main: improve error reporting while running go test

This commit switches integration tests to use the same error reporting
mechanism as the tinygo compiler normally uses. It replaces errors like
this:

    main_test.go:139: failed to build: interp: branch on a non-constant

With this:

    main.go:693: # math/rand
    main.go:695: interp: branch on a non-constant

In this particular case the error isn't much better (it gives the
relevant package, though) but other errors should also include the
source location where they happen.

main.go

@@ -675,38 +675,49 @@ func usage() {
 	flag.PrintDefaults()
 }
 
-func handleCompilerError(err error) {
-	if err != nil {
+// printCompilerError prints compiler errors using the provided logger function
+// (similar to fmt.Println).
+//
+// There is one exception: interp errors may print to stderr unconditionally due
+// to limitations in the LLVM bindings.
+func printCompilerError(logln func(...interface{}), err error) {
 	switch err := err.(type) {
 	case *interp.Unsupported:
 		// hit an unknown/unsupported instruction
-			fmt.Fprintln(os.Stderr, "#", err.ImportPath)
+		logln("#", err.ImportPath)
 		msg := "unsupported instruction during init evaluation:"
 		if err.Pos.String() != "" {
 			msg = err.Pos.String() + " " + msg
 		}
-			fmt.Fprintln(os.Stderr, msg)
+		logln(msg)
 		err.Inst.Dump()
-			fmt.Fprintln(os.Stderr)
+		logln()
 	case types.Error, scanner.Error:
-			fmt.Fprintln(os.Stderr, err)
+		logln(err)
 	case interp.Error:
-			fmt.Fprintln(os.Stderr, "#", err.ImportPath)
+		logln("#", err.ImportPath)
 		for _, err := range err.Errs {
-				fmt.Fprintln(os.Stderr, err)
+			logln(err)
 		}
 	case loader.Errors:
-			fmt.Fprintln(os.Stderr, "#", err.Pkg.ImportPath)
+		logln("#", err.Pkg.ImportPath)
 		for _, err := range err.Errs {
-				fmt.Fprintln(os.Stderr, err)
+			logln(err)
 		}
 	case *builder.MultiError:
 		for _, err := range err.Errs {
-				fmt.Fprintln(os.Stderr, err)
+			logln(err)
 		}
 	default:
-			fmt.Fprintln(os.Stderr, "error:", err)
+		logln("error:", err)
 	}
+}
+
+func handleCompilerError(err error) {
+	if err != nil {
+		printCompilerError(func(args ...interface{}) {
+			fmt.Fprintln(os.Stderr, args...)
+		}, err)
 		os.Exit(1)
 	}
 }

main_test.go

@@ -17,7 +17,6 @@ import (
 	"time"
 
 	"github.com/tinygo-org/tinygo/compileopts"
-	"github.com/tinygo-org/tinygo/loader"
 )
 
 const TESTDATA = "testdata"
@@ -131,13 +130,7 @@ func runTest(path, target string, t *testing.T) {
 	binary := filepath.Join(tmpdir, "test")
 	err = runBuild("./"+path, binary, config)
 	if err != nil {
-		if errLoader, ok := err.(loader.Errors); ok {
-			for _, err := range errLoader.Errs {
-				t.Log("failed to build:", err)
-			}
-		} else {
-			t.Log("failed to build:", err)
-		}
+		printCompilerError(t.Log, err)
 		t.Fail()
 		return
 	}