Missing warning on pure collection initializer methods

[Ilchert]

Missing warning when Add method with PureAttiribute used for collection initializer.

Rise warning when C# collection initializer method is marked as Pure.

public static class Ext
{
    [Pure]
    public static int Add<T>(this A<T> c, T item) => 1;

    public static void Do()
    {
        var a = new A<int>() { 1, 23, 4 }; // must warn
        a.Add(1); // warning CA1806
    }
}

public class A<T> : IEnumerable<T>
{
    public IEnumerator<T> GetEnumerator() => throw new NotImplementedException();

    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

This warning can help with immutable collections (marking as pure methods like this) to prevent unexpected behaviour.

var a = new ImmutableArray<int> { 1 }; // throws NulRef

[Youssef1313]

@jinujoseph This is a CA1806 report. Can you move to dotnet/roslyn-analyzers?

@stephentoub Per your comment (dotnet/runtime#34098 (comment)), you were concerned about using PureAttribute for that purpose. It's currently already the case for CA1806, but the analyzer is missing implicit Add calls for collection initializer. Do you think this should be handled?

cc @mavasani as well.

[stephentoub]

It's currently already the case for CA1806, but the analyzer is missing implicit Add calls for collection initializer. Do you think this should be handled?

If CA1806 is already special-casing Pure and it's just buggy in how it's doing so / missing some cases, I'm ok if we want to fix those inconsistencies.