fix(es/transform/compat): Preserve more span (#2766)

swc_ecma_transforms_compat:
 - `async_to_generator`: Use correct span for async methods. (Closes #2701)

crates/swc/tests/fixture/issue-2701/1/input/.swcrc

@@ -0,0 +1,13 @@
+{
+    "jsc": {
+      "externalHelpers": false,
+      "parser": {
+        "syntax": "ecmascript",
+        "jsx": false
+      },
+      "target": "es2016"
+    },
+    "module": {
+      "type": "es6"
+    }
+  }

crates/swc/tests/fixture/issue-2701/1/input/index.js

@@ -0,0 +1,22 @@
+const x = {
+    // i am some comment1
+    async hello() {
+      console.log("Hello")
+    },
+    // i am some comment2
+    async "hello"(){
+      console.log("Hello")
+    },
+    // i am some comment3
+    async 1(){
+      console.log("Hello")
+    },
+    // i am some comment4
+    async [Date.now()](){
+      console.log("Hello")
+    },
+    // i am some comment5
+    async 1n() {
+      console.log("Hello")
+    }
+};

crates/swc/tests/fixture/issue-2701/1/output/index.js

@@ -0,0 +1,61 @@
+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);
+        });
+    };
+}
+const x = {
+    // i am some comment1
+    hello () {
+        return _asyncToGenerator(function*() {
+            console.log("Hello");
+        })();
+    },
+    // i am some comment2
+    "hello" () {
+        return _asyncToGenerator(function*() {
+            console.log("Hello");
+        })();
+    },
+    // i am some comment3
+    1 () {
+        return _asyncToGenerator(function*() {
+            console.log("Hello");
+        })();
+    },
+    // i am some comment4
+    [Date.now()] () {
+        return _asyncToGenerator(function*() {
+            console.log("Hello");
+        })();
+    },
+    // i am some comment5
+    1n () {
+        return _asyncToGenerator(function*() {
+            console.log("Hello");
+        })();
+    }
+};

crates/swc_ecma_transforms_compat/src/es2017/async_to_generator.rs

@@ -1,5 +1,5 @@
 use std::{iter, mem::replace};
-use swc_common::{util::take::Take, Mark, Span, Spanned, DUMMY_SP};
+use swc_common::{util::take::Take, BytePos, Mark, Span, Spanned, SyntaxContext, DUMMY_SP};
 use swc_ecma_ast::*;
 use swc_ecma_transforms_base::{helper, perf::Check};
 use swc_ecma_transforms_macros::fast_path;
@@ -429,9 +429,28 @@ impl VisitMut for Actual {
             })
         };
 
+        let func_span_lo = {
+            let key_span_lo = match &prop.key {
+                PropName::Ident(ident) => ident.span().lo(),
+                PropName::Str(str) => str.span().lo(),
+                PropName::Num(num) => num.span().lo(),
+                PropName::Computed(computed) => computed.span().lo(),
+                PropName::BigInt(bigint) => bigint.span().lo(),
+            };
+
+            // sub length of "async " from prop's key span
+            key_span_lo - BytePos(6)
+        };
+
+        let func_span = Span::new(
+            func_span_lo,
+            func_span_lo + BytePos(1), // dummy pos
+            SyntaxContext::empty(),
+        );
+
         prop.function = Function {
             params: original_fn_params,
-            span: prop_method_span,
+            span: func_span,
             is_async: false,
             is_generator: false,
             body: Some(BlockStmt {