What expressions are allowed as the array length N in [_; N]?
As of Rust 1.24.1, the array length basically needs to either be a numeric literal or a "regular" constant that is a usize. There's a small amount of constant evaluation that exists today, but it's more-or-less limited to basic math.
a perfectly valid expression in other contexts, just not as a length parameter to an array
Array lengths don't support generic parameters. (#43408)
this is not a particularly good error message
Error message should be improved for associated consts in array lengths (#44168)
I would expect some restriction akin to
constexpr
This is essentially the restriction, the problem is that what is allowed to be used in a const is highly restricted at the moment. Notably, these aren't allowed:
- functions (except to construct enums or structs)
- loops
- multiple statements / blocks
Work on good constant / compile-time evaluation is still ongoing. There are a large amount of RFCs, issues, and PRs improving this. A sample:
- Const fn tracking issue (RFC 911)
- Allow locals and destructuring in const fn (RFC 2341)
- Allow
ifandmatchin constants (RFC 2342)