add @serdeProxyCast, @serdeEnumProxy UDAs

[WebFreak001]

PR on the mir-ion side will follow shortly.

Idea is that you can do this:

@serdeEnumProxy!uint
// equivalent to @serdeProxyCast @serdeEnumProxy!uint
enum Foo
{
    a = 1,
    b = 2
}

and Foo will be

1. serialized by int (as usual with serdeProxy)
2. allows other values than 1 and 2 to be (de)serialized

[9il]

Why it is needed? The following code works:

    import mir.ser.text;
    import mir.deser.text;

    @serdeProxy!int
    enum Foo
    {
        a = 1,
        b = 2
    }

    assert(Foo.b.serializeText == "2");
    assert("2".deserializeText!Foo == Foo.b);

[9il]

Ah, got it.

[9il]

But that works as well:

    import mir.ser.text;
    import mir.deser.text;

    @serdeProxy!int
    enum Foo
    {
        a = 1,
        b = 2
    }

    assert((cast(Foo)6).serializeText == "6", Foo.b.serializeText);
    assert("6".deserializeText!Foo == 6);

[WebFreak001]

But that works as well:

    import mir.ser.text;
    import mir.deser.text;

    @serdeProxy!int
    enum Foo
    {
        a = 1,
        b = 2
    }

    assert((cast(Foo)6).serializeText == "6", Foo.b.serializeText);
    assert("6".deserializeText!Foo == 6);

there is no change in how serialization works here, it partly worked before and was kind of inconsistent, that's why I added a new UDA instead of trying to change behavior in one way or the other.

The mir-ion unittest also checks for this for known currently broken/valid cases:
https://github.com/libmir/mir-ion/pull/22/files#diff-b21fe8bb968514207405f1eb867afb267ce2c6faaa9e8d0ffd9579795f4cdfb8R1270-R1273

https://github.com/libmir/mir-ion/pull/22/files#diff-b21fe8bb968514207405f1eb867afb267ce2c6faaa9e8d0ffd9579795f4cdfb8R1270-R1273

[9il]

A special user-defined conversion logic can be done using serdeProxy:

template MyEnumProxyImpl(T)
{
    @serdeProxy!T
    struct MyEnumProxyImpl
    {
        T value;
        // implement conversion To/From Enum's of T types AND base T
    }
}

alias MyEnumProxy(T) = serdeProxy!(MyEnumProxyImpl!T))

@MyEnumProxy!int
enum E { a, b }

Can this work for you?

[WebFreak001]

ok this worked, but imo this is something worth adding to mir:

template MyEnumProxyImpl(T)
{
	@serdeProxy!T
	struct MyEnumProxyImpl
	{
		T value;
		alias value this;

		this(T value)
		{
			this.value = value;
		}
	}
}

alias serdeEnumProxy(T) = serdeProxy!(MyEnumProxyImpl!T);

[WebFreak001]

nvm that serdeEnumProxy implementation breaks with variants as it's a struct and doesn't get matched. I can't add serdeFallbackStruct as I already have it on another thing. serdeFallbackStruct also doesn't fix it.

I still think the addition with this PR (and the one in mir-ion) would be the most stable and most efficient.

at least this is an example exception I get from this:

mir.ion.exception.IonException@../../.dub/packages/mir-ion-1.0.50/mir-ion/source/mir/deser/package.d(1211): Unexpected IonTypeCode for MirAlgebraic!(NoneType, FailureHandlingKind)

[WebFreak001]

@9il can we get this in? The user defined struct breaks Mir's Algebraic and fixing that is hard, while the proxy cast has a super simple implementation (just using cast instead of to) which allows for better enum control.

I currently need this for my serve-d mir-ion port where I have a bunch of Algebraic!(void, T) and T being a string enum that might also contain unknown values due to protocol upgrades.