In order to reflect nullability in constraint types, compiler emits Nullable attribute for rows in GenericParamConstraint metadata table

[AlekseyTs]

It looks like there is no syntax to represent this in IL. Is this a problem?

Example: class C<T> where T : Interface?

• extend Cecil to support such metadata (PR Add support for custom attributes on generic constraints jbevain/cecil#594)
• ILLinker should take new version of Cecil
• corefx should pick up new ILLinker (this is a corefx internal decision, the public facing reference assemblies will be correct in Core 3)
• extend IL/ILAsm/ILDasm to allow representing such metadata (issue https://github.com/dotnet/coreclr/issues/25488)

Relates to dotnet/linker#626

[jcouv]

Tagging @safern @stephentoub @agocke

The issue is that we're using metadata representation which is legal but not representable in IL.
We did this in the past for tuple names, but we're doing this more for nullability in constraints and nullability in method interface implementations (thx Aleksey for correction).

At this point in the release cycle, I think we will stick with this design unless there is a major problem and we have a viable alternative. I'll keep this issue open for another week or two to give you the chance to chime in, then I'll close.

[stephentoub]

cc: @jkotas

[jkotas]

Does this mean that you won't be able to round-trip these assemblies using ildasm/ilasm? Number of folks out there who depend on ilasm/ildasm round-tripping as part of their build...

[jcouv]

That's correct. We would need to update ildasm/ilasm to allow such round-tripping.

@AlekseyTs Could you add details on the method implementation scenario (where is the attribute placed?). Thanks

[jkotas]

Also, what about Cecil and all tools build on top of it? (e.g. https://github.com/mono/linker that is officially supported tool for .NET Core 3.0)

[AlekseyTs]

@jcouv

Could you add details on the method implementation scenario (where is the attribute placed?).

I was talking about interface implementations. I do not think we emit any attributes for MethodIlmpls.

[AlekseyTs]

As simple as class C : MyInterface<string?>

[jbevain]

@jcouv @jkotas That would be a breaking change in Cecil albeit a similar one we did to InterfaceImplementation in 0.10.

That would need to go in 0.11 and should be an easy change in Cecil upstream, the linker would need to be updated to adopt the change.

[jcouv]

It looks like the interface implementation scenario is supported by IL/ilasm/ildasm after all. See details below.
This means we only have to worry about the type constraint scenario (such as where T : Interface?).

Interface implementation appears to be supported:

From the following C#:

#nullable enable
public interface I<T> { }
public interface I2<T> { }
public class D<T, U> : I<T?>, I2<U> where T : class where U : class
{ 
}

ildasm produces the following IL:

.class public auto ansi beforefieldinit D`2<class T,class U>
       extends [mscorlib]System.Object
       implements class I`1<!T>,
                  class I2`1<!U>
{
  .interfaceimpl type class I`1<!T>
  .custom instance void System.Runtime.CompilerServices.NullableAttribute::.ctor(uint8[]) = ( 01 00 02 00 00 00 00 02 00 00 ) 
  .interfaceimpl type class I2`1<!U>
  .custom instance void System.Runtime.CompilerServices.NullableAttribute::.ctor(uint8[]) = ( 01 00 02 00 00 00 00 01 00 00 ) 
  .param type T 
  .custom instance void System.Runtime.CompilerServices.NullableAttribute::.ctor(uint8) = ( 01 00 01 00 00 ) 
  .param type U 
  .custom instance void System.Runtime.CompilerServices.NullableAttribute::.ctor(uint8) = ( 01 00 01 00 00 ) 
} // end of class D`2

And ildasm produces the following MetaInfo:

TypeDef #6 (02000007)
-------------------------------------------------------
	TypDefName: D`2  (02000007)
	Flags     : [Public] [AutoLayout] [Class] [AnsiClass] [BeforeFieldInit]  (00100001)
	Extends   : 01000007 [TypeRef] System.Object
	2 Generic Parameters
		(0) GenericParamToken : (2a000004) Name : T flags: 00000004 Owner: 02000007
		CustomAttribute #1 (0c00000d)
		-------------------------------------------------------
			CustomAttribute Type: 06000002
			CustomAttributeName: System.Runtime.CompilerServices.NullableAttribute :: instance void .ctor(unsigned int8)
			Length: 5
			Value : 01 00 01 00 00                                   >                <
			ctor args: (1)

		(1) GenericParamToken : (2a000005) Name : U flags: 00000004 Owner: 02000007
		CustomAttribute #1 (0c00000e)
		-------------------------------------------------------
			CustomAttribute Type: 06000002
			CustomAttributeName: System.Runtime.CompilerServices.NullableAttribute :: instance void .ctor(unsigned int8)
			Length: 5
			Value : 01 00 01 00 00                                   >                <
			ctor args: (1)

	Method #1 (06000006) 
	-------------------------------------------------------
		MethodName: .ctor (06000006)
		Flags     : [Public] [HideBySig] [ReuseSlot] [SpecialName] [RTSpecialName] [.ctor]  (00001886)
		RVA       : 0x00002085
		ImplFlags : [IL] [Managed]  (00000000)
		CallCnvntn: [DEFAULT]
		hasThis 
		ReturnType: Void
		No arguments.

	InterfaceImpl #1 (09000001)
	-------------------------------------------------------
		Class     : D`2
		Token     : 1B000001 [TypeSpec] [TypeSpec]
		CustomAttribute #1 (0c000001)
		-------------------------------------------------------
			CustomAttribute Type: 06000003
			CustomAttributeName: System.Runtime.CompilerServices.NullableAttribute :: instance void .ctor(unsigned int8[])
			Length: 10
			Value : 01 00 02 00 00 00 00 02  00 00                   >                <
			ctor args: ( <can not decode> )


	InterfaceImpl #2 (09000002)
	-------------------------------------------------------
		Class     : D`2
		Token     : 1B000002 [TypeSpec] [TypeSpec]
		CustomAttribute #1 (0c000009)
		-------------------------------------------------------
			CustomAttribute Type: 06000003
			CustomAttributeName: System.Runtime.CompilerServices.NullableAttribute :: instance void .ctor(unsigned int8[])
			Length: 10
			Value : 01 00 02 00 00 00 00 01  00 00                   >                <
			ctor args: ( <can not decode> )

[sbomer]

Are the attributes on generic param constraints going to be emitted in 3.0?

[jcouv]

Are the attributes on generic param constraints going to be emitted in 3.0?

Yes. They will be emitted by anyone using C# 8.0 (nullability analysis feature), which ships with Core 3.0.

[jbevain]

@jcouv is there a scenario where in the metadata you could generate a CustomAttr owned by a TypeRef or a TypeSpec?

[AlekseyTs]

is there a scenario where in the metadata you could generate a CustomAttr owned by a TypeRef or a TypeSpec?

In general, yes. See AssemblyAttributesGoHere type and the like, for example? However, compiler never attaches NullableAttribute to a TypeRef.

[jbevain]

@AlekseyTs I know the metadata format allows it. Just wanted to make sure that adding support for custom attributes on generic parameter constraints is enough to support this feature we're discussing.

[jaredpar]

Closing as the runtime team added support for this in the ilasm / ildasm toolset. Can re-open if it's not sufficient.