merging is not simple

crates/swc_common/src/syntax_pos.rs

@@ -11,6 +11,7 @@ use std::{
 
 #[cfg(feature = "parking_lot")]
 use parking_lot::Mutex;
+use rustc_hash::FxHashMap;
 use serde::{Deserialize, Serialize};
 use url::Url;
 
@@ -116,10 +117,11 @@ impl Globals {
     pub fn merge(globals: Vec<Lrc<Globals>>) -> MergeResult {
         let mut hygiene_data = hygiene::HygieneData::new();
         let mut marks = vec![];
+        let mut result = MergeResult::default();
 
         for g in &globals {
             let g = g.hygiene_data.lock();
-            hygiene_data.merge(&g);
+            hygiene_data.merge(&g, &mut result);
         }
 
         for g in &globals {
@@ -127,12 +129,14 @@ impl Globals {
             marks.extend(g.iter().cloned());
         }
 
-        MergeResult {}
+        result
     }
 }
 
-#[derive(Debug)]
-pub struct MergeResult {}
+#[derive(Debug, Default)]
+pub struct MergeResult {
+    pub ctxts: FxHashMap<SyntaxContext, SyntaxContext>,
+}
 
 better_scoped_tls::scoped_tls!(
 

crates/swc_common/src/syntax_pos/hygiene.rs

@@ -23,7 +23,7 @@ use std::{
 
 use serde::{Deserialize, Serialize};
 
-use super::GLOBALS;
+use super::{MergeResult, GLOBALS};
 use crate::{collections::AHashMap, EqIgnoreSpan};
 
 /// A SyntaxContext represents a chain of macro expansions (represented by
@@ -327,12 +327,7 @@ impl HygieneData {
         }
     }
 
-    pub(crate) fn merge(&mut self, other: &Self) {
-        self.syntax_contexts
-            .extend_from_slice(&other.syntax_contexts);
-        self.markings
-            .extend(other.markings.iter().map(|(k, v)| (*k, *v)));
-    }
+    pub(crate) fn merge(&mut self, other: &Self, result: &mut MergeResult) {}
 
     fn with<T, F: FnOnce(&mut HygieneData) -> T>(f: F) -> T {
         GLOBALS.with(|globals| {