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Update sizes.md (#228)

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Adding more details in how to control Rust enum size and discriminant values.
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Igor Petruk 2023-01-23 11:33:23 +00:00 committed by GitHub
parent 8dd749da60
commit 473fd02be7
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src/enums/sizes.md
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@ -17,8 +17,16 @@ enum Foo {
B, B,
} }
#[repr(u32)]
enum Bar {
A, // 0
B = 10000,
C, // 10001
}
fn main() { fn main() {
dbg_size!(Foo); dbg_size!(Foo);
dbg_size!(Bar);
dbg_size!(bool); dbg_size!(bool);
dbg_size!(Option<bool>); dbg_size!(Option<bool>);
dbg_size!(&i32); dbg_size!(&i32);
@ -32,6 +40,7 @@ fn main() {
Key Points: Key Points:
* Internally Rust is using a field (discriminant) to keep track of the enum variant. * Internally Rust is using a field (discriminant) to keep track of the enum variant.
* `Bar` enum demonstrates that there is a way to control the discriminant value and type. If `repr` is removed, the discriminant type takes 2 bytes, becuase 10001 fits 2 bytes.
* As a niche optimization an enum discriminant is merged with the pointer so that `Option<&Foo>` is the same size as `&Foo`. * 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. * `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>>()`. * For [some types](https://doc.rust-lang.org/std/option/#representation), Rust guarantees that `size_of::<T>()` equals `size_of::<Option<T>>()`.