(Unmanaged) Structs are not considered blittable by Emscripten / Blazor WASM AOT

[HurricanKai]

Description

Unmanaged structs are not considered blittable and therefore cannot be used in pinvoke callbacks.

Reproduction Steps

Attempt to compile code like

public struct S { public int Value; }

[UnmanagedCallersOnly]
public void M(S myStruct) { }

using Emscripten / WASM AOT

Expected behavior

The above code should compile.

Actual behavior

An error is shown, structs are not considered blittable.

Regression?

No response

Known Workarounds

No response

Configuration

Tested on .NET 6 RC2

Other information

No response

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Issue Details

---

Description

Unmanaged structs are not considered blittable and therefore cannot be used in pinvoke callbacks.

Reproduction Steps

Attempt to compile code like

public struct S { public int Value; }

[UnmanagedCallersOnly]
public void M(S myStruct) { }

using Emscripten / WASM AOT

Expected behavior

The above code should compile.

Actual behavior

An error is shown, structs are not considered blittable.

Regression?

No response

Known Workarounds

No response

Configuration

Tested on .NET 6 RC2

Other information

No response

Author:	HurricanKai
Assignees:	-
Labels:	arch-wasm, untriaged, area-Interop-mono
Milestone:	-

[lambdageek]

This is an error from the AOT tasks, maybe? Console sample using Mono works (see https://gist.github.com/lambdageek/f289259398a33cdf19811d2ab4eefd35):

$ dotnet run
Hello, World! from Mono
Called with 42

/cc @radical

[lambdageek]

Note that this is also related to #60639 - in particular we will need to follow the runtime's (updated) per-assembly rules for what is blittable

[lambdageek]

This could be considered an enhancement.

One current workaround is to pass a pointer to the struct:

public struct S {
    public int i;
}


public class P {
    public static void Main () {
	unsafe {
	    Q (&M);
	} 
    }

    [UnmanagedCallersOnly]
    public unsafe static void M (IntPtr myStruct) {
	ref S s = ref Unsafe.AsRef<S>((void*)myStruct);
	Console.WriteLine ($"Called with {s.i}");
    }

    [DllImport("Foo")]
    public static extern unsafe void Q (delegate *unmanaged<IntPtr,void> fn);
}

And correspondingly use a pointer on the native side:

typedef struct S {
	int i;
} S;

typedef void (*fn)(S* s);

void Q (fn f);

void Q (fn f) {
	S s;
	s.i = 42;
	f (&s);
}

[HurricanKai]

Given the workaround requires changes to the native side, which isn't always possible (ie binding to standardized or otherwise externally defined APIs), I would not consider this as an enhancement, but rather a missing feature that is important to making interop in WASM fully functional.