[swift interop] Cannot call instance methods with at least one parameter when using SwiftSelf<T>

[jkurdek]

Our current interop implementation only allows SwiftSelf (representing the self parameter in Swift instance methods) to be specified as the first parameter in function signatures. This design choice leads to issues when interfacing with Swift instance methods that accept additional parameters.

Let's consider following example

@frozen public struct SampleStruct
{
    public var x1: Int
    public var x2: Int

    public func addWithExtraArgs(x3: Float, x4: Float) -> Float {
        return Float(x1 + x2) + x3 + x4
    }
}

addWithExtraArgs results in the following LLVM-IR:

define protected swiftcc float @"output.SampleStruct.addWithExtraArgs(x3: Swift.Float, x4: Swift.Float) -> Swift.Float"(float %0, float %1, i64 %2, i64 %3) ...

SampleStruct is expected to be lowered into two i64 which are passed as last parameters.
This would correspond to following csharp signature:

static extern void AddWithExtraArgs(float a, float b, SwiftSelf<SampleStruct> struct)

We should either allow SwiftSelf<T> argument to be specified as the last parameter or we should reorder the parameters in runtime.

\cc: @kotlarmilos, @jakobbotsch, @amanasifkhalid, @matouskozak

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[jakobbotsch]

We should either allow SwiftSelf<T> argument to be specified as the last parameter or we should reorder the parameters in runtime.

My impression (from playing around on Godbolt, IIRC) was that the Swift compiler always put this first. Does it actually always put it last? Do you know what the rules are for how the lowering of the self arg is placed?

[jkurdek]

I am unable to find any piece of Apple documentation which describes this. Playing around in Godbolt: https://godbolt.org/z/1qj8vsf1h it seems that for both frozen and non-frozen structs it is passed as last argument.

Do you remember any example where it was passed as first argument?

[jakobbotsch]

I am unable to find any piece of Apple documentation which describes this. Playing around in Godbolt: https://godbolt.org/z/1qj8vsf1h it seems that for both frozen and non-frozen structs it is passed as last argument.

Do you remember any example where it was passed as first argument?

I don't. Looks like we should just switch the requirement to be the last argument then.

[stephen-hawley]

I am unable to find any piece of Apple documentation which describes this. Playing around in Godbolt: https://godbolt.org/z/1qj8vsf1h it seems that for both frozen and non-frozen structs it is passed as last argument.
Do you remember any example where it was passed as first argument?

I don't. Looks like we should just switch the requirement to be the last argument then.

It was might have been the case in Swift 2.2 when self was an implicit first argument like C++.