fix(mangler): exclude non-manglable symbols from slot assignment (#25539)

Slots were assigned to non-manglable symbols as well. This made the slot re-use non-optimal.

This PR fixes that.

I used AI to find out this bug and to generate the code and then reviewed the code.

crates/oxc_mangler/src/lib.rs

@@ -557,6 +557,19 @@ impl<'a, 's> Constraints<'a, 's> {
     fn is_kept_name(&self, name: &str) -> bool {
         is_special_name(name) || (!self.reserved.is_empty() && self.reserved.contains(name))
     }
+
+    /// Whether `symbol_id` will receive a mangled name.
+    #[inline]
+    fn is_mangle_candidate(&self, symbol_id: SymbolId, scoping: &Scoping) -> bool {
+        let scope_id = scoping.symbol_scope_id(symbol_id);
+
+        !(scope_id == scoping.root_scope_id()
+            && (!self.top_level
+                || self.exported_symbols.as_ref().is_some_and(|e| e.has_bit(symbol_id.index())))
+            || scoping.scope_flags(scope_id).contains_direct_eval()
+            || self.is_kept_name(scoping.symbol_name(symbol_id))
+            || self.keep_name_symbols.as_ref().is_some_and(|keep| keep.has_bit(symbol_id.index())))
+    }
 }
 
 impl<'a, 's> SlotAssignment<'a, 's> {
@@ -574,7 +587,6 @@ impl<'a, 's> SlotAssignment<'a, 's> {
         ast_nodes: &AstNodes,
         constraints: &Constraints,
     ) -> Self {
-        let keep_name_symbols = constraints.keep_name_symbols.as_ref();
         // Names of bindings in direct-`eval` scopes — collected here, reserved in Phase 4.
         // TODO: eval reservation is conservative — ideally we'd reserve names per-slot.
         let mut eval_reserved_names: FxHashSet<&'s str> = FxHashSet::default();
@@ -610,9 +622,12 @@ impl<'a, 's> SlotAssignment<'a, 's> {
 
             // Sort `bindings` in declaration order.
             tmp_bindings.clear();
-            tmp_bindings.extend(bindings.values().copied().filter(|binding| {
-                !keep_name_symbols.is_some_and(|keep| keep.has_bit(binding.index()))
-            }));
+            tmp_bindings.extend(
+                bindings
+                    .values()
+                    .copied()
+                    .filter(|&binding| constraints.is_mangle_candidate(binding, scoping)),
+            );
             if tmp_bindings.is_empty() {
                 continue;
             }
@@ -715,6 +730,7 @@ impl<'a, 's> SlotAssignment<'a, 's> {
                 && let Some(id) = &func.id
                 && let Some(&shadower) = bindings.get(&id.name)
                 && shadower != id.symbol_id()
+                && constraints.is_mangle_candidate(id.symbol_id(), scoping)
                 && slots[shadower.index()] != SLOT_UNASSIGNED
             {
                 slots[id.symbol_id().index()] = slots[shadower.index()];
@@ -727,17 +743,13 @@ impl<'a, 's> SlotAssignment<'a, 's> {
 }
 
 impl<'a> SlotRanking<'a> {
-    /// Phase 3: count references per slot and sort hottest-first, skipping slots whose only
-    /// symbols are kept, exported (at top level), eval-visible, or special (`arguments`).
+    /// Phase 3: count references per candidate slot and sort hottest-first.
     fn tally(
         allocator: &'a Allocator,
         scoping: &Scoping,
         constraints: &Constraints,
         slots: &SlotAssignment,
     ) -> Self {
-        let exported_symbols = constraints.exported_symbols.as_ref();
-        let keep_name_symbols = constraints.keep_name_symbols.as_ref();
-        let root_scope_id = scoping.root_scope_id();
         let mut frequencies = ArenaVec::from_iter_in(
             repeat_with(|| SlotFrequency::new(allocator)).take(slots.total_slots),
             &allocator,
@@ -748,20 +760,7 @@ impl<'a> SlotRanking<'a> {
                 continue;
             }
             let symbol_id = SymbolId::from_usize(symbol_id);
-            let symbol_scope_id = scoping.symbol_scope_id(symbol_id);
-            if symbol_scope_id == root_scope_id
-                && (!constraints.top_level
-                    || exported_symbols.is_some_and(|e| e.has_bit(symbol_id.index())))
-            {
-                continue;
-            }
-            if scoping.scope_flags(symbol_scope_id).contains_direct_eval() {
-                continue;
-            }
-            if constraints.is_kept_name(scoping.symbol_name(symbol_id)) {
-                continue;
-            }
-            if keep_name_symbols.is_some_and(|keep| keep.has_bit(symbol_id.index())) {
+            if !constraints.is_mangle_candidate(symbol_id, scoping) {
                 continue;
             }
             let index = slot as usize;

crates/oxc_minifier/tests/mangler/mod.rs

@@ -339,8 +339,8 @@ fn reserved_names() {
                  ? t(exports)
                  : t((e.lib = {}));
          })(this, function (exports) {
-             function t(e) { return e; }
-             exports.queue = t;
+             function e(e) { return e; }
+             exports.queue = e;
          });",
         &options,
     );
@@ -353,9 +353,9 @@ fn reserved_names() {
              exports.other = helper;
          })(m, m.exports);",
         "(function (module, exports) {
-             function t(e) { return e; }
-             module.exports.helper = t;
-             exports.other = t;
+             function e(e) { return e; }
+             module.exports.helper = e;
+             exports.other = e;
          })(m, m.exports);",
         &options,
     );
@@ -375,6 +375,17 @@ fn reserved_names() {
     );
 }
 
+#[test]
+fn non_mangleable_symbols_do_not_consume_slots() {
+    let options = MangleOptions { debug: true, ..MangleOptions::default() };
+
+    test(
+        "var A, B; { use(A); let x; } { use(B); let y; }",
+        "var A, B; { use(A); let slot_0; } { use(B); let slot_0; }",
+        &options,
+    );
+}
+
 /// Annex B.3.2.1: In sloppy mode, function declarations inside blocks have var-like hoisting.
 /// The mangler must not assign the same name to such a function and an outer `var` binding.
 #[test]

tasks/minsize/minsize.snap

@@ -23,5 +23,5 @@ Original   | minified   | minified   | gzip       | gzip       | Iterations | Fi
 
 6.69 MB    | 2.12 MB    | 2.31 MB    | 453.79 kB  | 488.28 kB  |          5 | antd.js    
 
-10.95 MB   | 3.33 MB    | 3.49 MB    | 852.64 kB  | 915.50 kB  |          4 | typescript.js 
+10.95 MB   | 3.33 MB    | 3.49 MB    | 852.09 kB  | 915.50 kB  |          4 | typescript.js