cmd/compile: check width of embedded interfaces in expandiface

The code in #20162 contains an embedded interface.

It didn't get dowidth'd by the frontend,
and during DWARF generation, ngotype asked
for a string description of it,
which triggered a request for the number of fields
in the interface, which triggered a dowidth,
which is disallowed in the backend.

The other changes in this CL are to support the test.

Fixes #20162

Change-Id: I4d0be5bd949c361d4cdc89a8ed28b10977e40cf9
Reviewed-on: https://go-review.googlesource.com/42131
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -31,6 +31,7 @@ func expandiface(t *types.Type) {
 	for _, m := range t.Methods().Slice() {
 		if m.Sym != nil {
 			fields = append(fields, m)
+			checkwidth(m.Type)
 			continue
 		}
 

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

@@ -1076,6 +1076,7 @@ func IsAlias(sym *types.Sym) bool {
 var concurrentFlagOK = [256]bool{
 	'B': true, // disabled bounds checking
 	'C': true, // disable printing of columns in error messages
+	'e': true, // no limit on errors; errors all come from non-concurrent code
 	'I': true, // add `directory` to import search path
 	'N': true, // disable optimizations
 	'l': true, // disable inlining

test/fixedbugs/issue20162.go

+// compile -c=4
+
+// Copyright 2017 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.
+
+// Issue 20162: embedded interfaces weren't dowidth-ed by the front end,
+// leading to races in the backend.
+
+package p
+
+func Foo() {
+	_ = (make([]func() interface {
+		M(interface{})
+	}, 1))
+}

test/run.go

@@ -193,8 +193,9 @@ func goFiles(dir string) []string {
 
 type runCmd func(...string) ([]byte, error)
 
-func compileFile(runcmd runCmd, longname string) (out []byte, err error) {
+func compileFile(runcmd runCmd, longname string, flags []string) (out []byte, err error) {
 	cmd := []string{"go", "tool", "compile", "-e"}
+	cmd = append(cmd, flags...)
 	if *linkshared {
 		cmd = append(cmd, "-dynlink", "-installsuffix=dynlink")
 	}
@@ -609,7 +610,7 @@ func (t *test) run() {
 		return
 
 	case "compile":
-		_, t.err = compileFile(runcmd, long)
+		_, t.err = compileFile(runcmd, long, flags)
 
 	case "compiledir":
 		// Compile all files in the directory in lexicographic order.