don't flag funcs passed as params via closures

Like in the previous case involving fields, do this with a bit of SSA
code instead of relying on callgraph. The trick here was to go past
MakeClosure.

Fixes #36.

Author: mvdan

check/check.go

@@ -238,21 +238,26 @@ func (c *Checker) Check() ([]Issue, error) {
 	for fn := range allFuncs {
 		for _, b := range fn.Blocks {
 			for _, instr := range b.Instrs {
-				faddr, ok := instr.(*ssa.FieldAddr)
-				if !ok {
-					continue
-				}
-				ftype := faddr.Type().(*types.Pointer).Elem()
-				if _, ok := ftype.(*types.Signature); !ok {
-					continue
-				}
-				for _, ref := range *faddr.Referrers() {
-					store, ok := ref.(*ssa.Store)
-					if !ok || store.Addr != faddr {
+				switch instr := instr.(type) {
+				case *ssa.Call:
+					for _, arg := range instr.Call.Args {
+						if fn := findFunction(arg); fn != nil {
+							c.funcUsedAs[fn] = "param"
+						}
+					}
+				case *ssa.FieldAddr:
+					ftype := instr.Type().(*types.Pointer).Elem()
+					if _, ok := ftype.(*types.Signature); !ok {
 						continue
 					}
-					if fn, _ := store.Val.(*ssa.Function); fn != nil {
-						c.funcUsedAs[fn] = "field"
+					for _, ref := range *instr.Referrers() {
+						store, ok := ref.(*ssa.Store)
+						if !ok || store.Addr != instr {
+							continue
+						}
+						if fn := findFunction(store.Val); fn != nil {
+							c.funcUsedAs[fn] = "field"
+						}
 					}
 				}
 			}
@@ -298,6 +303,17 @@ func (c *Checker) Check() ([]Issue, error) {
 	return c.issues, nil
 }
 
+// findFunction returns the function that is behind a value, if any.
+func findFunction(value ssa.Value) *ssa.Function {
+	switch value := value.(type) {
+	case *ssa.Function:
+		return value
+	case *ssa.MakeClosure:
+		return value.Fn.(*ssa.Function)
+	}
+	return nil
+}
+
 // addIssue records a newly found unused parameter.
 func (c *Checker) addIssue(fn *ssa.Function, pos token.Pos, format string, args ...interface{}) {
 	c.issues = append(c.issues, Issue{

testdata/scripts/usedas.txt

@@ -153,3 +153,19 @@ var usedAsGlobalField = http.Client{
                 return nil
         },
 }
+
+var mux = http.NewServeMux()
+
+func UsedAsGlobalParam(someBool bool) {
+	mux.HandleFunc("/inline", func(w http.ResponseWriter, r *http.Request) {
+		if r.Method == "GET" {
+			println(r.Host)
+		}
+	})
+	closure := func(w http.ResponseWriter, r *http.Request) {
+		if r.Method == "POST" && someBool {
+			println(r.Host)
+		}
+	}
+	mux.HandleFunc("/closureVar", closure)
+}