Fix a crash when truncating note text

String::truncate() is a bit of a footgun, and I've hit this before
too :-)

rslib/src/import_export/package/apkg/import/notes.rs

@@ -17,7 +17,10 @@ use crate::{
         ImportProgress,
     },
     prelude::*,
-    text::{replace_media_refs, strip_html_preserving_media_filenames, CowMapping},
+    text::{
+        replace_media_refs, strip_html_preserving_media_filenames, truncate_to_char_boundary,
+        CowMapping,
+    },
 };
 
 struct NoteContext<'a> {
@@ -73,7 +76,7 @@ impl Note {
                     let mut reduced = strip_html_preserving_media_filenames(&field)
                         .get_owned()
                         .unwrap_or(field);
-                    reduced.truncate(80);
+                    truncate_to_char_boundary(&mut reduced, 80);
                     reduced
                 })
                 .collect(),

rslib/src/media/files.rs

@@ -241,18 +241,18 @@ fn split_and_truncate_filename(fname: &str, max_bytes: usize) -> (&str, &str) {
     };
 
     // cap extension to 10 bytes so stem_len can't be negative
-    ext = truncate_to_char_boundary(ext, 10);
+    ext = truncated_to_char_boundary(ext, 10);
 
     // cap stem, allowing for the . and a trailing _
     let stem_len = max_bytes - ext.len() - 2;
-    stem = truncate_to_char_boundary(stem, stem_len);
+    stem = truncated_to_char_boundary(stem, stem_len);
 
     (stem, ext)
 }
 
-/// Trim a string on a valid UTF8 boundary.
+/// Return a substring on a valid UTF8 boundary.
 /// Based on a funtion in the Rust stdlib.
-fn truncate_to_char_boundary(s: &str, mut max: usize) -> &str {
+fn truncated_to_char_boundary(s: &str, mut max: usize) -> &str {
     if max >= s.len() {
         s
     } else {

rslib/src/text.rs

@@ -206,6 +206,17 @@ pub fn strip_html_for_tts(html: &str) -> Cow<str> {
         .map_cow(strip_html)
 }
 
+/// Truncate a String on a valid UTF8 boundary.
+pub(crate) fn truncate_to_char_boundary(s: &mut String, mut max: usize) {
+    if max >= s.len() {
+        return;
+    }
+    while !s.is_char_boundary(max) {
+        max -= 1;
+    }
+    s.truncate(max);
+}
+
 #[derive(Debug)]
 pub(crate) struct MediaRef<'a> {
     pub full_ref: &'a str,
@@ -589,4 +600,14 @@ mod test {
             "<img src=spam>[sound:spam]<img src=baz.jpg>",
         );
     }
+
+    #[test]
+    fn truncate() {
+        let mut s = "日本語".to_string();
+        truncate_to_char_boundary(&mut s, 6);
+        assert_eq!(&s, "日本");
+        let mut s = "日本語".to_string();
+        truncate_to_char_boundary(&mut s, 1);
+        assert_eq!(&s, "");
+    }
 }