js_parser: stop the [x][0] / {f:x}.f folds from changing optional-chain / this / assignment semantics

src/ast/expr.rs

@@ -56,6 +56,15 @@ impl Expr {
             // https://github.com/oven-sh/bun/issues/2594
             Data::ESpread(_) => false,
             Data::EMissing(_) => false,
+            // Inlining `a?.b` as the target of a surrounding member/call
+            // expression can splice two unrelated optional chains together:
+            // `[[a?.b]][0]?.[0].c` would become `a?.b.c`, which short-circuits
+            // past `.c` instead of throwing. The printer can only insert the
+            // `(a?.b)` wrapper when the parent's own optional_chain is None,
+            // which we cannot observe here, so bail out conservatively.
+            Data::EDot(e) => e.optional_chain.is_none(),
+            Data::EIndex(e) => e.optional_chain.is_none(),
+            Data::ECall(e) => e.optional_chain.is_none(),
             _ => true,
         }
     }

src/js_parser/visit/visit_expr.rs

@@ -1057,7 +1057,14 @@
         let target = e_.target.unwrap_inlined();
         let index = e_.index.unwrap_inlined();
 
-        if p.options.features.minify_syntax {
+        // These folds replace an index expression (a Reference into a temporary
+        // string/array) with a plain value. That is only sound when the parent
+        // uses the value, not the Reference: `delete [x][0]` and `[x][0] = v`
+        // act on the temporary, and inlining would redirect them at `x`.
+        if p.options.features.minify_syntax
+            && !is_delete_target
+            && in_.assign_target == js_ast::AssignTarget::None
+        {
             if let Some(number) = index.data.as_e_number() {
                 if number.value() >= 0.0
                     && number.value() < (usize::MAX as f64)
@@ -1085,29 +1092,34 @@
                             }
                         }
                     } else if let Some(array) = target.data.as_e_array() {
+                        let int: usize = number.value() as usize;
                         // [x][0] -> x
-                        if array.items.len_u32() == 1 && number.value() == 0.0 {
-                            let inlined = *array.items.at(0);
-                            if inlined.can_be_inlined_from_property_access() {
-                                *e = inlined;
-                                return;
-                            }
-                        }
-
                         // ['a', 'b', 'c'][1] -> 'b'
-                        let int: usize = number.value() as usize;
-                        if int < array.items.len_u32() as usize
+                        let inlined = if array.items.len_u32() == 1 && int == 0 {
+                            Some(*array.items.at(0))
+                        } else if int < array.items.len_u32() as usize
                             && p.expr_can_be_removed_if_unused(&target)
                         {
-                            let inlined = *array.items.at(int);
+                            Some(*array.items.at(int))
+                        } else {
+                            None
+                        };
+                        if let Some(inlined) = inlined {
                             // ['a', , 'c'][1] -> undefined
                             if matches!(inlined.data, Data::EMissing(..)) {
                                 *e = p.new_expr(E::Undefined {}, inlined.loc);
                                 return;
                             }
-                            debug_assert!(inlined.can_be_inlined_from_property_access());
-                            *e = inlined;
-                            return;
+                            if inlined.can_be_inlined_from_property_access() {
+                                // "[obj.m][0]()" => "(0, obj.m)()"
+                                *e = if is_call_target && inlined.has_value_for_this_in_call() {
+                                    p.new_expr(E::Number::new(0.0), expr.loc)
+                                        .join_with_comma(inlined)
+                                } else {
+                                    inlined
+                                };
+                                return;
+                            }
                         }
                     }
                 }

