Add ToImmutable*Async extension methods to System.Linq.Async

[bash]

I would like to propose the addition of new extension methods that allow collecting an IAsyncEnumerable into the immutable collections from System.Collections.Immutable.

The overloads below are copied from System.Collections.Immutable with a few changes to make them async friendly.

• Add Async suffix
• Change return type to ValueTask
• Add CancellationToken parameter

public static ValueTask<ImmutableArray<TSource>> ToImmutableArrayAsync<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default);

public static ValueTask<ImmutableDictionary<TKey, TValue>> ToImmutableDictionaryAsync<TSource, TKey, TValue>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TValue> elementSelector, IEqualityComparer<TKey>? keyComparer, IEqualityComparer<TValue>? valueComparer, CancellationToken cancellationToken = default) where TKey : notnull;

public static ValueTask<ImmutableDictionary<TKey, TValue>> ToImmutableDictionaryAsync<TSource, TKey, TValue>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TValue> elementSelector, IEqualityComparer<TKey>? keyComparer, CancellationToken cancellationToken = default) where TKey : notnull;

public static ValueTask<ImmutableDictionary<TKey, TSource>> ToImmutableDictionaryAsync<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, CancellationToken cancellationToken = default) where TKey : notnull;

public static ValueTask<ImmutableDictionary<TKey, TSource>> ToImmutableDictionaryAsync<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? keyComparer, CancellationToken cancellationToken = default) where TKey : notnull;

public static ValueTask<ImmutableDictionary<TKey, TValue>> ToImmutableDictionaryAsync<TSource, TKey, TValue>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TValue> elementSelector, CancellationToken cancellationToken = default) where TKey : notnull;

public static ValueTask<ImmutableDictionary<TKey, TValue>> ToImmutableDictionaryAsync<TKey, TValue>(this IAsyncEnumerable<KeyValuePair<TKey, TValue>> source, IEqualityComparer<TKey>? keyComparer, IEqualityComparer<TValue>? valueComparer, CancellationToken cancellationToken = default) where TKey : notnull;

public static ValueTask<ImmutableDictionary<TKey, TValue>> ToImmutableDictionaryAsync<TKey, TValue>(this IAsyncEnumerable<KeyValuePair<TKey, TValue>> source, IEqualityComparer<TKey>? keyComparer, CancellationToken cancellationToken = default) where TKey : notnull;

public static ValueTask<ImmutableDictionary<TKey, TValue>> ToImmutableDictionaryAsync<TKey, TValue>(this IAsyncEnumerable<KeyValuePair<TKey, TValue>> source, CancellationToken cancellationToken = default) where TKey : notnull;

public static ValueTask<ImmutableHashSet<TSource>> ToImmutableHashSetAsync<TSource>(this IAsyncEnumerable<TSource> source, IEqualityComparer<TSource>? equalityComparer, CancellationToken cancellationToken = default);

public static ValueTask<ImmutableHashSet<TSource>> ToImmutableHashSetAsync<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default);

public static ValueTask<ImmutableList<TSource>> ToImmutableListAsync<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default);

public static ValueTask<ImmutableSortedDictionary<TKey, TValue>> ToImmutableSortedDictionaryAsync<TSource, TKey, TValue>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TValue> elementSelector, IComparer<TKey>? keyComparer, IEqualityComparer<TValue>? valueComparer, CancellationToken cancellationToken = default) where TKey : notnull;

public static ValueTask<ImmutableSortedDictionary<TKey, TValue>> ToImmutableSortedDictionaryAsync<TSource, TKey, TValue>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TValue> elementSelector, IComparer<TKey>? keyComparer, CancellationToken cancellationToken = default) where TKey : notnull;

public static ValueTask<ImmutableSortedDictionary<TKey, TValue>> ToImmutableSortedDictionaryAsync<TSource, TKey, TValue>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TValue> elementSelector, CancellationToken cancellationToken = default) where TKey : notnull;

public static ValueTask<ImmutableSortedDictionary<TKey, TValue>> ToImmutableSortedDictionaryAsync<TKey, TValue>(this IAsyncEnumerable<KeyValuePair<TKey, TValue>> source, IComparer<TKey>? keyComparer, IEqualityComparer<TValue>? valueComparer, CancellationToken cancellationToken = default) where TKey : notnull;

public static ValueTask<ImmutableSortedDictionary<TKey, TValue>> ToImmutableSortedDictionaryAsync<TKey, TValue>(this IAsyncEnumerable<KeyValuePair<TKey, TValue>> source, IComparer<TKey>? keyComparer, CancellationToken cancellationToken = default) where TKey : notnull;

public static ValueTask<ImmutableSortedDictionary<TKey, TValue>> ToImmutableSortedDictionaryAsync<TKey, TValue>(this IAsyncEnumerable<KeyValuePair<TKey, TValue>> source, CancellationToken cancellationToken = default) where TKey : notnull;

public static ValueTask<ImmutableSortedSet<TSource>> ToImmutableSortedSetAsync<TSource>(this IAsyncEnumerable<TSource> source, IComparer<TSource>? comparer, CancellationToken cancellationToken = default);

public static ValueTask<ImmutableSortedSet<TSource>> ToImmutableSortedSetAsync<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default);

Pros/Cons

• Pro: Feature parity with the extensions available in the .NET SDK.
• Con: Requires a new NuGet dependency for older target frameworks because they don't include System.Collections.Immutable.
• others?

I'd happily create a PR for this feature.

[bartdesmet]

A PR to add this functionality is welcome. For down-level platform support, as long as we're only conditionally adding references for these target platforms, we should be fine.