[release-branch.go1.12] cmd/cgo: fix inappropriate array copy

Ensure that during rewriting of expressions that take the address of
an array, that we properly recognize *ast.IndexExpr as an operation
to create a pointer variable and thus assign the proper addressOf
and deference operators as "&" and "*" respectively.

This fixes a regression from CL 142884.

This is a backport of CLs 183458 and 183778 to the 1.12 release branch.
It is not a cherry pick because the code in misc/cgo/test has changed.

Updates #32579
Fixes #32756

Change-Id: I0daa75ec62cccbe82ab658cb2947f51423e0c235
Reviewed-on: https://go-review.googlesource.com/c/go/+/183627
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

misc/cgo/test/cgo_test.go

@@ -95,6 +95,7 @@ func Test26213(t *testing.T)                 { test26213(t) }
 func Test27660(t *testing.T)                 { test27660(t) }
 func Test28896(t *testing.T)                 { test28896(t) }
 func Test30065(t *testing.T)                 { test30065(t) }
+func Test32579(t *testing.T)                 { test32579(t) }
 
 func BenchmarkCgoCall(b *testing.B)  { benchCgoCall(b) }
 func BenchmarkGoString(b *testing.B) { benchGoString(b) }

misc/cgo/test/issue32579.go

@@ -0,0 +1,22 @@
+// 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 cgotest
+
+// #include <string.h>
+// typedef struct S32579 { unsigned char data[1]; } S32579;
+import "C"
+
+import (
+	"testing"
+	"unsafe"
+)
+
+func test32579(t *testing.T) {
+	var s [1]C.struct_S32579
+	C.memset(unsafe.Pointer(&s[0].data[0]), 1, 1)
+	if s[0].data[0] != 1 {
+		t.Errorf("&s[0].data[0] failed: got %d, want %d", s[0].data[0], 1)
+	}
+}

src/cmd/cgo/gcc.go

@@ -1256,6 +1256,8 @@ func (p *Package) isVariable(x ast.Expr) bool {
 		return true
 	case *ast.SelectorExpr:
 		return p.isVariable(x.X)
+	case *ast.IndexExpr:
+		return true
 	}
 	return false
 }