bind: fallback to const.Value.String for pre go1.6

Fixes golang/go#14615

Change-Id: I75e130e5b7b2534660098907fa1f044390c01d01
Reviewed-on: https://go-review.googlesource.com/20164
Reviewed-by: Elias Naur <elias.naur@gmail.com>

bind/gen.go

@@ -245,3 +245,18 @@ func paramName(params *types.Tuple, pos int) string {
 	}
 	return name
 }
+
+func constExactString(o *types.Const) string {
+	// TODO(hyangah): this is a temporary fix for golang.org/issues/14615.
+	// Clean this up when we can require at least go 1.6 or above.
+
+	type exactStringer interface {
+		ExactString() string
+	}
+	v := o.Val()
+	if v, ok := v.(exactStringer); ok {
+		return v.ExactString()
+	}
+	// TODO: warning?
+	return v.String()
+}

bind/genjava.go

@@ -535,7 +535,7 @@ func (g *javaGen) genConst(o *types.Const) {
 	// TODO(hyangah): should const names use upper cases + "_"?
 	// TODO(hyangah): check invalid names.
 	jType := g.javaType(o.Type())
-	val := o.Val().ExactString()
+	val := constExactString(o)
 	switch b := o.Type().(*types.Basic); b.Kind() {
 	case types.Int64, types.UntypedInt:
 		i, exact := constant.Int64Val(o.Val())

bind/genobjc.go

@@ -276,7 +276,7 @@ func (g *objcGen) genConstM(o *types.Const) {
 		g.Printf("const BOOL %s = %s;\n", cName, v)
 
 	case types.String, types.UntypedString:
-		g.Printf("NSString* const %s = @%s;\n", cName, o.Val().ExactString())
+		g.Printf("NSString* const %s = @%s;\n", cName, constExactString(o))
 
 	case types.Int, types.Int8, types.Int16, types.Int32:
 		g.Printf("const %s %s = %s;\n", objcType, cName, o.Val())