Examples, with placeholder attributes:

// ensures arg is true
static void Assert([EnsuresTrue] bool b);

// false if arg is not null
static bool IsNullOrEmpty([NotNullWhenFalse] string? s)

// reference equality for null
[NullEquals] static bool Equals(object? x, object? y)

// result nullability matches arg
static string? NormalizePath([NullInNullOut] string? path)

// ref arg not reset to null
static void InitializeAndAdd<T>([NotNulled] ref List<T>? list, T item)

(jcouv updated): We also need to handle:

• Interlocked.CompareExchange
• a ref parameter which has a non-null inbound value, but possibly null outbound value (see FilterAndAppendAndFreeDiagnostics). This way the caller would be warned if passing a maybe-null argument. (Debug.Assert would not do that)
• TryGetValue on WeakReference
• object.ReferenceEquals
• Would it be possible to hint that AsNode() is safe if IsNode is true?
• Should those attribute affect the inferred type? var x = EnsuresNotNull(something);
• What about M(x = y, y) where EnsuresNotNull is on the second parameter, or M(y, x = y) where it is on the first?

Filed https://github.com/dotnet/corefx/issues/33880 to track work to annotate the BCL and collect ideas of APIs needing annotations.

We should confirm whether the nullable attributes should be added to mono or not.