never warn on funcs used as interfaces

These often lead to false positives, as the interface value is often
called via type switches, reflection, or other methods.

As before, don't rely on callgraph for this. It doesn't deal well with
that kind of runtime type conversion.

We had a few test cases which were "correct positives", but looking at
them again, they don't seem particularly useful or stable. Better to be
on the safe side and not warn on those.

Fixes #23.

Author: mvdan

check/check.go

@@ -246,10 +246,6 @@ func (c *Checker) Check() ([]Issue, error) {
 						}
 					}
 				case *ssa.FieldAddr:
-					ftype := instr.Type().(*types.Pointer).Elem()
-					if _, ok := ftype.(*types.Signature); !ok {
-						continue
-					}
 					for _, ref := range *instr.Referrers() {
 						store, ok := ref.(*ssa.Store)
 						if !ok || store.Addr != instr {
@@ -309,7 +305,11 @@ func findFunction(value ssa.Value) *ssa.Function {
 	case *ssa.Function:
 		return value
 	case *ssa.MakeClosure:
+		// closure of a func
 		return value.Fn.(*ssa.Function)
+	case *ssa.MakeInterface:
+		// wrapping a func as an interface
+		return findFunction(value.X)
 	}
 	return nil
 }

testdata/scripts/usedas.txt

@@ -6,8 +6,7 @@ cmp stdout stdout.golden
 -- go.mod --
 module testdata.tld/foo
 -- stdout.golden --
-foo.go:55:22: UsedAsArg$4 - u is unused
-foo.go:73:36: usedAsIfaceNothing - i is unused
+foo.go:9:19: oneUnused - b is unused
 -- foo.go --
 package foo
 
@@ -17,6 +16,10 @@ type FooType int
 
 var DoWork func()
 
+func oneUnused(a, b int) int {
+        return a + 123
+}
+
 func funcAsParam(fn func(FooType) string) { fn(0) }
 
 func passedAsParam(f FooType) string {
@@ -77,29 +80,8 @@ func usedAsGlobalArg(f FooType, i int8) {
 	println(f)
 }
 
-func paramIfaceNothing(f interface{}) {
-	println(f)
-}
-
-func usedAsIfaceNothing(f FooType, i int16) {
-	DoWork()
-	println(f)
-}
-
-func paramIfaceAssert(f interface{}) {
-	fn := f.(func(FooType, *rune))
-	fn(2, nil)
-}
-
-func usedAsIfaceAssert(f FooType, i *rune) {
-	DoWork()
-	println(f)
-}
-
 func GlobArgUse() {
 	globalParam(usedAsGlobalArg)
-	paramIfaceNothing(usedAsIfaceNothing)
-	paramIfaceAssert(usedAsIfaceAssert)
 }
 
 type barIface interface {
@@ -169,3 +151,15 @@ func UsedAsGlobalParam(someBool bool) {
 	}
 	mux.HandleFunc("/closureVar", closure)
 }
+
+type FuncInterfaceField struct {
+	Func interface{}
+}
+
+var usedAsGlobalInterfaceField = FuncInterfaceField {
+	Func: func(i int, bs []byte) {
+		if i == 0 {
+			DoWork()
+		}
+	},
+}