Artifact in anon_target

I'm attempting to pass an artifact through a anon_target call (the artifacts are projected)

```
return ctx.actions.anon_target(native.cxx_library, {
"srcs": pb_cc,
}).map(k)
```
```
Expected value of type `string`, got value with type `artifact`
```

@dtolnay suggested that passing an "artifact" through to an attrs.source is likely intended to work. Have I just hit an edge-case of type coercion or something like that?