Eenable LanguageFeature.UnmanagedConstraintCsharpInterop for F# 10.0

[Copilot]

Summary

Re-enables the UnmanagedConstraintCsharpInterop language feature for F#109.0 to fix a TypeLoadException that occurs when using unmanaged constraints in lambda expressions.

Problem

open System

let printTypeConstraintsNative<'T when 'T : unmanaged> () = 
    printf $"Hello: {typeof<'T>.FullName} is unmanaged"

let Main() =
    let func (x:int) : 'T when 'T : unmanaged = Unchecked.defaultof<'T>
    let initFinite = Seq.init<nativeint> 3 func
    printfn "%A" initFinite
 
printTypeConstraintsNative<nativeint>()
Main()

Runtime Error:

System.TypeLoadException: Method 'Specialize' on type 'func@8' from assembly 'Program, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' tried to implicitly override a method with weaker type parameter constraints.

Solution

Well, copilot tried many times to hack it around by disabling bunch of closure related codegen in IlxGen, but it only kept making things worse.

The (human discovered) solution makes sure the right closure generation style is picked (there are two main forms, well documented in comments in IL generation and closure lowering).

The function isCompiledOrWitnessPassingConstraint asks "Which constraints actually get compiled to .NET constraints?" Unmanaged constraints absolutely DO compile to IL (modreq(System.Runtime.InteropServices.UnmanagedType)), so they belong in this list alongside:

class constraints
struct constraints
new() constraints
base type constraints

[github-actions]

⚠️ Release notes required, but author opted out

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cc @dotnet/fsharp-team-msft

[T-Gro]

The feature UnmanagedConstraintCsharpInterop generates invalid IL in the Specialize method.
Add tests in the ComponentTests projects.

Add a test to verify the exact reported issue by compiling and running.

For debugging this, also add a test which will verifyIL of that piece of code.
First run it with an abcxyz nonsense assertion. Then you take the produced IL and change the test assertion.

Make sure to run the tests with previewVersion language feature, otherwise it will not kick in.

[Copilot]

The feature UnmanagedConstraintCsharpInterop generates invalid IL in the Specialize method.
Add tests in the ComponentTests projects.

...

I've added the requested tests to reproduce issue #17509. The tests now demonstrate the exact problem: when UnmanagedConstraintCsharpInterop is enabled with preview version, it generates invalid IL in the Specialize method for lambda expressions with unmanaged constraints.

The key issue is in the generated IL:

.method public strict virtual instance object Specialize<valuetype (class [runtime]System.ValueType modreq([runtime]System.Runtime.InteropServices.UnmanagedType)) T>() cil managed

The modreq([runtime]System.Runtime.InteropServices.UnmanagedType) constraint creates incompatible method signatures causing the TypeLoadException at runtime.

Tests added in commit 9f670d0:

1. Test that reproduces the original lambda issue
2. Test that verifies the invalid IL generation with actual IL output

Both tests run with previewVersion as requested. The tests now clearly show the root cause of the issue.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• 1k9vsblobprodcus379.vsblob.vsassets.io
  • Triggering command: dotnet restore ./FSharp.Compiler.Service.sln (dns block)
• 4vyvsblobprodcus361.vsblob.vsassets.io
  • Triggering command: dotnet restore ./FSharp.Compiler.Service.sln (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list

[T-Gro]

/run fantomas

[github-actions]

🔧 CLI Command Report

• Command: /run fantomas
• Outcome: success

✅ Command succeeded, no changes needed.