cmd/cgo: use type aliases for #define type macros

Cgo's initial design for handling "#define foo int*" involved
rewriting "C.foo" to "*_Ctype_int" everywhere. But now that we have
type aliases, we can declare "type _Ctype_foo = *_Ctype_int" once, and
then rewrite "C.foo" to just "_Ctype_foo".

This is important for go/types's UsesCgo mode, where go/types needs to
be able to figure out a type for each C.foo identifier using only the
information written into _cgo_gotypes.go.

Fixes golang#38649.

Change-Id: Ia0f8c2d82df81efb1be5bc26195ea9154c0af871
Reviewed-on: https://go-review.googlesource.com/c/go/+/230037
Run-TryBot: Matthew Dempsky <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

misc/cgo/test/test.go

@@ -897,6 +897,10 @@ static uint16_t issue31093F(uint16_t v) { return v; }
 
 // issue 32579
 typedef struct S32579 { unsigned char data[1]; } S32579;
+
+// issue 38649
+// Test that #define'd type aliases work.
+#define netbsd_gid unsigned int
 */
 import "C"
 
@@ -2192,3 +2196,7 @@ func test32579(t *testing.T) {
 		t.Errorf("&s[0].data[0] failed: got %d, want %d", s[0].data[0], 1)
 	}
 }
+
+// issue 38649
+
+var issue38649 C.netbsd_gid = 42

misc/cgo/testgodefs/testdata/issue38649.go

@@ -0,0 +1,15 @@
+// Copyright 2020 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
+
+/*
+struct Issue38649 { int x; };
+#define issue38649 struct Issue38649
+*/
+import "C"
+
+type issue38649 C.issue38649

misc/cgo/testgodefs/testdata/main.go

@@ -19,5 +19,8 @@ var v6 = B{}
 // Test that S is fully defined
 var v7 = S{}
 
+// Test that #define'd type is fully defined
+var _ = issue38649{X: 0}
+
 func main() {
 }

misc/cgo/testgodefs/testgodefs_test.go

@@ -23,6 +23,7 @@ var filePrefixes = []string{
 	"fieldtypedef",
 	"issue37479",
 	"issue37621",
+	"issue38649",
 }
 
 func TestGoDefs(t *testing.T) {

src/cmd/cgo/gcc.go

@@ -182,6 +182,9 @@ func (p *Package) Translate(f *File) {
 		numTypedefs = len(p.typedefs)
 		// Also ask about any typedefs we've seen so far.
 		for _, info := range p.typedefList {
+			if f.Name[info.typedef] != nil {
+				continue
+			}
 			n := &Name{
 				Go: info.typedef,
 				C:  info.typedef,
@@ -710,6 +713,9 @@ func (p *Package) prepareNames(f *File) {
 			}
 		}
 		p.mangleName(n)
+		if n.Kind == "type" && typedef[n.Mangle] == nil {
+			typedef[n.Mangle] = n.Type
+		}
 	}
 }
 
@@ -1348,6 +1354,9 @@ func (p *Package) rewriteRef(f *File) {
 
 		if *godefs {
 			// Substitute definition for mangled type name.
+			if r.Name.Type != nil {
+				expr = r.Name.Type.Go
+			}
 			if id, ok := expr.(*ast.Ident); ok {
 				if t := typedef[id.Name]; t != nil {
 					expr = t.Go
@@ -1413,9 +1422,7 @@ func (p *Package) rewriteName(f *File, r *Ref) ast.Expr {
 				r.Context = ctxType
 				if r.Name.Type == nil {
 					error_(r.Pos(), "invalid conversion to C.%s: undefined C type '%s'", fixGo(r.Name.Go), r.Name.C)
-					break
 				}
-				expr = r.Name.Type.Go
 				break
 			}
 			error_(r.Pos(), "call of non-function C.%s", fixGo(r.Name.Go))
@@ -1472,9 +1479,7 @@ func (p *Package) rewriteName(f *File, r *Ref) ast.Expr {
 			// Okay - might be new(T)
 			if r.Name.Type == nil {
 				error_(r.Pos(), "expression C.%s: undefined C type '%s'", fixGo(r.Name.Go), r.Name.C)
-				break
 			}
-			expr = r.Name.Type.Go
 		case "var":
 			expr = &ast.StarExpr{Star: (*r.Expr).Pos(), X: expr}
 		case "macro":
@@ -1493,8 +1498,6 @@ func (p *Package) rewriteName(f *File, r *Ref) ast.Expr {
 			// Use of C.enum_x, C.struct_x or C.union_x without C definition.
 			// GCC won't raise an error when using pointers to such unknown types.
 			error_(r.Pos(), "type C.%s: undefined C type '%s'", fixGo(r.Name.Go), r.Name.C)
-		} else {
-			expr = r.Name.Type.Go
 		}
 	default:
 		if r.Name.Kind == "func" {