Extend IL/ILAsm/ILDasm to represent attributes on type constraints

[jcouv]

We have not yet discussed a specific proposal for how to represent this in IL. Tagging @RussKeldorph.

Relates to dotnet/roslyn#29997

[jbevain]

That's going to be a bit annoying as the constraints are currently inlined in the generic parameter declaration. What about introducing a new .constraint keyword for scenarios where you want a custom attribute or do no want to add the constraint inline in the brackets?

.class private auto ansi Library`1<T>
{
    .param type T
    .constraint class Foo.IInterface
    .custom instance void class System.Runtime.CompilerServices.NullableAttribute::'.ctor'(unsigned int8)
}

.constraint could be limited to class and valuetype arguments.

[dseefeld]

This is impacting source-build. We rely on round-tripping some assemblies using ildasm/ilasm to eliminate prebuilts. Not having the ability to round-trip these assemblies with nullable attributes will increasingly impact our ability to get to zero prebuilts, especially as usage of the nullable attribute increases.

[briansull]

Ok I have implemented the following ildasm:

#nullable enable
public class C2 { }
public interface I2<T> { }

public class C<T> where  T : C2?, I2<(int a, int b)> 
{ 
    void M<U>(C2? x) where U : C2? { }
}

Produces this IL:

.class public auto ansi beforefieldinit C`1<(C2, class I2`1<valuetype System.ValueTuple`2<int32,int32>>) T>
       extends [mscorlib]System.Object
{
  .param constraint   T, C2
    .custom instance void System.Runtime.CompilerServices.NullableAttribute::.ctor(uint8) = ( 01 00 02 00 00 ) 
  .param constraint   T, class I2`1<valuetype System.ValueTuple`2<int32,int32>>
    .custom instance void [mscorlib]System.Runtime.CompilerServices.TupleElementNamesAttribute::.ctor(string[]) = ( 01 00 02 00 00 00 01 61 01 62 00 00 )             // .......a.b..
    .custom instance void System.Runtime.CompilerServices.NullableAttribute::.ctor(uint8[]) = ( 01 00 02 00 00 00 01 00 00 00 ) 
  .method private hidebysig instance void 
          M(class C2 x) cil managed
  {
    .custom instance void System.Runtime.CompilerServices.NullableContextAttribute::.ctor(uint8) = ( 01 00 02 00 00 ) 
    // Code size       2 (0x2)
    .maxstack  8
    IL_0000:  nop
    IL_0001:  ret
  } // end of method C`1::M

  .method public hidebysig specialname rtspecialname 
          instance void  .ctor() cil managed
  {
    // Code size       8 (0x8)
    .maxstack  8
    IL_0000:  ldarg.0
    IL_0001:  call       instance void [mscorlib]System.Object::.ctor()
    IL_0006:  nop
    IL_0007:  ret
  } // end of method C`1::.ctor

} // end of class C`1

[BruceForstall]

I don't believe this meets the bar for 3.0, so moving Milestone to 5.0.

[briansull]

Example syntax:

#nullable enable
public class C2 { }
public interface I2<T> { }

public class C<T> where  T : C2?, I2<(int a, int b)> 
{ 
    void M<U>(C2? x) where U : C2? { }
}

---

.class public auto ansi beforefieldinit C`1<(C2, class I2`1<valuetype System.ValueTuple`2<int32,int32>>) T>
       extends [mscorlib]System.Object
{
  .param constraint   T, C2
    .custom instance void System.Runtime.CompilerServices.NullableAttribute::.ctor(uint8) = ( 01 00 02 00 00 ) 
  .param constraint   T, class I2`1<valuetype System.ValueTuple`2<int32,int32>>
    .custom instance void [mscorlib]System.Runtime.CompilerServices.TupleElementNamesAttribute::.ctor(string[]) = ( 01 00 02 00 00 00 01 61 01 62 00 00 )             // .......a.b..
    .custom instance void System.Runtime.CompilerServices.NullableAttribute::.ctor(uint8[]) = ( 01 00 02 00 00 00 01 00 00 00 ) 
  .method private hidebysig instance void 
          M(class C2 x) cil managed
  {

[briansull]

Fixed with:

Implement ilasm and ildasm support for .param constraint with Custom attributes dotnet/coreclr#26219