[rust-dev] Compile-time function evaluation in Rust

Kevin Ballard kevin at sb.org
Tue Jan 28 16:18:44 PST 2014

On Jan 28, 2014, at 4:01 PM, Pierre Talbot <ptalbot at hyc.io> wrote:

> On 01/29/2014 12:45 AM, Kevin Ballard wrote:
>> Yes, I was using #[ctfe] to mean something slightly different than you were. In my case, it meant "mark this function as eligible for CTFE, and impose all the CTFE restrictions". And it does fix the problem I mentioned, because #[ctfe] would be considered part of the function signature, not the function implementation. Everyone is already used to the idea that modifying the function signature may cause compiler errors at the call site. But the only example I can think of right now for when changing a function's _implementation_ causes call site compiler errors is when you're using C++ templates.
>> Not only that, but with your approach, changing the implementation of one function could accidentally cause a whole host of other functions to become ineligible for CTFE. And the farther apart the actual source of the problem, and the resulting error, the harder it is to diagnose and fix such errors.
>> That said, I was not aware that D already takes this approach, of allowing CTFE by default. I'm curious how it works for them, and how they handle these problems.
>> -Kevin
> They do not allow it by default, they rely on the context (as I also do) to activate it. If a function is designed with CTFE requirements, these might not change much over the time (speculation). I mean, did you ever re-factorize a function code to add unsafe, extern or I/O statements? I have the feeling that it doesn't arrive so often. But it's just an intuition...

Sure. Unsafe is sometimes used to optimize code for performance that was otherwise safe.

I could write a function intended for CTFE, using a perfectly harmless API (say, std::vec), and then have an update to the perfectly harmless API introduce unsafe code for the purposes of optimization (as has happened in std::vec in the past), and suddenly my function is no longer CTFE-eligible.


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