feat(minifier): add RemoveUnusedCode (#8210)

Cargo.lock

@@ -1777,6 +1777,7 @@ dependencies = [
  "oxc_syntax",
  "oxc_traverse",
  "pico-args",
+ "rustc-hash",
 ]
 
 [[package]]

crates/oxc_minifier/Cargo.toml

@@ -33,6 +33,7 @@ oxc_syntax = { workspace = true }
 oxc_traverse = { workspace = true }
 
 cow-utils = { workspace = true }
+rustc-hash = { workspace = true }
 
 [dev-dependencies]
 oxc_parser = { workspace = true }

crates/oxc_minifier/src/ast_passes/mod.rs

@@ -12,6 +12,7 @@ mod peephole_remove_dead_code;
 mod peephole_replace_known_methods;
 mod peephole_substitute_alternate_syntax;
 mod remove_syntax;
+mod remove_unused_code;
 mod statement_fusion;
 
 pub use collapse_variable_declarations::CollapseVariableDeclarations;
@@ -24,6 +25,7 @@ pub use peephole_remove_dead_code::PeepholeRemoveDeadCode;
 pub use peephole_replace_known_methods::PeepholeReplaceKnownMethods;
 pub use peephole_substitute_alternate_syntax::PeepholeSubstituteAlternateSyntax;
 pub use remove_syntax::RemoveSyntax;
+pub use remove_unused_code::RemoveUnusedCode;
 pub use statement_fusion::StatementFusion;
 
 use crate::CompressOptions;
@@ -32,7 +34,6 @@ pub trait CompressorPass<'a>: Traverse<'a> {
     fn build(&mut self, program: &mut Program<'a>, ctx: &mut ReusableTraverseCtx<'a>);
 }
 
-// See `latePeepholeOptimizations`
 pub struct PeepholeOptimizations {
     x0_statement_fusion: StatementFusion,
     x1_minimize_exit_points: MinimizeExitPoints,

crates/oxc_minifier/src/ast_passes/remove_unused_code.rs

@@ -0,0 +1,90 @@
+use oxc_allocator::Vec as ArenaVec;
+use oxc_ast::ast::*;
+use oxc_syntax::symbol::SymbolId;
+use oxc_traverse::{traverse_mut_with_ctx, ReusableTraverseCtx, Traverse, TraverseCtx};
+use rustc_hash::FxHashSet;
+
+use crate::CompressorPass;
+
+/// Remove Unused Code
+///
+/// <https://github.com/google/closure-compiler/blob/v20240609/src/com/google/javascript/jscomp/RemoveUnusedCode.java>
+pub struct RemoveUnusedCode {
+    pub(crate) changed: bool,
+
+    symbol_ids_to_remove: FxHashSet<SymbolId>,
+}
+
+impl<'a> CompressorPass<'a> for RemoveUnusedCode {
+    fn build(&mut self, program: &mut Program<'a>, ctx: &mut ReusableTraverseCtx<'a>) {
+        self.changed = false;
+        traverse_mut_with_ctx(self, program, ctx);
+    }
+}
+
+impl<'a> Traverse<'a> for RemoveUnusedCode {
+    fn enter_program(&mut self, _node: &mut Program<'a>, ctx: &mut TraverseCtx<'a>) {
+        let symbols = ctx.symbols();
+        for symbol_id in symbols.symbol_ids() {
+            if symbols.get_resolved_references(symbol_id).count() == 0 {
+                self.symbol_ids_to_remove.insert(symbol_id);
+            }
+        }
+    }
+
+    fn exit_statements(
+        &mut self,
+        stmts: &mut ArenaVec<'a, Statement<'a>>,
+        _ctx: &mut TraverseCtx<'a>,
+    ) {
+        if self.changed {
+            stmts.retain(|stmt| !matches!(stmt, Statement::EmptyStatement(_)));
+        }
+    }
+
+    fn exit_statement(&mut self, stmt: &mut Statement<'a>, ctx: &mut TraverseCtx<'a>) {
+        if let Statement::VariableDeclaration(decl) = stmt {
+            decl.declarations.retain(|d| {
+                if let BindingPatternKind::BindingIdentifier(ident) = &d.id.kind {
+                    if d.init.is_none() && self.symbol_ids_to_remove.contains(&ident.symbol_id()) {
+                        return false;
+                    }
+                }
+                true
+            });
+            if decl.declarations.is_empty() {
+                self.changed = true;
+                *stmt = ctx.ast.statement_empty(decl.span);
+            }
+        }
+    }
+}
+
+impl RemoveUnusedCode {
+    pub fn new() -> Self {
+        Self { changed: false, symbol_ids_to_remove: FxHashSet::default() }
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use oxc_allocator::Allocator;
+
+    use crate::tester;
+
+    fn test(source_text: &str, expected: &str) {
+        let allocator = Allocator::default();
+        let mut pass = super::RemoveUnusedCode::new();
+        tester::test(&allocator, source_text, expected, &mut pass);
+    }
+
+    fn test_same(source_text: &str) {
+        test(source_text, source_text);
+    }
+
+    #[test]
+    fn simple() {
+        test("var x", "");
+        test_same("var x = 1");
+    }
+}

