feat(swc): Add comment preservation

crates/swc/src/config/mod.rs

@@ -65,6 +65,7 @@ use swc_ecma_visit::{Fold, VisitMutWith};
 use self::util::BoolOrObject;
 use crate::{
     builder::PassBuilder,
+    dropped_comments_preserver::dropped_comments_preserver,
     plugin::{PluginConfig, PluginContext},
     SwcImportResolver,
 };
@@ -296,6 +297,7 @@ impl Options {
             minify: mut js_minify,
             experimental,
             lints,
+            preserve_all_comments,
             ..
         } = config.jsc;
 
@@ -368,7 +370,11 @@ impl Options {
 
         let regenerator = transform.regenerator.clone();
 
-        let preserve_comments = js_minify.as_ref().map(|v| v.format.comments.clone());
+        let preserve_comments = if preserve_all_comments {
+            Some(BoolOrObject::from(true))
+        } else {
+            js_minify.as_ref().map(|v| v.format.comments.clone())
+        };
 
         if syntax.typescript() {
             transform.legacy_decorator = true;
@@ -498,7 +504,11 @@ impl Options {
                 syntax.jsx()
             ),
             pass,
-            Optional::new(jest::jest(), transform.hidden.jest)
+            Optional::new(jest::jest(), transform.hidden.jest),
+            Optional::new(
+                dropped_comments_preserver(comments.cloned()),
+                preserve_all_comments
+            ),
         );
 
         Ok(BuiltInput {
@@ -1031,6 +1041,9 @@ pub struct JscConfig {
 
     #[serde(default)]
     pub lints: LintConfig,
+
+    #[serde(default)]
+    pub preserve_all_comments: bool,
 }
 
 /// `jsc.experimental` in `.swcrc`
@@ -1567,6 +1580,8 @@ impl Merge for JscConfig {
         self.paths.merge(&from.paths);
         self.minify.merge(&from.minify);
         self.experimental.merge(&from.experimental);
+        self.preserve_all_comments
+            .merge(&from.preserve_all_comments)
     }
 }
 

