Versionable breaking changes

[SupinePandora43]

Replacing int Length with nuint or ulong is a binary and source breaking change, but it can be fixed.

XXX can be replaced with .NET version, or some other versioning

Requirements

Assembly

• XXX version

Language

• A way to describe behaviour on previous XXX versions

public struct Span<T> {
    // ...
    public nuint Length => _length;
    public version(0) int Length => checked((int) _length); // behaviour for XXX version 0
}

Runtime

• Call methods with respect to XXX version of callee's assembly version

// XXX is 0
span.Length // replaced with a call to Span<T>.get_Length_version_0() by a runtime

// XXX is 1
span.Length

Cons

• adds performance overhead for old programs

[huoyaoyuan]

I did consider similar idea. In fact, IL allows overload by return type. The two versions does not have to be different names.

adds performance overhead for old programs

Unfortunately, array.Length and span.Length are extremely hot paths. Any performance overhead can hurt a lot. This makes the problem more difficult to handle.