@value decorator generates constructors whose arguments are owned

[armgabrielyan]

The @value decorator section in the documentation provides the following example, describing how the @value decorator generates missing constructors for a struct.

@value
struct MyPet:
    var name: String
    var age: Int

fn __init__(inout self, owned name: String, age: Int):
    self.name = name^
    self.age = age

fn __copyinit__(inout self, existing: Self):
    self.name = existing.name
    self.age = existing.age

fn __moveinit__(inout self, owned existing: Self):
    self.name = existing.name^
    self.age = existing.age

I am having trouble figuring out why name is passed as owned to the constructors. Could you please elaborate on how the compiler determines which parameters should be passed as owned in this case?

[lattner]

Sure, no problem. It passes /all/ of the arguments as owned. The reason Int isn't passed as owned is just because it doesn't matter either way - Int is a register_passable(trivial) type, and trivial types have no ownership. I added this statement to clarify, thanks!

The arguments to__init__ are all passed as owned arguments since the struct
takes ownership and stores the value. This is a useful micro-optimization and
enables the use of move-only types. Trivial types like Int are also passed
as owned values, but since that doesn't mean anything for them, we elide the
marker and the transfer operator (^) for clarity.

[armgabrielyan]

Sounds great. It all makes sense, thank you!