crates/swc/src/dropped_comments_preserver.rs

@@ -0,0 +1,138 @@
+use swc_common::{
+    comments::{Comment, Comments, SingleThreadedComments},
+    BytePos, Span, DUMMY_SP,
+};
+use swc_ecma_ast::{Module, Script};
+use swc_ecma_visit::{as_folder, noop_visit_mut_type, Fold, VisitMut, VisitMutWith};
+
+/// Preserves comments that would otherwise be dropped.
+///
+/// If during compilation an ast node associated with
+/// a comment is dropped, the comment will not appear in the final emitted
+/// output. This can create problems in the JavaScript ecosystem, particularly
+/// around instanbul coverage and other tooling that relies on comment
+/// directives.
+///
+/// This transformer shifts orphaned comments to the next closest known span
+/// while making a best-effort to preserve the "general orientation" of
+/// comments.
+
+pub fn dropped_comments_preserver(
+    comments: Option<SingleThreadedComments>,
+) -> impl Fold + VisitMut {
+    as_folder(DroppedCommentsPreserver {
+        comments,
+        is_first_span: true,
+        known_spans: Vec::new(),
+    })
+}
+
+struct DroppedCommentsPreserver {
+    comments: Option<SingleThreadedComments>,
+    is_first_span: bool,
+    known_spans: Vec<Span>,
+}
+
+type CommentEntries = Vec<(BytePos, Vec<Comment>)>;
+
+impl VisitMut for DroppedCommentsPreserver {
+    noop_visit_mut_type!();
+
+    fn visit_mut_module(&mut self, module: &mut Module) {
+        module.visit_mut_children_with(self);
+        self.shift_comments_to_known_spans();
+    }
+
+    fn visit_mut_script(&mut self, script: &mut Script) {
+        script.visit_mut_children_with(self);
+        self.shift_comments_to_known_spans();
+    }
+
+    fn visit_mut_span(&mut self, span: &mut Span) {
+        if span.is_dummy() || self.is_first_span {
+            self.is_first_span = false;
+            return;
+        }
+
+        self.known_spans.push(*span);
+        span.visit_mut_children_with(self)
+    }
+}
+
+impl DroppedCommentsPreserver {
+    fn shift_comments_to_known_spans(&self) {
+        if let Some(comments) = &self.comments {
+            let trailing_comments = self.shift_leading_comments(comments);
+
+            self.shift_trailing_comments(trailing_comments);
+        }
+    }
+
+    /// We'll be shifting all comments to known span positions, so drain the
+    /// current comments first to limit the amount of look ups needed into
+    /// the hashmaps.
+    ///
+    /// This way, we only need to take the comments once, and then add them back
+    /// once.
+    fn collect_existing_comments(&self, comments: &SingleThreadedComments) -> CommentEntries {
+        let (mut leading_comments, mut trailing_comments) = comments.borrow_all_mut();
+        let mut existing_comments: CommentEntries = leading_comments
+            .drain()
+            .chain(trailing_comments.drain())
+            .collect();
+
+        existing_comments.sort_by(|(bp_a, _), (bp_b, _)| bp_a.cmp(bp_b));
+
+        existing_comments
+    }
+
+    /// Shift all comments to known leading positions.
+    /// This prevents trailing comments from ending up associated with
+    /// nodes that will not emit trailing comments, while
+    /// preserving any comments that might show up after all code positions.
+    ///
+    /// This maintains the highest fidelity between existing comment positions
+    /// of pre and post compiled code.
+    fn shift_leading_comments(&self, comments: &SingleThreadedComments) -> CommentEntries {
+        let mut existing_comments = self.collect_existing_comments(comments);
+
+        for span in self.known_spans.iter() {
+            let (comments_to_move, next_byte_positions): (CommentEntries, CommentEntries) =
+                existing_comments
+                    .drain(..)
+                    .partition(|(bp, _)| *bp <= span.lo);
+
+            existing_comments.extend(next_byte_positions);
+
+            let collected_comments = comments_to_move.into_iter().flat_map(|(_, c)| c).collect();
+
+            self.comments
+                .add_leading_comments(span.lo, collected_comments)
+        }
+
+        existing_comments
+    }
+
+    /// These comments trail all known span lo byte positions.
+    /// Therefore, by shifting them to trail the highest known hi position, we
+    /// ensure that any remaining trailing comments are emitted in a
+    /// similar location
+    fn shift_trailing_comments(&self, remaining_comment_entries: CommentEntries) {
+        let last_trailing = self
+            .known_spans
+            .iter()
+            .copied()
+            .fold(
+                DUMMY_SP,
+                |acc, span| if span.hi > acc.hi { span } else { acc },
+            );
+
+        self.comments.add_trailing_comments(
+            last_trailing.hi,
+            remaining_comment_entries
+                .into_iter()
+                .flat_map(|(_, c)| c)
+                .collect(),
+        );
+    }
+}

crates/swc/src/lib.rs

@@ -170,6 +170,7 @@ use crate::config::{
 
 mod builder;
 pub mod config;
+mod dropped_comments_preserver;
 mod plugin;
 pub mod resolver {
     use std::path::PathBuf;

crates/swc/tests/fixture/issue-2964.case-1/input/.swcrc

@@ -0,0 +1,16 @@
+{
+  "jsc": {
+    "parser": {
+      "syntax": "typescript",
+      "tsx": false,
+      "decorators": false,
+      "dynamicImport": false
+    },
+    "transform": null,
+    "target": "es5",
+    "loose": false,
+    "externalHelpers": false,
+    "keepClassNames": false,
+    "preserveAllComments": true
+  }
+}

crates/swc/tests/fixture/issue-2964.case-1/input/index.ts

@@ -0,0 +1,11 @@
+/* istanbul ignore next */
+type Z = number;
+// preserved comment
+const x = 1;
+
+// Stacked Comment
+// Another comment
+type Y = string;
+const a = "";
+
+// trailing comment

crates/swc/tests/fixture/issue-2964.case-1/output/index.ts

@@ -0,0 +1,5 @@
+/* istanbul ignore next */ // preserved comment
+var x = 1;
+// Stacked Comment
+// Another comment
+var a = ""; // trailing comment

