cmd/vet: use type info to detect the atomic funcs

Simply checking if a name is "atomic" isn't enough, as that might be a
var or another imported package. Now that vet requires type information,
we can do better. And add a simple regression test.

Change-Id: Ibd2004428374e3628cd3cd0ffb5f37cedaf448ea
Reviewed-on: https://go-review.googlesource.com/91795
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/vet/atomic.go

@@ -7,6 +7,7 @@ package main
 import (
 	"go/ast"
 	"go/token"
+	"go/types"
 )
 
 func init() {
@@ -36,8 +37,9 @@ func checkAtomicAssignment(f *File, node ast.Node) {
 		if !ok {
 			continue
 		}
-		pkg, ok := sel.X.(*ast.Ident)
+		pkgIdent, _ := sel.X.(*ast.Ident)
-		if !ok || pkg.Name != "atomic" {
+		pkgName, ok := f.pkg.uses[pkgIdent].(*types.PkgName)
+		if !ok || pkgName.Imported().Path() != "sync/atomic" {
 			continue
 		}
 

src/cmd/vet/testdata/atomic.go

@@ -50,3 +50,13 @@ func AtomicTests() {
 		_ = w
 	}
 }
+
+type T struct{}
+
+func (T) AddUint64(addr *uint64, delta uint64) uint64 { return 0 }
+
+func NonAtomic() {
+	x := uint64(1)
+	var atomic T
+	x = atomic.AddUint64(&x, 1) // ok; not the imported pkg
+}