feat(es/testing): Improve comment testing story (#9150)

**Description:**

This PR makes `PluginCommentProxy` work by default in `test_fixture()`. This PR adds a scoped-local to `swc_common`, and uses it from `swc_ecma_transforms_testing`.

It can't be tested in the main repository, though. We cannot enable `plugin_mode` of `swc_common` in the main SWC repository.


**Related issue:**

 - Closes https://github.com/swc-project/swc/issues/9149.

crates/swc_common/src/comments.rs

@@ -690,3 +690,9 @@ pub trait CommentsExt: Comments {
 
 #[allow(deprecated)]
 impl<C> CommentsExt for C where C: Comments {}
+
+better_scoped_tls::scoped_tls!(
+    /// **This is not a public API**. Used to handle comments while **testing**.
+    #[doc(hidden)]
+    pub static COMMENTS: Box<dyn Comments>
+);

crates/swc_ecma_transforms_testing/src/lib.rs

@@ -7,7 +7,7 @@ use std::{
     env,
     fs::{self, create_dir_all, read_to_string, OpenOptions},
     io::Write,
-    mem::take,
+    mem::{take, transmute},
     panic,
     path::{Path, PathBuf},
     process::Command,
@@ -21,14 +21,14 @@ use serde::de::DeserializeOwned;
 use sha2::{Digest, Sha256};
 use swc_common::{
     chain,
-    comments::SingleThreadedComments,
+    comments::{Comments, SingleThreadedComments},
     errors::{Handler, HANDLER},
     source_map::SourceMapGenConfig,
     sync::Lrc,
     FileName, Mark, SourceMap, DUMMY_SP,
 };
 use swc_ecma_ast::*;
-use swc_ecma_codegen::{to_code_default, Emitter};
+use swc_ecma_codegen::to_code_default;
 use swc_ecma_parser::{lexer::Lexer, Parser, StringInput, Syntax};
 use swc_ecma_testing::{exec_node_js, JsExecOptions};
 use swc_ecma_transforms_base::{
@@ -63,13 +63,22 @@ impl<'a> Tester<'a> {
     where
         F: FnOnce(&mut Tester<'_>) -> Result<Ret, ()>,
     {
+        let comments = Rc::new(SingleThreadedComments::default());
+
         let out = ::testing::run_test(false, |cm, handler| {
             HANDLER.set(handler, || {
                 HELPERS.set(&Default::default(), || {
-                    op(&mut Tester {
-                        cm,
-                        handler,
-                        comments: Default::default(),
+                    let cmts = comments.clone();
+                    let c = Box::new(unsafe {
+                        // Safety: This is unsafe but it's used only for testing.
+                        transmute::<&dyn Comments, &'static dyn Comments>(&*cmts)
+                    }) as Box<dyn Comments>;
+                    swc_common::comments::COMMENTS.set(&c, || {
+                        op(&mut Tester {
+                            cm,
+                            handler,
+                            comments,
+                        })
                     })
                 })
             })
@@ -863,14 +872,17 @@ fn test_fixture_inner<'a>(
     let mut sourcemap = None;
 
     let (actual_src, stderr) = Tester::run_captured(|tester| {
-        println!("----- {} -----\n{}", Color::Green.paint("Input"), input);
+        eprintln!("----- {} -----\n{}", Color::Green.paint("Input"), input);
 
         let tr = tr(tester);
 
-        println!("----- {} -----", Color::Green.paint("Actual"));
+        eprintln!("----- {} -----", Color::Green.paint("Actual"));
 
         let actual = tester.apply_transform(tr, "input.js", syntax, input)?;
 
+        eprintln!("----- {} -----", Color::Green.paint("Comments"));
+        eprintln!("{:?}", tester.comments);
+
         match ::std::env::var("PRINT_HYGIENE") {
             Ok(ref s) if s == "1" => {
                 let hygiene_src = tester.print(
@@ -893,23 +905,6 @@ fn test_fixture_inner<'a>(
         let actual_src = {
             let module = &actual;
             let comments: &Rc<SingleThreadedComments> = &tester.comments.clone();
-            let mut buf = vec![];
-            {
-                let mut emitter = Emitter {
-                    cfg: Default::default(),
-                    cm: tester.cm.clone(),
-                    wr: Box::new(swc_ecma_codegen::text_writer::JsWriter::new(
-                        tester.cm.clone(),
-                        "\n",
-                        &mut buf,
-                        src_map.as_mut(),
-                    )),
-                    comments: Some(comments),
-                };
-
-                // println!("Emitting: {:?}", module);
-                emitter.emit_module(module).unwrap();
-            }
 
             if let Some(src_map) = &mut src_map {
                 sourcemap = Some(tester.cm.build_source_map_with_config(
@@ -919,8 +914,7 @@ fn test_fixture_inner<'a>(
                 ));
             }
 
