Add `Option::as_deref_inner: Option<&T> -> Option<&T::Target> where T: Deref`

[tesuji]

Proposal

Problem statement

Currently, we have Option::as_deref to transform &Option<T> to Option<&T::Target>. But because there is impl Deref for &T with Target = T. as_deref(opt: &Option<&T>) where T: Deref always returns Option<&T>, not what we look for, Option<&T::Target>.

Motivating examples or use cases

Given this code:

pub fn get(index: &HashMap<usize, Vec<i32>>, id: usize) -> &[i32] {
     if let Some(names) = index.get(&id) { names } else { &[] }
}

User could try to rewrite it by using combinators:

index.get(&id).unwrap_or(&[]) // compile errors: wrong type
// they try to fix the above error
index.get(&id).as_deref().unwrap_or(&[]) // the error remains: wrong type
// okay, that's unexpected, now try again
index.get(&id).map(|v| &**v).unwrap_or(&[]) // finally!

That was frustrating because as_deref promises that &Option<T> -> Option<&T::Target>. But they don't see that when T=&U, it will become &Option<&U> -> Option<&U>, which is not always what they want.

Solution sketch

Add this implementation

impl<T: Deref> Option<&T> {
    fn as_deref_inner(self) -> Option<&T::Target>; // may require a better name when stabilizing
    // mut version maybe
}
// maybe impl for Result

Or add a lint to war about this pitfall if the build passes. If the build fails, guide people to use map(|v| &**v) instead.

Alternatives

Links and related work

Zulip: https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Make.20.60Option.3C.26T.3E.3A.3Aas_deref.28.29.20returns.20Option.3C.26T.3A.3ATarget.3E.60

What happens now?

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[tgross35]

Original tracking issue and stabilization for as_deref, might be worth some time digging to figure out what the motivation for the current design were.

Any chance we could update the existing as_deref to be more flexible in a backward-compatible way, rather than adding a new method?

[tesuji]

I don't think we could because &T implements Deref: &T -> T. So this holds true for the signature of as_deref:

impl<T> Option<T> {
    fn as_deref(&self) -> Option<&T::Target> where T: Deref;
}
// replace T = &U
impl<U> Option<&U> {
    fn as_deref(&self) -> Option<&U>;
}

[programmerjake]

can we add a mut version at the same time for consistency?

[m-ou-se]

We discussed this in the libs-api meeting. We don't think this method outweighs the cost in readability: it's relatively uncommon and it's not immediately clear what it does when encountering this in code, and it doesn't seem to provide enough value over the alternative over map with &**.

[tesuji]

Ty for letting me know that. Is improving compile errors when the build fail with as_deref also considered?

[dtolnay]

Improving compile errors would be welcome.

Current error, for the motivating use case above:

error[E0308]: mismatched types
   --> src/lib.rs:5:30
    |
5   |     index.get(&id).unwrap_or(&[])
    |                    --------- ^^^ expected `&Vec<i32>`, found `&[_; 0]`
    |                    |
    |                    arguments to this method are incorrect
    |
    = note: expected reference `&Vec<i32>`
               found reference `&[_; 0]`
help: the return type of this call is `&[_; 0]` due to the type of the argument passed
   --> src/lib.rs:5:5
    |
5   |     index.get(&id).unwrap_or(&[])
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^---^
    |                              |
    |                              this argument influences the return type of `unwrap_or`

It should be possible for rustc to suggest replacing this specific instantiation of unwrap_or with map_or(&[], Vec::as_slice). Maybe similarly for some other common cases: map_or("", String::as_str).

[Skgland]

I could have also used this several times recently, the name as_deref_inner is a bit of a mouth full, and my suggestion would have been as_reref as it acts similar to &* where one dereferences and the re-references a reference. (play-ground)