Adds a check in tracing_appender

tracing-appender/src/rolling/builder.rs

@@ -208,6 +208,10 @@ impl Builder {
     /// Files matching these criteria may be deleted if the maximum number of
     /// log files in the directory has been reached.
     ///
+    /// # Panics
+    ///
+    /// This method panics if and only if `n` is 0.
+    ///
     /// [`filename_prefix`]: Self::filename_prefix
     /// [`filename_suffix`]: Self::filename_suffix
     ///
@@ -227,6 +231,7 @@ impl Builder {
     /// ```
     #[must_use]
     pub fn max_log_files(self, n: usize) -> Self {
+        assert!(n != 0, "must keep at least one log file");
         Self {
             max_files: Some(n),
             ..self
@@ -271,3 +276,15 @@ impl Default for Builder {
         Self::new()
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::Builder;
+
+    /// Test that [`Builder::max_log_files`] panics if `n` is 0.
+    #[test]
+    #[should_panic(expected = "must keep at least one log file")]
+    fn test_zero_max_log_files() {
+        let _ = Builder::new().max_log_files(0);
+    }
+}