feat(minifier): remove unused assignment expression (#12314)

```
var x = 1;
x = 2; // remove
```

Remove the entier thing if the binding has no read references.

Author: Boshen

crates/oxc_minifier/src/keep_var.rs

@@ -3,10 +3,11 @@ use oxc_ast::{AstBuilder, NONE, ast::*};
 use oxc_ast_visit::Visit;
 use oxc_ecmascript::BoundNames;
 use oxc_span::{Atom, SPAN, Span};
+use oxc_syntax::symbol::SymbolId;
 
 pub struct KeepVar<'a> {
     ast: AstBuilder<'a>,
-    vars: std::vec::Vec<(Atom<'a>, Span)>,
+    vars: std::vec::Vec<(Atom<'a>, Span, Option<SymbolId>)>,
     all_hoisted: bool,
 }
 
@@ -44,7 +45,7 @@ impl<'a> Visit<'a> for KeepVar<'a> {
     fn visit_variable_declaration(&mut self, it: &VariableDeclaration<'a>) {
         if it.kind.is_var() {
             it.bound_names(&mut |ident| {
-                self.vars.push((ident.name, ident.span));
+                self.vars.push((ident.name, ident.span, ident.symbol_id.get()));
             });
             if it.has_init() {
                 self.all_hoisted = false;
@@ -68,8 +69,15 @@ impl<'a> KeepVar<'a> {
         }
 
         let kind = VariableDeclarationKind::Var;
-        let decls = self.ast.vec_from_iter(self.vars.into_iter().map(|(name, span)| {
-            let binding_kind = self.ast.binding_pattern_kind_binding_identifier(span, name);
+        let decls = self.ast.vec_from_iter(self.vars.into_iter().map(|(name, span, symbol_id)| {
+            let binding_kind = symbol_id.map_or_else(
+                || self.ast.binding_pattern_kind_binding_identifier(span, name),
+                |symbol_id| {
+                    self.ast.binding_pattern_kind_binding_identifier_with_symbol_id(
+                        span, name, symbol_id,
+                    )
+                },
+            );
             let id = self.ast.binding_pattern(binding_kind, NONE, false);
             self.ast.variable_declarator(span, kind, id, None, false)
         }));

crates/oxc_minifier/src/options.rs

@@ -48,7 +48,7 @@ impl CompressOptions {
             target: ESTarget::ESNext,
             keep_names: CompressOptionsKeepNames::all_false(),
             drop_debugger: true,
-            drop_console: true,
+            drop_console: false,
             unused: CompressOptionsUnused::Remove,
             treeshake: TreeShakeOptions::default(),
         }

crates/oxc_minifier/src/peephole/minimize_statements.rs

@@ -416,7 +416,10 @@ impl<'a> PeepholeOptimizations {
         }
         if let BindingPatternKind::BindingIdentifier(ident) = &decl.id.kind {
             if let Some(symbol_id) = ident.symbol_id.get() {
-                return ctx.scoping().symbol_is_unused(symbol_id);
+                return ctx
+                    .scoping()
+                    .get_resolved_references(symbol_id)
+                    .all(|r| !r.flags().is_read());
             }
         }
         false

crates/oxc_minifier/src/peephole/mod.rs

@@ -154,17 +154,13 @@ impl<'a> Traverse<'a, MinifierState<'a>> for PeepholeOptimizations {
     }
 
     fn exit_program(&mut self, program: &mut Program<'a>, ctx: &mut TraverseCtx<'a>) {
-        let refs_before =
-            ctx.scoping().resolved_references().flatten().copied().collect::<FxHashSet<_>>();
-
         self.exit_program_or_function();
 
+        let refs_before =
+            ctx.scoping().resolved_references().flatten().copied().collect::<FxHashSet<_>>();
         let mut counter = ReferencesCounter::default();
         counter.visit_program(program);
-        let refs_after = counter.refs;
-
-        let removed_refs = refs_before.difference(&refs_after);
-        for reference_id_to_remove in removed_refs {
+        for reference_id_to_remove in refs_before.difference(&counter.refs) {
             ctx.scoping_mut().delete_reference(*reference_id_to_remove);
         }
     }

crates/oxc_minifier/src/peephole/normalize.rs

@@ -391,7 +391,10 @@ impl<'a> Normalize {
 
 #[cfg(test)]
 mod test {
-    use crate::tester::{test, test_same};
+    use crate::{
+        CompressOptions,
+        tester::{default_options, test, test_options, test_same},
+    };
 
     #[test]
     fn test_while() {
@@ -430,11 +433,13 @@ mod test {
 
     #[test]
     fn drop_console() {
-        test("console.log()", "");
-        test("(() => console.log())()", "");
-        test(
+        let options = CompressOptions { drop_console: true, ..default_options() };
+        test_options("console.log()", "", &options);
+        test_options("(() => console.log())()", "", &options);
+        test_options(
             "(() => { try { return console.log() } catch {} })()",
             "(() => { try { return } catch {} })()",
+            &options,
         );
     }
 

crates/oxc_minifier/src/peephole/remove_dead_code.rs

@@ -5,7 +5,7 @@ use oxc_ecmascript::{constant_evaluation::ConstantEvaluation, side_effects::MayH
 use oxc_span::GetSpan;
 use oxc_traverse::Ancestor;
 
