Infer compile time GOPATH from fn.Name().

Author: ChrisHines

errors.go

@@ -65,11 +65,45 @@ func (l loc) Location() (string, int) {
 		return "unknown", 0
 	}
 
-	_, prefix, _, _ := runtime.Caller(0)
 	file, line := fn.FileLine(pc)
-	if i := strings.LastIndex(prefix, "github.com/pkg/errors"); i > 0 {
-		file = file[i:]
+
+	// Here we want to get the source file path relative to the compile time
+	// GOPATH. As of Go 1.6.x there is no direct way to know the compiled
+	// GOPATH at runtime, but we can infer the number of path segments in the
+	// GOPATH. We note that fn.Name() returns the function name qualified by
+	// the import path, which does not include the GOPATH. Thus we can trim
+	// segments from the beginning of the file path until the number of path
+	// separators remaining is one more than the number of path separators in
+	// the function name. For example, given:
+	//
+	//    GOPATH     /home/user
+	//    file       /home/user/src/pkg/sub/file.go
+	//    fn.Name()  pkg/sub.Type.Method
+	//
+	// We want to produce:
+	//
+	//    pkg/sub/file.go
+	//
+	// From this we can easily see that fn.Name() has one less path separator
+	// than our desired output. We count separators from the end of the file
+	// path until it finds two more than in the function name and then move
+	// one character forward to preserve the initial path segment without a
+	// leading separator.
+	const sep = "/"
+	goal := strings.Count(fn.Name(), sep) + 2
+	pathCnt := 0
+	i := len(file)
+	for pathCnt < goal {
+		i = strings.LastIndex(file[:i], sep)
+		if i == -1 {
+			i = -len(sep)
+			break
+		}
+		pathCnt++
 	}
+	// get back to 0 or trim the leading seperator
+	file = file[i+len(sep):]
+
 	return file, line
 }