fix(es/compat): Fix syntax context of async-to-generator (#6741)

**Description:**

Previously, the `async-to-generator` produced invalid AST, in the aspect of span hygiene.

[Playground](https://play.swc.rs/?version=1.3.24&code=H4sIAAAAAAAAAz1MbQqAIBT77yn2UyG6gNQJuoSZRCAa7xkk4d1TicZgH7C5%2B4yUYGPgBE7ZuyWazREmSIVpxiMAcumi0C3ANZxyvQ6%2Fqa8CehxjaGNpOAcLaWjn%2F6KhKN1dGaoULSpfHPTdxn8AAAA%3D&config=H4sIAAAAAAAAA0WOSwrDMAxE76K1F22hXfgE3fQQxlWCi39ICsQY3z12cMlOjObNTIUfW9AVsiFGGheXKGYHDVIysiWXBRQId2kxnrEpwF2QovFv9BmJQQtt2D2GVpROIj9u92enfEqMk1MQXHRLGR02hUzIfL1MXP3f2XpFSN9tCPWccWa%2BoF0Zk3P8mcYxoR3Kj7IYzwAAAA%3D%3D).
It generate two bindings for `args` so it's invalid.


**Related issue:**

 - Closes #6730.

Author: kdy1

Cargo.lock

@@ -0,0 +1,26 @@
+{
+    "jsc": {
+        "parser": {
+            "syntax": "typescript",
+            "tsx": false
+        },
+        "target": "es2015",
+        "loose": false,
+        "minify": {
+            "compress": false,
+            "mangle": {
+                "toplevel": false,
+                "keep_classnames": false,
+                "keep_fnames": false,
+                "keep_private_props": false,
+                "ie8": false,
+                "safari10": false
+            }
+        }
+    },
+    "module": {
+        "type": "es6"
+    },
+    "minify": false,
+    "isModule": true
+}

crates/swc/tests/fixture/issues-6xxx/6730/input/.swcrc

@@ -0,0 +1,11 @@
+import { Plugin, PluginBuild } from 'esbuild';
+
+export const styleLoader = (): Plugin => {
+    return {
+        setup(build: PluginBuild) {
+            build.onLoad(async (args) => {
+
+            });
+        },
+    };
+};

crates/swc/tests/fixture/issues-6xxx/6730/input/index.ts

@@ -0,0 +1,13 @@
+import r from "@swc/helpers/src/_async_to_generator.mjs";
+export const styleLoader = ()=>{
+    return {
+        setup (t) {
+            t.onLoad(function() {
+                var t = r(function*(r) {});
+                return function(r) {
+                    return t.apply(this, arguments);
+                };
+            }());
+        }
+    };
+};

crates/swc/tests/fixture/issues-6xxx/6730/output/index.ts

@@ -812,6 +812,10 @@ fn should_visit() {
 
 #[testing::fixture("tests/fixture/**/input/")]
 #[testing::fixture("tests/vercel/**/input/")]
+fn fixture(input_dir: PathBuf) {
+    tests(input_dir)
+}
+
 fn tests(input_dir: PathBuf) {
     let output = input_dir.parent().unwrap().join("output");
 

crates/swc/tests/projects.rs

@@ -64,7 +64,7 @@ export var listOfUser = function(t) {
                 2
             ];
         });
-    }), function(r, e) {
+    }), function(r, n) {
         return e.apply(this, arguments);
     }));
 };

crates/swc/tests/vercel/full/next-31419/1/output/index.js

@@ -19,7 +19,7 @@ export const listOfUser = function(n) {
         postgreSQL.query(t, null, function(n, t) {
             n ? e(n) : r(t.rows);
         });
-    }), function(r, e) {
+    }), function(r, n) {
         return e.apply(this, arguments);
     }));
 };

crates/swc/tests/vercel/full/next-31419/2/output/index.js

@@ -5,15 +5,15 @@ use swc_atoms::js_word;
 use swc_common::{pass::Either, util::take::Take, Mark, Spanned, SyntaxContext, DUMMY_SP};
 use swc_ecma_ast::*;
 use swc_ecma_utils::{
-    contains_arguments, contains_this_expr, find_pat_ids, undefined, ExprFactory,
+    contains_arguments, contains_this_expr, find_pat_ids, undefined, ExprFactory, Remapper,
 };
 use swc_ecma_visit::VisitMutWith;
 
 use super::{util::NormalMultiReplacer, Optimizer};
 #[cfg(feature = "debug")]
 use crate::debug::dump;
 use crate::{
-    compress::optimize::{util::Remapper, Ctx},
+    compress::optimize::Ctx,
     mode::Mode,
     util::{idents_captured_by, idents_used_by, make_number},
 };
@@ -578,7 +578,7 @@ where
                         if self.vars.inline_with_multi_replacer(body) {
                             self.changed = true;
                         }
-                        body.visit_mut_with(&mut Remapper { vars: remap });
+                        body.visit_mut_with(&mut Remapper::new(&remap));
                         exprs.push(body.take());
 