test/bundler/transpiler/transpiler.test.js

@@ -139,6 +139,111 @@ describe("Bun.Transpiler", () => {
     it("works nested", () => {
       ts.expectPrintedMin_('const a = ["hey"][0][0];', 'const a = "h"');
     });
+    it("bails out when the array item is an optional chain", () => {
+      // Folding `[a?.b][0]` to `a?.b` is unsafe when the result lands as the
+      // target of a surrounding optional-chain continuation: the two chains
+      // would be spliced into one. `[[a?.b]][0]?.[0].c` must not become
+      // `a?.b.c`, which short-circuits to `undefined` for `a == null` instead
+      // of throwing on the trailing `.c`.
+      ts.expectPrintedMin_("x = [[a?.b]][0]?.[0].c", "x = [a?.b]?.[0].c");
+      ts.expectPrintedMin_("x = [[a?.b]][0]?.[0]()", "x = [a?.b]?.[0]()");
+      ts.expectPrintedMin_("x = [[a?.b]][0]?.[0][c]", "x = [a?.b]?.[0][c]");
+      ts.expectPrintedMin_("x = ({ f: [a?.b] }).f?.[0].c", "x = [a?.b]?.[0].c");
+      ts.expectPrintedMin_("x = [[a?.[b]]][0]?.[0].c", "x = [a?.[b]]?.[0].c");
+      ts.expectPrintedMin_("x = [[a?.()]][0]?.[0].c", "x = [a?.()]?.[0].c");
+      // Continuation (not Start) on the inlined item's outermost node:
+      ts.expectPrintedMin_("x = [[a?.b.c]][0]?.[0].d", "x = [a?.b.c]?.[0].d");
+      ts.expectPrintedMin_("x = [[a?.b[c]]][0]?.[0].d", "x = [a?.b[c]]?.[0].d");
+      ts.expectPrintedMin_("x = [[a?.b()]][0]?.[0].d", "x = [a?.b()]?.[0].d");
+      // Multi-item path (expr_can_be_removed_if_unused): a @__PURE__ optional
+      // call is removable, so the second fold arm sees it.
+      ts.expectPrintedMin_("x = [[0, /* @__PURE__ */ a?.()]][0]?.[1].c", "x = [0, a?.()]?.[1].c");
+      ts.expectPrintedMin_("x = [0, /* @__PURE__ */ a?.()][1]", "x = [0, a?.()][1]");
+
+      // The outer `?.` on an array literal is dropped at parse time, so these
+      // reach the fold with `optional_chain == None` on the index and the
+      // printer adds the `(a?.b)` wrapper itself. Keep bailing on the fold so
+      // the wrapper isn't load-bearing.
+      ts.expectPrintedMin_("x = [a?.b][0]", "x = [a?.b][0]");
+      ts.expectPrintedMin_("x = [a?.b]?.[0]", "x = [a?.b][0]");
+      ts.expectPrintedMin_("x = [a?.b][0].c", "x = [a?.b][0].c");
+      ts.expectPrintedMin_("x = [a?.b]?.[0].c", "x = [a?.b][0].c");
+      ts.expectPrintedMin_("x = [a?.b][0]()", "x = [a?.b][0]()");
+
+      // Same bailout protects LHS / delete / new / tagged-template positions
+      // from becoming `a?.b = v` / `delete a?.b` / `new a?.b()`.
+      ts.expectPrintedMin_("[a?.b][0] = v", "[a?.b][0] = v");
+      ts.expectPrintedMin_("delete [a?.b][0]", "delete [a?.b][0]");
+      ts.expectPrintedMin_("x = new [a?.b][0]()", "x = new [a?.b][0]");
+      ts.expectPrintedMin_("[a?.b][0]`x`", "[a?.b][0]`x`");
+
+      // Non-chain items are still inlined.
+      ts.expectPrintedMin_("x = [[y]][0]?.[0].c", "x = y.c");
+      ts.expectPrintedMin_("x = [a.b][0].c", "x = a.b.c");
+      ts.expectPrintedMin_("x = [(a?.b)][0]", "x = [a?.b][0]");
+    });
+    it("bails out or strips `this` when the index is a call/assignment target", () => {
+      // `[obj.m][0]()` calls through a Reference into the temporary array,
+      // so `this` is the array; inlining to `obj.m()` would bind `this` to
+      // `obj`. Match the sibling folds and emit `(0, obj.m)()`.
+      ts.expectPrintedMin_("x = [obj.m][0]()", "x = (0, obj.m)()");
+      ts.expectPrintedMin_("x = [obj[m]][0]()", "x = (0, obj[m])()");
+      ts.expectPrintedMin_("x = [obj.m][0]", "x = obj.m");
+      ts.expectPrintedMin_("x = [y][0]()", "x = y()");
+      ts.expectPrintedMin_("x = [() => y][0]()", "x = (() => y)()");
+
+      // `[x][0] = v` writes into the temporary, not `x`. Same for `"s"[n]`.
+      ts.expectPrintedMin_("[obj.p][0] = 5", "[obj.p][0] = 5");
+      ts.expectPrintedMin_("[obj.p][0] += 5", "[obj.p][0] += 5");
+      ts.expectPrintedMin_("[obj.p][0]++", "[obj.p][0]++");
+      ts.expectPrintedMin_("[x][0] = 1", "[x][0] = 1");
+      ts.expectPrintedMin_("[,][0] = 1", "[,][0] = 1");
+      ts.expectPrintedMin_('"foo"[2] = 1', '"foo"[2] = 1');
+      ts.expectPrintedMin_('["a", "b"][1] = 1', '["a", "b"][1] = 1');
+      ts.expectPrintedMin_("({ a: [obj.p][0] } = {})", "({ a: [obj.p][0] } = {})");
+    });
+    it("preserves runtime semantics when inlining from a literal index", async () => {
+      const src = `
+        var a = null;
+        function check(label, fn, expected) {
+          var got;
+          try { got = "=> " + fn(); } catch (e) { got = e.constructor.name; }
+          console.log(label + ": " + (got === expected ? "ok" : got + " (want " + expected + ")"));
+        }
+        check("chain .c",   () => [[a?.b]][0]?.[0].c, "TypeError");
+        check("chain ()",   () => [[a?.b]][0]?.[0](), "TypeError");
+        check("chain [0]",  () => [[a?.b]][0]?.[0][0], "TypeError");
+        check("chain obj",  () => ({ f: [a?.b] }).f?.[0].c, "TypeError");
+        check("chain flat", () => [a?.b][0].c, "TypeError");
+        check("chain ?.[", () => [a?.b]?.[0].c, "TypeError");
+        check("chain cont", () => [[a?.b.c]][0]?.[0].d, "TypeError");
+        check("chain pure", () => [[0, /* @__PURE__ */ a?.()]][0]?.[1].c, "TypeError");
+        var obj = { n: "obj", m() { return this === obj; } };
+        check("this",       () => [obj.m][0](), "=> false");
+        var o2 = { p: 1 };
+        check("assign",     () => ([o2.p][0] = 5, o2.p), "=> 1");
+      `;
+      await using proc = Bun.spawn({
+        cmd: [bunExe(), "-e", src],
+        env: bunEnv,
+        stderr: "pipe",
+      });
+      const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+      expect(stderr).toBe("");
+      expect(stdout.trim().split("\n")).toEqual([
+        "chain .c: ok",
+        "chain (): ok",
+        "chain [0]: ok",
+        "chain obj: ok",
+        "chain flat: ok",
+        "chain ?.[: ok",
+        "chain cont: ok",
+        "chain pure: ok",
+        "this: ok",
+        "assign: ok",
+      ]);
+      expect(exitCode).toBe(0);
+    });
     it("bails out on optional-chain index into enum", () => {
       const pre = "enum Foo { A }\nenum Bar { 'a-b' = 1 }\n";
       const lastLine = out => out.trimEnd().split("\n").at(-1);