CODEWITHSUNDEEP
rust
phip1611
phip1611

Reputation: 6140

Does implementing the Sync trait change the compiler output?

If I mark my struct as Sync will the compiler output differ? Will the compiler implement some Mutex-like magic?

struct MyStruct {
    data: RefCell<u32>,
}
unsafe impl Sync for MyStruct {}
unsafe impl Send for MyStruct {}

Upvotes: 2

Views: 181

Answers (1)

Francis Gagn&#233;
Francis Gagn&#233;

Reputation: 65732

The compiler uses a mechanism named "language items" to reference items (types, traits, etc.) that are defined in a library (usually core) but are used by the compiler, whether that be in code generated by the compiler, for validating the code or for producing specialized error messages.

Send and Sync are defined in the core library. Sync is a language item, but Send isn't. The only reference to Sync I could find in the compiler is where it checks that the type of a static variable implements Sync. (Send and Sync used to be more special to the compiler. Before auto traits were added to the language, they were implemented as "auto traits" explicitly.)

Other than that, the compiler doesn't care about what Send and Sync mean. It's the libraries (specifically, types/functions that are generic over Send/Sync types) that give the traits their meaning.

Neither trait influences what code is emitted by the compiler regarding a particular type. Making a type "thread-safe" is not something that can be done automatically. Consider a struct with many fields: even if the fields are all atomic types, a partially updated struct might not be in a valid state. The compiler doesn't know about the invariants of a particular type; only the programmer knows them. Therefore, it's the programmer's responsibility to make the type thread-safe.

Upvotes: 3

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