feat(es/minifier): Implement more rules for dropping unused assignemnts (#4171)

crates/swc/tests/tsc-references/ES5For-of16_es2015.2.minified.js

@@ -1 +1 @@
-for (let v of [])for (let v1 of [])v1++;
+for (let v of [])for (let v1 of []);

crates/swc/tests/tsc-references/ES5For-of16_es5.2.minified.js

@@ -4,10 +4,7 @@ try {
         _step.value;
         var _iteratorNormalCompletion1 = !0, _didIteratorError1 = !1, _iteratorError1 = void 0;
         try {
-            for(var _step1, _iterator1 = [][Symbol.iterator](); !(_iteratorNormalCompletion1 = (_step1 = _iterator1.next()).done); _iteratorNormalCompletion1 = !0){
-                var v = _step1.value;
-                v++;
-            }
+            for(var _step1, _iterator1 = [][Symbol.iterator](); !(_iteratorNormalCompletion1 = (_step1 = _iterator1.next()).done); _iteratorNormalCompletion1 = !0)_step1.value;
         } catch (err) {
             _didIteratorError1 = !0, _iteratorError1 = err;
         } finally{

crates/swc/tests/tsc-references/compoundAdditionAssignmentLHSCanBeAssigned_es2015.2.minified.js

@@ -1,4 +1,4 @@
-var E, a, b, x1, x2, x3, x4, x5, x6, x7;
+var E, a, b, x1, x2, x3, x4, x6;
 !function(E) {
     E[E.a = 0] = "a", E[E.b = 1] = "b";
-}(E || (E = {})), x1 += a, x1 += b, x1 += !0, x1 += 0, x1 += '', x1 += E.a, x1 += {}, x1 += null, x1 += void 0, x2 += a, x2 += b, x2 += !0, x2 += 0, x2 += '', x2 += E.a, x2 += {}, x2 += null, x2 += void 0, x3 += a, x3 += 0, x3 += E.a, x3 += null, x3 += void 0, x4 += a, x4 += 0, x4 += E.a, x4 += null, x4 += void 0, x5 += a, x6 += a, x6 += '', x7 += a;
+}(E || (E = {})), x1 += a, x1 += b, x1 += !0, x1 += 0, x1 += '', x1 += E.a, x1 += {}, x1 += null, x1 += void 0, x2 += a, x2 += b, x2 += !0, x2 += 0, x2 += '', x2 += E.a, x2 += {}, x2 += null, x2 += void 0, x3 += a, x3 += 0, x3 += E.a, x3 += null, x3 += void 0, x4 += a, x4 += 0, x4 += E.a, x4 += null, x4 += void 0, x6 += a, x6 += '';

crates/swc/tests/tsc-references/compoundAdditionAssignmentLHSCanBeAssigned_es5.2.minified.js

@@ -1,4 +1,4 @@
-var E, a, b, x1, x2, x3, x4, x5, x6, x7;
+var E, a, b, x1, x2, x3, x4, x6;
 !function(E) {
     E[E.a = 0] = "a", E[E.b = 1] = "b";
-}(E || (E = {})), x1 += a, x1 += b, x1 += !0, x1 += 0, x1 += "", x1 += E.a, x1 += {}, x1 += null, x1 += void 0, x2 += a, x2 += b, x2 += !0, x2 += 0, x2 += "", x2 += E.a, x2 += {}, x2 += null, x2 += void 0, x3 += a, x3 += 0, x3 += E.a, x3 += null, x3 += void 0, x4 += a, x4 += 0, x4 += E.a, x4 += null, x4 += void 0, x5 += a, x6 += a, x6 += "", x7 += a;
+}(E || (E = {})), x1 += a, x1 += b, x1 += !0, x1 += 0, x1 += "", x1 += E.a, x1 += {}, x1 += null, x1 += void 0, x2 += a, x2 += b, x2 += !0, x2 += 0, x2 += "", x2 += E.a, x2 += {}, x2 += null, x2 += void 0, x3 += a, x3 += 0, x3 += E.a, x3 += null, x3 += void 0, x4 += a, x4 += 0, x4 += E.a, x4 += null, x4 += void 0, x6 += a, x6 += "";

crates/swc/tests/tsc-references/compoundAdditionAssignmentLHSCannotBeAssigned_es2015.2.minified.js

@@ -1,4 +1,4 @@
-var E, x1, x2, x3, x4, x5;
+var E;
 !function(E) {
     E[E.a = 0] = "a", E[E.b = 1] = "b", E[E.c = 2] = "c";
-}(E || (E = {})), x1 += '', x2 += '', x3 += '', x4 += '', x5 += '';
+}(E || (E = {}));

crates/swc/tests/tsc-references/compoundAdditionAssignmentLHSCannotBeAssigned_es5.2.minified.js

@@ -1,4 +1,4 @@
-var E, x1, x2, x3, x4, x5;
+var E;
 !function(E) {
     E[E.a = 0] = "a", E[E.b = 1] = "b", E[E.c = 2] = "c";
-}(E || (E = {})), x1 += "", x2 += "", x3 += "", x4 += "", x5 += "";
+}(E || (E = {}));

