Type checking of protocol implementations

[josevalim]

This pull request adds type checking of protocol implementations.

In other words, if you implement a protocol FooBar for a struct T, we will validate that the arguments given to the implementation are said structs. Here is an example:

The to_charlist callback expects a URI struct, which does not have the .unknown field, and therefore a typing violation is reported. This will help ensure protocol implementations are kept in sync with struct definitions. In order to make this all work, we now warn when implementing a protocol for an undefined struct.

[zachdaniel]

Wasn’t something done recently to ensure that using defimpl didn’t require a compile time dependency on modules specified on the module passed into the for option? Is that no longer the case with this change or does type checking like this happen in a later pass that doesn’t incur compile dependencies?

[josevalim]

We still do not require a compile-time dependency. A later commit to main pushed a test too.