Add `Option::take_if`

[kellerkindt]

Feature gate:
#![feature(option_take_if)]

This is a tracking issue for adding Option::retain_if similar to Option::take but requires an additional predicate.
It takes() the value if the predicate evaluates to true. If the predicate evaluates to false, the value of the Option remains Some(...). The predicate (Fn(&mut T) -> bool) also has the chance to modify the value.

struct MyStruct {
    pub some_value: Option<u32>,
    // ...
}

fn main() {
    let mut my_struct: MyStruct = out_of_thin_air();

    do_something_before(&my_struct);

    // on non-copy values, this would require a multiple line long match or if-let instead
    my_struct.some_value.take_if(|x| *x == 42);

    // also allows this
    if let Some(prev) = my_struct.some_value.take_if(|x| *x != 42) {
        // do something with the previous value
    }

    do_something_after(&mut my_struct);
    pass_on(my_struct);
}

Steps / History

• ACP: Add Option::retain libs-team#70
• Implementation: Implement Option::take_if #98935
• Final comment period (FCP)¹
• Stabilization PR

Unresolved Questions

• Provide both immutable and mutable retain methods (keeping it analogue to Vec)?
  Consensus seems to be to only implement retain take_if but with &mut T

Footnotes

1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩

[ivan770]

Is there a reason to have both retain and retain_mut when we already have &mut self in retain?

[kellerkindt]

To be honest, personally I can't think of a concrete reason at the moment. Vec has both (although Vec::retain redirects to Vec::retain_mut internally) and I used it as impl orientation.

The question would then also be which name to keep: retain or retain_mut. A *_mut suffixed method without the corresponding not-suffixed one seems odd, but retain and a &mut T in the callback might be unexpected after using Vec::retain{,_mut}?

I am also not sure about all the ~const stuff which I copied over from the Option::filter impl.

edit: I am totally fine with only retain and &mut T

[zirconium-n]

Vec::retain is a mistake and should not be followed. I prefer keep 'retain' only with '&mut' signature.​

[kellerkindt]

Done :)

[kellerkindt]

@ivan770 Do you think there are further changes needed?

[ivan770]

@ivan770 Do you think there are further changes needed?

Signature looks good now, can't say anything about trait bounds though.

[mirror-kt]

Hi! Any progress on this unstable-feature?
It's not that big a feature and I'd like to use it so much that I'd like to move forward with the stabilization process if there are no major issues. Any concerns come to mind?