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

@@ -1,17 +1,14 @@
 import * as a from "@swc/helpers";
 Promise.all(assignAll).then(function() {
-    var b = a.asyncToGenerator(function*(e) {
-        let b = 'DELETE FROM "TABLE" WHERE "UUID" IN ( ';
-        for(let c in obj){
-            let a = obj[c];
-            b += `'${a.id}', `;
-            let d = yield listOfUser(a.id);
-            d.forEach((b)=>{
+    var b = a.asyncToGenerator(function*(c) {
+        for(let b in obj){
+            let a = obj[b];
+            `'${a.id}', `, (yield listOfUser(a.id)).forEach((b)=>{
                 insertQuery += `INSERT INTO "TABLE"("UUID", id, other_ids_here) VALUES ('${uuidv4()}', '${a.id}', now());`;
             });
         }
     });
-    return function(e) {
+    return function(c) {
         return b.apply(this, arguments);
     };
 }());

crates/swc_ecma_minifier/src/analyzer/mod.rs

@@ -76,6 +76,9 @@ pub(crate) struct VarUsageInfo {
 
     pub assign_count: usize,
     pub mutation_by_call_count: usize,
+    /// ## Things to note
+    ///
+    /// - Update is counted as usage
     pub usage_count: usize,
 
     /// The variable itself is modified.

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

@@ -1,7 +1,8 @@
 use std::{collections::HashMap, mem::swap};
 
+use rustc_hash::FxHashMap;
 use swc_atoms::js_word;
-use swc_common::{collections::AHashMap, pass::Either, util::take::Take, Spanned, DUMMY_SP};
+use swc_common::{pass::Either, util::take::Take, Spanned, DUMMY_SP};
 use swc_ecma_ast::*;
 use swc_ecma_utils::{contains_arguments, ident::IdentLike, undefined, ExprFactory, Id};
 use swc_ecma_visit::VisitMutWith;
@@ -246,8 +247,8 @@ where
         }
     }
 
-    #[cfg_attr(feature = "debug", tracing::instrument(skip(self, n, vars)))]
-    pub(super) fn inline_vars_in_node<N>(&mut self, n: &mut N, vars: AHashMap<Id, Box<Expr>>)
+    #[cfg_attr(feature = "debug", tracing::instrument(skip_all))]
+    pub(super) fn inline_vars_in_node<N>(&mut self, n: &mut N, vars: FxHashMap<Id, Box<Expr>>)
     where
         N: VisitMutWith<MultiReplacer>,
     {

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

@@ -195,7 +195,7 @@ struct Optimizer<'a, M> {
     /// Used for inlining.
     lits: AHashMap<Id, Box<Expr>>,
 
-    vars_for_inlining: AHashMap<Id, Box<Expr>>,
+    vars_for_inlining: FxHashMap<Id, Box<Expr>>,
 
     vars_for_prop_hoisting: AHashMap<Id, Box<Expr>>,
     /// Used for `hoist_props`.
@@ -703,6 +703,10 @@ where
 
         self.compress_cond_to_logical_ignoring_return_value(e);
 
+        self.drop_unused_update(e);
+
+        self.drop_unused_op_assign(e);
+
         match e {
             Expr::This(_) | Expr::Invalid(_) | Expr::Lit(..) => {
                 if cfg!(feature = "debug") {

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

@@ -508,16 +508,39 @@ where
         }
     }
 
+    /// This should be only called from ignore_return_value
     #[cfg_attr(feature = "debug", tracing::instrument(skip_all))]
-    pub(super) fn drop_unused_assignments(&mut self, e: &mut Expr) {
-        let assign = match e {
-            Expr::Assign(e) => e,
+    pub(super) fn drop_unused_update(&mut self, e: &mut Expr) {
+        if !self.options.unused {
+            return;
+        }
+
+        let update = match e {
+            Expr::Update(u) => u,
             _ => return,
         };
 
-        let has_mark = assign.span.has_mark(self.marks.non_top_level);
+        if let Expr::Ident(arg) = &*update.arg {
+            if let Some(var) = self.data.vars.get(&arg.to_id()) {
+                // Update is counted as usage
+                if var.declared && var.is_fn_local && var.usage_count == 1 {
+                    self.changed = true;
+                    tracing::debug!(
+                        "unused: Dropping an update '{}{:?}' because it is not used",
+                        arg.sym,
+                        arg.span.ctxt
+                    );
+                    // This will remove the update.
+                    e.take();
+                }
+            }
+        }
+    }
 
-        if !has_mark && !self.options.unused {
+    /// This should be only called from ignore_return_value
+    #[cfg_attr(feature = "debug", tracing::instrument(skip_all))]
+    pub(super) fn drop_unused_op_assign(&mut self, e: &mut Expr) {
+        if !self.options.unused {
             return;
         }
 
