go/packages: make LoadFiles the zero mode

Also:
- make a nil *Config equivalent to new(Config).
- add a test for nil *Config.
- document that Load may return an empty list of packages.

Change-Id: I642133abe18553ca8c7f46b7bd2709a03eda0b28
Reviewed-on: https://go-review.googlesource.com/128875
Reviewed-by: Michael Matloob <matloob@golang.org>

go/packages/packages.go

@@ -30,11 +30,9 @@ import (
 type LoadMode int
 
 const (
-	_ LoadMode = iota
-
 	// LoadFiles finds the packages and computes their source file lists.
 	// Package fields: ID, Name, Errors, GoFiles, OtherFiles.
-	LoadFiles
+	LoadFiles LoadMode = iota
 
 	// LoadImports adds import information for each package
 	// and its dependencies.
@@ -144,6 +142,14 @@ type Config struct {
 }
 
 // Load and returns the Go packages named by the given patterns.
+//
+// Config specifies loading options;
+// nil behaves the same as an empty Config.
+//
+// Load returns an error if any of the patterns was invalid
+// as defined by the underlying build system.
+// It may return an empty list of packages without an error,
+// for instance for an empty expansion of a valid wildcard.
 func Load(cfg *Config, patterns ...string) ([]*Package, error) {
 	l := newLoader(cfg)
 	rawCfg := newRawConfig(&l.Config)
@@ -247,8 +253,6 @@ func newLoader(cfg *Config) *loader {
 	ld := &loader{}
 	if cfg != nil {
 		ld.Config = *cfg
-	} else {
-		ld.Config.Mode = LoadAllSyntax
 	}
 	if ld.Context == nil {
 		ld.Context = context.Background()

go/packages/packages_test.go

@@ -45,6 +45,25 @@ var usesOldGolist = false
 //   - import cycles are gracefully handled in type checker.
 //   - test typechecking of generated test main and cgo.
 
+// The zero-value of Config has LoadFiles mode.
+func TestLoadZeroConfig(t *testing.T) {
+	initial, err := packages.Load(nil, "hash")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(initial) != 1 {
+		t.Fatalf("got %s, want [hash]", initial)
+	}
+	hash := initial[0]
+	// Even though the hash package has imports,
+	// they are not reported.
+	got := fmt.Sprintf("name=%s srcs=%v imports=%v", hash.Name, srcs(hash), hash.Imports)
+	want := "name=hash srcs=[hash.go] imports=map[]"
+	if got != want {
+		t.Fatalf("got %s, want %s", got, want)
+	}
+}
+
 func TestLoadImportsGraph(t *testing.T) {
 	tmp, cleanup := makeTree(t, map[string]string{
 		"src/a/a.go":             `package a; const A = 1`,