Warn for Vec<Box<T>> where T: Sized

[Kampfkarren]

A fellow Rust programmer pointed me to this mistake in my code:

I'm quite new to Rust, but is there ever a point where Vec<Box<T: Sized>> is correct?

[sinkuu]

For example, it might make sense when T is a very large type. Calling .swap(0, 1) on Vec<T> will memcpy size_of::<T>() * 3, but it would be pointer_size * 3 for Vec<Box<T>>.

[benma]

Another example: reallocation of the vector when growing it is slower and copies more bytes when the size of T is large.

[wiiznokes]

Could this warning be deactivate when the struct is large ?

[y21]

@wiiznokes The lint already checks that the type's size is lower than a threshold to avoid linting on large types. You can override vec-box-size-threshold to a lower value in config.toml (default is 4096 bytes)