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

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.