New lint: Prefer mem::take over Vec::split_off(0)

[tesuji]

let mut vec = vec![1,2,3];
let vec2 = vec.split_off(0);

Should be written as

let vec2  = std::mem::take(&mut vec);

[tesuji]

Oh, wait. The semantics between two functions are different.
Vec::split_off leave the caller with same capacity while mem::take leave call with default value
(in this case an empty Vec).

[basil-cow]

I think vec.drain(..).collect() better represents the intention behind vec.split_off(0) (assuming it doesn't change capacity, which I don't think it is but seems like it's not specified either).

[Zalathar]

Note that the current implementation of split_off has special handling for the 0 case, which avoids the overhead of a memcpy but also results in the returned vector potentially having much more capacity than necessary.

So code that deliberately uses split_off(0) instead of mem::take might not actually be managing capacity in the way the author expects.

See rust-lang/rust#119913 for more context.

[andre-digiexam]

@rustbot claim