Doc Cookbook Auto Interface Implementation

[bdbonazz]

Added documentation of IncrementalGenerators Cookbook about Auto interface implementation

#72149

Working solution: https://github.com/bdbonazz/AutoImplementGenerator

[bdbonazz]

@dotnet-policy-service agree

[jaredpar]

@333fred, @CyrusNajmabadi PTAL

[333fred]

Unfortunately, this type of generator is not something that we want to put in the cookbook or recommend that people do. Going through all interfaces implemented by a type, particularly the way that ends up being implemented here (using a Compilation as an incremental parameter) is not actually incremental in any fashion and will kill IDE performance. As of yet, we have no ideas for how to implement this type of generator efficiently enough to suggest that people should write a generator in this fashion.

A restructuring of the generator that attributes the types that get their interfaces auto-implemented, rather than the interfaces themselves, would be acceptable to include. But the current structure is something that we explicitly recommend against doing in this very document.

[bdbonazz]

@333fred
I agree with you about performance issues, and I agree with @sharwell about using "ForAttributeWithMetadataName "
Do you think a solution of this type would be acceptable?

[AutoImplement(nameof(IUserInterface), nameof(IUserInterface2))]
public partial class UserClass : IUserInterface, IUserInterface2
{
    public string UserProp { get; set; }
}

Moving the attribute to the class would let me use "ForAttributeWithMetadataName" to get only the decorated classes.
I would no longer cycle through all the interfaces of a class, but only the interfaces passed as an argument to the attribute.
Bonus point: the developer could decide when a class should auto-implement an interface and when to implement the interface manually

If at least one of you gives me the ok, I will start implementing this solution

[333fred]

Yes, that's exactly the type of structure I meant when I said a change could be acceptable to include.

[bdbonazz]

@333fred
Perfect
Just one last thing:
You rightly had doubts about "using a Compilation as an incremental parameter".
Still I think I need Compilation in order to get INamedTypeSymbol interfaceSymbol from the interface name (and I need interfaceSymbol in order to have interfaceUsings, interfaceProperties, etc.)
I don't know if there is a more efficient way to do this.
Would you prefer I get the interface infos in the RegisterSourceOutput method like I'm doing now? Or should I get that infos in the transform argument of the ForAttributeWithMetadataName method?

[333fred]

That info is already available in the context of your transform callback in FAWMN. See https://github.com/dotnet/roslyn/blob/main/docs/features/incremental-generators.cookbook.md#augment-user-code.

[bdbonazz]

@333fred You are right, now it should be more performant

[333fred]

Done review pass. Large step in the right direction, but there's still plenty of room for improvement.

[333fred]

@CyrusNajmabadi @chsienki for reviews as well please.

[333fred]

Thanks @bdbonazz!

[bdbonazz]

Thanks @bdbonazz!
@333fred
You are welcome
Thank you too, I learned a lot from your comments