feat(minifier): remove unused function declaration (#12318)

Author: Boshen

crates/oxc_minifier/src/peephole/remove_dead_code.rs

@@ -28,6 +28,7 @@ impl<'a> PeepholeOptimizations {
             Statement::ForStatement(s) => self.try_fold_for(s, state, ctx),
             Statement::TryStatement(s) => Self::try_fold_try(s, ctx),
             Statement::LabeledStatement(s) => Self::try_fold_labeled(s, ctx),
+            Statement::FunctionDeclaration(f) => Self::remove_unused_function_declaration(f, ctx),
             _ => None,
         } {
             *stmt = new_stmt;
@@ -565,6 +566,21 @@ impl<'a> PeepholeOptimizations {
             _ => false,
         }
     }
+
+    fn remove_unused_function_declaration(
+        f: &Function<'a>,
+        ctx: &mut Ctx<'a, '_>,
+    ) -> Option<Statement<'a>> {
+        if ctx.state.options.unused == CompressOptionsUnused::Keep {
+            return None;
+        }
+        let id = f.id.as_ref()?;
+        let symbol_id = id.symbol_id.get()?;
+        if ctx.scoping().symbol_is_unused(symbol_id) {
+            return Some(ctx.ast.statement_empty(f.span));
+        }
+        None
+    }
 }
 
 impl<'a> LatePeepholeOptimizations {
@@ -587,7 +603,10 @@ impl<'a> LatePeepholeOptimizations {
 /// <https://github.com/google/closure-compiler/blob/v20240609/test/com/google/javascript/jscomp/PeepholeRemoveDeadCodeTest.java>
 #[cfg(test)]
 mod test {
-    use crate::tester::{test, test_same};
+    use crate::{
+        CompressOptions,
+        tester::{test, test_options, test_same},
+    };
 
     #[test]
     fn test_fold_block() {
@@ -794,4 +813,10 @@ mod test {
     fn remove_constant_value() {
         test("const foo = false; if (foo) { console.log('foo') }", "const foo = !1;");
     }
+
+    #[test]
+    fn remove_unused_function_declaration() {
+        let options = CompressOptions::smallest();
+        test_options("function foo() {}", "", &options);
+    }
 }

crates/oxc_minifier/src/peephole/remove_unused_expression.rs

@@ -665,7 +665,7 @@ impl<'a> PeepholeOptimizations {
 mod test {
     use crate::{
         CompressOptions, TreeShakeOptions,
-        tester::{test, test_options, test_same, test_same_options},
+        tester::{default_options, test, test_options, test_same, test_same_options},
     };
 
     #[test]
@@ -948,14 +948,14 @@ mod test {
     fn treeshake_options_annotations_false() {
         let options = CompressOptions {
             treeshake: TreeShakeOptions { annotations: false, ..TreeShakeOptions::default() },
-            ..CompressOptions::smallest()
+            ..default_options()
         };
         test_same_options("function test() {} /* @__PURE__ */ test()", &options);
         test_same_options("function test() {} /* @__PURE__ */ new test()", &options);
 
         let options = CompressOptions {
             treeshake: TreeShakeOptions { annotations: true, ..TreeShakeOptions::default() },
-            ..CompressOptions::smallest()
+            ..default_options()
         };
         test_options("function test() {} /* @__PURE__ */ test()", "function test() {}", &options);
         test_options(

crates/oxc_minifier/tests/peephole/dead_code_elimination.rs

@@ -7,6 +7,8 @@ use oxc_minifier::Compressor;
 use oxc_parser::Parser;
 use oxc_span::SourceType;
 
+use super::default_options;
+
 #[track_caller]
 fn run(source_text: &str, source_type: SourceType, options: Option<CompressOptions>) -> String {
     let allocator = Allocator::default();
@@ -26,7 +28,7 @@ fn test(source_text: &str, expected: &str) {
     let source_text = source_text.cow_replace("false", f);
 
     let source_type = SourceType::default();
-    let result = run(&source_text, source_type, Some(CompressOptions::default()));
+    let result = run(&source_text, source_type, Some(default_options()));
     let expected = run(expected, source_type, None);
     assert_eq!(result, expected, "\nfor source\n{source_text}\nexpect\n{expected}\ngot\n{result}");
 }
@@ -229,6 +231,7 @@ fn dce_from_terser() {
         "#,
         r#"function f() {
             g();
+            x = 10;
             throw new Error("foo");
             var x;
         }

crates/oxc_minifier/tests/peephole/mod.rs

@@ -7,15 +7,17 @@ mod statement_fusion;
 
 use oxc_minifier::{CompressOptions, CompressOptionsUnused};
 
-#[track_caller]
-fn test(source_text: &str, expected: &str) {
-    let options = CompressOptions {
+pub fn default_options() -> CompressOptions {
+    CompressOptions {
         drop_debugger: false,
-        drop_console: false,
         unused: CompressOptionsUnused::Keep,
         ..CompressOptions::smallest()
-    };
-    crate::test(source_text, expected, options);
+    }
+}
+
+#[track_caller]
+fn test(source_text: &str, expected: &str) {
+    crate::test(source_text, expected, default_options());
 }
 
 #[track_caller]

napi/minify/test/terser.test.ts

@@ -3385,19 +3385,17 @@ test('inline_script_on', () => {
 });
 
 test('test_unexpected_crash', () => {
-  const prepend = 'x();';
   const code =
-    'function x(){var getsInlined=function(){var leakedVariable1=3;var leakedVariable2=1+2*leakedVariable1;console.log(leakedVariable1);console.log(leakedVariable2)};var getsDropped=getsInlined()}';
+    'x(); function x(){var getsInlined=function(){var leakedVariable1=3;var leakedVariable2=1+2*leakedVariable1;console.log(leakedVariable1);console.log(leakedVariable2)};var getsDropped=getsInlined()}';
   const expected = ['3', '7'];
-  run(code, expected, prepend);
+  run(code, expected);
 });
 
 test('test_unexpected_crash_2', () => {
-  const prepend = 'x();';
   const code =
-    'function x(){var getsInlined=function(){var leakedVariable1=3;var leakedVariable2=1+leakedVariable1[0];console.log(leakedVariable1);console.log(leakedVariable2)};var getsDropped=getsInlined()}';
+    'x(); function x(){var getsInlined=function(){var leakedVariable1=3;var leakedVariable2=1+leakedVariable1[0];console.log(leakedVariable1);console.log(leakedVariable2)};var getsDropped=getsInlined()}';
   const expected = ['3', 'NaN'];
-  run(code, expected, prepend);
+  run(code, expected);
 });
 
 test('issue_1724', () => {

tasks/minsize/minsize.snap

@@ -21,7 +21,7 @@ Original   | minified   | minified   | gzip       | gzip       | Fixture
 
 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    | 463.18 kB  | 488.28 kB  | antd.js   
+6.69 MB    | 2.24 MB    | 2.31 MB    | 462.45 kB  | 488.28 kB  | antd.js   
 
 10.95 MB   | 3.34 MB    | 3.49 MB    | 856.90 kB  | 915.50 kB  | typescript.js