F# RFC FS-1081 - Extend fixed expressions to support byref types and types implementing GetPinnableReference()
The design suggestion Extend fixed expressions to support more types has been marked "approved in principle". This RFC covers the detailed proposal for this suggestion.
fixed expression should be extended to support
byref<'a> types and any type that implements the C#-style
It should be possible to pin and take native references of types like
fixed statements, as this is useful especially in native interop scenarios. At the moment, there are no workarounds in pure F#: you can do this in C#, but it's impossible in F#. The best you can do is write that part of your code in C#.
Currently, statements of the following form:
use ptr = fixed expr are allowed when
expr is one of:
- Address of an array element
- Address of a field
FS-1081 adds the following to the list of allowed types for
- any 'a when 'a implements
GetPinnableReference() : unit -> byref<'t>
The code generator will need to generate code that pins these references and takes their addresses. For reference, here is the C# proposal: https://github.com/dotnet/csharplang/blob/master/proposals/csharp-7.3/pattern-based-fixed.md.
This feature would introduce more special rules into the language.
- Don't do this, providing no alternative for those who want to use Span<'T> and friends with native code from F#
- Only extend
- Only extend
Please address all necessary compatibility questions:
- Is this a breaking change?
- What happens when previous versions of the F# compiler encounter this design addition as source code?
- A pre-FS1081 F# compiler will generate a compiler error.
- What happens when previous versions of the F# compiler encounter this design addition in compiled binaries?
- A pre-FS1081 F# compiler will not encounter any issues because any differences in codegen that this design addition creates will be contained to method/function body IL, and will not affect the public interface of user code.
- If this is a change or extension to FSharp.Core, what happens when previous versions of the F# compiler encounter this construct?
- N/A - no changes to FSharp.Core are prescribed by FS-1081.