Tracking Issue for `io::const_error!`

[joboet]

Feature gate: #![feature(io_const_error)]

This is a tracking issue for const_error!, a macro to create io::Errors from a string literal without allocating.

Public API

// std::io

macro const_error($kind:expr, $message:expr $(,)?) { ... }

Steps / History

• ACP: Expose the internal SimpleMessage struct and allow creating std::io::Errors with them libs-team#205
• Implementation: #...
• Final comment period (FCP)¹
• Stabilization PR

Unresolved Questions

• None yet.

Footnotes

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

[clarfonthey]

Is there any particular reason why this is a macro instead of a const fn?

[joboet]

io::Error is pointer sized on 64-bit platforms, which is not enough to store a &'static str, let alone an additional ErrorKind. Making this a macro is the only way¹ of achieving the no-allocation guarantee while keeping the size small (which is probably very important for performance).

Footnotes

1. At least without const_allocate, but that would only work if users explicitly requested a const call. ↩

[clarfonthey]

I see, the main issue is that you can't guarantee that your const structure is static without it. Fair enough!