Fixed length arrays - Why (Question about syntax)

RustyYato · December 4, 2018, 10:24pm

This has been asked before

github.com/rust-lang/rust

Array length inference?

opened 12:21AM - 05 Mar 17 UTC

closed 04:07PM - 07 Mar 19 UTC

SimonSapin

A-const-fn A-inference C-feature-request

It would be nice if `_` were allowed instead of an expression for the length in …array types and array expressions, in cases where the length can be inferred based on context: ```rust fn main() { let a: [u8; 4] = [0; _]; let b: [u8; _] = a; } ``` rustc 1.17.0-nightly (b1e31766d 2017-03-03) ```rust error: expected expression, found `_` --> a.rs:2:26 | 2 | let a: [u8; 4] = [0; _]; | ^ error: expected expression, found `_` --> a.rs:3:17 | 3 | let b: [u8; _] = a; | ^ error: aborting due to 2 previous error ``` In comparison, the item type in array expressions and array types can be inferred today: ```rust fn main() { let a: [u8; 4] = ["0".parse::<_>().unwrap(); 4]; let b: [_; 4] = a; } ```

and const generics may help with it

github.com/rust-lang/rust

Tracking issue for const generics (RFC 2000)

opened 11:17PM - 14 Sep 17 UTC

closed 03:26PM - 21 Mar 22 UTC

withoutboats

A-typesystem B-RFC-approved T-lang T-compiler A-const-fn C-tracking-issue A-const-generics requires-nightly F-const_generics S-tracking-impl-incomplete

Tracking issue for rust-lang/rfcs#2000 Updates: - 2 May 2019: https://github….com/rust-lang/rust/issues/44580#issuecomment-488819344 - 19 Oct 2019: https://github.com/rust-lang/rust/issues/44580#issuecomment-544155666 - 2 Jan 2020: https://github.com/rust-lang/rust/issues/44580#issuecomment-570191702 - 22 Jul 2020: https://github.com/rust-lang/rust/issues/44580#issuecomment-662543117 - 17 Nov 2020: https://github.com/rust-lang/rust/issues/44580#issuecomment-728913127 - 11 Dez 2021: https://github.com/rust-lang/rust/issues/44580#issuecomment-991782799 If you want to help out, take a look at the [open const generics issues](https://github.com/rust-lang/rust/labels/A-const-generics) and feel free to ping @varkor, @eddyb, @yodaldevoid, @oli-obk or @lcnr for help in getting started! --- Blocking stabilization: - [ ] Design: - [x] Resolving ordering of const and type parameters, with default parameters - [ ] Decide what the best UX / implementation cost balance is for unifying abstract const expressions. - [ ] How we determine well formedness of const expressions. - [x] Implementation - [ ] Documentation - [ ] rustc guide --- Remaining implementation issues: - [ ] Resolve various `FIXME(const_generics)` comments. - [ ] Resolve concerns with canonicalisation / lazy normalisation. - [ ] Investigate handling of const parameters in patterns. - [ ] Add more tests. - [ ] Implement defaults for const parameters (`FIXME(const_generics_defaults)`). - [ ] Fix other [A-const-generics issues](https://github.com/rust-lang/rust/labels/A-const-generics). - [ ] Audit uses of `has_infer_types`. - [x] Forbid complex expressions for const arguments involving parameters (for now), e.g. `{X * 2}`. - [ ] Audit diagnostics (e.g. https://github.com/rust-lang/rust/pull/76401#discussion_r484819320).

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Fixed length arrays - Why (Question about syntax)

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