Feature Name: remove-static-assert
Start Date: 2015-04-28
RFC PR: rust-lang/rfcs#1096
Rust Issue: https://github.com/rust-lang/rust/pull/24910

Summary

Remove the static_assert feature.

Motivation

To recap, static_assert looks like this:

#![feature(static_assert)]
#[static_assert]
static assertion: bool = true;

If assertion is false instead, this fails to compile:

error: static assertion failed
static assertion: bool = false;
                         ^~~~~

If you don’t have the feature flag, you get another interesting error:

error: `#[static_assert]` is an experimental feature, and has a poor API

Throughout its life, static_assert has been… weird. Graydon suggested it in May of 2013, and it was implemented shortly after. Another issue was created to give it a ‘better interface’. Here’s why:

The biggest problem with it is you need a static variable with a name, that goes through trans and ends up in the object file.

In other words, assertion above ends up as a symbol in the final output. Not something you’d usually expect from some kind of static assertion.

So why not improve static_assert? With compile time function evaluation, the idea of a ‘static assertion’ doesn’t need to have language semantics. Either const functions or full-blown CTFE is a useful feature in its own right that we’ve said we want in Rust. In light of it being eventually added, static_assert doesn’t make sense any more.

static_assert isn’t used by the compiler at all.

Detailed design

Remove static_assert. Implementation submitted here.

Drawbacks

Why should we not do this?

Alternatives

This feature is pretty binary: we either remove it, or we don’t. We could keep the feature, but build out some sort of alternate version that’s not as weird.

Unresolved questions