truncate docs on char boundary using floor_char_boundary

Author: leighmcculloch

soroban-sdk-macros/src/doc.rs

@@ -20,11 +20,12 @@ pub fn docs_from_attrs(attrs: &[Attribute]) -> StringM<DOCS_MAX_LEN> {
             _ => None,
         })
         .map(|s| s.trim().to_string())
-        .join("\n")
-        .as_bytes()
-        .to_vec();
-    docs.truncate(DOCS_MAX_LEN as usize);
-    docs.try_into().unwrap()
+        .join("\n");
+    // Truncate on a char boundary to avoid splitting multi-byte UTF-8 codepoints.
+    let max = DOCS_MAX_LEN as usize;
+    let safe_len = docs.floor_char_boundary(max);
+    docs.truncate(safe_len);
+    docs.into_bytes().try_into().unwrap()
 }
 
 #[cfg(test)]
@@ -34,10 +35,10 @@ mod test {
 
     #[test]
     fn test_truncation_does_not_split_multibyte_utf8() {
-        // 1023 ASCII bytes followed by 'é' (2 bytes: 0xC3 0xA9) = 1025 bytes.
-        // Truncation at 1024 keeps the 0xC3 but drops the 0xA9, producing
-        // invalid UTF-8.
-        let padding = "a".repeat(1023);
+        // (DOCS_MAX_LEN - 1) ASCII bytes followed by 'é' (2 bytes: 0xC3
+        // 0xA9) = DOCS_MAX_LEN + 1 bytes. Truncation at DOCS_MAX_LEN keeps
+        // the 0xC3 but drops the 0xA9, producing invalid UTF-8.
+        let padding = "a".repeat(DOCS_MAX_LEN as usize - 1);
         let doc_value = format!("{padding}é");
         let attr: Attribute = parse_quote!(#[doc = #doc_value]);
         let result = docs_from_attrs(&[attr]);