[nmilo]

There should be a from_raw_parts-like interface for Arc that can construct one out of a pointer to a T + control block. That way you can shift the string data over and stick the control block inline (which is how I assume it's laid out in memory) and convert String to Arc<str> for free (well not free, shifting takes O(n), but so does the copying in Arc::new, so in the end you do end up avoiding an allocation for free)

[LegionMammal978]

Unfortunately, this can't really be done efficiently with how allocation works in Rust. A String is always allocated with alignment 1, but an Arc's control block requires usize alignment. An allocated pointer can't be deallocated with a different alignment than it was allocated with, nor can its alignment be changed when it is reallocated. Thus, the from_raw_parts() idea would be unusable for any types with less than usize alignment.

(And the control block really does need to be aligned, since otherwise we couldn't perform atomic operations on it.)

[Gibbon1]

> A String is always allocated with alignment 1, but an Arc's control block requires usize alignment.

I'll add this to my list of reasons why alignment is terrible.

[nmilo]

Wow that is annoying, especially since any malloc is going to give you 8-byte aligned allocations anyways.