size_of cannot be called outside of a function

[dymk]

I realize this isn't an issue that can be fixed by changing the definition of sys::size_of, however it seems like Rust (like any systems programming language) should have a mechanism for determining the size of a type.

a-la:

pub type MyType = u32;
// Couldn't really be done like this, but you get the idea
pub static MyTypeBits = sys::sizeof::<MyType>() * 8; 

Could be implemented with a macro? Something like size_of!(MyType).

[thestinger]

It can't be implemented as a macro or syntax extension, because they have no knowledge of types. They just generate an AST.

[catamorphism]

Not sure what changes would need to be made to make this work, but as it would likely entail changing the constant language, nominating for milestone 1, well-defined

[catamorphism]

Accepted for milestone 1, well-defined

[pcwalton]

Nominating as I believe this is totally out of scope for 1.0.

[pnkfelix]

We are probably not going to get to this for 1.0; we've reserved the sizeof keyword so we should be able to put something in post-1.0.

[thestinger]

Extending constant expressions or adding CTFE would need to go through an RFC, and I don't think there's consensus for adding an ever increasing number of special cases (like size_of, align_of and friends). It's sad that Rust can't do this, but it's not a backwards compatibility risk, it's not a bug and a good solution is unclear - not bug tracker material.