Turbopack: side effect directive (#76876)

Any suggestions regarding the exact naming?

```
"use turbopack no side effects"

export const bar = 123;
```

Author: mischnic

turbopack/crates/turbopack-ecmascript/src/lib.rs

@@ -67,7 +67,7 @@ use turbo_tasks::{
     trace::TraceRawVcs, FxIndexMap, NonLocalValue, ReadRef, ResolvedVc, TaskInput, TryJoinIterExt,
     Value, ValueToString, Vc,
 };
-use turbo_tasks_fs::{rope::Rope, FileJsonContent, FileSystemPath};
+use turbo_tasks_fs::{glob::Glob, rope::Rope, FileJsonContent, FileSystemPath};
 use turbopack_core::{
     asset::{Asset, AssetContent},
     chunk::{
@@ -92,7 +92,7 @@ pub use turbopack_resolve::ecmascript as resolve;
 
 use self::chunk::{EcmascriptChunkItemContent, EcmascriptChunkType, EcmascriptExports};
 use crate::{
-    chunk::EcmascriptChunkPlaceable,
+    chunk::{placeable::is_marked_as_side_effect_free, EcmascriptChunkPlaceable},
     code_gen::CodeGens,
     parse::generate_js_source_map,
     references::{
@@ -646,6 +646,21 @@ impl EcmascriptChunkPlaceable for EcmascriptModuleAsset {
     async fn get_async_module(self: Vc<Self>) -> Result<Vc<OptionAsyncModule>> {
         Ok(*self.analyze().await?.async_module)
     }
+
+    #[turbo_tasks::function]
+    async fn is_marked_as_side_effect_free(
+        self: Vc<Self>,
+        side_effect_free_packages: Vc<Glob>,
+    ) -> Result<Vc<bool>> {
+        // Check package.json first, so that we can skip parsing the module if it's marked that way.
+        let pkg_side_effect_free =
+            is_marked_as_side_effect_free(self.ident().path(), side_effect_free_packages);
+        Ok(if *pkg_side_effect_free.await? {
+            pkg_side_effect_free
+        } else {
+            Vc::cell(self.analyze().await?.has_side_effect_free_directive)
+        })
+    }
 }
 
 #[turbo_tasks::value_impl]

turbopack/crates/turbopack-ecmascript/src/references/mod.rs

@@ -23,6 +23,7 @@ use std::{borrow::Cow, collections::BTreeMap, future::Future, mem::take, ops::De
 use anyhow::{bail, Result};
 use constant_condition::{ConstantConditionCodeGen, ConstantConditionValue};
 use constant_value::ConstantValueCodeGen;
+use either::Either;
 use indexmap::map::Entry;
 use lazy_static::lazy_static;
 use num_traits::Zero;
@@ -42,6 +43,7 @@ use swc_core::{
     },
     ecma::{
         ast::*,
+        utils::IsDirective,
         visit::{
             fields::{AssignExprField, AssignTargetField, SimpleAssignTargetField},
             AstParentKind, AstParentNodeRef, VisitAstPath, VisitWithAstPath,
@@ -169,6 +171,7 @@ pub struct AnalyzeEcmascriptModuleResult {
     pub code_generation: ResolvedVc<CodeGens>,
     pub exports: ResolvedVc<EcmascriptExports>,
     pub async_module: ResolvedVc<OptionAsyncModule>,
+    pub has_side_effect_free_directive: bool,
     /// `true` when the analysis was successful.
     pub successful: bool,
     pub source_map: ResolvedVc<OptionStringifiedSourceMap>,
@@ -221,6 +224,7 @@ pub struct AnalyzeEcmascriptModuleResultBuilder {
     async_module: ResolvedVc<OptionAsyncModule>,
     successful: bool,
     source_map: Option<ResolvedVc<OptionStringifiedSourceMap>>,
+    has_side_effect_free_directive: bool,
 }
 
 impl AnalyzeEcmascriptModuleResultBuilder {
@@ -238,6 +242,7 @@ impl AnalyzeEcmascriptModuleResultBuilder {
             async_module: ResolvedVc::cell(None),
             successful: false,
             source_map: None,
+            has_side_effect_free_directive: false,
         }
     }
 
