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Don't copy in From<String> for InternedString. #14674

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Don't copy in From<String> for InternedString. #14674

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9 changes: 8 additions & 1 deletion src/cargo/util/interning.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -46,7 +46,14 @@ impl<'a> From<&'a String> for InternedString {

impl From<String> for InternedString {
fn from(item: String) -> Self {
InternedString::new(&item)
let mut cache = interned_storage();
let s = cache.get(item.as_str()).copied().unwrap_or_else(|| {
let s = item.leak();
cache.insert(s);
s
});

InternedString { inner: s }
Comment on lines +49 to +56
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Without performance data, this falls to being a refactor. My main concern is that we're duplicating the logic for creating an InternedString.

Unsure whether thats sufficient to block this or not. Will think more on this

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Another way to handle this is having a Cow variant of new(), and then both new() and from() call it.

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I agree, it just looked like an attempted optimisation — the impl for an already-owned String — that wasn't quite complete. I couldn't work out a way to avoid the duplication without allocating on a common path.

Cow would work but turns a statically known property into a runtime check, which might be reasonable, but requires benchmarking to check.

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Uncertainty about the additional complexity is why this was not done originally.

If we want to make it part of the type system without duplicating the core logic we could do something like:

fn new_inner<S: AsRef<str>>(str: S, make_stat: impl Fn(S) -> &'static str) -> InternedString {
    let mut cache = interned_storage();
    let s = cache.get(str.as_ref()).copied().unwrap_or_else(|| {
        let s = make_stat(str);
        cache.insert(s);
        s
    });
    InternedString { inner: s }
}

I don't know if it's worth the additional complexity.

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imo cow offers us a fairly clean design and I don't expect the check will impact performance all that much

}
}

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