crates/oxc_minifier/src/compressor.rs

@@ -4,7 +4,9 @@ use oxc_semantic::{ScopeTree, SemanticBuilder, SymbolTable};
 use oxc_traverse::ReusableTraverseCtx;
 
 use crate::{
-    ast_passes::{DeadCodeElimination, Normalize, PeepholeOptimizations, RemoveSyntax},
+    ast_passes::{
+        DeadCodeElimination, Normalize, PeepholeOptimizations, RemoveSyntax, RemoveUnusedCode,
+    },
     CompressOptions, CompressorPass,
 };
 
@@ -32,6 +34,7 @@ impl<'a> Compressor<'a> {
     ) {
         let mut ctx = ReusableTraverseCtx::new(scopes, symbols, self.allocator);
         RemoveSyntax::new(self.options).build(program, &mut ctx);
+        RemoveUnusedCode::new().build(program, &mut ctx);
         Normalize::new().build(program, &mut ctx);
         PeepholeOptimizations::new(true, self.options).run_in_loop(program, &mut ctx);
         PeepholeOptimizations::new(false, self.options).build(program, &mut ctx);

tasks/coverage/snapshots/minifier_test262.snap

@@ -1,5 +1,7 @@
 commit: c4317b0c
 
 minifier_test262 Summary:
-AST Parsed     : 44101/44101 (100.00%)
-Positive Passed: 44101/44101 (100.00%)
+AST Parsed     : 41696/41696 (100.00%)
+Positive Passed: 41694/41696 (100.00%)
+Compress: tasks/coverage/test262/test/language/module-code/instn-local-bndng-for-dup.js
+Compress: tasks/coverage/test262/test/language/types/undefined/S8.1_A1_T1.js

tasks/coverage/src/runtime/mod.rs

@@ -153,8 +153,8 @@ impl Case for Test262RuntimeCase {
             return;
         }
 
-        // Unable to minify `script`, which may contain syntaxes that the minifier do not support (e.g. `with`).
-        if !self.base.is_module() {
+        // Unable to minify non-strict code, which may contain syntaxes that the minifier do not support (e.g. `with`).
+        if self.base.is_no_strict() {
             self.base.set_result(TestResult::Passed);
             return;
         }

tasks/coverage/src/test262/mod.rs

@@ -84,6 +84,10 @@ impl Test262Case {
         self.meta.flags.contains(&TestFlag::OnlyStrict)
     }
 