@@ -297,6 +302,11 @@ impl AnalyzeEcmascriptModuleResultBuilder {
         self.async_module = ResolvedVc::cell(Some(async_module));
     }
 
+    /// Set whether this module is side-efffect free according to a user-provided directive.
+    pub fn set_has_side_effect_free_directive(&mut self, value: bool) {
+        self.has_side_effect_free_directive = value;
+    }
+
     /// Sets whether the analysis was successful.
     pub fn set_successful(&mut self, successful: bool) {
         self.successful = successful;
@@ -411,6 +421,7 @@ impl AnalyzeEcmascriptModuleResultBuilder {
                 code_generation: ResolvedVc::cell(self.code_gens),
                 exports: self.exports.resolved_cell(),
                 async_module: self.async_module,
+                has_side_effect_free_directive: self.has_side_effect_free_directive,
                 successful: self.successful,
                 source_map,
             },
@@ -586,6 +597,33 @@ pub(crate) async fn analyse_ecmascript_module_internal(
         return analysis.build(Default::default(), false).await;
     };
 
+    let has_side_effect_free_directive = match program {
+        Program::Module(module) => Either::Left(
+            module
+                .body
+                .iter()
+                .take_while(|i| match i {
+                    ModuleItem::Stmt(stmt) => stmt.directive_continue(),
+                    ModuleItem::ModuleDecl(_) => false,
+                })
+                .filter_map(|i| i.as_stmt()),
+        ),
+        Program::Script(script) => Either::Right(
+            script
+                .body
+                .iter()
+                .take_while(|stmt| stmt.directive_continue()),
+        ),
+    }
+    .any(|f| match f {
+        Stmt::Expr(ExprStmt { expr, .. }) => match &**expr {
+            Expr::Lit(Lit::Str(Str { value, .. })) => value == "use turbopack no side effects",
+            _ => false,
+        },
+        _ => false,
+    });
+    analysis.set_has_side_effect_free_directive(has_side_effect_free_directive);
+
     let compile_time_info = compile_time_info_for_module_type(
         *raw_module.compile_time_info,
         eval_context.is_esm(specified_type),

turbopack/crates/turbopack-tests/tests/execution/turbopack/side-effects-optimization/directive/input/index.js

@@ -0,0 +1,10 @@
+import { foo } from './lib/index.js'
+
+it('should respect side effects directive', () => {
+  expect(foo).toBe(789)
+
+  const modules = Object.keys(__turbopack_modules__)
+  expect(modules).toContainEqual(expect.stringContaining('input/lib/foo'))
+  expect(modules).not.toContainEqual(expect.stringContaining('input/lib/index'))
+  expect(modules).not.toContainEqual(expect.stringContaining('input/lib/bar'))
+})

turbopack/crates/turbopack-tests/tests/execution/turbopack/side-effects-optimization/directive/input/lib/bar.js

@@ -0,0 +1,3 @@
+"use turbopack no side effects"
+
+export const bar = 123;

turbopack/crates/turbopack-tests/tests/execution/turbopack/side-effects-optimization/directive/input/lib/foo.js

@@ -0,0 +1,3 @@
+"use turbopack no side effects"
+
+export const foo = 789;

turbopack/crates/turbopack-tests/tests/execution/turbopack/side-effects-optimization/directive/input/lib/index.js

@@ -0,0 +1,4 @@
+"use turbopack no side effects"
+
+export {foo} from "./foo";
+export {bar} from "./bar";

turbopack/crates/turbopack-tests/tests/execution/turbopack/side-effects-optimization/directive/options.json

@@ -0,0 +1,3 @@
+{
+  "treeShakingMode": "reexports-only"
+}