Avoid copy when convert owned String to Arc<str>

[tisonkun]

This is almost Arc::<str>::from(Box::<str>::from(owned_string)) that could avoid an extra copy.

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[tisonkun]

Currently it calls ptr::copy_nonoverlapping internally so there is still a copy. But it's semantically move and at least avoid iteratively calling ptr::write.

[hanna-kruppe]

This implementation seems worse for copying: String::as_boxed_slice has to reallocate, i.e., copy, if there’s excess capacity, and Box<str> -> Arc<str> also has to reallocate to add refcounts. I don’t know what the current implementation compiles down to, but one copy/reallocation (for the refcounts and the alignment they imply) seems unavoidable.

[tisonkun]

This implementation seems worse for copying: String::as_boxed_slice has to reallocate, i.e., copy, if there’s excess capacity, and Box<str> -> Arc<str> also has to reallocate to add refcounts. I don’t know what the current implementation compiles down to, but one copy/reallocation (for the refcounts and the alignment they imply) seems unavoidable.

Thanks for your analyze. I get the copy internal wrong.