`unnecessary_box_returns` don't lint on large objects

[tgross35]

Description

A lot of the reason you may Box something is if it is too large to move cheaply. For large types it wouldn't be very unusual to return a Box<T>, especially if the box existed before returning it (as in the example here) to prevent some unnecessary moves or reallocations.

It could be a good idea to not raise this lint if T is e.g. >64 bytes (or reuse the threashold for suggesting boxed enum variants).

Some relevant zulip discussion: https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/returning.20boxed.20values

#![allow(unused)]
#![warn(clippy::pedantic)]

struct Huge([u8; 500]);
struct HasHuge(Box<Huge> /* ... */);

impl HasHuge {
  fn into_huge(self) -> Box<Huge> {
    self.0
  }
}

Version

rustc 1.68.2 stable

Additional Labels

@rustbot label +C-enhancement