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

Pierre Talbot ptalbot at hyc.io
Tue Jan 28 15:41:53 PST 2014

The way it is implemented in Rust is by using the libc, but the 
requirement #6 says we can't call external function, so implicitly the 
problem is solved. I'm agree that it isn't formal, but I can't come up 
with a better solution for now.

You made me think of another requirements (so basic that I forgot to 
mention it):

* A function is eligible for CTFE if every function calls inside this 
function are themselves eligible.

On 01/28/2014 11:27 PM, Josh Matthews wrote:
> Out of that list of requirements, #5 (doesn’t perform I/O actions) is 
> the one that strikes me as least well-defined. Could you elaborate on 
> how you would enforce it?
> Cheers,
> Josh
> On 28 January 2014 14:15, Pierre Talbot <ptalbot at hyc.io 
> <mailto:ptalbot at hyc.io>> wrote:
>     Hi,
>     The Mozilla foundation proposes research internships [1] and the
>     CTFE optimization in the Rust compiler seems to be a really
>     exciting project. I wrote a proposal [2] that I'll send with my
>     application and so I'd like to share it with you and discuss about
>     bringing CTFE inside Rust.
>     Here a non-exhaustive summary of key points in my proposal.
>     First of all, we need to establish when CTFE is triggered, I found
>     two contexts (denoted as a hole []):
>     * Inside a immutable static variable ("static" ident ’:’ type ’=’
>     [] ’;’).
>     * In a vector expression (’[’ expr ’,’ ".." [] ’]’).
>     Next in a similar way than with "inline attributes" we might want
>     to add these new attributes:
>     * #[ctfe] hints the compiler to perform CTFE.
>     * #[ctfe(always)] asks the compiler to always perform CTFE
>     resulting in a
>     compiler error if it’s impossible.
>     * #[ctfe(never)] asks the compiler to never perform CTFE resulting
>     in a compiler
>     error if this function is called in a CTFE context.
>     The rational behind this is that some functions might want to
>     disallow CTFE, for example if they manipulate machine-dependent
>     data (such as playing with endianness). Some might want to be
>     designed only for compile-time and so we want to disable run-time
>     execution. Finally others might hints the compiler to "try to
>     optimize whenever you can", of course if the function contains
>     infinite loop for some input, the compilation might not terminate.
>     I propose some requirements on function eligible for CTFE (see the
>     proposal for references to the Rust manual):
>     1. Its parameters are evaluable at compile-time.
>     2. It isn’t a diverging function.
>     3. It isn’t an unsafe function.
>     4. It doesn’t contain unsafe block.
>     5. It doesn’t perform I/O actions.
>     6. The function source code is available to the compiler. It
>     mustn’t be in an external
>     block, however it can be an extern function.
>     In this proposal, you'll also find a pseudo-coded algorithm,
>     related work (in D and C++), and much more :-)
>     If you have any suggestions or corrections, do not hesitate. Also,
>     feel free to ask questions.
>     Regards,
>     Pierre Talbot
>     [1] https://careers.mozilla.org/en-US/position/oZO7XfwB
>     [2] http://hyc.io/rust-ctfe-proposal.pdf
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