Impl AsRef, Borrow for Ref, RefMut

[dhardy]

Summary: Implement AsRef, Borrow for std::cell::Ref, RefMut.

Sorry if I didn't exactly follow a guide. This is too small to need an RFC?

I'm not certain about the #[stable] attrs, but it seems there's little choice in the matter (given that one can't have unstable impls of existing traits).

Motivation

Why? Because we can. This is what AsRef and Borrow are for? (More generally: any impl of Deref should impl AsRef too?)

Further motivation:

trait Foo<T: ?Sized> {
    type Ref<'b>: std::borrow::Borrow<T>;
    fn borrow(&self) -> Self::Ref<'_>;
}

struct MyCell(std::cell::RefCell<String>);
impl Foo<str> for MyCell {
    type Ref<'b> = std::cell::Ref<'b>; // avoids the need to construct a new type here
    fn borrow(&self) -> Self::Ref<'_> {
        std::cell::Ref::map(self.0.borrow(), String::as_str)
    }
}

ACP: rust-lang/libs-team#215

[rust-highfive]

r? @m-ou-se

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[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[dhardy]

Hmm, the Borrow impls may violate the "soft requirements" on the Borrow type:

x.borrow() == y.borrow() should give the same result as x == y

But Ref does not impl PartialEq.

---

Unfortunately neither AsRef nor Borrow are exactly the ideal trait for the above example: AsRef does not have a blanket impl<T> AsRef<T> for T while Borrow has the above "soft requirement on behaviour" in its documentation.

[dhardy]

@rustbot label +T-libs-api -T-libs

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

    Checking rustc_resolve v0.0.0 (/checkout/compiler/rustc_resolve)
error[E0507]: cannot move out of a shared reference
   --> compiler/rustc_infer/src/infer/resolve.rs:209:32
    |
209 |               ty::ReVar(_) => Ok(self
210 | |                 .infcx
211 | |                 .lexical_region_resolutions
212 | |                 .borrow()
213 | |                 .as_ref()
213 | |                 .as_ref()
214 | |                 .expect("region resolution not performed")
    | |__________________|_______________________________________|
    | |__________________|_______________________________________|
    |                    |                                       move occurs because value has type `Option<LexicalRegionResolutions<'_>>`, which does not implement the `Copy` trait
    |                    value moved due to this method call
    |
note: this function takes ownership of the receiver `self`, which moves value
    |
735 |     pub const fn expect(self, msg: &str) -> T {
    |                         ^^^^
    |                         ^^^^
help: consider calling `.as_ref()` to borrow the type's contents
    |
214 |                 .as_ref().expect("region resolution not performed")

For more information about this error, try `rustc --explain E0507`.
error: could not compile `rustc_infer` due to previous error
warning: build failed, waiting for other jobs to finish...
warning: build failed, waiting for other jobs to finish...
error: could not compile `rustc_infer` due to previous error
error[E0507]: cannot move out of a shared reference
    --> compiler/rustc_resolve/src/lib.rs:1577:44
     |
1577 |         for (trait_name, trait_binding) in traits.as_ref().unwrap().iter() {
     |                                            ^^^^^^^^^^^^^^^^--------
     |                                            |               |
     |                                            |               value moved due to this method call
     |                                            move occurs because value has type `Option<std::boxed::Box<[(rustc_span::symbol::Ident, &NameBinding<'_>)]>>`, which does not implement the `Copy` trait
     |
note: this function takes ownership of the receiver `self`, which moves value
     |
772  |     pub const fn unwrap(self) -> T {
     |                         ^^^^
     |                         ^^^^
help: consider calling `.as_ref()` to borrow the type's contents
     |
1577 |         for (trait_name, trait_binding) in traits.as_ref().as_ref().unwrap().iter() {

error: could not compile `rustc_resolve` due to previous error
error: could not compile `rustc_resolve` due to previous error
Build completed unsuccessfully in 0:02:28

[bors]

☔ The latest upstream changes (presumably #105951) made this pull request unmergeable. Please resolve the merge conflicts.

[anden3]

Hello @dhardy! I noticed that there's a merge conflict for this PR, what's the status of it? :)

[dhardy]

Status: no feedback. I'm not going to waste time resolving CI/merge issues on a PR that no one wants to comment on.

[Noratrieb]

Hi, thank you for your PR! The process for adding new library APIs was changed and if you want to push your PR forward, you should open an ACP instead to get additional visibility. The libs API team does not have enough time to go though everything, but your change seems simple enough that it could get approved.

[dhardy]

Closing (see the ACP, rust-lang/libs-team#215).