⚡ Bolt: Optimize string concatenation by avoiding format! overhead

.jules/bolt.md

@@ -23,3 +23,6 @@
 ## 2024-05-24 - Optimize rusqlite extraction in query_map
 **Learning:** When mapping `rusqlite` query results to dynamic types like `IndexMap<String, RuntimeValue>` in `techscript_stdlib`, use `row.get_ref_unwrap(i)` to map the underlying `rusqlite::types::ValueRef` directly to the corresponding `RuntimeValue` (e.g., `Int`, `Float`, `Str`), avoiding the overhead and type issues of coercing all numeric and null columns to `String`s.
 **Action:** When extracting data from rusqlite database rows, inspect the `ValueRef` directly with `get_ref` or `get_ref_unwrap` instead of eagerly copying out a target Rust type like `String`, significantly reducing allocations and retaining correct native types for numeric and null data.
+## 2024-05-18 - String Concatenation Optimization in VM
+**Learning:** String concatenation using `format!("{}{}", a, b)` creates unnecessary intermediate string allocations. Replacing it with `String::with_capacity(a.len() + b.len())` and `push_str()` significantly improves performance by allocating the exact required size once. Even better, if the first string is an owned `String` that is no longer needed (e.g., from popping a stack), reusing its buffer via `mut a` and `a.push_str(&b)` avoids allocating a new buffer entirely.
+**Action:** When concatenating strings in hot paths like VM execution loops or native interop, prefer in-place buffer reuse (`a.push_str(&b)`) or `String::with_capacity` over `format!`.

runtime/native_runtime/src/lib.rs

@@ -505,10 +505,11 @@ pub unsafe extern "C" fn ts_add(left: *mut TsValue, right: *mut TsValue) -> *mut
 
     // Concatenation if either is a string
     if l.tag == TsTag::String as u32 || r.tag == TsTag::String as u32 {
-        let l_str = value_to_string(left);
+        let mut l_str = value_to_string(left);
         let r_str = value_to_string(right);
-        let combined = format!("{}{}", l_str, r_str);
-        let combined_cstr = CString::new(combined).unwrap();
+        // Bolt ⚡: In-place string append avoids additional buffer allocations and format! overhead
+        l_str.push_str(&r_str);
+        let combined_cstr = CString::new(l_str).unwrap();
         return ts_alloc_string(combined_cstr.as_ptr());
     }
 

runtime/vm/src/executor.rs

@@ -161,8 +161,10 @@ impl VM {
                         (RuntimeValue::Float(a), RuntimeValue::Float(b)) => {
                             RuntimeValue::Float(a + b)
                         }
-                        (RuntimeValue::Str(a), RuntimeValue::Str(b)) => {
-                            RuntimeValue::Str(format!("{}{}", a, b))
+                        (RuntimeValue::Str(mut a), RuntimeValue::Str(b)) => {
+                            // Bolt ⚡: In-place string append reuses buffer capacity and avoids fresh allocation
+                            a.push_str(&b);
+                            RuntimeValue::Str(a)
                         }
                         _ => {
                             return Err(VMError::TypeError {