-            let s = String::from_utf8_lossy(&buf);
-            s.to_string()
+            to_code_default(tester.cm.clone(), Some(comments), module)
         };
 
         Ok(actual_src)
@@ -935,11 +929,11 @@ fn test_fixture_inner<'a>(
     }
 
     if let Some(actual_src) = actual_src {
-        println!("{}", actual_src);
+        eprintln!("{}", actual_src);
 
         if let Some(sourcemap) = &sourcemap {
-            println!("----- ----- ----- ----- -----");
-            println!("SourceMap: {}", visualizer_url(&actual_src, sourcemap));
+            eprintln!("----- ----- ----- ----- -----");
+            eprintln!("SourceMap: {}", visualizer_url(&actual_src, sourcemap));
         }
 
         if actual_src != expected_src {

crates/swc_plugin_proxy/src/comments/plugin_comments_proxy.rs

@@ -73,7 +73,77 @@ impl PluginCommentsProxy {
     }
 }
 
-#[cfg(feature = "__plugin_mode")]
+#[cfg(all(feature = "__plugin_mode", not(target_arch = "wasm32")))]
+#[swc_trace]
+impl Comments for PluginCommentsProxy {
+    fn add_leading(&self, pos: BytePos, cmt: Comment) {
+        swc_common::comments::COMMENTS.with(|comments| {
+            comments.add_leading(pos, cmt);
+        });
+    }
+
+    fn add_leading_comments(&self, pos: BytePos, comments: Vec<Comment>) {
+        swc_common::comments::COMMENTS.with(|comments| {
+            comments.add_leading_comments(pos, comments);
+        });
+    }
+
+    fn has_leading(&self, pos: BytePos) -> bool {
+        swc_common::comments::COMMENTS.with(|comments| comments.has_leading(pos))
+    }
+
+    fn move_leading(&self, from: BytePos, to: BytePos) {
+        swc_common::comments::COMMENTS.with(|comments| {
+            comments.move_leading(from, to);
+        });
+    }
+
+    fn take_leading(&self, pos: BytePos) -> Option<Vec<Comment>> {
+        swc_common::comments::COMMENTS.with(|comments| comments.take_leading(pos))
+    }
+
+    fn get_leading(&self, pos: BytePos) -> Option<Vec<Comment>> {
+        swc_common::comments::COMMENTS.with(|comments| comments.get_leading(pos))
+    }
+
+    fn add_trailing(&self, pos: BytePos, cmt: Comment) {
+        swc_common::comments::COMMENTS.with(|comments| {
+            comments.add_trailing(pos, cmt);
+        });
+    }
+
+    fn add_trailing_comments(&self, pos: BytePos, comments: Vec<Comment>) {
+        swc_common::comments::COMMENTS.with(|comments| {
+            comments.add_trailing_comments(pos, comments);
+        });
+    }
+
+    fn has_trailing(&self, pos: BytePos) -> bool {
+        swc_common::comments::COMMENTS.with(|comments| comments.has_trailing(pos))
+    }
+
+    fn move_trailing(&self, from: BytePos, to: BytePos) {
+        swc_common::comments::COMMENTS.with(|comments| {
+            comments.move_trailing(from, to);
+        });
+    }
+
+    fn take_trailing(&self, pos: BytePos) -> Option<Vec<Comment>> {
+        swc_common::comments::COMMENTS.with(|comments| comments.take_trailing(pos))
+    }
+
+    fn get_trailing(&self, pos: BytePos) -> Option<Vec<Comment>> {
+        swc_common::comments::COMMENTS.with(|comments| comments.get_trailing(pos))
+    }
+
+    fn add_pure_comment(&self, pos: BytePos) {
+        swc_common::comments::COMMENTS.with(|comments| {
+            comments.add_pure_comment(pos);
+        });
+    }
+}
+
+#[cfg(all(feature = "__plugin_mode", target_arch = "wasm32"))]
 #[cfg_attr(not(target_arch = "wasm32"), allow(unused))]
 #[swc_trace]
 impl Comments for PluginCommentsProxy {