[vm_service] FieldRef.library.uri points to the wrong library for mixins

[rrousselGit]

Consider:

// a.dart

mixin Mixin {
  int _private = 42;
}

// b.dart

class Class with Mixin {
  int _private = 42;
}

When iterating over Instance.fields on a Class instance, this will correctly report two _private fields but:
Mixin._private's FieldRef.owner.uri will point to b.dart instead of a.dart

This is problematic because that can cause evaluations to be performed on Class._private instead of Mixin._private.
An example of such evaluation could be classInstance._private++

[jacob314]

This is a tricky edge case. Good news there is a way out without any VM (or more accurately mixin kernel transformer changes)
The sourceLocation from a BoundField should tell you the library the private symbol is private relative to.
https://github.com/dart-lang/sdk/blob/master/runtime/vm/service/service.md#field
https://github.com/dart-lang/sdk/blob/master/runtime/vm/service/service.md#sourcelocation

[jacob314]

Fyi @bkonyi

[bkonyi]

After talking with @rmacnak-google, I think this is WAI as the mixed in field is owned by the class which the mixin was applied to, not the original mixin declaration. I've gone ahead and updated the documentation here to explain some of the situations where the location of owner may not match the location of a field/function's origin.

[rrousselGit]

@bkonyi Unless I'm missing something, I don't agree with this getting closed.

The root of the issue is that this behavior prevents mutating private properties of mixins as defined in the issue.

To make the expression mixin._property = value valid, we need to know the URI in which the mixin is defined – which we can't do because of this issue.

[bkonyi]

It's still possible to get access to the declaration location of the properties from their sourceLocation, which is more accurate than going off of the location of the property's owner. In general, sourceLocation of a service object should be used to determine where the corresponding property was declared.

[rrousselGit]

I see, thanks for sharing. I didn't know about this property

[bkonyi]

Not a problem! :-)