Go generate is like a manual step for having generics, I imagine one day soon its functionality or something similar will be part of go build.
In response to your particular example, a map plus a mutex seems sufficient.
I dig that it's a few extra lines of code, but I guess I'm looking for a really mean example.
Well, if you don't care about repeating yourself, mistakes by omission, needless boilerplate, and/or loss of type safety, then, sure, everything is possible in "a few extra lines of code" in a turing complete language...
The problem with your mutex solution is at least two:
1) it is more complex
* more lines to type and read and check and thus harder to understand
* you can forget the mutex
* if you have data structures of hastables, you will have to pass mutexes around or store them together with the tables.
2) it is not nearly as performent as a lock free hash table
(edit indentation)
So the first version of BIDS, Borland's C++ data structures library, used pre-processor tricks where one defined the types before including the respective data structure.
So go generate as workaround feels quite old.