constify len and is_empty functions of string and vec

[Luro02]

No description provided.

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[Centril]

cc @oli-obk r? @alexcrichton

[oli-obk]

Iirc we had a PR for this before and closed it because you can't use these because you can't really create constant strings

[Luro02]

Is it fundamentally impossible to create a const String/Vec or just not possible right now? I looked a bit through the function calls and landed at

rust/src/libcore/intrinsics.rs

Line 1602 in d28a9c3

pub unsafe fn write_bytes<T>(dst: *mut T, val: u8, count: usize) {

, which seems to be some external function? So does this mean, that you can't make a memset const?

[Centril]

You can create an empty string / vector but not one which has allocated, so you'll always get .is_empty() == true which isn't very exciting. That said, it seems obvious to me that we will want these to be const eventually so I don't personally mind the order in which we stabilize.

[memoryruins]

There were also concerns about uniformity in the previous PR #55278 (comment)

[Luro02]

So just for my own understanding, a const function cannot allocate memory?
For example this could be quite useful:

const fn takes_string(value: String) -> usize {
     value.len() + 420 / 10
} // the size of value is already known, therfore it can be const?

let mut string = String::new();
string.push_str("Hello");

takes_string(string);

[Centril]

So just for my own understanding, a const function cannot allocate memory?

Not yet, but there are plans.

[alexcrichton]

Is there a clear use case that this is enabling at this time? If not I would personally prefer to lean on the side of not merging this at this time.

[Luro02]

There is no reason anymore. I just thought it was weird, that len wasn't const, because it just returns a number/variable.

[alexcrichton]

Ok in that case I'm gonna go ahead and close this, and we can reconsider once these can be useful in a const context.