Step 3

At this point your project should still build without errors.

Next, let’s put together the datetime format string: dt_fmt.

After looking into the crono crates specifiers, we come up with this string:

"%Y-%m-%d %H:%M:%S%.6f"

which should produce something like:

2025-06-23 13:10:45.123456

Now, let’s build the main format string: fmt_string.

First, look at the provided method: FormatTrait::ft_fmt(). It provides the following variables that can be included in your format string via interpolation:

• dt - The datetime formatted with: dt_fmt.
• mod_path - The module path, possibly supplied via: module_path!().
• fn_name - The name of the function/method inside which the log entry was generated. Supplied by the #[logger] macro, or manually with the set_fn_name() method.
• level - The log Level for which the entry was created.
• message - The text of the log entry.

Using the specifiers available in std:fmt, we produce this:

"{dt},{mod_path}->{fn_name},{level},\"{message}\""

with a possible output of:

2025-06-23 13:10:45.123456,my_project::csv_formatter::tests->csv_format,INFO,"trait methods"

Using these format strings, we can now modify our new custom formatter as follows:

impl CsvFormatter {
    ///
    /// Creates a new instance of `CsvFormatter`.
    ///
    pub fn new() -> Self {
        Self {
-            dt_fmt: "%s.%f".to_string(),
+            dt_fmt: "%Y-%m-%d %H:%M:%S%.6f".to_string(),
-            fmt_string: "{dt} {mod_path}->{fn_name} [{level:7}] {message}".to_string(),
+            fmt_string: "{dt},{mod_path}->{fn_name},{level},\"{message}\"".to_string(),
        }
    }