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

docs/errors/E0720.md

@@ -0,0 +1,18 @@
+# E0720: function 'let' call may be confused for destructuring; remove parentheses to declare a variable
+
+In JavaScript, variables can be named `let` and interpreted as a function
+call if it is followed by parentheses. This code calls function `let`
+instead of destructuring an object:
+
+```javascript
+const words = {first: "hello", second: "world"};
+let ({first} = words);
+```
+
+If you want to declare a variable, remove the parentheses:
+
+```javascript
+const words = {first: "hello", second: "world"};
+let {first} = words;
+
+```

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 "function 'let' call may be confused for destructuring; remove parentheses to declare a variable"
+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_Confusing_Let_Call
+    {
+      .code = 720,
+      .severity = Diagnostic_Severity::warning,
+      .message_formats = {
+        QLJS_TRANSLATABLE("function 'let' call may be confused for destructuring; remove parentheses to declare a variable"),
+      },
+      .message_args = {
+        {
+          Diagnostic_Message_Arg_Info(offsetof(Diag_Confusing_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_Confusing_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_Confusing_Let_Call {
+  [[qljs::diag("E0720", Diagnostic_Severity::warning)]]  //
+  [
+      [qljs::message("function 'let' call may be confused for destructuring; "
+                     "remove parentheses to declare a variable",
+                     ARG(let_function_call))]]  //
+  Source_Code_Span let_function_call;
+};
+
 }
 QLJS_WARNING_POP
 

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

@@ -162,6 +162,21 @@ 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);  // Confusing, 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_0()->kind() == Expression_Kind::Variable &&
+            expression_cast<Expression::Variable *>(call->child_0())->type_ ==
+                Token_Type::kw_let &&
+            call->child_1()->kind() == Expression_Kind::Assignment &&
+            call->child_1()->child_0()->kind() == Expression_Kind::Object) {
+          this->diag_reporter_->report(
+              Diag_Confusing_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

@@ -317,7 +317,8 @@ const Translation_Table translation_data = {
         {61, 26, 69, 43, 41, 46},            //
         {53, 51, 0, 0, 0, 51},               //
         {0, 0, 0, 0, 0, 25},                 //
-        {27, 25, 64, 54, 59, 9},             //
+        {0, 0, 0, 0, 0, 9},                  //
+        {27, 25, 64, 54, 59, 96},            //
         {29, 17, 31, 33, 0, 27},             //
         {68, 28, 70, 45, 30, 55},            //
         {0, 0, 0, 24, 0, 23},                //
@@ -2188,6 +2189,7 @@ const Translation_Table translation_data = {
         u8"forwarding exports are only allowed in export-from\0"
         u8"free {1} and {0} {1} {2}\0"
         u8"function\0"
+        u8"function 'let' call may be confused for destructuring; remove parentheses to declare a variable\0"
         u8"function call started here\0"
         u8"function called before declaration in block scope: {0}\0"
         u8"function declared here\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 = 82504;
 constexpr std::size_t translation_table_locale_table_size = 35;
 
 QLJS_CONSTEVAL std::uint16_t translation_table_const_look_up(
@@ -332,6 +332,7 @@ QLJS_CONSTEVAL std::uint16_t translation_table_const_look_up(
           "forwarding exports are only allowed in export-from"sv,
           "free {1} and {0} {1} {2}"sv,
           "function"sv,
+          "function 'let' call may be confused for destructuring; remove parentheses to declare a variable"sv,
           "function call started here"sv,
           "function called before declaration in block scope: {0}"sv,
           "function declared here"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",
@@ -3394,6 +3394,17 @@ inline const Translated_String test_translation_table[604] = {
             u8"function",
         },
     },
+    {
+        "function 'let' call may be confused for destructuring; remove parentheses to declare a variable"_translatable,
+        u8"function 'let' call may be confused for destructuring; remove parentheses to declare a variable",
+        {
+            u8"function 'let' call may be confused for destructuring; remove parentheses to declare a variable",
+            u8"function 'let' call may be confused for destructuring; remove parentheses to declare a variable",
+            u8"function 'let' call may be confused for destructuring; remove parentheses to declare a variable",
+            u8"function 'let' call may be confused for destructuring; remove parentheses to declare a variable",
+            u8"function 'let' call may be confused for destructuring; remove parentheses to declare a variable",
+        },
+    },
     {
         "function call started here"_translatable,
         u8"function call started here",

test/test-parse-warning.cpp

@@ -507,6 +507,13 @@ TEST_F(Test_Parse_Warning, early_exit_does_not_trigger_fallthrough_warning) {
         no_diags);
   }
 }
+
+TEST_F(Test_Parse_Warning, warn_on_confusing_let_use) {
+  test_parse_and_visit_statement(u8"let ({x} = y);"_sv,  //
+                                 u8"^^^ Diag_Confusing_Let_Call"_diag);
+  test_parse_and_visit_statement(u8"let.prop({x} = y);"_sv, no_diags);
+  test_parse_and_visit_statement(u8"let (x = y);"_sv, no_diags);
+}
 }
 }