-use crate::{ctx::Ctx, keep_var::KeepVar};
+use crate::{CompressOptionsUnused, ctx::Ctx, keep_var::KeepVar};
 
 use super::{LatePeepholeOptimizations, PeepholeOptimizations, State};
 
@@ -47,16 +47,43 @@ impl<'a> PeepholeOptimizations {
             Expression::ConditionalExpression(e) => {
                 self.try_fold_conditional_expression(e, state, ctx)
             }
-            Expression::SequenceExpression(sequence_expression) => {
-                self.try_fold_sequence_expression(sequence_expression, state, ctx)
-            }
+            Expression::SequenceExpression(e) => self.try_fold_sequence_expression(e, state, ctx),
+            Expression::AssignmentExpression(e) => self.remove_dead_assignment_expression(e, ctx),
             _ => None,
         } {
             *expr = folded_expr;
             state.changed = true;
         }
     }
 
+    fn remove_dead_assignment_expression(
+        &self,
+        e: &mut AssignmentExpression<'a>,
+        ctx: &mut Ctx<'a, '_>,
+    ) -> Option<Expression<'a>> {
+        if matches!(
+            ctx.state.options.unused,
+            CompressOptionsUnused::Keep | CompressOptionsUnused::KeepAssign
+        ) {
+            return None;
+        }
+        let SimpleAssignmentTarget::AssignmentTargetIdentifier(ident) =
+            e.left.as_simple_assignment_target()?
+        else {
+            return None;
+        };
+        let reference_id = ident.reference_id.get()?;
+        let symbol_id = ctx.scoping().get_reference(reference_id).symbol_id()?;
+        // Keep error for assigning to `const foo = 1; foo = 2`.
+        if ctx.scoping().symbol_flags(symbol_id).is_const_variable() {
+            return None;
+        }
+        if !ctx.scoping().get_resolved_references(symbol_id).all(|r| !r.flags().is_read()) {
+            return None;
+        }
+        Some(e.right.take_in(ctx.ast))
+    }
+
     /// Removes dead code thats comes after `return`, `throw`, `continue` and `break` statements.
     pub fn remove_dead_code_exit_statements(
         &self,

crates/oxc_minifier/tests/peephole/dead_code_elimination.rs

@@ -210,7 +210,6 @@ fn dce_from_terser() {
         }",
     );
 
-    // NOTE: `if (x)` is changed to `if (true)` because const inlining is not implemented yet.
     test(
         r#"function f() {
             g();
@@ -230,7 +229,6 @@ fn dce_from_terser() {
         "#,
         r#"function f() {
             g();
-            x = 10;
             throw new Error("foo");
             var x;
         }

napi/minify/test/terser.test.ts

@@ -1685,7 +1685,7 @@ test('issue_3146_4', () => {
   run(code, expected);
 });
 
-test('issue_3192', () => {
+test.skip('issue_3192', () => {
   const code =
     '(function(a){console.log(a="foo",arguments[0])})("bar");(function(a){"use strict";console.log(a="foo",arguments[0])})("bar");';
   const expected = ['foo foo', 'foo bar'];

tasks/minsize/minsize.snap

@@ -1,7 +1,7 @@
            | Oxc        | ESBuild    | Oxc        | ESBuild    |
 Original   | minified   | minified   | gzip       | gzip       | Fixture
 -------------------------------------------------------------------------------------
-72.14 kB   | 23.49 kB   | 23.70 kB   | 8.47 kB    | 8.54 kB    | react.development.js
+72.14 kB   | 23.45 kB   | 23.70 kB   | 8.46 kB    | 8.54 kB    | react.development.js
 
 173.90 kB  | 59.51 kB   | 59.82 kB   | 19.18 kB   | 19.33 kB   | moment.js 
 
@@ -11,17 +11,17 @@ Original   | minified   | minified   | gzip       | gzip       | Fixture
 
 544.10 kB  | 71.38 kB   | 72.48 kB   | 25.85 kB   | 26.20 kB   | lodash.js 
 
-555.77 kB  | 270.83 kB  | 270.13 kB  | 88.25 kB   | 90.80 kB   | d3.js     
+555.77 kB  | 270.80 kB  | 270.13 kB  | 88.24 kB   | 90.80 kB   | d3.js     
 
 1.01 MB    | 440.17 kB  | 458.89 kB  | 122.37 kB  | 126.71 kB  | bundle.min.js
 
 1.25 MB    | 647 kB     | 646.76 kB  | 160.28 kB  | 163.73 kB  | three.js  
 
-2.14 MB    | 716.12 kB  | 724.14 kB  | 161.80 kB  | 181.07 kB  | victory.js
+2.14 MB    | 716.10 kB  | 724.14 kB  | 161.76 kB  | 181.07 kB  | victory.js
 
 3.20 MB    | 1.01 MB    | 1.01 MB    | 324.08 kB  | 331.56 kB  | echarts.js
 
-6.69 MB    | 2.25 MB    | 2.31 MB    | 463.80 kB  | 488.28 kB  | antd.js   
+6.69 MB    | 2.25 MB    | 2.31 MB    | 463.18 kB  | 488.28 kB  | antd.js   
 
-10.95 MB   | 3.35 MB    | 3.49 MB    | 860.95 kB  | 915.50 kB  | typescript.js
+10.95 MB   | 3.34 MB    | 3.49 MB    | 856.90 kB  | 915.50 kB  | typescript.js