feat(fe): add warning for 'let({x} = y);'

docs/errors/E0720.md

@@ -0,0 +1,39 @@
+# E0720: calling `let` on an assignment is ambiguous
+
+If `let` is declared as a function, then this code:
+
+```javascript
+function let(arg) {
+  console.log(arg);
+}
+
+const y = {x: "hello", z: "world"};
+let ({x} = y);
+```
+
+is actually equivalent to:
+
+```javascript
+function let(arg) {
+  console.log(arg);
+}
+
+const y = {x: "hello", z: "world"};
+let(y);
+```
+
+instead of destructuring `y`.
+
+To remove the ambiguity, separate the declaration and the assignment into two
+statements:
+
+```javascript
+function let(arg) {
+  console.log(arg);
+}
+
+const y = {x: "hello", z: "world"};
+let x;
+({x} = y);
+
+```

po/messages.pot

@@ -2413,6 +2413,10 @@ msgstr ""
 msgid "namespace alias cannot use 'import type'"
 msgstr ""
 
+#: src/quick-lint-js/diag/diagnostic-metadata-generated.cpp
+msgid "ambiguous use of the keyword 'let'; let is a function in this scope"
+msgstr ""
+
 #: test/test-diagnostic-formatter.cpp
 #: test/test-vim-qflist-json-diag-reporter.cpp
 msgid "something happened"

src/quick-lint-js/diag/diagnostic-metadata-generated.cpp

@@ -6912,6 +6912,20 @@ const QLJS_CONSTINIT Diagnostic_Info all_diagnostic_infos[] = {
         },
       },
     },
+
+    // Diag_Ambiguous_Let_Call
+    {
+      .code = 720,
+      .severity = Diagnostic_Severity::warning,
+      .message_formats = {
+        QLJS_TRANSLATABLE("ambiguous use of the keyword 'let'; let is a function in this scope"),
+      },
+      .message_args = {
+        {
+          Diagnostic_Message_Arg_Info(offsetof(Diag_Ambiguous_Let_Call, let_function_call), Diagnostic_Arg_Type::source_code_span),
+        },
+      },
+    },
 };
 }
 

src/quick-lint-js/diag/diagnostic-metadata-generated.h

@@ -472,10 +472,11 @@ namespace quick_lint_js {
   QLJS_DIAG_TYPE_NAME(Diag_Multiple_Export_Defaults) \
   QLJS_DIAG_TYPE_NAME(Diag_Unintuitive_Bitshift_Precedence) \
   QLJS_DIAG_TYPE_NAME(Diag_TypeScript_Namespace_Alias_Cannot_Use_Import_Type) \
+  QLJS_DIAG_TYPE_NAME(Diag_Ambiguous_Let_Call) \
   /* END */
 // clang-format on
 
-inline constexpr int Diag_Type_Count = 461;
+inline constexpr int Diag_Type_Count = 462;
 
 extern const Diagnostic_Info all_diagnostic_infos[Diag_Type_Count];
 }

src/quick-lint-js/diag/diagnostic-types-2.h

@@ -3589,6 +3589,16 @@ struct Diag_TypeScript_Namespace_Alias_Cannot_Use_Import_Type {
                   ARG(type_keyword))]]  //
   Source_Code_Span type_keyword;
 };
+
+struct Diag_Ambiguous_Let_Call {
+  [[qljs::diag("E0720", Diagnostic_Severity::warning)]]  //
+  [
+      [qljs::message("ambiguous use of the keyword 'let'; "
+                     "let is a function in this scope",
+                     ARG(let_function_call))]]  //
+  Source_Code_Span let_function_call;
+};
+
 }
 QLJS_WARNING_POP
 

src/quick-lint-js/fe/parse-statement.cpp

@@ -162,6 +162,18 @@ bool Parser::parse_and_visit_statement(Parse_Visitor_Base &v,
       Expression *ast =
           this->parse_expression(v, Precedence{.in_operator = true});
       this->visit_expression(ast, v, Variable_Context::rhs);
+
+      // let ({x} = y);  // Ambiguous if 'let' is a function
+      if (ast->kind() == Expression_Kind::Call) {
+        Expression::Call *call = expression_cast<Expression::Call *>(ast);
+        if (call->child_count() == 2 &&
+            call->child_1()->kind() == Expression_Kind::Assignment &&
+            call->child_1()->child_0()->kind() == Expression_Kind::Object) {
+          this->diag_reporter_->report(
+              Diag_Ambiguous_Let_Call{.let_function_call = let_token.span()});
+        }
+      }
+
       this->parse_end_of_expression_statement();
     } else {
       // Variable declaration.

