Support optional input for MapAction

[Kahbazi]

• Add support for optional input
• Add support for ValueTask output
• Pass HttpContext.RequestAborted to WriteAsJsonAsync(), WriteAsync(), ReadFromJsonAsync(), ReadFormAsync()

Addresses #30248

[Kahbazi]

Ping @halter73!

[davidfowl]

Pass HttpContext.RequestAborted to WriteAsJsonAsync(), WriteAsync(), ReadFromJsonAsync(), ReadFormAsync()

I'm not sure we want to do this. This will result in LOTS of allocations (see #30390).

[Kahbazi]

Pass HttpContext.RequestAborted to WriteAsJsonAsync(), WriteAsync(), ReadFromJsonAsync(), ReadFormAsync()

I'm not sure we want to do this. This will result in LOTS of allocations (see #30390).

What if the HttpContext.RequestAborted is used in the user code? That would cause allocation too, right? Is it possible to have a state to see if a cts is created and then pass the HttpContext.RequestAborted to WriteAsync?

[davidfowl]

We're going to make it cheap but not as part of this PR. We also don't pass the token anywhere except when doing big IO transfers (like copying files) so I'd leave it for now

[halter73]

Nit: Any particular reason for using local functions for FromRouteResult but not FromRouteOptionalResult?

[Kahbazi]

It looks like local functions are not generating the optional parameter and handles then in compile time.
According to Sharplab.io, this code:

public void M()
{
    Test();
    void Test(int a = 2)
    {
    }
}

will be compiled to this:

public void M()
{
    <M>g__Test|0_0(2);
}

[CompilerGenerated]
internal static void <M>g__Test|0_0(int a)
{
}

Perhaps this is another feature that could be asked from C# team?

[halter73]

Issue filed: dotnet/roslyn#51518

I also requested that lambdas add support for optional parameters as part of the C# 10 lambda improvements: dotnet/csharplang#4451 (comment)

[ghost]

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