fix(es/isolated-dts): Preserve comments (#9572)

**Related issue:**

 - Closes #9550

Author: kdy1

.changeset/itchy-socks-marry.md

@@ -0,0 +1,6 @@
+---
+swc_core: patch
+swc: patch
+---
+
+fix(es/isolated-dts): Preserve comments

crates/swc/src/lib.rs

@@ -113,6 +113,7 @@ pub extern crate swc_atoms as atoms;
 extern crate swc_common as common;
 
 use std::{
+    cell::RefCell,
     fs::{read_to_string, File},
     io::ErrorKind,
     path::{Path, PathBuf},
@@ -136,7 +137,7 @@ use swc_common::{
 pub use swc_compiler_base::{PrintArgs, TransformOutput};
 pub use swc_config::config_types::{BoolConfig, BoolOr, BoolOrDataConfig};
 use swc_ecma_ast::{EsVersion, Program};
-use swc_ecma_codegen::{to_code, Node};
+use swc_ecma_codegen::{to_code_with_comments, Node};
 use swc_ecma_loader::resolvers::{
     lru::CachingResolver, node::NodeModulesResolver, tsc::TsConfigResolver,
 };
@@ -724,7 +725,7 @@ impl Compiler {
                 None
             };
 
-            self.apply_transforms(handler, fm.clone(), orig.as_ref(), config)
+            self.apply_transforms(handler, comments.clone(), fm.clone(), orig.as_ref(), config)
         })
     }
 
@@ -944,6 +945,7 @@ impl Compiler {
     fn apply_transforms(
         &self,
         handler: &Handler,
+        comments: SingleThreadedComments,
         fm: Arc<SourceFile>,
         orig: Option<&sourcemap::SourceMap>,
         config: BuiltInput<impl swc_ecma_visit::Fold>,
@@ -971,6 +973,12 @@ impl Compiler {
             };
 
             let dts_code = if emit_dts && program.is_module() {
+                let (leading, trailing) = comments.borrow_all();
+
+                let leading = std::rc::Rc::new(RefCell::new(leading.clone()));
+                let trailing = std::rc::Rc::new(RefCell::new(trailing.clone()));
+
+                let comments = SingleThreadedComments::from_leading_and_trailing(leading, trailing);
                 let mut checker = FastDts::new(fm.name.clone());
                 let mut module = program.clone().expect_module();
 
@@ -983,7 +991,7 @@ impl Compiler {
                         .struct_span_err(range.span, &issue.to_string())
                         .emit();
                 }
-                let dts_code = to_code(&module);
+                let dts_code = to_code_with_comments(Some(&comments), &module);
                 Some(dts_code)
             } else {
                 None

crates/swc/tests/ts-isolated-declaration/issues/input/.swcrc

@@ -0,0 +1,11 @@
+{
+    "jsc": {
+        "parser": {
+            "syntax": "typescript"
+        },
+        "experimental": {
+            "emitIsolatedDts": true
+        }
+    },
+    "isModule": true
+}

crates/swc/tests/ts-isolated-declaration/issues/input/swc-issue-9550.ts

@@ -0,0 +1,9 @@
+/**
+ * Sample JSDoc that I wish was in the swc.transform output
+ * @param a - string param
+ * @param b - number param
+ * @returns - object with a and b
+ */
+function sampleFunc(a: string, b: number) {
+    return { a, b };
+}

crates/swc/tests/ts-isolated-declaration/issues/output/swc-issue-9550.d.ts

@@ -0,0 +1,6 @@
+/**
+ * Sample JSDoc that I wish was in the swc.transform output
+ * @param a - string param
+ * @param b - number param
+ * @returns - object with a and b
+ */ declare function sampleFunc(a: string, b: number);

crates/swc/tests/ts-isolated-declaration/issues/output/swc-issue-9550.ts

@@ -0,0 +1,11 @@
+/**
+ * Sample JSDoc that I wish was in the swc.transform output
+ * @param a - string param
+ * @param b - number param
+ * @returns - object with a and b
+ */ function sampleFunc(a, b) {
+    return {
+        a: a,
+        b: b
+    };
+}

crates/swc/tests/ts-isolated-declaration/oxc/output/async-function.d.ts

@@ -1,4 +1,7 @@
+// Correct
 declare async function asyncFunctionGood(): Promise<number>;
 declare const asyncFunctionGoo2: () => Promise<number>;
+// Need to explicit return type for async functions
+// Incorrect
 declare async function asyncFunction();
 declare const asyncFunction2: () => any;

crates/swc/tests/ts-isolated-declaration/oxc/output/function-parameters.d.ts

@@ -1,10 +1,12 @@
+// Correct
 export declare function fnDeclGood(p?: T, rParam?: string): void;
 export declare function fnDeclGood2(p?: T, rParam?: number): void;
 export declare function fooGood([a, b]?: any[]): number;
 export declare const fooGood2: (_dts_1: {
     a: number;
     b: number;
 }) => number;
+// Incorrect
 export declare function fnDeclBad<T>(p?: T, rParam?: T, r2: T): void;
 export declare function fnDeclBad2<T>(p?: T, r2: T): void;
 export declare function fnDeclBad3<T>(p?: T, rParam?: T, r2: T): void;

crates/swc/tests/ts-isolated-declaration/oxc/output/generator.d.ts

@@ -1,2 +1,5 @@
+// Correct
 declare function* generatorGood(): Generator<number>;
+// Need to explicit return type for async functions
+// Incorrect
 declare function* generatorGoodBad();

crates/swc/tests/ts-isolated-declaration/oxc/output/infer-expression.d.ts

@@ -1,3 +1,4 @@
+// ParenthesizedExpression
 declare const n: any;
 declare const s: any;
 declare const t: any;

crates/swc/tests/ts-isolated-declaration/oxc/output/infer-return-type.d.ts

@@ -1,4 +1,7 @@
 declare function foo();
+// inferred type is number
 declare function bar();
+// inferred type is number | undefined
 declare function baz();
+// We can't infer return type if there are multiple return statements with different types
 declare function qux();

crates/swc/tests/ts-isolated-declaration/oxc/output/non-exported-binding-elements.d.ts

@@ -1,5 +1,7 @@
+// Correct
 declare const [A, B];
 export declare function foo(): number;
+// Incorrect
 declare const { c, d };
 declare const [e];
 export { c, d, e };