src/quick-lint-js/i18n/translation-table-generated.cpp

@@ -196,7 +196,8 @@ const Translation_Table translation_data = {
         {0, 0, 0, 58, 0, 50},                //
         {0, 0, 0, 0, 0, 57},                 //
         {0, 0, 0, 0, 0, 37},                 //
-        {14, 14, 0, 64, 0, 29},              //
+        {0, 0, 0, 0, 0, 29},                 //
+        {14, 14, 0, 64, 0, 68},              //
         {0, 0, 0, 0, 0, 18},                 //
         {0, 0, 0, 0, 0, 8},                  //
         {0, 0, 0, 0, 0, 16},                 //
@@ -2067,6 +2068,7 @@ const Translation_Table translation_data = {
         u8"abstract properties are only allowed in abstract classes\0"
         u8"abstract properties cannot be static\0"
         u8"accessors cannot be optional\0"
+        u8"ambiguous use of the keyword 'let'; let is a function in this scope\0"
         u8"an 'if' statement\0"
         u8"an enum\0"
         u8"an import alias\0"

src/quick-lint-js/i18n/translation-table-generated.h

@@ -18,8 +18,8 @@ namespace quick_lint_js {
 using namespace std::literals::string_view_literals;
 
 constexpr std::uint32_t translation_table_locale_count = 5;
-constexpr std::uint16_t translation_table_mapping_table_size = 605;
-constexpr std::size_t translation_table_string_table_size = 82408;
+constexpr std::uint16_t translation_table_mapping_table_size = 606;
+constexpr std::size_t translation_table_string_table_size = 82476;
 constexpr std::size_t translation_table_locale_table_size = 35;
 
 QLJS_CONSTEVAL std::uint16_t translation_table_const_look_up(
@@ -211,6 +211,7 @@ QLJS_CONSTEVAL std::uint16_t translation_table_const_look_up(
           "abstract properties are only allowed in abstract classes"sv,
           "abstract properties cannot be static"sv,
           "accessors cannot be optional"sv,
+          "ambiguous use of the keyword 'let'; let is a function in this scope"sv,
           "an 'if' statement"sv,
           "an enum"sv,
           "an import alias"sv,

src/quick-lint-js/i18n/translation-table-test-generated.h

@@ -27,7 +27,7 @@ struct Translated_String {
 };
 
 // clang-format off
-inline const Translated_String test_translation_table[604] = {
+inline const Translated_String test_translation_table[605] = {
     {
         "\"global-groups\" entries must be strings"_translatable,
         u8"\"global-groups\" entries must be strings",
@@ -2063,6 +2063,17 @@ inline const Translated_String test_translation_table[604] = {
             u8"accessors cannot be optional",
         },
     },
+    {
+        "ambiguous use of the keyword 'let'; let is a function in this scope"_translatable,
+        u8"ambiguous use of the keyword 'let'; let is a function in this scope",
+        {
+            u8"ambiguous use of the keyword 'let'; let is a function in this scope",
+            u8"ambiguous use of the keyword 'let'; let is a function in this scope",
+            u8"ambiguous use of the keyword 'let'; let is a function in this scope",
+            u8"ambiguous use of the keyword 'let'; let is a function in this scope",
+            u8"ambiguous use of the keyword 'let'; let is a function in this scope",
+        },
+    },
     {
         "an 'if' statement"_translatable,
         u8"an 'if' statement",

test/test-parse-warning.cpp

@@ -507,6 +507,11 @@ TEST_F(Test_Parse_Warning, early_exit_does_not_trigger_fallthrough_warning) {
         no_diags);
   }
 }
+
+TEST_F(Test_Parse_Warning, warn_on_ambiguous_let_use) {
+  test_parse_and_visit_statement(u8"let ({x} = y);"_sv,  //
+                                 u8"^^^ Diag_Ambiguous_Let_Call"_diag);
+}
 }
 }