reflect: let StructOf define unexported fields

This was missing from the original StructOf CL because I couldn't
think of a use for it. Now I can: even with types used entirely
by reflect, unexported fields can be set using UnsafeAddr.

Change-Id: I5e7e3d81d16e8817cdd69d85796ce33930ef523b
Reviewed-on: https://go-review.googlesource.com/c/go/+/85661
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.14.html

@@ -115,6 +115,19 @@ TODO
 
 </dl><!-- plugin -->
 
+<dl id="reflect">
+
+<dt><a href="/pkg/reflect/">reflect</a></dt>
+  <dd>
+    <p><!-- CL 85661 -->
+      <a href="/pkg/reflect#StructOf"><code>StructOf</code></a> now
+      supports creating struct types with unexported fields, by
+      setting the <code>PkgPath</code> field in
+      a <code>StructField</code> element.
+    </p>
+
+</dl><!-- reflect -->
+
 <dl id="runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
   <dd>
     <p><!-- CL 187739 -->

src/reflect/all_test.go

@@ -4733,17 +4733,14 @@ func TestStructOfExportRules(t *testing.T) {
 			mustPanic: true,
 		},
 		{
-			field:     StructField{Name: "s2", Type: TypeOf(int(0)), PkgPath: "other/pkg"},
-			mustPanic: true,
+			field: StructField{Name: "s2", Type: TypeOf(int(0)), PkgPath: "other/pkg"},
 		},
 		{
-			field:     StructField{Name: "s2", Type: TypeOf(int(0)), PkgPath: "other/pkg"},
-			mustPanic: true,
+			field: StructField{Name: "s2", Type: TypeOf(int(0)), PkgPath: "other/pkg"},
 		},
 		{
-			field:     StructField{Name: "S", Type: TypeOf(S1{})},
-			mustPanic: false,
-			exported:  true,
+			field:    StructField{Name: "S", Type: TypeOf(S1{})},
+			exported: true,
 		},
 		{
 			field:    StructField{Name: "S", Type: TypeOf((*S1)(nil))},
@@ -4774,20 +4771,16 @@ func TestStructOfExportRules(t *testing.T) {
 			mustPanic: true,
 		},
 		{
-			field:     StructField{Name: "s", Type: TypeOf(S1{}), PkgPath: "other/pkg"},
-			mustPanic: true, // TODO(sbinet): creating a name with a package path
+			field: StructField{Name: "s", Type: TypeOf(S1{}), PkgPath: "other/pkg"},
 		},
 		{
-			field:     StructField{Name: "s", Type: TypeOf((*S1)(nil)), PkgPath: "other/pkg"},
-			mustPanic: true, // TODO(sbinet): creating a name with a package path
+			field: StructField{Name: "s", Type: TypeOf((*S1)(nil)), PkgPath: "other/pkg"},
 		},
 		{
-			field:     StructField{Name: "s", Type: TypeOf(s2{}), PkgPath: "other/pkg"},
-			mustPanic: true, // TODO(sbinet): creating a name with a package path
+			field: StructField{Name: "s", Type: TypeOf(s2{}), PkgPath: "other/pkg"},
 		},
 		{
-			field:     StructField{Name: "s", Type: TypeOf((*s2)(nil)), PkgPath: "other/pkg"},
-			mustPanic: true, // TODO(sbinet): creating a name with a package path
+			field: StructField{Name: "s", Type: TypeOf((*s2)(nil)), PkgPath: "other/pkg"},
 		},
 		{
 			field:     StructField{Name: "", Type: TypeOf(ΦType{})},

src/reflect/type.go

@@ -2731,15 +2731,18 @@ func StructOf(fields []StructField) Type {
 }
 
 func runtimeStructField(field StructField) structField {
-	if field.PkgPath != "" {
-		panic("reflect.StructOf: StructOf does not allow unexported fields")
+	if field.Anonymous && field.PkgPath != "" {
+		panic("reflect.StructOf: field \"" + field.Name + "\" is anonymous but has PkgPath set")
 	}
 
-	// Best-effort check for misuse.
-	// Since PkgPath is empty, not much harm done if Unicode lowercase slips through.
-	c := field.Name[0]
-	if 'a' <= c && c <= 'z' || c == '_' {
-		panic("reflect.StructOf: field \"" + field.Name + "\" is unexported but missing PkgPath")
+	exported := field.PkgPath == ""
+	if exported {
+		// Best-effort check for misuse.
+		// Since this field will be treated as exported, not much harm done if Unicode lowercase slips through.
+		c := field.Name[0]
+		if 'a' <= c && c <= 'z' || c == '_' {
+			panic("reflect.StructOf: field \"" + field.Name + "\" is unexported but missing PkgPath")
+		}
 	}
 
 	offsetEmbed := uintptr(0)
@@ -2749,7 +2752,7 @@ func runtimeStructField(field StructField) structField {
 
 	resolveReflectType(field.Type.common()) // install in runtime
 	return structField{
-		name:        newName(field.Name, string(field.Tag), true),
+		name:        newName(field.Name, string(field.Tag), exported),
 		typ:         field.Type.common(),
 		offsetEmbed: offsetEmbed,
 	}