cgo: better error for no C symbols

R=rsc, iant
CC=golang-dev, graeme.perrow
https://golang.org/cl/1193046

diff --git a/src/cmd/cgo/main.go b/src/cmd/cgo/main.go
index 070146c9acb2e955c6c5b8a3a34db924eb7b604c..ac82ae91841ba360e00b689e8359387e78fcb5d1 100644 (file)
--- a/src/cmd/cgo/main.go
+++ b/src/cmd/cgo/main.go
@@ -83,6 +83,11 @@ func main() {
                // Reset p.Preamble so that we don't end up with conflicting headers / defines
                p.Preamble = builtinProlog
                openProg(input, p)
+
+               if len(p.Vardef) == 0 && len(p.Funcdef) == 0 && len(p.Enumdef) == 0 && len(p.Constdef) == 0 {
+                       fatal("no C symbols were used in cgo file " + input)
+               }
+
                for _, cref := range p.Crefs {
                        // Convert C.ulong to C.unsigned long, etc.
                        if expand, ok := expandName[cref.Name]; ok {