TextChange.NewText has an unexpected nullable annotation

[jasonmalinowski]

This member:

roslyn/src/Compilers/Core/Portable/Text/TextChange.cs

Line 31 in 97b4837

public string? NewText { get; }

is marked as nullable, but there's no clear indication what we intend by that. The constructor throws if you try to pass null, so it seems like it's only allowing null to handle the case of a default-constructed TextChange. SourceText.WithChanges has some code to handle that being null and treat that as an empty string. (But if the only way you can get this is a default-constructed struct, then what's the point since you're replacing nothing with nothing?)

But some other code has odd asserts that we expect non-null values:

roslyn/src/Compilers/Core/Portable/Text/TextChange.cs

Lines 89 to 93 in 0c1dd5b

	public static implicit operator TextChangeRange(TextChange change)
	{
	Debug.Assert(change.NewText is object);
	return new TextChangeRange(change.Span, change.NewText.Length);
	}

But not if you call this other method that should do the same thing!

roslyn/src/Compilers/Core/Portable/Text/TextChangeRangeExtensions.cs

Lines 103 to 106 in 0c1dd5b

	public static TextChangeRange ToTextChangeRange(this TextChange textChange)
	{
	return new TextChangeRange(textChange.Span, textChange.NewText?.Length ?? 0);
	}

If the intent of the nullable annotation was to ensure we're "safe" in regards to default constructed structs, it seems like in practice this didn't result in good safety.