Is var x: T an ExistentialContainer?

[Azoy]

Cross posting: https://github.com/Azoy/Echo/discussions/24#discussioncomment-625539

NSExceptional asks:

If var x: Any is an AnyExistentialContainer, is var y: T an ExistentialContainer?

Also, are ExistentialContainers only for classes and structs, strictly? Or can it house anything that can conform to a protocol? (So, enums too?)

[Azoy]

Cross posting: https://github.com/Azoy/Echo/discussions/24#discussioncomment-625549

Azoy answers:

var y: T is simply whatever type it is at runtime, so if its an Int it will literally be Int.

ExistentialContainers store whatever thing conforms to some protocol, so classes, structs, enums, etc.

I think for the future it would be beneficial for new discussion threads because it allows for Github's answering feature to properly work on a question vs. a massive FAQ thread.

[Azoy]

Cross posting: https://github.com/Azoy/Echo/discussions/24#discussioncomment-625617

NSExceptional replies:

var y: T is simply whatever type it is at runtime, so if its an Int it will literally be Int.

Really? Isn't the variable boxed somehow, like Any? I guess local variables wouldn't have to be, but what about parameters?

func foo<T>(bar: T)

Is bar boxed somehow? Or is it passed in as a pointer under the hood, sorta like this in C?

void foo(TypeParam t, void *bar)

[Azoy]

Cross posting: https://github.com/Azoy/Echo/discussions/24#discussioncomment-625626

Azoy replies:

Haha you're getting into some pretty interesting calling convention information that I'm giddy to talk about. Surprisingly, there is no boxing! For your example foo<T>(bar: T) the C-ish definition looks like: void foo(opaque *bar, Metadata *T). The reason why Swift can do this is because of the value witness table which allows these functions to have complete knowledge on how to copy, destroy, etc. generically.

[Azoy]

Cross posting: https://github.com/Azoy/Echo/discussions/24#discussioncomment-625832

NSExceptional replies:

Gotcha, that's what I meant with my last comment :) Good stuff. It's not like Any I guess, where the value itself has to carry that information with it at all times