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

Pierre Talbot ptalbot at hyc.io
Tue Jan 28 14:15:03 PST 2014


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 
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.

Pierre Talbot

[1] https://careers.mozilla.org/en-US/position/oZO7XfwB
[2] http://hyc.io/rust-ctfe-proposal.pdf

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