feat(es/minifier): Drop unused properties (#7534)

**Related issue:**

 - Closes #7472.

crates/swc/tests/projects.rs

@@ -1095,7 +1095,7 @@ function test() {
 
 #[test]
 fn issue_7513_2() {
-    static INPUT: &str = "const cachedTextDecoder = { ignoreBOM: true, fatal: true };";
+    static INPUT: &str = "export const cachedTextDecoder = { ignoreBOM: true, fatal: true };";
 
     let cm = Lrc::<SourceMap>::default();
     let c = swc::Compiler::new(cm.clone());
@@ -1129,7 +1129,10 @@ fn issue_7513_2() {
         .unwrap();
 
     println!("{}", output.code);
-    assert_eq!(output.code, "const a={ignoreBOM:!0,fatal:!0};");
+    assert_eq!(
+        output.code,
+        "export const cachedTextDecoder={ignoreBOM:!0,fatal:!0};"
+    );
 }
 
 #[testing::fixture("tests/minify/**/input.js")]

crates/swc/tests/tsc-references/checkJsdocSatisfiesTag10.2.minified.js

@@ -2,7 +2,6 @@
 //// [/a.js]
 var p = {
     a: 0,
-    b: "hello",
-    x: 8
+    b: "hello"
 };
 p.a.toFixed(), p.b.substring(1), p.d;

crates/swc/tests/tsc-references/checkJsdocSatisfiesTag7.2.minified.js

@@ -2,7 +2,6 @@
 //// [/a.js]
 var p = {
     a: 0,
-    b: "hello",
-    x: 8
+    b: "hello"
 };
 p.a.toFixed(), p.b.substring(1), p.d;

crates/swc/tests/tsc-references/destructuringObjectBindingPatternAndAssignment1ES5.2.minified.js

@@ -1,8 +1,6 @@
 //// [destructuringObjectBindingPatternAndAssignment1ES5.ts]
 (void 0).a1;
-var tmp = {
-    b21: "world"
-};
+var tmp = {};
 (void 0 === tmp ? {
     b21: "string"
 } : tmp).b21;

crates/swc/tests/tsc-references/destructuringVariableDeclaration1ES5.2.minified.js

@@ -1,9 +1,7 @@
 //// [destructuringVariableDeclaration1ES5.ts]
 import { _ as _sliced_to_array } from "@swc/helpers/_/_sliced_to_array";
 import { _ as _to_consumable_array } from "@swc/helpers/_/_to_consumable_array";
-var tmp = {
-    b11: "world"
-};
+var tmp = {};
 (void 0 === tmp ? {
     b11: "string"
 } : tmp).b11;

crates/swc/tests/tsc-references/destructuringVariableDeclaration1ES5iterable.2.minified.js

@@ -1,9 +1,7 @@
 //// [destructuringVariableDeclaration1ES5iterable.ts]
 import { _ as _sliced_to_array } from "@swc/helpers/_/_sliced_to_array";
 import { _ as _to_consumable_array } from "@swc/helpers/_/_to_consumable_array";
-var tmp = {
-    b11: "world"
-};
+var tmp = {};
 (void 0 === tmp ? {
     b11: "string"
 } : tmp).b11;

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

@@ -6,9 +6,7 @@ import { _ as _inherits } from "@swc/helpers/_/_inherits";
 import { _ as _create_super } from "@swc/helpers/_/_create_super";
 export var inModule = 1;
 inmodule.toFixed();
-var object = {
-    spaaace: 3
-};
+var object = {};
 object.spaaaace, object.spaace = 12, object.fresh = 12, other.puuuce, new Date().getGMTDate(), setIntegral(function() {
     return console.log("ok");
 }, 500), AudioBuffin, Jimmy, Jon;

crates/swc/tests/tsc-references/stringLiteralTypesAsTags01.2.minified.js

@@ -1,6 +1,5 @@
 //// [stringLiteralTypesAsTags01.ts]
 var x = {
-    kind: "A",
-    a: 100
+    kind: "A"
 };
 x.kind, x.kind;

crates/swc/tests/tsc-references/stringLiteralTypesAsTags02.2.minified.js

@@ -1,6 +1,5 @@
 //// [stringLiteralTypesAsTags02.ts]
 var x = {
-    kind: "A",
-    a: 100
+    kind: "A"
 };
 x.kind, x.kind;

crates/swc/tests/tsc-references/stringLiteralTypesAsTags03.2.minified.js

@@ -1,6 +1,5 @@
 //// [stringLiteralTypesAsTags03.ts]
 var x = {
-    kind: "A",
-    a: 100
+    kind: "A"
 };
 x.kind, x.kind;

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

@@ -30,7 +30,6 @@ function implicitThis(n) {
     return this.m + n + 12;
 }
 var impl = {
-    a: 12,
     explicitVoid2: function() {
         return _this.a;
     },

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

@@ -1,8 +1,7 @@
 //// [typeGuardsOnClassProperty.ts]
 import { _ as _class_call_check } from "@swc/helpers/_/_class_call_check";
 var o = {
-    prop1: "string",
-    prop2: !0
+    prop1: "string"
 };
 "string" == typeof o.prop1 && o.prop1.toLowerCase();
 var prop1 = o.prop1;

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

@@ -1,5 +1,3 @@
 //// [typedefTagExtraneousProperty.js]
-var y = {
-    bye: "no"
-};
+var y = {};
 y.ignoreMe = "ok but just because of the index signature", y.hi = "yes";

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

@@ -2877,6 +2877,8 @@ impl VisitMut for Optimizer<'_> {
 
         self.store_var_for_prop_hoisting(var);
 
