Extend fixed expressions to Span, Memory, refs, and GetPinnableReference

There should be some way to pin instances of Span, Memory, and certain kinds of types to take their native addresses for native interop purposes. In C# you can just use the fixed statement, but F# only supports a limited set of constructs, none of which you can get to work with Spans.

This can all be accomplished in F# by making fixed expressions work with anything that implements GetPinnableReference() just like C#.

Pros and Cons

Advantages:

• You can use spans in native interop situations without being forced to copy it to an array first
• Feature parity with C#

Disadvantages:

• It's work
• More special language rules

Extra information

Estimated cost (XS, S, M, L, XL, XXL): M? Just a guess.

Related suggestions: (put links to related suggestions here)

Affidavit (please submit!)

Please tick this by placing a cross in the box:

• This is not a question (e.g. like one you might ask on stackoverflow) and I have searched stackoverflow for discussions of this issue
• I have searched both open and closed suggestions on this site and believe this is not a duplicate
• This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it.

Please tick all that apply:

• This is not a breaking change to the F# language design
• I or my company would be willing to help implement and/or test this