cmd/cgo: for -godefs, promote first field of anonymous union

Update #6677

When a struct contains an anonymous union, use the type and
name of the first field in the union.

This should make the glibc <sys/resource.h> file work; in that
file struct rusage has fields like

__extension__ union
{
        long int ru_maxrss;
        __syscall_slong_t __ru_maxrss_word;
};

in which the field that matters is ru_maxrss and
__ru_maxrss_word just exists to advance to the next field on
systems where the kernel uses long long fields but userspace
expects long fields.

LGTM=mikioh.mikioh
R=golang-codereviews, mikioh.mikioh
CC=golang-codereviews
https://golang.org/cl/106260044

misc/cgo/testgodefs/anonunion.go

+// Copyright 2014 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.
+//
+// +build ignore
+
+package main
+
+// This file tests that when cgo -godefs sees a struct with a field
+// that is an anonymous union, the first field in the union is
+// promoted to become a field of the struct.  See issue 6677 for
+// background.
+
+/*
+typedef struct {
+	union {
+		long l;
+		int c;
+	};
+} t;
+*/
+import "C"
+
+// Input for cgo -godefs.
+
+type T C.t

misc/cgo/testgodefs/main.go

+// Copyright 2014 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 main
+
+// Test that the struct field in anonunion.go was promoted.
+var v1 T
+var v2 = v1.L
+
+func main() {
+}

misc/cgo/testgodefs/test.bash

+# Copyright 2014 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.
+
+# We are testing cgo -godefs, which translates Go files that use
+# import "C" into Go files with Go definitions of types defined in the
+# import "C" block.  Add more tests here.
+FILE_PREFIXES="anonunion"
+
+RM=
+for FP in $FILE_PREFIXES
+do
+  go tool cgo -godefs ${FP}.go > ${FP}_defs.go
+  RM="${RM} ${FP}_defs.go"
+done
+
+go build . && ./testgodefs
+EXIT=$?
+rm -rf _obj testgodefs ${RM}
+exit $EXIT

src/cmd/cgo/gcc.go

@@ -1548,7 +1548,27 @@ func (c *typeConv) Struct(dt *dwarf.StructType, pos token.Pos) (expr *ast.Struct
 			fld = c.pad(fld, f.ByteOffset-off)
 			off = f.ByteOffset
 		}
-		t := c.Type(f.Type, pos)
+
+		name := f.Name
+		ft := f.Type
+
+		// In godefs or cdefs mode, if this field is a C11
+		// anonymous union then treat the first field in the
+		// union as the field in the struct.  This handles
+		// cases like the glibc <sys/resource.h> file; see
+		// issue 6677.
+		if *godefs || *cdefs {
+			if st, ok := f.Type.(*dwarf.StructType); ok && name == "" && st.Kind == "union" && len(st.Field) > 0 && !used[st.Field[0].Name] {
+				name = st.Field[0].Name
+				ident[name] = name
+				ft = st.Field[0].Type
+			}
+		}
+
+		// TODO: Handle fields that are anonymous structs by
+		// promoting the fields of the inner struct.
+
+		t := c.Type(ft, pos)
 		tgo := t.Go
 		size := t.Size
 		talign := t.Align
@@ -1577,7 +1597,6 @@ func (c *typeConv) Struct(dt *dwarf.StructType, pos token.Pos) (expr *ast.Struct
 		}
 		n := len(fld)
 		fld = fld[0 : n+1]
-		name := f.Name
 		if name == "" {
 			name = fmt.Sprintf("anon%d", anon)
 			anon++