all: add testing for compiler error messages

This is needed for some improvements I'm going to make next.

This commit also refactors error handling slightly to make it more
easily testable, this should hopefully not result in any actual changes
in behavior.

errors_test.go

@@ -0,0 +1,84 @@
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/tinygo-org/tinygo/compileopts"
+)
+
+// Test the error messages of the TinyGo compiler.
+func TestErrors(t *testing.T) {
+	for _, name := range []string{
+		"cgo",
+		"syntax",
+		"types",
+	} {
+		t.Run(name, func(t *testing.T) {
+			testErrorMessages(t, "./testdata/errors/"+name+".go")
+		})
+	}
+}
+
+func testErrorMessages(t *testing.T, filename string) {
+	// Parse expected error messages.
+	expected := readErrorMessages(t, filename)
+
+	// Try to build a binary (this should fail with an error).
+	tmpdir := t.TempDir()
+	err := Build(filename, tmpdir+"/out", &compileopts.Options{
+		Target:        "wasip1",
+		Semaphore:     sema,
+		InterpTimeout: 180 * time.Second,
+		Debug:         true,
+		VerifyIR:      true,
+		Opt:           "z",
+	})
+	if err == nil {
+		t.Fatal("expected to get a compiler error")
+	}
+
+	// Get the full ./testdata/errors directory.
+	wd, absErr := filepath.Abs("testdata/errors")
+	if absErr != nil {
+		t.Fatal(absErr)
+	}
+
+	// Write error message out as plain text.
+	var buf bytes.Buffer
+	printCompilerError(err, func(v ...interface{}) {
+		fmt.Fprintln(&buf, v...)
+	}, wd)
+	actual := strings.TrimRight(buf.String(), "\n")
+
+	// Check whether the error is as expected.
+	if actual != expected {
+		t.Errorf("expected error:\n%s\ngot:\n%s", indentText(expected, "> "), indentText(actual, "> "))
+	}
+}
+
+// Indent the given text with a given indentation string.
+func indentText(text, indent string) string {
+	return indent + strings.ReplaceAll(text, "\n", "\n"+indent)
+}
+
+// Read "// ERROR:" prefixed messages from the given file.
+func readErrorMessages(t *testing.T, file string) string {
+	data, err := os.ReadFile(file)
+	if err != nil {
+		t.Fatal("could not read input file:", err)
+	}
+
+	var errors []string
+	for _, line := range strings.Split(string(data), "\n") {
+		if strings.HasPrefix(line, "// ERROR: ") {
+			errors = append(errors, strings.TrimRight(line[len("// ERROR: "):], "\r\n"))
+		}
+	}
+	return strings.Join(errors, "\n")
+}