InternalsVisibleTo + internal module + inline function error message could be improved

[ForNeVeR]

Repro steps

1. Create two F# projects, LibProject and TestProject, where TestProject references LibProject
2. In LibProject, write the following code:

   module internal LibProject.Tools
   
   open System.Net
   open System.Runtime.CompilerServices
   
   [<assembly: InternalsVisibleTo("TestProject")>]
   do ()
   
   let inline int32FromBigEndian(num : int) =
       IPAddress.NetworkToHostOrder(num)

3. In TestProject, write the following code:

   module TestProject.Tests
   
   open LibProject.Tools
   
   let testTheFunction() =
       let xx = int32FromBigEndian 100
       0

The repro solution is attached: ReproTestProject.zip

Expected behavior

I think that the code should compile successfully. But even if it isn't, then the compiler should print a sensible error message.

Actual behavior

The code doesn't compile and the compiler provides rather cryptic messages:

LibProject\LibProject.fs(9,12): Error FS1113 : The value 'int32FromBigEndian' was marked inline but its implementation makes use of an internal or private function which is not sufficiently accessible
TestProject\TestProject.fs(6,14): Warning FS1116 : A value marked as 'inline' has an unexpected value
TestProject\TestProject.fs(6,14): Error FS1118 : Failed to inline the value 'int32FromBigEndian' marked 'inline', perhaps because a recursive value was marked 'inline'

Known workarounds

Well, don't use inline functions within internal modules with the InternalsVisibleTo attribute :)

Related information

Reproduced using .NET Core SDK:

$ dotnet --info
.NET Core SDK (reflecting any global.json):
 Version:   2.2.300
 Commit:    73efd5bd87

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.18362
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\2.2.300\

Host (useful for support):
  Version: 2.2.5
  Commit:  0a3c9209c0

[dsyme]

Well, don't use inline functions within internal modules with the InternalsVisibleTo attribute :)

This is correct - InternalsVisibleTo doesn't allow inlining across assembly boundaries and if you try to use both of these you will get the above errors. So this is by design.

[ForNeVeR]

I think that the error message could be improved. Let's consider the first one:

The value 'int32FromBigEndian' was marked inline but its implementation makes use of an internal or private function which is not sufficiently accessible

I can't see which "internal or private function" do int32FromBigEndian use. It uses only IPAddress.NetworkToHostOrder which is a public static method. But probably "the value 'int32FromBigEndian'" isn't the function body but something else, and I don't understand the error message?

Maybe we could change the text so it's more clear?

The value 'int32FromBigEndian' refers to an internal or private function which is not sufficiently accessible

The second message looks okay if you don't read the "perhaps" part, and the warning is probably okay, too.

[dsyme]

The private/internal thing is int32FromBigEndian itself. So you can access it via InternalsVisibleTo but compilation will fail because the inlining information is thrown away at the assembly boundary.

We could indeed add a compilation error for that specific case (direct access to an internal inline function from another assembly via IVT).

Note we could really choose to keep the "internal" inlining information in the assembly whenever there is an IVT just in case the inline functions get used. But we don't do that to ensure assemblies are not bloated.

[ForNeVeR]

Okay, I understand that int32FromBigEndian is internal. But error message states that the implementation of int32FromBigEndian makes use of some other "private/internal symbol", and that part is confusing (I was legitimately confused by it). The message hints me that I should read the implementation of this function, and that isn't helpful, because the problem isn't in the implementation, but in the function itself (or even in the declaring module).

[dsyme]

Okay, I understand that int32FromBigEndian is internal. But error message states that the implementation of int32FromBigEndian refers to some other "private/internal symbol", and that part is confusing (I was legitimately confused by it). The message hints me that I should read the implementation of this function, and that isn't helpful, because the problem isn't in the implementation, but in the function itself (or even in the declaring module) .

Yes, absolutely, the error message is rotten

[cartermp]

Tagging as a feature improvement

[g5becks]

Ran into this today and this today and spent 30 min trying to figure out what the issue was. Had to come here to the repo, search for the specific error message, sift through these issues just to figure out exactly what was going wrong. Not user friendly at all imo.