fix(mangler): keep exported symbols for top_level: true (#7927)

~~I'm not going to work on a fix for a while so feel free to fix it if
anyone wants to work on it.~~

Exported symbols are now not mangled.

Cargo.lock

@@ -1833,6 +1833,7 @@ dependencies = [
  "oxc_index",
  "oxc_semantic",
  "oxc_span",
+ "rustc-hash",
 ]
 
 [[package]]

crates/oxc_mangler/Cargo.toml

@@ -26,3 +26,4 @@ oxc_ast = { workspace = true }
 oxc_index = { workspace = true }
 oxc_semantic = { workspace = true }
 oxc_span = { workspace = true }
+rustc-hash = { workspace = true }

crates/oxc_mangler/src/lib.rs

@@ -1,8 +1,9 @@
 use itertools::Itertools;
-use oxc_ast::ast::Program;
+use oxc_ast::ast::{Declaration, Program, Statement};
 use oxc_index::{index_vec, Idx, IndexVec};
 use oxc_semantic::{ReferenceId, ScopeTree, SemanticBuilder, SymbolId, SymbolTable};
 use oxc_span::CompactStr;
+use rustc_hash::FxHashSet;
 
 type Slot = usize;
 
@@ -85,6 +86,12 @@ impl Mangler {
     pub fn build<'a>(mut self, program: &'a Program<'a>) -> Mangler {
         let semantic = SemanticBuilder::new().build(program).semantic;
 
+        let (exported_names, exported_symbols) = if self.options.top_level {
+            Mangler::collect_exported_symbols(program)
+        } else {
+            Default::default()
+        };
+
         // Mangle the symbol table by computing slots from the scope tree.
         // A slot is the occurrence index of a binding identifier inside a scope.
         let (mut symbol_table, scope_tree) = semantic.into_symbol_table_and_scope_tree();
@@ -126,8 +133,13 @@ impl Mangler {
             }
         }
 
-        let frequencies =
+        let frequencies = self.tally_slot_frequencies(
-            self.tally_slot_frequencies(&symbol_table, &scope_tree, total_number_of_slots, &slots);
+            &symbol_table,
+            &exported_symbols,
+            &scope_tree,
+            total_number_of_slots,
+            &slots,
+        );
 
         let root_unresolved_references = scope_tree.root_unresolved_references();
         let root_bindings = scope_tree.get_bindings(scope_tree.root_scope_id());
@@ -145,7 +157,8 @@ impl Mangler {
                 if !is_keyword(n)
                     && !is_special_name(n)
                     && !root_unresolved_references.contains_key(n)
-                    && (self.options.top_level || !root_bindings.contains_key(n))
+                    && !(root_bindings.contains_key(n)
+                        && (!self.options.top_level || exported_names.contains(n)))
                 {
                     break name;
                 }
@@ -206,6 +219,7 @@ impl Mangler {
     fn tally_slot_frequencies(
         &self,
         symbol_table: &SymbolTable,
+        exported_symbols: &FxHashSet<SymbolId>,
         scope_tree: &ScopeTree,
         total_number_of_slots: usize,
         slots: &IndexVec<SymbolId, Slot>,
@@ -213,7 +227,9 @@ impl Mangler {
         let root_scope_id = scope_tree.root_scope_id();
         let mut frequencies = vec![SlotFrequency::default(); total_number_of_slots];
         for (symbol_id, slot) in slots.iter_enumerated() {
-            if !self.options.top_level && symbol_table.get_scope_id(symbol_id) == root_scope_id {
+            if symbol_table.get_scope_id(symbol_id) == root_scope_id
+                && (!self.options.top_level || exported_symbols.contains(&symbol_id))
+            {
                 continue;
             }
             if is_special_name(symbol_table.get_name(symbol_id)) {
@@ -228,6 +244,29 @@ impl Mangler {
         frequencies.sort_unstable_by_key(|x| std::cmp::Reverse(x.frequency));
         frequencies
     }
+
+    fn collect_exported_symbols(program: &Program) -> (FxHashSet<CompactStr>, FxHashSet<SymbolId>) {
+        program
+            .body
+            .iter()
+            .filter_map(|statement| {
+                let Statement::ExportNamedDeclaration(v) = statement else { return None };
+                v.declaration.as_ref()
+            })
+            .flat_map(|decl| {
+                if let Declaration::VariableDeclaration(decl) = decl {
+                    itertools::Either::Left(
+                        decl.declarations
+                            .iter()
+                            .filter_map(|decl| decl.id.get_binding_identifier()),
+                    )
+                } else {
+                    itertools::Either::Right(decl.id().into_iter())
+                }
+            })
+            .map(|id| (id.name.to_compact_str(), id.symbol_id()))
+            .collect()
+    }
 }
 
 fn is_special_name(name: &str) -> bool {

crates/oxc_minifier/tests/mangler/mod.rs

@@ -26,7 +26,13 @@ fn mangler() {
         "import { x } from 's'; export { x }",
         "function _ (exports) { Object.defineProperty(exports, '__esModule', { value: true }) }",
     ];
-    let top_level_cases = ["function foo(a) {a}"];
+    let top_level_cases = [
+        "function foo(a) {a}",
+        "export function foo() {}; foo()",
+        "export default function foo() {}; foo()",
+        "export const foo = 1; foo",
+        "const foo = 1; foo; export { foo }",
+    ];
 
     let mut snapshot = String::new();
     cases.into_iter().fold(&mut snapshot, |w, case| {

crates/oxc_minifier/tests/mangler/snapshots/mangler.snap

@@ -35,3 +35,22 @@ function foo(a) {a}
 function a(b) {
 	b;
 }
+
+export function foo() {}; foo()
+export function foo() {}
+;
+foo();
+
+export default function foo() {}; foo()
+export default function a() {}
+;
+a();
+
+export const foo = 1; foo
+export const foo = 1;
+foo;
+
+const foo = 1; foo; export { foo }
+const a = 1;
+a;
+export { a as foo };