+    pub fn is_no_strict(&self) -> bool {
+        self.meta.flags.contains(&TestFlag::NoStrict)
+    }
+
     pub fn is_raw(&self) -> bool {
         self.meta.flags.contains(&TestFlag::Raw)
     }

tasks/coverage/src/tools/minifier.rs

@@ -41,17 +41,14 @@ impl Case for MinifierTest262Case {
 
     fn skip_test_case(&self) -> bool {
         self.base.should_fail() || self.base.skip_test_case()
+            // Unable to minify non-strict code, which may contain syntaxes that the minifier do not support (e.g. `with`).
+            || self.base.is_no_strict()
     }
 
     fn run(&mut self) {
         let source_text = self.base.code();
         let is_module = self.base.is_module();
         let source_type = SourceType::default().with_module(is_module);
-        // Unable to minify `script`, which may contain syntaxes that the minifier do not support (e.g. `with`).
-        if source_type.is_script() {
-            self.base.set_result(TestResult::Passed);
-            return;
-        }
         let result = get_result(source_text, source_type);
         self.base.set_result(result);
     }

tasks/minsize/minsize.snap

@@ -21,7 +21,7 @@ Original   | minified   | minified   | gzip       | gzip       | Fixture
 
 3.20 MB    | 1.01 MB    | 1.01 MB    | 332.13 kB  | 331.56 kB  | echarts.js
 
-6.69 MB    | 2.32 MB    | 2.31 MB    | 492.99 kB  | 488.28 kB  | antd.js   
+6.69 MB    | 2.32 MB    | 2.31 MB    | 493.00 kB  | 488.28 kB  | antd.js   
 
 10.95 MB   | 3.51 MB    | 3.49 MB    | 910.06 kB  | 915.50 kB  | typescript.js
 

tasks/minsize/src/lib.rs

@@ -10,7 +10,7 @@ use flate2::{write::GzEncoder, Compression};
 use humansize::{format_size, DECIMAL};
 use oxc_allocator::Allocator;
 use oxc_codegen::{CodeGenerator, CodegenOptions};
-use oxc_minifier::{CompressOptions, MangleOptions, Minifier, MinifierOptions};
+use oxc_minifier::{Minifier, MinifierOptions};
 use oxc_parser::Parser;
 use oxc_semantic::SemanticBuilder;
 use oxc_span::SourceType;
@@ -126,17 +126,13 @@ pub fn run() -> Result<(), io::Error> {
 
 fn minify_twice(file: &TestFile) -> String {
     let source_type = SourceType::from_path(&file.file_name).unwrap();
-    let options = MinifierOptions {
-        mangle: Some(MangleOptions::default()),
-        compress: CompressOptions::default(),
-    };
-    let source_text1 = minify(&file.source_text, source_type, options);
-    let source_text2 = minify(&source_text1, source_type, options);
-    assert_eq_minified_code(&source_text1, &source_text2, &file.file_name);
-    source_text2
+    let code1 = minify(&file.source_text, source_type);
+    let code2 = minify(&code1, source_type);
+    assert_eq_minified_code(&code1, &code2, &file.file_name);
+    code2
 }
 
-fn minify(source_text: &str, source_type: SourceType, options: MinifierOptions) -> String {
+fn minify(source_text: &str, source_type: SourceType) -> String {
     let allocator = Allocator::default();
     let ret = Parser::new(&allocator, source_text, source_type).parse();
     let mut program = ret.program;
@@ -147,7 +143,7 @@ fn minify(source_text: &str, source_type: SourceType, options: MinifierOptions)
         ReplaceGlobalDefinesConfig::new(&[("process.env.NODE_ENV", "'development'")]).unwrap(),
     )
     .build(symbols, scopes, &mut program);
-    let ret = Minifier::new(options).build(&allocator, &mut program);
+    let ret = Minifier::new(MinifierOptions::default()).build(&allocator, &mut program);
     CodeGenerator::new()
         .with_options(CodegenOptions { minify: true, ..CodegenOptions::default() })
         .with_mangler(ret.mangler)
@@ -164,13 +160,9 @@ fn gzip_size(s: &str) -> usize {
 
 fn assert_eq_minified_code(s1: &str, s2: &str, filename: &str) {
     if s1 != s2 {
-        let normalized_left = normalize_minified_code(s1);
-        let normalized_right = normalize_minified_code(s2);
-        similar_asserts::assert_eq!(
-            normalized_left,
-            normalized_right,
-            "Minification failed for {filename}"
-        );
+        let left = normalize_minified_code(s1);
+        let right = normalize_minified_code(s2);
+        similar_asserts::assert_eq!(left, right, "Minification failed for {filename}");
     }
 }