cmd/vet: check for copying of array of locks

Updates #14664

Change-Id: I1f7b1116cfe91466816c760f136ce566da3e80a9
Reviewed-on: https://go-review.googlesource.com/24340
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/copylock.go

@@ -210,6 +210,14 @@ func lockPath(tpkg *types.Package, typ types.Type) typePath {
 		return nil
 	}
 
+	for {
+		atyp, ok := typ.Underlying().(*types.Array)
+		if !ok {
+			break
+		}
+		typ = atyp.Elem()
+	}
+
 	// We're only interested in the case in which the underlying
 	// type is a struct. (Interfaces and pointers are safe to copy.)
 	styp, ok := typ.Underlying().(*types.Struct)

src/cmd/vet/testdata/copylock.go

@@ -68,6 +68,24 @@ func BadFunc() {
 	// override 'new' keyword
 	new := func(interface{}) {}
 	new(t) // ERROR "function call copies lock value: testdata.Tlock contains sync.Once contains sync.Mutex"
+
+	// copy of array of locks
+	var muA [5]sync.Mutex
+	muB := muA        // ERROR "assignment copies lock value to muB: sync.Mutex"
+	muA = muB         // ERROR "assignment copies lock value to muA: sync.Mutex"
+	muSlice := muA[:] // OK
+
+	// multidimensional array
+	var mmuA [5][5]sync.Mutex
+	mmuB := mmuA        // ERROR "assignment copies lock value to mmuB: sync.Mutex"
+	mmuA = mmuB         // ERROR "assignment copies lock value to mmuA: sync.Mutex"
+	mmuSlice := mmuA[:] // OK
+
+	// slice copy is ok
+	var fmuA [5][][5]sync.Mutex
+	fmuB := fmuA        // OK
+	fmuA = fmuB         // OK
+	fmuSlice := fmuA[:] // OK
 }
 
 // SyncTypesCheck checks copying of sync.* types except sync.Mutex