Override clone_from method for PathBuf and OsString

[a1phyr]

This was not the case before because #[derive(Clone)] do not do it.

[rust-highfive]

r? @Mark-Simulacrum

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[jyn514]

What is the goal of this change? Does it improve performance at all? If not I don't see much point to it.

[a1phyr]

The default implementation of clone_from is

fnclone_from(&mutself,source:&Self){*self = source.clone();}

In case of an OsString and a PathBuf, it drops the old buffer and allocates a new one, which defeats the purpose of this method. Overriding it allows to reuse the old buffer. Currently this method is already overridden for Vec and String.

Example comparison between String and PathBuf: playground.

[jyn514]

Overriding it allows to reuse the old buffer.

I don't understand why that's the case? You're still calling clone on the buffer.

[a1phyr]

You're still calling clone on the buffer.

No, the new implementation calls self.inner.clone_from(&source.inner), which ultimately calls Vec::clone_from, which reuses the buffer. This is what String does today (and it works, cf playground link above).

[jyn514]

Ok, I found why I was confused: this reuses the allocation of the destination, not the source: https://doc.rust-lang.org/nightly/src/alloc/vec/mod.rs.html#2337-2366

LGTM but I'm not on T-libs and I'd prefer someone else to take a look.

[Mark-Simulacrum]

Seems likely pretty harmless but we also likely don't want to do this for everything as there is some compile-time overhead to their existence I suspect.

@bors r+ rollup=never

[bors]

📌 Commit 4a8671a has been approved by Mark-Simulacrum

[a1phyr]

Seems likely pretty harmless but we also likely don't want to do this for everything as there is some compile-time overhead to their existence I suspect.

Yes, for derives #27939 was closed because of compile time. For individual items in std, I don't think there is a noticeable change, especially for downstream users.

[bors]

⌛ Testing commit 4a8671a with merge 50ca3ac...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 50ca3ac to master...