`SkipLocalsInit` is an unsafe operation, but does not give warning FS0009

[abelbraaksma]

Applying the attribute SkipLocalsInitAttribute is an unsafe operation, but does not raise the FS0009 warning.

Repro steps

type Rain() =
    [<SkipLocalsInit>]
    member _.ItPours(?msg: string) =
        let message= defaultArg mname "absent"
        $"Message: {message}"

Expected behavior

According to the docs, this is an unsafe operation.

F# issues the FS0009 warning for unsafe operations (like fixed and using nativeptr and the like). This warning should be thrown when using SkipLocalsInit as well.

This is akin to using the unsafe keyword in C#, which is indeed required for this attribute.

Actual behavior

The warning is not thrown.

Related information

• Original implementation: Support SkipLocalsinit #11267, by @kerams, perhaps you know whether there was a decision here?
• Original suggestion: Support [<SkipLocalsInit>] fsharp/fslang-suggestions#990
• RFC: not found

The suggestion does not mention the term unsafe, but @dsyme mentioned

It's also in the realm of a "allow generation of unverifiable code for performance reasons" feature

which further cements that this is unsafe and that FS0009 should be shown here.

Since this is just a warning, I hope this can be added to F# still, without being considered a backward compatibility issue.

[abelbraaksma]

Actually, trying out some scenarios:

• In C#, if you use pointers to access locals, you will get an error for the unsafe pointer code
• If you do not use other unsafe constructs, the attribute alone does not raise the unsafe error and you do not need the unsafe keyword
• If you do not use the unsafe switch, by default, C# prevents you from using unassigned variables (but there are workarounds with structs).
• In F#, if you use any pointers, you will of course get the FS0009
• In F#, if you do not use pointers, F# prevents you from using unassigned (mutable) let bindings. I am not aware of a constructs that would access uninitialized stack memory without getting an FS0009

In other words, the raised issue may not be an issue in practice, as the FS0009 will be raised as soon as you would access the unassigned memory.

[kerams]

perhaps you know whether there was a decision here?

No decision on my part. It never occurred to me to add a warning.

I am not aware of a constructs that would access uninitialized stack memory without getting an FS0009

Uninitialized local.

> [<System.Runtime.CompilerServices.SkipLocalsInit>]
let a () =
    match Map.empty.TryGetValue "" with
    | false, (x: float) -> printfn "%A" x
    | _ -> ();;
val a: unit -> unit

> a ();;
6.951863497e-310

[abelbraaksma]

That looks like a bug, @kerams. I believe F# normally initializes any let bound value, and here it's implicitly let bound (I don't think this is possible with normal let or val keywords).

I would have expected pointers or references to be required to access initialized memory.

@dsyme even mentioned something along the lines to use this attribute in F# internally, as F# itself forces initialization.

[abelbraaksma]

Oh wait. This is actually an outref, right?