Reject fixed expressions when used with static GetPinnableReference method

Author: jwosty

src/Compiler/Checking/CheckExpressions.fs

@@ -10343,7 +10343,7 @@ and TcAndBuildFixedExpr (cenv: cenv) env (overallPatTy, fixedExpr, overallExprTy
             |> List.tryFind (fun mInfo ->
                 // GetPinnableReference must be a parameterless method with a byref or inref return value
                 match mInfo.GetParamDatas(cenv.amap, mBinding, mInfo.FormalMethodInst), mInfo.GetFSharpReturnType(cenv.amap, mBinding, mInfo.FormalMethodInst) with
-                | [[]], retTy when isByrefTy g retTy -> true
+                | [[]], retTy when isByrefTy g retTy && mInfo.IsInstance -> true
                 | _ -> false
             )
 

tests/FSharp.Compiler.ComponentTests/EmittedIL/FixedExpressionTests.fs

@@ -606,3 +606,55 @@ let main _ =
     IL_0016:  ldobj      [runtime]System.Int32
     IL_001b:  ret
   } """ ]
+        
+    [<Fact>]
+    let ``Pin type with extension method GetPinnableReference : unit -> byref<T>`` () =
+        Fsx """
+module FixedExpressions
+open System.Runtime.CompilerServices
+open Microsoft.FSharp.NativeInterop
+
+type RefField<'T> = { mutable _value: 'T }
+
+[<Extension>]
+type RefFieldExtensions =
+    [<Extension>]
+    static member GetPinnableReference(refField: RefField<'T>) : byref<'T> = &refField._value 
+
+let pinIt (thing: RefField<'T>) =
+    use ptr = fixed thing
+    NativePtr.get ptr 0
+    
+[<EntryPoint>]
+let main _ =
+    let mutable x = 42
+    let refToX = { _value = x }
+    let y = pinIt refToX
+    if y <> x then failwith "y did not equal x"
+    0
+"""
+        |> withOptions ["--nowarn:9"]
+        |> compileExeAndRun
+        |> shouldSucceed
+        |> verifyIL ["""
+  .method public static !!a  pinIt<T,a>(class FixedExpressions/RefField`1<!!T> thing) cil managed
+  {
+    
+    .maxstack  5
+    .locals init (native int V_0,
+             !!a& pinned V_1)
+    IL_0000:  ldarg.0
+    IL_0001:  ldflda     !0 class FixedExpressions/RefField`1<!!a>::_value@
+    IL_0006:  stloc.1
+    IL_0007:  ldloc.1
+    IL_0008:  conv.i
+    IL_0009:  stloc.0
+    IL_000a:  ldloc.0
+    IL_000b:  ldc.i4.0
+    IL_000c:  conv.i
+    IL_000d:  sizeof     !!a
+    IL_0013:  mul
+    IL_0014:  add
+    IL_0015:  ldobj      !!a
+    IL_001a:  ret
+  } """ ]

tests/FSharp.Compiler.ComponentTests/Language/FixedExpressionTests.fs

@@ -367,3 +367,45 @@ let pinIt (thing: RefField<'T>) =
             (Warning 9, Line 11, Col 9, Line 11, Col 12, """Uses of this construct may result in the generation of unverifiable .NET IL code. This warning can be disabled using '--nowarn:9' or '#nowarn "9"'.""")
             (Warning 9, Line 12, Col 5, Line 12, Col 18, """Uses of this construct may result in the generation of unverifiable .NET IL code. This warning can be disabled using '--nowarn:9' or '#nowarn "9"'.""")
         ]
+        
+    [<Fact>]
+    let ``Pin type with private method GetPinnableReference - illegal`` () =
+        Fsx """
+open Microsoft.FSharp.NativeInterop
+
+type StrangeType<'T>(_value) =
+    let mutable _value = _value
+    member private this.GetPinnableReference() : byref<'T> = _value
+
+let pinIt (thing: StrangeType<'T>) =
+    use ptr = fixed thing
+    NativePtr.get ptr 0
+"""
+        |> ignoreWarnings
+        |> typecheck
+        |> shouldFail
+        |> withDiagnostics [
+            (Warning 9, Line 9, Col 9, Line 9, Col 12, """Uses of this construct may result in the generation of unverifiable .NET IL code. This warning can be disabled using '--nowarn:9' or '#nowarn "9"'.""")
+            (Error 3207, Line 9, Col 9, Line 9, Col 12, """Invalid use of 'fixed'. 'fixed' may only be used in a declaration of the form 'use x = fixed expr' where the expression is an array, the address of a field, the address of an array element or a string'""")
+        ]
+
+    [<Fact>]
+    let ``Pin type with static method GetPinnableReference - illegal`` () =
+        Fsx """
+open Microsoft.FSharp.NativeInterop
+
+type StrangeType<'T>(_value) =
+    let mutable _value = _value
+    static member GetPinnableReference() : byref<'T> = Unchecked.defaultof<'T>
+
+let pinIt (thing: StrangeType<'T>) =
+    use ptr = fixed thing
+    NativePtr.get ptr 0
+"""
+        |> ignoreWarnings
+        |> typecheck
+        |> shouldFail
+        |> withDiagnostics [
+            (Warning 9, Line 9, Col 9, Line 9, Col 12, """Uses of this construct may result in the generation of unverifiable .NET IL code. This warning can be disabled using '--nowarn:9' or '#nowarn "9"'.""")
+            (Error 3207, Line 9, Col 9, Line 9, Col 12, """Invalid use of 'fixed'. 'fixed' may only be used in a declaration of the form 'use x = fixed expr' where the expression is an array, the address of a field, the address of an array element or a string'""")
+        ]