If I have two threads concurrently modifying a string field on a struct, will I always see one or the other string assigned to the field, but nothing else?
No. If you need atomic operations, there is sync/atomic.
The Go Memory Model will have all the related details. From the top of the Memory Model document:
Programs that modify data being simultaneously accessed by multiple goroutines must serialize such access.
To serialize access, protect the data with channel operations or other synchronization primitives such as those in the
syncandsync/atomicpackages.