Implement str to [u8] conversion for refcounted containers

[udoprog]

This seems motivated to complete the APIs for shared containers since we already have similar allocation-free conversions for strings like From<Box<[u8]>> for Box<str>.

Insta-stable since it's a new trait impl?

[rust-highfive]

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[rust-highfive]

r? @thomcc

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[thomcc]

The implementation seems fine and I'm not opposed, but

r? rust-lang/libs-api @rustbot label +T-libs-api

[dtolnay]

@rust-lang/libs-api:

impl From<Rc<str>> for Rc<[u8]> {…}
impl From<Arc<str>> for Arc<[u8]> {…}

This is the Rc and Arc version of what already exists for Box:

rust/library/alloc/src/boxed.rs

Lines 1508 to 1509 in 1e6fe58

	#[stable(feature = "boxed_str_conv", since = "1.19.0")]
	impl<A: Allocator> From<Box<str, A>> for Box<[u8], A> {

@rfcbot fcp merge

[rfcbot]

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

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se
• @yaahc

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. 🔔

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[dtolnay]

@bors r+

[bors]

📌 Commit 100006b has been approved by dtolnay

[bors]

⌛ Testing commit 100006b with merge f75d884...

[bors]

☀️ Test successful - checks-actions
Approved by: dtolnay
Pushing f75d884 to master...

[rust-timer]

Finished benchmarking commit (f75d884): comparison url.

Summary: This benchmark run did not return any relevant results.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

[petertodd]

If we're going to allow this, we should also update the Safety notes for get_mut_unchecked to warn about the situation where a non-cloning conversion has been performed.

Also note how this conversion is safe only because both types are Freeze.