use .to_string_lossy() member of Path, CStr etc.

instead of calling to_str().unwrap_or_default().

mmime/src/charconv.rs

@@ -16,12 +16,9 @@ pub unsafe fn charconv(
 ) -> libc::c_int {
     assert!(!fromcode.is_null(), "invalid fromcode");
     assert!(!s.is_null(), "invalid input string");
-    if let Some(encoding) = charset::Charset::for_label(
-        CStr::from_ptr(fromcode)
-            .to_str()
-            .unwrap_or_default()
-            .as_bytes(),
-    ) {
+    if let Some(encoding) =
+        charset::Charset::for_label(CStr::from_ptr(fromcode).to_string_lossy().as_bytes())
+    {
         let data = std::slice::from_raw_parts(s as *const u8, strlen(s));
 
         let (res, _, _) = encoding.decode(data);

mmime/src/mailmime/write_generic.rs

@@ -1515,8 +1515,8 @@ pub unsafe fn mailmime_data_write_driver(
         }
         1 => {
             let filename = CStr::from_ptr((*mime_data).dt_data.dt_filename)
-                .to_str()
-                .unwrap_or_default();
+                .to_string_lossy()
+                .into_owned();
             if let Ok(file) = std::fs::File::open(filename) {
                 if let Ok(mut text) = memmap::MmapOptions::new().map_copy(&file) {
                     if 0 != (*mime_data).dt_encoded {