Changing the interface 1) is obviously not relevant.

Re-wrapping works only for the toy example. In the real world, the reader isn't some local variable, but there could be many, across different structs, behind private fields.

To cirle back, and not focus too much on the io.Reader example: the virality of ctx is real, and making wrapper structs is not a good solution. Updating stale references may not be possible, and would quickly become overwhelming. Not to forget the performance overhead.

Personally I think it's okay, go is fine as a "webservices" language. The go gospel is, You can have your cake and eat it too, but it's almost never true unless you twist the meaning of "cake" and "eat".