[rust-dev] How to assert or get sizeof(T) at compile time?

Patrick Walton pwalton at mozilla.com
Fri Dec 28 11:19:20 PST 2012

On 12/28/12 1:40 AM, James Gao wrote:
> Hi,
> How can I static_assert(expr) or evaluate sizeof(T) at compile time?
> Having these operators/keywords, we can optmize or check many code when
> compiling. See the following snippet:
> fn dump_memory<T>(t : T) {
>    ...
>    let buff : [u8 * sizeof T] = ....;
>    statc_assert (sizeof T < 1024*1024);
>    ....
> }

You can't do either of these today, unless you write a syntax extension 
and build it into the compiler.

`sizeof()` might be nice to build into the language, for the reason you 
suggest. That said, there's a real limit to how complex we can make the 
constant evaluation system without going off the cliff of language 
complexity. Even the ability to say `[u8 * <constant expression>]` 
interacts with other language features in surprisingly complex ways 
(although I think we should support it anyway).

I suspect `static_assert` wouldn't be useful unless we added things like 
D's compile-time function evaluation or C++'s `constexpr`, which strikes 
me as crossing the line.

We already have a system planned for doing arbitrary code evaluation at 
compile time: the syntax extension system. When we get the ability to 
dynamically link syntax extensions into the compiler, you'll be able to 
perform arbitrary static assertions at compile time by writing a syntax 
extension that reports an error if some condition doesn't hold. (To make 
it really convenient to use, we'll probably want to allow syntax 
extensions to be written within the same crate that they're used.) In 
general, I think this is a more flexible and less ad-hoc system than 
compile-time function evaluation; Lisp and Scheme dialects use it to 
great effect, for instance.


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