cmd/vet: include function name or value in copylock message

Given
	var t struct{ lock sync.Mutex }
	var fntab []func(t)
	f(a(), b(&t), c(), fntab[0](t))

Before:
	function call copies lock value: struct{lock sync.Mutex} contains sync.Mutex

After:
	call of fntab[0] copies lock value: struct{lock sync.Mutex} contains sync.Mutex

This will make diagnosis easier when there are multiple function calls per line.

Change-Id: I9881713c5671b847b84a0df0115f57e7cba17d72
Reviewed-on: https://go-review.googlesource.com/34730Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/vet/copylock.go

@@ -101,7 +101,7 @@ func checkCopyLocksCallExpr(f *File, ce *ast.CallExpr) {
 	}
 	for _, x := range ce.Args {
 		if path := lockPathRhs(f, x); path != nil {
-			f.Badf(x.Pos(), "function call copies lock value: %v", path)
+			f.Badf(x.Pos(), "call of %s copies lock value: %v", f.gofmt(ce.Fun), path)
 		}
 	}
 }

src/cmd/vet/testdata/copylock.go

@@ -67,7 +67,7 @@ func BadFunc() {
 
 	// override 'new' keyword
 	new := func(interface{}) {}
-	new(t) // ERROR "function call copies lock value: testdata.Tlock contains sync.Once contains sync.Mutex"
+	new(t) // ERROR "call of new copies lock value: testdata.Tlock contains sync.Once contains sync.Mutex"
 
 	// copy of array of locks
 	var muA [5]sync.Mutex
@@ -96,10 +96,10 @@ func LenAndCapOnLockArrays() {
 	// override 'len' and 'cap' keywords
 
 	len := func(interface{}) {}
-	len(a) // ERROR "function call copies lock value: sync.Mutex"
+	len(a) // ERROR "call of len copies lock value: sync.Mutex"
 
 	cap := func(interface{}) {}
-	cap(a) // ERROR "function call copies lock value: sync.Mutex"
+	cap(a) // ERROR "call of cap copies lock value: sync.Mutex"
 }
 
 // SyncTypesCheck checks copying of sync.* types except sync.Mutex

src/cmd/vet/testdata/copylock_func.go

@@ -86,8 +86,10 @@ func FuncCallInterfaceArg(f func(a int, b interface{})) {
 	f(1, "foo")
 	f(2, &t)
 	f(3, &sync.Mutex{})
-	f(4, m) // ERROR "function call copies lock value: sync.Mutex"
-	f(5, t) // ERROR "function call copies lock value: struct{lock sync.Mutex} contains sync.Mutex"
+	f(4, m) // ERROR "call of f copies lock value: sync.Mutex"
+	f(5, t) // ERROR "call of f copies lock value: struct{lock sync.Mutex} contains sync.Mutex"
+	var fntab []func(t)
+	fntab[0](t) // ERROR "call of fntab.0. copies lock value: struct{lock sync.Mutex} contains sync.Mutex"
 }
 
 // Returning lock via interface value