Result<T, E> where size_of::<T>() < 128 and size_of::<E> >= 128

[llogiq]

Those will cause LLVM to issue memcpy calls to copy them around. Boxing the error type will push them below LLVM's size threshold, thus possibly improving performance, unless the Err case is more likely than the Ok case.

[andjo403]

feels like the large_enum_variant check but with a limit of 128 by default instead of 200.

[llogiq]

Sort of, but specialized to Result, because in most cases the Err case isn't quite as hot as the Ok case, so the wins on the latter can outweigh the losses from boxing on the former.

[thomcc]

Also large_enum_variant doesn't fire on Result<T, E>, since (I think?) it's looking at the definition of the enum.