perf(plugin): Avoid data copy when transformation finished (#11223)

**Description:**

When the transformation is done, we can make sure that the
`transformed_result` won't be used by wasm imports. So it's safe to
fetch the data from it without cloning the bytes.

Author: CPunisher

.changeset/gentle-owls-count.md

@@ -0,0 +1,6 @@
+---
+swc_plugin_runner: patch
+swc_core: patch
+---
+
+perf(plugin): avoid data copy when transformation finished

crates/swc_plugin_runner/src/transform_executor.rs

@@ -38,7 +38,7 @@ struct PluginTransformState {
 #[cfg(feature = "encoding-impl")]
 impl PluginTransformState {
     fn run(
-        &mut self,
+        mut self,
         program: &PluginSerializedBytes,
         unresolved_mark: swc_common::Mark,
         should_enable_comments_proxy: Option<bool>,
@@ -67,10 +67,21 @@ impl PluginTransformState {
             should_enable_comments_proxy,
         )?;
 
-        // Copy guest's memory into host, construct serialized struct from raw
-        // bytes.
-        let transformed_result = &(*self.transform_result.lock());
-        let ret = PluginSerializedBytes::from_bytes(transformed_result.clone());
+        self.instance
+            .caller()?
+            .free(guest_program_ptr.0, guest_program_ptr.1)?;
+        self.instance.cleanup()?;
+
+        // Construct serialized struct from raw bytes.
+        // Since we have finished transformation, it's safe to fetch the data from
+        // Arc<Mutex<T>>
+        drop(self.instance);
+        let transformed_result = Arc::try_unwrap(self.transform_result)
+            .map_err(|_| {
+                anyhow!("Failed to unwrap Arc: other references to transform_result exist")
+            })?
+            .into_inner();
+        let ret = PluginSerializedBytes::from_bytes(transformed_result);
 
         let ret = if returned_ptr_result == 0 {
             Ok(ret)
@@ -89,11 +100,6 @@ impl PluginTransformState {
             }
         };
 
-        self.instance
-            .caller()?
-            .free(guest_program_ptr.0, guest_program_ptr.1)?;
-        self.instance.cleanup()?;
-
         ret
     }