Recreates Rust's Compiler Error reporting style in Golang.

errdiagnostic.go

// colorful strings for printing.
const (
	COLOR_RED     = "\x1B[31m"    // used for errors.
	COLOR_GREEN   = "\x1B[32m"
	COLOR_YELLOW  = "\x1B[33m"
	COLOR_BLUE    = "\x1B[34m"
	COLOR_MAGENTA = "\x1B[35m"    // used for warnings.
	COLOR_CYAN    = "\x1B[36m"
	COLOR_WHITE   = "\x1B[37m"
	COLOR_RESET   = "\033[0m"     // used to reset the color.
)


/* Example:
 * {error | warning} [{ErrCode}]: {Msg}
 * --> {file, line, col}
 * Line1 | Code1
 * Span1 | ^^^^^ note1
 * Line2 | Code2
 * Span1 | ----- note2
 * ...
 * LineN | CodeN
 * SpanN | ~~~~~ noteN
 */
 
type MsgSpan struct {
	note_line, note_col []uint32
	notes, snippets     []string
	code               *[]string
}

func (m *MsgSpan) AddNote(line, col uint32, snippet, note_fmt string, args ...any) { 
	m.note_line = append(m.note_line, line)
	m.note_col  = append(m.note_col, col)
	m.snippets  = append(m.snippets, snippet)
	m.notes     = append(m.notes, fmt.Sprintf(note_fmt, args...))
}

func (m *MsgSpan) Report(msgtype, errcode, color, msg_fmt, filename string, line, col *uint32, args ...any) string {
	var sb strings.Builder
	sb.WriteString(color)
	sb.WriteString(msgtype)
	if errcode != "" {
		sb.WriteRune('[')
		sb.WriteString(errcode)
		sb.WriteRune(']')
	}
	sb.WriteString(COLOR_RESET)
	sb.WriteString(": " + fmt.Sprintf(msg_fmt, args...))
	if filename != "" {
		sb.WriteRune('\n')
		sb.WriteString("--> ")
		sb.WriteString(filename)
		if line != nil {
			sb.WriteString(fmt.Sprintf(":%d", *line))
		}
		if col != nil {
			sb.WriteString(fmt.Sprintf(":%d", *col))
		}
	}
	
	if len(m.note_line) > 0 {
		sb.WriteRune('\n')
		largest := 0
		for _, l := range m.note_line {
			if largest < int(l) {
				largest = int(l)
			}
		}
		big_line_str := fmt.Sprintf("%d", largest)
		largest_span := len(big_line_str) + 1
		span_write := func (sb *strings.Builder, span int, c rune) {
			for i := 0; i < span; i++ {
				sb.WriteRune(c)
			}
		}
		span_write(&sb, largest_span, ' ')
		sb.WriteRune('|')
		sb.WriteRune('\n')
		for i := range m.note_line {
			line := m.note_line[i]
			line_num_str := fmt.Sprintf("%d", line)
			sb.WriteString(line_num_str)
			span_write(&sb, largest_span - len(line_num_str), ' ')
			sb.WriteRune('|')
			note_col := m.note_col[i]
			snippet := m.snippets[i]
			snippet_len := len(snippet)
			note := m.notes[i]
			code_line := (*m.code)[line]
			sb.WriteString(code_line)
			sb.WriteRune('\n')
			span_write(&sb, largest_span, ' ')
			sb.WriteRune('|')
			span_write(&sb, int(note_col), ' ')
			span_write(&sb, snippet_len, '^')
			sb.WriteRune(' ')
			sb.WriteString(COLOR_CYAN);
			sb.WriteString(note);
			sb.WriteString(COLOR_RESET)
		}
	}
	return sb.String()
}

Was able to reduce it into one single function.

func MsgReport(code []string, msgtype, errcode, color, msg_fmt, snippet, note, filename string, line, col uint32, args ...any) string {
	var sb strings.Builder
	sb.WriteString(color)
	sb.WriteString(msgtype)
	if errcode != "" {
		sb.WriteRune('[')
		sb.WriteString(errcode)
		sb.WriteRune(']')
	}
	sb.WriteString(COLOR_RESET)
	sb.WriteString(": |" + fmt.Sprintf(msg_fmt, args...))
	sb.WriteRune('|')
	if filename != "" {
		sb.WriteRune('\n')
		sb.WriteString("--> ")
		sb.WriteString(filename)
		if line > 0 {
			sb.WriteString(fmt.Sprintf(":%d", line))
			sb.WriteString(fmt.Sprintf(":%d", col))
		}
	}
	
	if len(note) > 0 {
		sb.WriteRune('\n')
		big_line_str := fmt.Sprintf("%d", line)
		largest_span := len(big_line_str) + 1
		span_write := func(sb *strings.Builder, span int, c rune) {
			for i := 0; i < span; i++ {
				sb.WriteRune(c)
			}
		}
		span_write(&sb, largest_span, ' ')
		sb.WriteRune('|')
		sb.WriteRune('\n')
		line_num_str := fmt.Sprintf("%d", line)
		sb.WriteString(line_num_str)
		span_write(&sb, largest_span - len(line_num_str), ' ')
		sb.WriteRune('|')
		snippet_len := len(snippet)
		code_line := code[line - 1]
		sb.WriteString(code_line)
		sb.WriteRune('\n')
		span_write(&sb, largest_span, ' ')
		sb.WriteRune('|')
		span_write(&sb, int(col), ' ')
		span_write(&sb, snippet_len, '^')
		sb.WriteRune(' ')
		sb.WriteString(COLOR_CYAN);
		sb.WriteString(note);
		sb.WriteString(COLOR_RESET)
	}
	return sb.String()
}