feat: add preserve_imports_with_side_effects option for simplify optimization

crates/swc/src/config/mod.rs

@@ -455,6 +455,10 @@ impl Options {
             .as_ref()
             .map(|v| v.simplify.into_bool())
             .unwrap_or_default();
+        let drop_unused_imports = optimizer
+            .as_ref()
+            .map(|v| v.drop_unused_imports.into_bool())
+            .unwrap_or_default();
 
         let optimization = {
             if let Some(opts) = optimizer.and_then(|o| o.globals) {
@@ -472,7 +476,7 @@ impl Options {
             optimization,
             Optional::new(export_default_from(), syntax.export_default_from()),
             Optional::new(
-                simplifier(top_level_mark, Default::default()),
+                simplifier(top_level_mark, drop_unused_imports, Default::default()),
                 enable_simplifier
             ),
             json_parse_pass
@@ -1481,6 +1485,9 @@ pub struct OptimizerConfig {
 
     #[serde(default)]
     pub jsonify: Option<JsonifyOption>,
+
+    #[serde(default)]
+    pub drop_unused_imports: BoolConfig<false>,
 }
 
 #[derive(Debug, Default, Clone, Copy, Serialize, Deserialize)]

crates/swc_bundler/src/bundler/optimize.rs

@@ -28,6 +28,7 @@ where
                         top_retain: Default::default(),
                     },
                     self.unresolved_mark,
+                    false,
                 )));
             }
             node

crates/swc_ecma_minifier/src/lib.rs

@@ -306,6 +306,7 @@ fn perform_dce(m: &mut Program, options: &CompressOptions, extra: &ExtraOptions)
             top_retain: options.top_retain.clone(),
         },
         extra.unresolved_mark,
+        false,
     );
 
     loop {

crates/swc_ecma_transforms_optimization/src/simplify/dce/mod.rs

@@ -30,6 +30,7 @@ use crate::debug_assert_valid;
 pub fn dce(
     config: Config,
     unresolved_mark: Mark,
+    drop_unused_imports: bool,
 ) -> impl Fold + VisitMut + Repeated + CompilerPass {
     as_folder(TreeShaker {
         expr_ctx: ExprCtx {
@@ -44,6 +45,7 @@ pub fn dce(
         var_decl_kind: None,
         data: Default::default(),
         bindings: Default::default(),
+        drop_unused_imports,
     })
 }
 
@@ -89,6 +91,8 @@ struct TreeShaker {
     data: Arc<Data>,
 
     bindings: Arc<AHashSet<Id>>,
+
+    drop_unused_imports: bool,
 }
 
 impl CompilerPass for TreeShaker {
@@ -820,6 +824,27 @@ impl VisitMut for TreeShaker {
         }
     }
 
+    fn visit_mut_import_specifiers(&mut self, ss: &mut Vec<ImportSpecifier>) {
+        ss.retain(|s| {
+            let local = match s {
+                ImportSpecifier::Named(l) => &l.local,
+                ImportSpecifier::Default(l) => &l.local,
+                ImportSpecifier::Namespace(l) => &l.local,
+            };
+
+            if self.drop_unused_imports && self.can_drop_binding(local.to_id(), false) {
+                debug!(
+                    "Dropping import specifier `{}` because it's not used",
+                    local
+                );
+                self.changed = true;
+                return false;
+            }
+
+            true
+        });
+    }
+
     fn visit_mut_module(&mut self, m: &mut Module) {
         debug_assert_valid(m);
 
@@ -883,7 +908,22 @@ impl VisitMut for TreeShaker {
     }
 
     fn visit_mut_module_item(&mut self, n: &mut ModuleItem) {
-        n.visit_mut_children_with(self);
+        match n {
+            ModuleItem::ModuleDecl(ModuleDecl::Import(i)) => {
+                let is_for_side_effect = i.specifiers.is_empty();
+
+                i.visit_mut_with(self);
+
+                if self.drop_unused_imports && !is_for_side_effect && i.specifiers.is_empty() {
+                    debug!("Dropping an import because it's not used");
+                    self.changed = true;
+                    *n = ModuleItem::Stmt(Stmt::Empty(EmptyStmt { span: DUMMY_SP }));
+                }
+            }
+            _ => {
+                n.visit_mut_children_with(self);
+            }
+        }
         debug_assert_valid(n);
     }
 

crates/swc_ecma_transforms_optimization/src/simplify/mod.rs

@@ -22,10 +22,14 @@ pub struct Config {
 
 /// Performs simplify-expr, inlining, remove-dead-branch and dce until nothing
 /// changes.
-pub fn simplifier(unresolved_mark: Mark, c: Config) -> impl RepeatedJsPass {
+pub fn simplifier(
+    unresolved_mark: Mark,
+    drop_unused_imports: bool,
+    c: Config,
+) -> impl RepeatedJsPass {
     Repeat::new(chain!(
         expr_simplifier(unresolved_mark, c.expr),
         dead_branch_remover(unresolved_mark),
-        dce::dce(c.dce, unresolved_mark)
+        dce::dce(c.dce, unresolved_mark, drop_unused_imports)
     ))
 }

