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Added speaker notes to enum sizes (#201)

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* Added speaker notes to enum sizes

* Update sizes.md

Just formatting

Co-authored-by: Fabian Bornhofen <fbornhofen@google.com>
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Charisee Chiw 2023-01-20 13:55:30 -08:00 committed by GitHub
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src/enums/sizes.md
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@ -29,9 +29,12 @@ fn main() {
* See the [Rust Reference](https://doc.rust-lang.org/reference/type-layout.html).
<details>
* `Option<bool>` is another example of tight packing.
* For [some types](https://doc.rust-lang.org/std/option/#representation), Rust guarantees that `size_of::<T>()` equals `size_of::<Option<T>>()`.
* Zero-sized types allow for efficient implementation of `HashSet` using `HashMap` with `()` as the value.
Key Points:
* Internally Rust is using a field (discriminant) to keep track of the enum variant.
* As a niche optimization an enum discriminant is merged with the pointer so that `Option<&Foo>` is the same size as `&Foo`.
* `Option<bool>` is another example of tight packing.
* For [some types](https://doc.rust-lang.org/std/option/#representation), Rust guarantees that `size_of::<T>()` equals `size_of::<Option<T>>()`.
* Zero-sized types allow for efficient implementation of `HashSet` using `HashMap` with `()` as the value.
</details>