According to the Go spec: "The function copy copies slice elements from a source
src to a destination dst and returns the number of elements copied. [...] The number of
elements copied is the minimum of len(src) and len(dst)."

This is normally handled correctly as demonstrated below, but when the race detector is
enabled, copy() returns 0 instead.  This is because gc switches to using slicecopy(),
which includes a "width == 0" short-circuit test, but then returns
"0" instead of "min(len(src), len(dst))".

(Doesn't seem terribly important, so I'll wait for the next release cycle to send a
patch.  Just wanted to note down the issue so I don't forget it later.)

$ cat copytest.go
package main
import "fmt"
func main() {
        var b [100][0]byte
        s := b[:]
        fmt.Println(len(s), copy(s, s))
}

$ go run copytest.go 
100 100

$ go run -race copytest.go 
100 0