                         report_change!("inline: Inlining a call to an arrow function");
@@ -889,7 +889,7 @@ where
         if self.vars.inline_with_multi_replacer(body) {
             self.changed = true;
         }
-        body.visit_mut_with(&mut Remapper { vars: remap });
+        body.visit_mut_with(&mut Remapper::new(&remap));
 
         let mut vars = Vec::new();
         let mut exprs = Vec::new();

crates/swc_ecma_minifier/src/compress/optimize/iife.rs

@@ -5,7 +5,7 @@ use std::{
 
 use rustc_hash::{FxHashMap, FxHashSet};
 use swc_atoms::js_word;
-use swc_common::{util::take::Take, Span, SyntaxContext, DUMMY_SP};
+use swc_common::{util::take::Take, Span, DUMMY_SP};
 use swc_ecma_ast::*;
 use swc_ecma_transforms_base::perf::{Parallel, ParallelExt};
 use swc_ecma_utils::{ExprCtx, ExprExt};
@@ -185,24 +185,6 @@ pub(crate) fn is_valid_for_lhs(e: &Expr) -> bool {
     !matches!(e, Expr::Lit(..) | Expr::Unary(..))
 }
 
-/// Variable remapper
-///
-/// - Used for evaluating IIFEs
-
-pub(crate) struct Remapper {
-    pub vars: FxHashMap<Id, SyntaxContext>,
-}
-
-impl VisitMut for Remapper {
-    noop_visit_mut_type!();
-
-    fn visit_mut_ident(&mut self, i: &mut Ident) {
-        if let Some(new_ctxt) = self.vars.get(&i.to_id()).copied() {
-            i.span.ctxt = new_ctxt;
-        }
-    }
-}
-
 /// A visitor responsible for inlining special kind of variables and removing
 /// (some) unused variables. Due to the order of visit, the main visitor cannot
 /// handle all edge cases and this type is the complement for it.

crates/swc_ecma_minifier/src/compress/optimize/util.rs

@@ -0,0 +1,3 @@
+{
+    "defaults": false
+}

crates/swc_ecma_minifier/tests/fixture/issues/6730/config.json

@@ -0,0 +1,45 @@
+function asyncGeneratorStep(gen, resolve, reject, _next, _throw, key, arg) {
+    try {
+        var info = gen[key](arg);
+        var value = info.value;
+    } catch (error) {
+        reject(error);
+        return;
+    }
+    if (info.done) {
+        resolve(value);
+    } else {
+        Promise.resolve(value).then(_next, _throw);
+    }
+}
+function _asyncToGenerator(fn) {
+    return function () {
+        var self = this, args = arguments;
+        return new Promise(function (resolve, reject) {
+            var gen = fn.apply(self, args);
+            function _next(value) {
+                asyncGeneratorStep(gen, resolve, reject, _next, _throw, "next", value);
+            }
+            function _throw(err) {
+                asyncGeneratorStep(gen, resolve, reject, _next, _throw, "throw", err);
+            }
+            _next(undefined);
+        });
+    };
+}
+export const styleLoader = () => {
+    return {
+        name: 'style-loader',
+        setup(build) {
+            build.onLoad({
+                filter: /.*/,
+                namespace: 'less'
+            }, function () {
+                var _ref = _asyncToGenerator(function* (args) { });
+                return function (args) {
+                    return _ref.apply(this, arguments);
+                };
+            }());
+        }
+    };
+};

