

[RalfJung]

This looks great. Could we do the same for the other char methods, like is_digit and is_alphabetic?

is_digit is tracked at #132241. Looks like nobody tried making is_alphabetic const-callable yet.

EDIT: is_alphabetic is tricky since it needs a quite complicated function that currently uses a whole bunch of non-const machinery, such as binary search and iterators. So that one is out of reach currently.

EDIT2: The only other one we can currently make const is is_whitespace: #132500

[rfcbot]

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