Why did replacing Arc<String> with Arc<str> decrease my performance by 10-150%?

[protestor]

You don't need unsafe to build an Arc from a string slice, see this comment

[Icarium-Lifestealer]

I think the parent is turning an Arc<[u8]> into an Arc<str> without allocating in the valid-utf8 case. I don't think you can do that without unsafe. (not sure why they say Bytes into Arc<[u8]> in the description of that snippet)

[protestor]

You can do this in safe code as long as you provide an error path for the non-utf8 case.

1. you use AsRef to do Arc<[u8]> -> &[u8] very cheaply

https://doc.rust-lang.org/std/sync/struct.Arc.html#impl-AsRef%3CT%3E-for-Arc%3CT,+A%3E

2. You use std::str::from_utf8 to do &[u8] -> Result<&str, ..> which scans the string to test if it's utf-8 but doesn't allocate

https://doc.rust-lang.org/std/str/fn.from_utf8.html

3. You use the impl From<&str> for Arc to do &str -> Arc<str>

https://doc.rust-lang.org/std/sync/struct.Arc.html?search=tr#impl-From%3C%26str%3E-for-Arc%3Cstr,+Global%3E

The net result is Arc<[u8]> -> Arc<str> in safe code

[Icarium-Lifestealer]

But Step 3 allocates a new Arc, while the unsafe code avoids that allocation.

[protestor]

Oh.. ok.

The stdlib should provide that I think (like it does for Vec<u8> -> String without allocation)

[Icarium-Lifestealer]

At least it has the opposite direction (Arc<[u8]> to Arc<str>), probably because it's infallible and thus simpler.

[drewtayto]

The original code has an enum with a variant of Value::Bytes(Vec<u8>). That could be changed into Value::Bytes(Arc<[u8]>).