cmd/compile: improve error when setting unexported fields

Improve the error user experience when users try to set/refer
to unexported fields and methods of struct literals, by directly saying

    "cannot refer to unexported field or method"

Fixes #31053

Change-Id: I6fd3caf64b7ca9f9d8ea60b7756875e340792d59
Reviewed-on: https://go-review.googlesource.com/c/go/+/201657
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/typecheck.go

@@ -2967,6 +2967,8 @@ func typecheckcomplit(n *Node) (res *Node) {
 					if ci := lookdot1(nil, l.Sym, t, t.Fields(), 2); ci != nil { // Case-insensitive lookup.
 						if visible(ci.Sym) {
 							yyerror("unknown field '%v' in struct literal of type %v (but does have %v)", l.Sym, t, ci.Sym)
+						} else if nonexported(l.Sym) && l.Sym.Name == ci.Sym.Name { // Ensure exactness before the suggestion.
+							yyerror("cannot refer to unexported field '%v' in struct literal of type %v", l.Sym, t)
 						} else {
 							yyerror("unknown field '%v' in struct literal of type %v", l.Sym, t)
 						}
@@ -3070,6 +3072,11 @@ func visible(sym *types.Sym) bool {
 	return sym != nil && (types.IsExported(sym.Name) || sym.Pkg == localpkg)
 }
 
+// nonexported reports whether sym is an unexported field.
+func nonexported(sym *types.Sym) bool {
+	return sym != nil && !types.IsExported(sym.Name)
+}
+
 // lvalue etc
 func islvalue(n *Node) bool {
 	switch n.Op {

test/fixedbugs/issue25727.go

@@ -9,13 +9,13 @@ package main
 import "net/http"
 
 var s = http.Server{}
-var _ = s.doneChan // ERROR "s.doneChan undefined .cannot refer to unexported field or method doneChan.$"
+var _ = s.doneChan                  // ERROR "s.doneChan undefined .cannot refer to unexported field or method doneChan.$"
-var _ = s.DoneChan // ERROR "s.DoneChan undefined .type http.Server has no field or method DoneChan.$"
+var _ = s.DoneChan                  // ERROR "s.DoneChan undefined .type http.Server has no field or method DoneChan.$"
 var _ = http.Server{tlsConfig: nil} // ERROR "unknown field 'tlsConfig' in struct literal.+ .but does have TLSConfig.$"
-var _ = http.Server{DoneChan: nil} // ERROR "unknown field 'DoneChan' in struct literal of type http.Server$"
+var _ = http.Server{DoneChan: nil}  // ERROR "unknown field 'DoneChan' in struct literal of type http.Server$"
 
 type foo struct {
-    bar int
+	bar int
 }
 
 var _ = &foo{bAr: 10} // ERROR "unknown field 'bAr' in struct literal.+ .but does have bar.$"

test/fixedbugs/issue31053.dir/f1.go

@@ -0,0 +1,18 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package f1
+
+type Foo struct {
+	doneChan chan bool
+	Name     string
+	fOO      int
+	hook     func()
+}
+
+func (f *Foo) Exported() {
+}
+
+func (f *Foo) unexported() {
+}

test/fixedbugs/issue31053.dir/main.go

@@ -0,0 +1,42 @@
+// errorcheck
+
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "./f1"
+
+func main() {
+	f := f1.Foo{
+		doneChan:      nil, // ERROR "cannot refer to unexported field 'doneChan' in struct literal of type f1.Foo"
+		DoneChan:      nil, // ERROR "unknown field 'DoneChan' in struct literal of type f1.Foo"
+		Name:          "hey",
+		name:          "there",   // ERROR "unknown field 'name' in struct literal of type f1.Foo .but does have Name."
+		noSuchPrivate: true,      // ERROR "unknown field 'noSuchPrivate' in struct literal of type f1.Foo"
+		NoSuchPublic:  true,      // ERROR "unknown field 'NoSuchPublic' in struct literal of type f1.Foo"
+		foo:           true,      // ERROR "unknown field 'foo' in struct literal of type f1.Foo"
+		hook:          func() {}, // ERROR "cannot refer to unexported field 'hook' in struct literal of type f1.Foo"
+		unexported:    func() {}, // ERROR "unknown field 'unexported' in struct literal of type f1.Foo"
+		Exported:      func() {}, // ERROR "unknown field 'Exported' in struct literal of type f1.Foo"
+	}
+	f.doneChan = nil // ERROR "f.doneChan undefined .cannot refer to unexported field or method doneChan."
+	f.DoneChan = nil // ERROR "f.DoneChan undefined .type f1.Foo has no field or method DoneChan."
+	f.name = nil     // ERROR "f.name undefined .type f1.Foo has no field or method name, but does have Name."
+
+	_ = f.doneChan // ERROR "f.doneChan undefined .cannot refer to unexported field or method doneChan."
+	_ = f.DoneChan // ERROR "f.DoneChan undefined .type f1.Foo has no field or method DoneChan."
+	_ = f.Name
+	_ = f.name          // ERROR "f.name undefined .type f1.Foo has no field or method name, but does have Name."
+	_ = f.noSuchPrivate // ERROR "f.noSuchPrivate undefined .type f1.Foo has no field or method noSuchPrivate."
+	_ = f.NoSuchPublic  // ERROR "f.NoSuchPublic undefined .type f1.Foo has no field or method NoSuchPublic."
+	_ = f.foo           // ERROR "f.foo undefined .type f1.Foo has no field or method foo."
+	_ = f.Exported
+	_ = f.exported    // ERROR "f.exported undefined .type f1.Foo has no field or method exported, but does have Exported."
+	_ = f.Unexported  // ERROR "f.Unexported undefined .type f1.Foo has no field or method Unexported."
+	_ = f.unexported  // ERROR "f.unexported undefined .cannot refer to unexported field or method f1..\*Foo..unexported."
+	f.unexported = 10 // ERROR "f.unexported undefined .cannot refer to unexported field or method f1..\*Foo..unexported."
+	f.unexported()    // ERROR "f.unexported undefined .cannot refer to unexported field or method f1..\*Foo..unexported."
+	_ = f.hook        // ERROR "f.hook undefined .cannot refer to unexported field or method hook."
+}

test/fixedbugs/issue31053.go

@@ -0,0 +1,7 @@
+// errorcheckdir
+
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ignored