main: use GOPATH from the environment

Be more compatible with the Go toolchain by setting GOPATH in the same
way. This makes it possible to flash and run examples from the standard
GOPATH instead of only from the source tree.

compiler/compiler.go

@@ -34,6 +34,7 @@ type Config struct {
 	DumpSSA   bool     // dump Go SSA, for compiler debugging
 	Debug     bool     // add debug symbols for gdb
 	RootDir   string   // GOROOT for TinyGo
+	GOPATH    string   // GOPATH, like `go env GOPATH`
 	BuildTags []string // build tags for TinyGo (empty means {runtime.GOOS/runtime.GOARCH})
 }
 
@@ -151,6 +152,15 @@ func (c *Compiler) Module() llvm.Module {
 func (c *Compiler) Compile(mainPath string) error {
 	tripleSplit := strings.Split(c.Triple, "-")
 
+	// Prefix the GOPATH with the system GOROOT, as GOROOT is already set to
+	// the TinyGo root.
+	gopath := c.GOPATH
+	if gopath == "" {
+		gopath = runtime.GOROOT()
+	} else {
+		gopath = runtime.GOROOT() + string(filepath.ListSeparator) + gopath
+	}
+
 	config := loader.Config{
 		TypeChecker: types.Config{
 			Sizes: &StdSizes{
@@ -163,7 +173,7 @@ func (c *Compiler) Compile(mainPath string) error {
 			GOARCH:      tripleSplit[0],
 			GOOS:        tripleSplit[2],
 			GOROOT:      c.RootDir,
-			GOPATH:      runtime.GOROOT(),
+			GOPATH:      gopath,
 			CgoEnabled:  true,
 			UseAllFiles: false,
 			Compiler:    "gc", // must be one of the recognized compilers

main.go

@@ -22,6 +22,7 @@ func Compile(pkgName, outpath string, spec *TargetSpec, printIR, dumpSSA, debug
 		Debug:     debug,
 		DumpSSA:   dumpSSA,
 		RootDir:   sourceDir(),
+		GOPATH:    getGopath(),
 		BuildTags: spec.BuildTags,
 	}
 	c, err := compiler.NewCompiler(pkgName, config)
@@ -198,6 +199,7 @@ func Flash(pkgName, target, port string, printIR, dumpSSA, debug bool, printSize
 func Run(pkgName string) error {
 	config := compiler.Config{
 		RootDir: sourceDir(),
+		GOPATH:  getGopath(),
 	}
 	c, err := compiler.NewCompiler(pkgName, config)
 	if err != nil {

target.go

@@ -58,3 +58,24 @@ func sourceDir() string {
 	_, path, _, _ := runtime.Caller(0)
 	return filepath.Dir(path)
 }
+
+func getGopath() string {
+	gopath := os.Getenv("GOPATH")
+	if gopath != "" {
+		return gopath
+	}
+
+	// fallback
+	var home string
+	if runtime.GOOS == "windows" {
+		home = os.Getenv("USERPROFILE")
+	} else {
+		home = os.Getenv("HOME")
+	}
+	if home == "" {
+		// This is very unlikely, so panic here.
+		// Not the nicest solution, however.
+		panic("no $HOME or %USERPROFILE% found")
+	}
+	return filepath.Join(home, "go")
+}