Optimize Rc::<str>::default() implementation

[Kijewski]

This PR lets impl Default for Rc<str> re-use the implementation for Rc::<[u8]>::default(). The previous version only calculted the memory layout at runtime, even though it should be known at compile time, resulting in an additional function call.

The same optimization is done for Rc<CStr>.

Generated byte code: https://godbolt.org/z/dfq73jsoP.

Resolves #135784.

Cc @Billy-Sheppard.

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[ibraheemdev]

Approving this without a perf run because the codegen is clearly better (and a perf run is unlikely to provide meaningful results). Thanks!

@bors r+

[bors]

📌 Commit e090db8 has been approved by ibraheemdev

It is now in the queue for this repository.

[mati865]

FWIW, there are guidelines on how to write comments regarding the use of unsafe functions: https://std-dev-guide.rust-lang.org/policy/safety-comments.html

[Kijewski]

Normally I follow that guideline, but I adapted to what I found in the file I changed: No "SAFETY" keywords and short sentences (and most unsafe blocks have no comments at all). IMHO someone should give the file some love and adapt the code to current guidelines.

[mati865]

Definitely there there are some SAFETY keywords in this file, although as you have noticed, they are not present everywhere they should be.

IMHO someone should give the file some love and adapt the code to current guidelines.

There were attempts in the past, like #63793, but as you can probably tell, this is not a trivial task. I wouldn't block this PR on that, but I hope to raise awareness for the future contributions.

[Kijewski]

Definitely there there are some SAFETY keywords in this file

Ah, I missed the 18 instances in a 4000+ lines file. But you are right, newer functions further down do follow the guideline. I'll do too on future contributions regardless if the rest of a file does not.