feat: add RUSTY_V8_REUSE_PREBUILT env var

When set, reuses existing prebuilt V8 library and binding files
instead of downloading. Useful when only Rust code changed but
V8 itself hasn't changed (e.g., version bump without V8 update).

Author: max-lt

build.rs

@@ -40,6 +40,7 @@ fn main() {
     "OUT_DIR",
     "RUSTY_V8_ARCHIVE",
     "RUSTY_V8_MIRROR",
+    "RUSTY_V8_REUSE_PREBUILT",
     "RUSTY_V8_SRC_BINDING_PATH",
     "SCCACHE",
     "V8_FORCE_DEBUG",
@@ -717,7 +718,20 @@ fn download_static_lib_binaries() {
   fs::create_dir_all(&dir).unwrap();
   println!("cargo:rustc-link-search={}", dir.display());
 
-  download_file(&url, &static_lib_path());
+  let lib_path = static_lib_path();
+
+  // If RUSTY_V8_REUSE_PREBUILT is set and the library already exists,
+  // skip the download. This is useful when only Rust code changed but
+  // V8 itself hasn't changed (e.g., version bump without V8 update).
+  if env::var("RUSTY_V8_REUSE_PREBUILT").is_ok() && lib_path.exists() {
+    println!(
+      "cargo:warning=Reusing existing prebuilt V8 library at {}",
+      lib_path.display()
+    );
+    return;
+  }
+
+  download_file(&url, &lib_path);
 }
 
 fn decompress_to_writer<R, W>(input: &mut R, output: &mut W) -> io::Result<()>
@@ -855,6 +869,37 @@ fn print_prebuilt_src_binding_path() {
 
   let src_binding_path = get_dirs().root.join("gen").join(name.clone());
 
+  // If RUSTY_V8_REUSE_PREBUILT is set, look for any existing binding file
+  // in the gen directory (ignoring version in path).
+  if env::var("RUSTY_V8_REUSE_PREBUILT").is_ok() {
+    let gen_dir = get_dirs().root.join("gen");
+
+    if let Ok(entries) = fs::read_dir(&gen_dir) {
+      for entry in entries.flatten() {
+        let path = entry.path();
+
+        if let Some(filename) = path.file_name().and_then(|f| f.to_str()) {
+          // Match pattern: src_binding{features}_{profile}_{target}.rs
+          if filename.starts_with("src_binding")
+            && filename.ends_with(".rs")
+            && filename.contains(&format!("_{profile}_"))
+            && filename.contains(&target)
+          {
+            println!(
+              "cargo:warning=Reusing existing binding file at {}",
+              path.display()
+            );
+            println!(
+              "cargo:rustc-env=RUSTY_V8_SRC_BINDING_PATH={}",
+              path.display()
+            );
+            return;
+          }
+        }
+      }
+    }
+  }
+
   if let Ok(base) = env::var("RUSTY_V8_MIRROR") {
     let version = env::var("CARGO_PKG_VERSION").unwrap();
     let url = format!("{base}/v{version}/{name}");