Add lint for mem::replace(.., None).

[JayKickliter]

This PR adds a lint suggesting Option::take() as an alternative to mem::replace(.., None):

error: replacing an `Option` with `None`
 --> $DIR/mem_replace.rs:8:13
  |
8 |     let _ = mem::replace(&mut an_option, None);
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider `Option::take()` instead: `an_option.take()`
  |
  = note: `-D clippy::mem-replace-option-with-none` implied by `-D warnings`

error: aborting due to previous error

I have run across this in both my company's and third party codebases. At least one guide suggests favoring take() as well:

Note, however, that if we are using an Option and want to replace its value with a None, Option’s take() method provides a shorter and more idiomatic alternative.

Since this is my first Clippy contribution, I'm all ears to any suggestions to improve this PR.

[JayKickliter]

One thing I’m not sure about is this early check to make sure the function call has two arguments. It seems like a low-cost check, but maybe it’s premature optimization?

[ghost]

I came across this same suggestion in a reddit thread a while ago: https://www.reddit.com/r/rust/comments/94elrg/blog_some_slight_improvements/

[JayKickliter]

It just occurred to me that this lint will match:

    let mut an_option = Some(1);
    let _ = mem::replace(&mut an_option, None);

but not:

    let mut_ref_an_option = &mut an_option;
    let _ = mem::replace(mut_ref_an_option, None);

Let figure out how to do that and update this PR...

[flip1995]

Oh good catch! I missed that one. The problem is that match_type(cx, expr_ty(replaced), path) fails due to the reference. There's already a function to solve this in utils.

[JayKickliter]

There's already a function to solve this in utils

Thanks for the suggestion. I looked through utils but nothing stood out. Any idea what the function is called?

[flip1995]

It should be
https://github.com/rust-lang-nursery/rust-clippy/blob/125907ad08853b92d35e86aecebcf0f784f348d5/clippy_lints/src/utils/mod.rs#L613-L618

or just walk_ptrs_ty. Not sure if expr_ty() returns a Ty::Ty or a hir::Ty.

EDIT: Here's an example on how to use it (it's walk_ptrs_ty):
https://github.com/rust-lang-nursery/rust-clippy/blob/f166b7d2f4fb184e757c52973054ec07a7b8be60/clippy_lints/src/bytecount.rs#L57-L59

[JayKickliter]

I updated the lint to match both &mut an_option and an_option. I think this solution is sound:
https://github.com/rust-lang-nursery/rust-clippy/blob/79cda3bb1ef7f4261bf0fb18bbb141127d31c5a5/clippy_lints/src/mem_replace.rs#L55-L59

[flip1995]

Oh the problem was something else. Oops, sorry for the wrong pointers! LGTM

[JayKickliter]

@flip1995 no problem, you pointed me to something useful I didn't know about. Thanks!

[JayKickliter]

@Manishearth not sure if necessary, but:

I license past and future contributions under the dual MIT/Apache-2.0 license, allowing licensees to chose either at their option.

[Manishearth]

Sweet, thanks. I was going to open a separate issue for the new contributors once the other issues closed.

[Manishearth]

but this works too

[Manishearth]

@JayKickliter could you also leave this comment on #3230 for completeness' sake?

[JayKickliter]

Done