truncate long slices instead of panicking

Author: joseph-gio

crates/bevy_utils/src/label.rs

@@ -66,23 +66,33 @@ pub struct StuffedStr<'a> {
 unsafe impl<'a> Send for StuffedStr<'a> {}
 unsafe impl<'a> Sync for StuffedStr<'a> {}
 
+/// Truncates the passed string slice to be of length `len`, or shorter.
+#[cold]
+#[inline(never)]
+fn truncate(str: &str, mut len: usize) -> &str {
+    // Just keep lowering the expected length until we hit a unicode boundary.
+    // This is messy but if the string is long enough that we need to truncate it,
+    // it's too long for anyone to notice what the end looks like.
+    loop {
+        if let Some(str) = str.get(..len) {
+            break str;
+        }
+        // In practice this won't be able to underflow, since  `str.get(..0)` will always succeed.
+        len -= 1;
+    }
+}
+
 impl<'a> StuffedStr<'a> {
     const DATA_MASK: usize = !Self::LEN_MASK;
     const LEN_MASK: usize = !0 << 16;
 
-    pub fn new(str: &'a str, data: u16) -> Self {
-        #[cold]
-        #[inline(never)]
-        fn panic() -> ! {
-            // needs a better message but its late
-            panic!("string exceeds {} bytes", StuffedStr::LEN_MASK >> 16);
-        }
-
-        let ptr = str.as_ptr();
+    pub fn new(mut str: &'a str, data: u16) -> Self {
         // Make sure there's enough room to store the data.
         if str.len().leading_zeros() < 16 {
-            panic();
+            str = truncate(str, Self::LEN_MASK >> 16);
+            debug_assert!(str.len().leading_zeros() < 16);
         }
+        let ptr = str.as_ptr();
         let meta = data as usize | str.len() << 16;
         Self {
             ptr,
@@ -202,3 +212,18 @@ macro_rules! define_label {
         }
     };
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_truncate() {
+        // Slice at byte '4'
+        assert_eq!(truncate("Hello, World!", 4), "Hell");
+
+        // Slicing off at byte '3' would be inside of the emoji,
+        // so instead the whole emoji gets sliced off.
+        assert_eq!(truncate("x😂", 3), "x");
+    }
+}