impl From<Cow> for Box, Rc, and Arc

[cuviper]

These forward Borrowed/Owned values to existing From impls.

• Box<T> is a fundamental type, so it would be a breaking change to add a blanket impl. Therefore, From<Cow> is only implemented for [T], str, CStr, OsStr, and Path.
• For Rc<T> and Arc<T>, From<Cow> is implemented for everything that implements From the borrowed and owned types separately.

[rust-highfive]

r? @cramertj

(rust_highfive has picked a reviewer for you, use r? to override)

[pickfire]

Looks good to me.

[pickfire]

I will probably create another pull request for Path.

[cuviper]

@pickfire What do you mean? I did implement for Box<Path> here.

[pickfire]

impl AsRef<Path> for Cow<'_, str> is that possible?

[cuviper]

impl AsRef<Path> for Cow<'_, str> is that possible?

Oh, sure, that seems fine. It's tangential to this PR though.

Let's try another reviewer here...

r? @dtolnay

[dtolnay]

I think I am on board with all of these. Thanks @cuviper.

[dtolnay]

@rust-lang/libs:
@rfcbot fcp merge

impl From<Cow<str>> for Box<str> {...}
impl From<Cow<CStr>> for Box<CStr> {...}
impl From<Cow<OsStr>> for Box<OsStr> {...}
impl From<Cow<Path>> for Box<Path> {...}

impl<T> From<Cow<[T]>> for Box<[T]>
where
    T: Copy,
{...}

impl<'a, B> From<Cow<'a, B>> for Rc<B>
where
    B: ToOwned + ?Sized,
    Rc<B>: From<&'a B> + From<B::Owned>,
{...}

impl<'a, B> From<Cow<'a, B>> for Arc<B>
where
    B: ToOwned + ?Sized,
    Arc<B>: From<&'a B> + From<B::Owned>,
{...}

All of the impls follow the same pattern:

fn from(cow: Cow<...>) -> Self {
    match cow {
        Cow::Borrowed(v) => Self::from(v),
        Cow::Owned(v) => Self::from(v),
    }
}

[rfcbot]

Team member @dtolnay has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @SimonSapin
• @dtolnay
• @sfackler
• @withoutboats

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[dtolnay]

@bors r+

[bors]

📌 Commit 22efd95 has been approved by dtolnay

[bors]

⌛ Testing commit 22efd95 with merge 914adf0...

[bors]

☀️ Test successful - checks-azure
Approved by: dtolnay
Pushing 914adf0 to master...

[jplatte]

This should get a relnotes label and milestone of 1.45, no?

[cuviper]

@jplatte as a stable library addition, yes, I suppose so.