crates/swc_ecma_minifier/tests/fixture/issues/6730/input.js

@@ -0,0 +1,3 @@
+{
+    "toplevel": true
+}

crates/swc_ecma_minifier/tests/fixture/issues/6730/mangle.json

@@ -0,0 +1,42 @@
+function n(n, e, t, r, o, u, i) {
+    try {
+        var a = n[u](i);
+        var c = a.value;
+    } catch (s) {
+        t(s);
+        return;
+    }
+    if (a.done) e(c);
+    else Promise.resolve(c).then(r, o);
+}
+function e(e) {
+    return function() {
+        var t = this, r = arguments;
+        return new Promise(function(o, u) {
+            var i = e.apply(t, r);
+            function a(e) {
+                n(i, o, u, a, c, "next", e);
+            }
+            function c(e) {
+                n(i, o, u, a, c, "throw", e);
+            }
+            a(void 0);
+        });
+    };
+}
+export const styleLoader = ()=>{
+    return {
+        name: 'style-loader',
+        setup (n) {
+            n.onLoad({
+                filter: /.*/,
+                namespace: 'less'
+            }, function() {
+                var n = e(function*(n) {});
+                return function(e) {
+                    return n.apply(this, arguments);
+                };
+            }());
+        }
+    };
+};

crates/swc_ecma_minifier/tests/fixture/issues/6730/output.js

@@ -6,9 +6,9 @@ use swc_ecma_ast::*;
 use swc_ecma_transforms_base::{helper, helper_expr, perf::Check};
 use swc_ecma_transforms_macros::fast_path;
 use swc_ecma_utils::{
-    contains_this_expr,
+    contains_this_expr, find_pat_ids,
     function::{FnEnvHoister, FnWrapperResult, FunctionWrapper},
-    private_ident, quote_ident, ExprFactory, StmtLike,
+    private_ident, quote_ident, ExprFactory, Remapper, StmtLike,
 };
 use swc_ecma_visit::{
     as_folder, noop_visit_mut_type, noop_visit_type, Fold, Visit, VisitMut, VisitMutWith, VisitWith,
@@ -393,6 +393,16 @@ impl<C: Comments> Actual<C> {
 /// `_asyncToGenerator(function*() {})` from `async function() {}`;
 #[tracing::instrument(level = "info", skip_all)]
 fn make_fn_ref(mut expr: FnExpr) -> Expr {
+    {
+        let param_ids: Vec<Id> = find_pat_ids(&expr.function.params);
+        let mapping = param_ids
+            .into_iter()
+            .map(|id| (id, SyntaxContext::empty().apply_mark(Mark::new())))
+            .collect();
+
+        expr.function.visit_mut_with(&mut Remapper::new(&mapping));
+    }
+
     expr.function.body.visit_mut_with(&mut AsyncFnBodyHandler {
         is_async_generator: expr.function.is_generator,
     });

crates/swc_ecma_transforms_compat/src/es2017/async_to_generator.rs

@@ -24,6 +24,7 @@ indexmap = "1.6.1"
 num_cpus = "1.13.1"
 once_cell = "1.10.0"
 rayon = {version = "1.5.1", optional = true}
+rustc-hash = "1.1.0"
 swc_atoms = {version = "0.4.32", path = "../swc_atoms"}
 swc_common = {version = "0.29.25", path = "../swc_common"}
 swc_ecma_ast = {version = "0.95.9", path = "../swc_ecma_ast"}

crates/swc_ecma_utils/Cargo.toml

@@ -18,6 +18,7 @@ use std::{
     ops::Add,
 };
 
+use rustc_hash::FxHashMap;
 use swc_atoms::{js_word, JsWord};
 use swc_common::{collections::AHashSet, Mark, Span, Spanned, SyntaxContext, DUMMY_SP};
 use swc_ecma_ast::*;
@@ -2825,6 +2826,31 @@ pub fn contains_top_level_await<V: VisitWith<TopLevelAwait>>(t: &V) -> bool {
     finder.found
 }
 
+/// Variable remapper
+///
+/// This visitor modifies [SyntaxContext] while preserving the symbol of
+/// [Ident]s.
+
+pub struct Remapper<'a> {
+    vars: &'a FxHashMap<Id, SyntaxContext>,
+}
+
+impl<'a> Remapper<'a> {
+    pub fn new(vars: &'a FxHashMap<Id, SyntaxContext>) -> Self {
+        Self { vars }
+    }
+}
+
+impl VisitMut for Remapper<'_> {
+    noop_visit_mut_type!();
+
+    fn visit_mut_ident(&mut self, i: &mut Ident) {
+        if let Some(new_ctxt) = self.vars.get(&i.to_id()).copied() {
+            i.span.ctxt = new_ctxt;
+        }
+    }
+}
+
 #[cfg(test)]
 mod test {
     use swc_common::{input::StringInput, BytePos};