+        self.drop_unused_properties(var);
+
         debug_assert_valid(&var.init);
     }
 

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

@@ -1,8 +1,8 @@
-use swc_atoms::js_word;
+use swc_atoms::{js_word, JsWord};
 use swc_common::{util::take::Take, Span, DUMMY_SP};
 use swc_ecma_ast::*;
 use swc_ecma_usage_analyzer::util::is_global_var_with_pure_property_access;
-use swc_ecma_utils::contains_ident_ref;
+use swc_ecma_utils::{contains_ident_ref, ExprExt};
 
 use super::Optimizer;
 #[cfg(feature = "debug")]
@@ -829,4 +829,73 @@ impl Optimizer<'_> {
             f.ident = None;
         }
     }
+
+    pub(super) fn drop_unused_properties(&mut self, v: &mut VarDeclarator) -> Option<()> {
+        if !self.options.hoist_props || self.ctx.is_exported {
+            return None;
+        }
+
+        let name = v.name.as_ident()?;
+        let obj = v.init.as_mut()?.as_mut_object()?;
+
+        let usage = self.data.vars.get(&name.to_id())?;
+
+        if usage.indexed_with_dynamic_key || usage.used_as_ref {
+            return None;
+        }
+
+        if obj.props.iter().any(|p| match p {
+            PropOrSpread::Spread(_) => true,
+            PropOrSpread::Prop(p) => match &**p {
+                Prop::Shorthand(_) => false,
+                Prop::KeyValue(p) => {
+                    p.key.is_computed() || p.value.may_have_side_effects(&self.expr_ctx)
+                }
+                Prop::Assign(_) => true,
+                Prop::Getter(p) => p.key.is_computed(),
+                Prop::Setter(p) => p.key.is_computed(),
+                Prop::Method(p) => p.key.is_computed(),
+            },
+        }) {
+            return None;
+        }
+
+        let should_preserve_property = |sym: &JsWord| {
+            if let "toString" = &**sym {
+                return true;
+            }
+            !usage.accessed_props.contains_key(sym)
+        };
+        let should_preserve = |key: &PropName| match key {
+            PropName::Ident(k) => should_preserve_property(&k.sym),
+            PropName::Str(k) => should_preserve_property(&k.value),
+            PropName::Num(..) => true,
+            PropName::Computed(..) => true,
+            PropName::BigInt(..) => true,
+        };
+
+        let len = obj.props.len();
+        obj.props.retain(|prop| match prop {
+            PropOrSpread::Spread(_) => {
+                unreachable!()
+            }
+            PropOrSpread::Prop(p) => match &**p {
+                Prop::Shorthand(p) => !should_preserve_property(&p.sym),
+                Prop::KeyValue(p) => !should_preserve(&p.key),
+                Prop::Assign(..) => {
+                    unreachable!()
+                }
+                Prop::Getter(p) => !should_preserve(&p.key),
+                Prop::Setter(p) => !should_preserve(&p.key),
+                Prop::Method(p) => !should_preserve(&p.key),
+            },
+        });
+
+        if obj.props.len() != len {
+            self.changed = true;
+            report_change!("unused: Removing unused properties");
+        }
+
+        None
+    }
 }

crates/swc_ecma_minifier/src/eval.rs

@@ -12,7 +12,7 @@ use swc_ecma_visit::VisitMutWith;
 use crate::{
     compress::{compressor, pure_optimizer, PureOptimizerConfig},
     mode::Mode,
-    option::CompressOptions,
+    option::{CompressOptions, TopLevelOptions},
 };
 
 pub struct Evaluator {
@@ -81,6 +81,7 @@ impl Evaluator {
                 marks,
                 &CompressOptions {
                     hoist_props: true,
+                    top_level: Some(TopLevelOptions { functions: true }),
                     ..Default::default()
                 },
                 &data,

crates/swc_ecma_minifier/src/program_data.rs

@@ -298,6 +298,7 @@ impl Storage for ProgramData {
 
                     e.get_mut().callee_count += var_info.callee_count;
                     e.get_mut().used_as_arg |= var_info.used_as_arg;
+                    e.get_mut().used_as_ref |= var_info.used_as_ref;
                     e.get_mut().indexed_with_dynamic_key |= var_info.indexed_with_dynamic_key;
 
                     e.get_mut().pure_fn |= var_info.pure_fn;
@@ -501,6 +502,10 @@ impl VarDataLike for VarUsageInfo {
         self.reassigned = true;
     }
 
+    fn mark_used_as_ref(&mut self) {
+        self.used_as_ref = true;
+    }
+
     fn add_infects_to(&mut self, other: Access) {
         self.infects_to.push(other);
     }

crates/swc_ecma_minifier/tests/TODO.txt

@@ -275,7 +275,6 @@ keep_names/keep_some_classnames/input.js
 keep_names/keep_some_fnames/input.js
 keep_names/keep_some_fnames_reduce/input.js
 keep_quoted_strict/keep_quoted_strict/input.js
-logical_assignment/assign_in_conditional_part/input.js
 logical_assignment/assignment_in_left_part_2/input.js
 loops/drop_if_else_break_1/input.js
 loops/issue_2740_3/input.js

crates/swc_ecma_minifier/tests/fixture/issues/2257/full/output.js

@@ -11055,7 +11055,6 @@
             }
             var kb = {
                 html: "http://www.w3.org/1999/xhtml",
-                mathml: "http://www.w3.org/1998/Math/MathML",
                 svg: "http://www.w3.org/2000/svg"
             };
             function lb(a) {