xtools$ git log | head -n 1
commit 40fb89cd55bdb68faa88c52593caafb733cdc880

xtools$ cat a.go
package tools

type E int

func (E) Error() string { return "" }

type MyError interface{ Error() string }

xtools$ go run ./gopls implementation ./a.go:#21
/Users/adonovan/w/xtools/a.go:7:6-13

xtools$ nl -ba /Users/adonovan/w/xtools/a.go | grep 7
     7	type MyError interface{ Error() string }