Champion "Permit attributes on local functions" (VS 16.8, .NET 5)

[gafter]

• Proposal added
• Discussed in LDM 2018-09-10
• Decision in LDM
• Finalized (done, rejected, inactive)
• Draft spec in Draft spec for local function attributes #3008

The idea is to permit attributes to be part of the declaration of a local function.

From discussion in LDM today (4/29/2019), we would like to allow attributes on local function parameters, which would help with async-iterator local functions that want to use [EnumeratorCancellation].

Speclet: https://github.com/dotnet/csharplang/blob/master/proposals/csharp-9.0/local-function-attributes.md

Note: we did include support for extern as part of this work after all.

[bbarry]

In addition to permitting attributes we would need to also be able to declare a local function without a body (CS8112). Perhaps only if the function is also extern?

[gafter]

@bbarry That is not part of this proposal.

[jcouv]

FYI @rynowak (from Razor) mentioned interest in this feature as well and helpfully provided a use case to illustrate:
https://gist.github.com/rynowak/4d4738a57fb482952056ca67573f1d50#file-attributes_for_local_functions-md

Another use case: Nullable reference types feature sometimes requires attributes on parameters. The following code currently fails to compile with C#8 preview (VS 16.1) but should compile:

using System;
using System.Runtime.CompilerServices;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");

            // Attributes can't be used here, but are sometimes required when nullable reference types is enabled
            //  CS8205 Attributes are not allowed on local function parameters or type parameters
            static bool LocalFunc([NotNullWhenTrue] out string? result)
            {
                result = null;
                return false;
            }
        }
    }
}

namespace System.Runtime.CompilerServices
{
    [AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false)]
    public class NotNullWhenTrueAttribute : Attribute
    {
        public NotNullWhenTrueAttribute() { }
    }
}