@@ -529,6 +552,58 @@ where
             return;
         }
 
+        let assign = match e {
+            Expr::Assign(AssignExpr { op: op!("="), .. }) => return,
+            // RHS may not be evaluated
+            Expr::Assign(AssignExpr {
+                op: op!("&&=") | op!("||=") | op!("&&="),
+                ..
+            }) => return,
+            Expr::Assign(e) => e,
+            _ => return,
+        };
+        assign.left.map_with_mut(|left| left.normalize_ident());
+
+        if let PatOrExpr::Pat(p) = &assign.left {
+            if let Pat::Ident(left) = &**p {
+                if let Some(var) = self.data.vars.get(&left.to_id()) {
+                    // TODO: We don't need fn_local check
+                    if var.declared && var.is_fn_local && var.usage_count == 1 {
+                        self.changed = true;
+                        tracing::debug!(
+                            "unused: Dropping an op-assign '{}{:?}' because it is not used",
+                            left.id.sym,
+                            left.id.span.ctxt
+                        );
+                        // This will remove the op-assign.
+                        *e = *assign.right.take();
+                    }
+                }
+            }
+        }
+    }
+
+    #[cfg_attr(feature = "debug", tracing::instrument(skip_all))]
+    pub(super) fn drop_unused_assignments(&mut self, e: &mut Expr) {
+        if self.ctx.is_delete_arg {
+            return;
+        }
+
+        if self.data.top.has_eval_call || self.data.top.has_with_stmt {
+            return;
+        }
+
+        let assign = match e {
+            Expr::Assign(e) => e,
+            _ => return,
+        };
+
+        let has_mark = assign.span.has_mark(self.marks.non_top_level);
+
+        if !has_mark && !self.options.unused {
+            return;
+        }
+
         let used_arguments = self
             .data
             .scopes

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

@@ -1,7 +1,8 @@
 use std::ops::{Deref, DerefMut};
 
+use rustc_hash::FxHashMap;
 use swc_atoms::JsWord;
-use swc_common::{collections::AHashMap, Span};
+use swc_common::Span;
 use swc_ecma_ast::*;
 use swc_ecma_utils::{ident::IdentLike, prop_name_eq, ExprExt, Id};
 use swc_ecma_visit::{noop_visit_mut_type, VisitMut, VisitMutWith};
@@ -168,7 +169,7 @@ pub(crate) fn is_valid_for_lhs(e: &Expr) -> bool {
 }
 
 pub(crate) struct MultiReplacer {
-    pub vars: AHashMap<Id, Box<Expr>>,
+    pub vars: FxHashMap<Id, Box<Expr>>,
     pub changed: bool,
 }
 

crates/swc_ecma_minifier/tests/TODO.txt

@@ -174,7 +174,6 @@ drop_unused/issue_2105_2/input.js
 drop_unused/issue_2136_2/input.js
 drop_unused/issue_2136_3/input.js
 drop_unused/issue_2163/input.js
-drop_unused/issue_2226_1/input.js
 drop_unused/issue_2226_2/input.js
 drop_unused/issue_2226_3/input.js
 drop_unused/issue_2288/input.js

crates/swc_ecma_minifier/tests/golden.txt

@@ -381,6 +381,7 @@ drop_unused/issue_1656/input.js
 drop_unused/issue_1715_1/input.js
 drop_unused/issue_1715_2/input.js
 drop_unused/issue_2136_1/input.js
+drop_unused/issue_2226_1/input.js
 drop_unused/issue_2995/input.js
 drop_unused/issue_3146_1/input.js
 drop_unused/issue_3146_2/input.js

crates/swc_ecma_minifier/tests/terser/compress/drop-unused/issue_2226_1/output.js

@@ -6,7 +6,6 @@ function f2(a) {
     b;
 }
 function f3(a) {
-    0;
 }
 function f4() {
     var a = b;

crates/swc_plugin/tests/js.rs

@@ -1,11 +0,0 @@
-//! Ensure that wrong macro definitions are caught by swc monorepo.
-
-use swc_ecma_visit::Fold;
-
-fn drop_console(_: ()) -> impl Fold {
-    DropConsole
-}
-
-struct DropConsole;
-
-impl Fold for DropConsole {}

crates/wasm/src/lib.rs

@@ -1,3 +1,5 @@
+#![allow(clippy::unused_unit)]
+
 use std::sync::Arc;
 
 use anyhow::{Context, Error};
@@ -8,7 +10,7 @@ use swc::{
 };
 use swc_common::{comments::Comments, FileName, FilePathMapping, SourceMap};
 use swc_ecmascript::ast::{EsVersion, Program};
-use wasm_bindgen::{prelude::*, JsCast};
+use wasm_bindgen::prelude::*;
 
 fn convert_err(err: Error) -> JsValue {
     format!("{:?}", err).into()
@@ -156,6 +158,7 @@ pub fn transform_sync(
     {
         if experimental_plugin_bytes_resolver.is_object() {
             use js_sys::{Array, Object, Uint8Array};
+            use wasm_bindgen::JsCast;
 
             // TODO: This is probably very inefficient, including each transform
             // deserializes plugin bytes.