crates/swc_ecma_transforms_optimization/tests/drop_unused_imports.rs

@@ -0,0 +1,76 @@
+use swc_common::{chain, pass::Repeat, Mark};
+use swc_ecma_parser::{EsConfig, Syntax};
+use swc_ecma_transforms_base::resolver;
+use swc_ecma_transforms_optimization::simplify::dce::{dce, Config};
+use swc_ecma_transforms_testing::test;
+use swc_ecma_visit::Fold;
+
+fn tr() -> impl Fold {
+    Repeat::new(dce(
+        Config {
+            top_level: true,
+            ..Default::default()
+        },
+        Mark::new(),
+        true,
+    ))
+}
+
+macro_rules! to {
+    ($name:ident, $src:expr, $expected:expr) => {
+        test!(
+            Syntax::Es(EsConfig {
+                decorators: true,
+                ..Default::default()
+            }),
+            |_| chain!(resolver(Mark::new(), Mark::new(), false), tr()),
+            $name,
+            $src,
+            $expected
+        );
+    };
+}
+
+macro_rules! optimized_out {
+    ($name:ident, $src:expr) => {
+        to!($name, $src, "");
+    };
+}
+
+macro_rules! noop {
+    ($name:ident, $src:expr) => {
+        to!($name, $src, $src);
+    };
+}
+
+to!(
+    single_pass,
+    "
+    const a = 1;
+
+    if (a) {
+        const b = 2;
+    }
+    ",
+    "
+    const a = 1;
+    if (a) {}
+    "
+);
+
+optimized_out!(import_default_unused, "import foo from 'foo'");
+
+optimized_out!(import_specific_unused, "import {foo} from 'foo'");
+
+optimized_out!(import_mixed_unused, "import foo, { bar } from 'foo'");
+
+noop!(
+    import_export_named,
+    "import foo from 'src'; export { foo };"
+);
+
+to!(
+    import_unused_export_named,
+    "import foo, { bar } from 'src'; export { foo }; ",
+    "import foo from 'src'; export { foo }; "
+);

crates/swc_ecma_transforms_optimization/tests/fixture.rs

@@ -25,7 +25,7 @@ fn dce_single_pass(input: PathBuf) {
         &|t| {
             let unresolved_mark = Mark::new();
 
-            chain!(remover(t), dce(Default::default(), unresolved_mark))
+            chain!(remover(t), dce(Default::default(), unresolved_mark, false))
         },
         &input,
         &output,
@@ -45,7 +45,7 @@ fn dce_repeated(input: PathBuf) {
         &|t| {
             chain!(
                 remover(t),
-                Repeat::new(dce(Default::default(), Mark::new()))
+                Repeat::new(dce(Default::default(), Mark::new(), false))
             )
         },
         &input,
@@ -64,7 +64,7 @@ fn dce_jsx(input: PathBuf) {
             jsx: true,
             ..Default::default()
         }),
-        &|t| chain!(remover(t), dce(Default::default(), Mark::new())),
+        &|t| chain!(remover(t), dce(Default::default(), Mark::new(), false)),
         &input,
         &output,
         Default::default(),

crates/swc_ecma_transforms_optimization/tests/simplify.rs

@@ -29,7 +29,7 @@ fn test(src: &str, expected: &str) {
                     expr_simplifier(unresolved_mark, Default::default()),
                     inlining::inlining(Default::default()),
                     dead_branch_remover(unresolved_mark),
-                    dce::dce(Default::default(), unresolved_mark)
+                    dce::dce(Default::default(), unresolved_mark, false)
                 )),
             )
         },
@@ -57,7 +57,7 @@ macro_rules! to {
                         expr_simplifier(unresolved_mark, Default::default()),
                         inlining::inlining(Default::default()),
                         dead_branch_remover(unresolved_mark),
-                        dce::dce(Default::default(), unresolved_mark)
+                        dce::dce(Default::default(), unresolved_mark, false)
                     )),
                 )
             },
@@ -514,7 +514,7 @@ test!(
                     ..Default::default()
                 }
             ),
-            dce(Default::default(), unresolved_mark),
+            dce(Default::default(), unresolved_mark, false),
             inlining(Default::default())
         )
     },
@@ -590,7 +590,7 @@ test!(
                 expr_simplifier(unresolved_mark, Default::default()),
                 inlining::inlining(Default::default()),
                 dead_branch_remover(unresolved_mark),
-                dce::dce(Default::default(), unresolved_mark)
+                dce::dce(Default::default(), unresolved_mark, false)
             )),
             es2018(Default::default()),
             es2017(

crates/swc_ecma_transforms_optimization/tests/simplify_dce.rs

@@ -15,6 +15,7 @@ fn tr() -> impl Fold {
             ..Default::default()
         },
         Mark::new(),
+        false,
     ))
 }