Fix SmoLStrBuilder pushing null bytes on heap spill

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## 0.3.2 - 2024-10-23
+
+- Fix `SmolStrBuilder::push` incorrectly padding null bytes when spilling onto the heap on a
+  multibyte character push
+
 ## 0.3.1 - 2024-09-04
 
 - Fix `SmolStrBuilder` leaking implementation details

src/lib.rs

@@ -763,7 +763,7 @@ impl SmolStrBuilder {
                     let mut heap = String::with_capacity(new_len);
                     // copy existing inline bytes over to the heap
                     // SAFETY: inline data is guaranteed to be valid utf8 for `old_len` bytes
-                    unsafe { heap.as_mut_vec().extend_from_slice(buf) };
+                    unsafe { heap.as_mut_vec().extend_from_slice(&buf[..*len]) };
                     heap.push(c);
                     self.0 = SmolStrBuilderRepr::Heap(heap);
                 }

tests/test.rs

@@ -255,6 +255,7 @@ fn test_to_smolstr() {
         assert_eq!(a, smol_str::format_smolstr!("{}", a));
     }
 }
+
 #[test]
 fn test_builder_push_str() {
     //empty
@@ -290,6 +291,14 @@ fn test_builder_push_str() {
     let s = builder.finish();
     assert!(s.is_heap_allocated());
     assert_eq!("a".repeat(46), s);
+
+    // heap push on multibyte char
+    let mut builder = SmolStrBuilder::new();
+    builder.push_str("ohnonononononononono!");
+    builder.push('🤯');
+    let s = builder.finish();
+    assert!(s.is_heap_allocated());
+    assert_eq!("ohnonononononononono!🤯", s);
 }
 
 #[test]