crates/swc/tests/fixture/issue-2964.case-2/input/.swcrc

@@ -0,0 +1,33 @@
+{
+  "sourceMaps": false,
+  "module": {
+    "type": "commonjs"
+  },
+  "jsc": {
+    "parser": {
+      "syntax": "typescript",
+      "tsx": true,
+      "dynamicImport": true,
+      "decorators": false
+    },
+    "transform": {
+      "react": {
+        "pragma": "React.createElement",
+        "pragmaFrag": "React.Fragment",
+        "throwIfNamespace": true,
+        "development": false,
+        "useBuiltins": false
+      }
+    },
+    "target": "es2015",
+    "loose": false,
+    "externalHelpers": false,
+    "keepClassNames": false,
+    "minify": {
+      "compress": false,
+      "mangle": false
+    },
+    "preserveAllComments": true
+  },
+  "minify": false
+}

crates/swc/tests/fixture/issue-2964.case-2/input/index.ts

@@ -0,0 +1,12 @@
+//top comment
+export const noop = () => {};
+/* istanbul ignore next */
+export const badIstanbul = (test: Record<string, unknown>) => {
+    const { value, ...pixelParams } = test;
+    console.log('fail');
+};
+
+/* istanbul ignore next: UI-5137 */
+export const downloadDocument = (): void => {
+    console.log('fail');
+};

crates/swc/tests/fixture/issue-2964.case-2/output/index.ts

@@ -0,0 +1,20 @@
+"use strict";
+Object.defineProperty(exports, "__esModule", {
+    value: true
+});
+exports.downloadDocument = exports.badIstanbul = exports.noop = void 0;
+var swcHelpers = require("@swc/helpers");
+//top comment
+const noop = ()=>{};
+exports.noop = noop;
+var /* istanbul ignore next */ badIstanbul = (test)=>{
+    const { value  } = test, pixelParams = swcHelpers.objectWithoutProperties(test, [
+        "value"
+    ]);
+    console.log('fail');
+};
+exports.badIstanbul = badIstanbul;
+/* istanbul ignore next: UI-5137 */ const downloadDocument = ()=>{
+    console.log('fail');
+};
+exports.downloadDocument = downloadDocument;

crates/swc/tests/fixture/issue-2964.case-3/input/.swcrc

@@ -0,0 +1,32 @@
+{
+  "sourceMaps": false,
+  "module": {
+    "type": "commonjs"
+  },
+  "jsc": {
+    "parser": {
+      "syntax": "typescript",
+      "tsx": false,
+      "dynamicImport": true,
+      "decorators": false
+    },
+    "transform": {
+      "react": {
+        "pragma": "React.createElement",
+        "pragmaFrag": "React.Fragment",
+        "throwIfNamespace": true,
+        "development": false,
+        "useBuiltins": false
+      },
+      "hidden": {
+        "jest": true
+      }
+    },
+    "target": "es2015",
+    "loose": false,
+    "externalHelpers": false,
+    "keepClassNames": false,
+    "preserveAllComments": true
+  },
+  "minify": false
+}

crates/swc/tests/fixture/issue-2964.case-3/input/index.ts

@@ -0,0 +1,7 @@
+// single line comment
+const x = ({y, ...rest}: /*todo: refactor any type*/ any) => {
+    return {
+        y, // another comment
+        z: rest.z // final comment
+    }
+}

crates/swc/tests/fixture/issue-2964.case-3/output/index.ts

@@ -0,0 +1,14 @@
+"use strict";
+var swcHelpers = require("@swc/helpers");
+// single line comment
+const x = (_param)=>/*todo: refactor any type*/ {
+    var { y  } = _param, rest = swcHelpers.objectWithoutProperties(_param, [
+        "y"
+    ]);
+    return {
+        y,
+        // another comment
+        z: rest.z
+    };
+} // final comment
+;

crates/swc/tests/rust_api.rs

@@ -149,6 +149,7 @@ fn shopify_2_same_opt() {
                     experimental: Default::default(),
                     lints: Default::default(),
                     assumptions: Default::default(),
+                    preserve_all_comments: false,
                 },
                 module: None,
                 minify: false,