cgo: avoid '"unsafe" imported but not used' error

This can happen when not all CGo features are used.

cgo/cgo.go

@@ -185,6 +185,7 @@ func Process(files []*ast.File, dir string, fset *token.FileSet, cflags []string
 			Name:    files[0].Name.Name,
 		},
 		Decls: []ast.Decl{
+			// import "unsafe"
 			&ast.GenDecl{
 				TokPos: p.generatedPos,
 				Tok:    token.IMPORT,
@@ -192,6 +193,32 @@ func Process(files []*ast.File, dir string, fset *token.FileSet, cflags []string
 					unsafeImport,
 				},
 			},
+			// var _ unsafe.Pointer
+			// This avoids type errors when the unsafe package is never used.
+			&ast.GenDecl{
+				Tok: token.VAR,
+				Specs: []ast.Spec{
+					&ast.ValueSpec{
+						Names: []*ast.Ident{
+							&ast.Ident{
+								Name: "_",
+								Obj: &ast.Object{
+									Kind: ast.Var,
+									Name: "_",
+								},
+							},
+						},
+						Type: &ast.SelectorExpr{
+							X: &ast.Ident{
+								Name: "unsafe",
+							},
+							Sel: &ast.Ident{
+								Name: "Pointer",
+							},
+						},
+					},
+				},
+			},
 		},
 		Imports: []*ast.ImportSpec{unsafeImport},
 	}

cgo/testdata/basic.out.go

@@ -2,6 +2,8 @@ package main
 
 import "unsafe"
 
+var _ unsafe.Pointer
+
 type C.int16_t = int16
 type C.int32_t = int32
 type C.int64_t = int64

cgo/testdata/types.out.go

@@ -2,6 +2,8 @@ package main
 
 import "unsafe"
 
+var _ unsafe.Pointer
+
 const C.option2A = 20
 const C.optionA = 0
 const C.optionB = 1