Generic struct type whose fields are all unmanaged types is unmanaged

[dzmitry-lahoda]

Generic struct type whose fields are all unmanaged types is unmanaged

A non-generic struct type whose fields are all unmanaged types is unmanaged.

into

A generic struct type whose fields are all unmanaged types is unmanaged.

as

The unmanaged constraint on generic type T enforces the type to be an 'unmanaged' struct that does not recursively contain reference type fields.

as for cases

type Cell<'T when 'T: unmanaged> =
    struct
        val element: 'T
        val next: nativeptr<Cell<'T>>
    end    

type UnmanagedCell<'T when 'T: unmanaged> =
    struct
    end  

[<EntryPoint>]
let main argv =
    let fine = UnmanagedCell<int>()
    let FS0001 = UnmanagedCell<UnmanagedCell<int>>()
    0

The existing way of approaching this problem in F# is prevent compilation with error

error FS0001: A generic construct requires that the type 'X' is an unmanaged type

I have asked the question on SO and found that it is F# limitation, not CLR.

Pros and Cons

See Related suggestions

Extra information

Related suggestions:
dotnet/csharplang#1504
dotnet/csharplang#1649

Affidavit (please submit!)

Please tick this by placing a cross in the box:

• [x ] 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

[TIHan]

This is actually quite easy to implement and it should not be a breaking change. We should make this change to follow C#.

[TIHan]

We would also need to interop with C# by looking at the IsUnmanagedAttribute on the type argument and be able to emit it as well.

[abelbraaksma]

For reference, here's a link to the original PR for C#, which is now merged: dotnet/roslyn#31148.

Edit: apparently more work was done on several PR's, here's all: https://github.com/dotnet/roslyn/commits/features/unmanaged-constructed-types.

[cartermp]

@TIHan as a part of this we should also update the Span 'n Friends RFC

[TIHan]

Yes and we would need to update docs as well.

[TIHan]

I have a PR for this: dotnet/fsharp#6064