first version of plugins working

Add a caveat about -trimpath too.

Fixes #18.

README.md

@@ -66,3 +66,6 @@ to document the current shortcomings of this tool.
 
 * Functions implemented outside Go, such as assembly, aren't garbled since we
   currently only transform the input Go source.
+
+* Since `garble` forces `-trimpath`, plugins built with `-garble` must be loaded
+  from Go programs built with `-trimpath` too.

main.go

@@ -477,13 +477,22 @@ func transformGo(file *ast.File, info *types.Info) *ast.File {
 		if pkg == nil {
 			return true // universe scope
 		}
+		if vr, ok := obj.(*types.Var); ok && vr.Embedded() {
+			// ObjectOf returns the field for embedded struct
+			// fields, not the type it uses. Use the type.
+			obj = objOf(obj.Type())
+			pkg = obj.Pkg()
+		}
+
+		if pkg.Name() == "main" && obj.Exported() && obj.Parent() == pkg.Scope() {
+			// TODO: only do this when -buildmode is plugin? what
+			// about other -buildmode options?
+			return true // could be a Go plugin API
+		}
 		// log.Printf("%#v %T", node, obj)
 		switch x := obj.(type) {
 		case *types.Var:
-			if x.Embedded() {
-				obj = objOf(obj.Type())
-				pkg = obj.Pkg()
-			} else if x.IsField() && x.Exported() {
+			if x.IsField() && x.Exported() {
 				// might be used for reflection, e.g.
 				// encoding/json without struct tags
 				return true

testdata/scripts/plugin.txt

@@ -0,0 +1,54 @@
+garble build -buildmode=plugin ./plugin
+binsubstr plugin.so 'PublicVar' 'PublicFunc'
+! binsubstr plugin.so 'privateFunc'
+
+# Note that we need -trimpath; see the caveat section in the README.
+go build -trimpath
+exec ./main
+cmp stdout main.stdout
+binsubstr main$exe 'PublicVar' 'PublicFunc'
+! binsubstr plugin.so 'privateFunc'
+
+[short] stop # no need to verify this with -short
+
+go build -buildmode=plugin ./plugin
+binsubstr plugin.so 'PublicVar' 'PublicFunc' 'privateFunc'
+go build
+exec ./main
+cmp stdout main.stdout
+
+-- go.mod --
+module test/main
+-- plugin/main.go --
+package main
+
+import "fmt"
+
+var PublicVar int
+
+func privateFunc(n int) { fmt.Printf("Hello, number %d\n", n) }
+
+func PublicFunc() { privateFunc(PublicVar) }
+-- main.go --
+package main
+
+import "plugin"
+
+func main() {
+	p, err := plugin.Open("plugin.so")
+	if err != nil {
+		panic(err)
+	}
+	v, err := p.Lookup("PublicVar")
+	if err != nil {
+		panic(err)
+	}
+	f, err := p.Lookup("PublicFunc")
+	if err != nil {
+		panic(err)
+	}
+	*v.(*int) = 7
+	f.(func())()
+}
+-- main.stdout --
+Hello, number 7

testdata/scripts/syntax.txt

@@ -30,6 +30,12 @@ type Embedding struct {
 	Embedded
 }
 
+type embedded int
+
+type embedding struct {
+	embedded
+}
+
 func main() {
 	switch V := V.(type) {
 	case int: