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Clarify behavior of HashSet::insert when there's existing key #107581

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stepancheg opened this issue Feb 2, 2023 · 2 comments · Fixed by #107619
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Clarify behavior of HashSet::insert when there's existing key #107581

stepancheg opened this issue Feb 2, 2023 · 2 comments · Fixed by #107619
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A-docs Area: documentation for any part of the project, including the compiler, standard library, and tools

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@stepancheg
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Location

https://doc.rust-lang.org/std/collections/struct.HashSet.html#method.insert

Summary

HashSet::insert is underspecified.

It says:

If the set already contained this value, false is returned.

But when the set already contains a value, is existing value replaced with new, or new value just discarded?

It does not make difference for primitive types, but can have significant effect when inserting Arc for example.
@stepancheg stepancheg added the A-docs Area: documentation for any part of the project, including the compiler, standard library, and tools label Feb 2, 2023
@GuillaumeGomez
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In case the entry already exists, it uses mem::replace() as you can see here (HashSet comes from the hashbrown crate and is in fact a HashMap without a value). But indeed, updating the docs to mention would be nice. Want to send a PR?

@stepancheg stepancheg mentioned this issue Feb 3, 2023
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@cuviper
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cuviper commented Feb 3, 2023

That mem::replace is only on the map value, which is () for a set. The key is not modified.

matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Jun 14, 2023
@matthiaskrgr
Rollup merge of rust-lang#107619 - stepancheg:hash-set-insert, r=Amanieu
d54bb50 
Specify behavior of HashSet::insert

`HashSet::insert` does not replace the value with equal value.

Fixes rust-lang#107581.
@bors bors closed this as completed in e800d5a Jun 14, 2023
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Specify behavior of HashSet::insert stepancheg/rust
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@stepancheg @cuviper @GuillaumeGomez