parser: add parse-only support for "import.meta"

src/parser/expression_parser.ml

@@ -70,6 +70,16 @@ module Expression
           } ) ->
       false
     (* #sec-static-semantics-static-semantics-isvalidsimpleassignmenttarget *)
+    | ( _,
+        MetaProperty
+          {
+            MetaProperty.meta = (_, { Identifier.name = "import"; comments = _ });
+            property = (_, { Identifier.name = "meta"; comments = _ });
+            comments = _;
+          } ) ->
+      false
+    (* #sec-static-semantics-static-semantics-isvalidsimpleassignmenttarget *)
+    (* draft spec: https://tc39.github.io/proposal-import-meta/ *)
     | (_, Array _)
     | (_, Identifier _)
     | (_, Member _)
@@ -255,6 +265,16 @@ module Expression
           } ) ->
       false
     (* #sec-static-semantics-static-semantics-isvalidsimpleassignmenttarget *)
+    | ( _,
+        MetaProperty
+          {
+            MetaProperty.meta = (_, { Identifier.name = "import"; comments = _ });
+            property = (_, { Identifier.name = "meta"; comments = _ });
+            comments = _;
+          } ) ->
+      false
+    (* #sec-static-semantics-static-semantics-isvalidsimpleassignmenttarget *)
+    (* draft spec: https://tc39.github.io/proposal-import-meta/ *)
     | (_, Identifier _)
     | (_, Member _)
     | (_, MetaProperty _) ->
@@ -670,21 +690,49 @@ module Expression
       super
 
   and import env =
-    with_loc
-      (fun env ->
-        let leading = Peek.comments env in
-        Expect.token env T_IMPORT;
-        let leading_arg = Peek.comments env in
-        Expect.token env T_LPAREN;
-        let argument = add_comments (assignment (with_no_in false env)) ~leading:leading_arg in
-        Expect.token env T_RPAREN;
+    let start_loc = Peek.loc env in
+    let leading = Peek.comments env in
+    Expect.token env T_IMPORT;
+
+    if Peek.token env = T_PERIOD then (
+      (* "import.meta" syntax (no other metaproperties are permitted) *)
+      Expect.token env T_PERIOD;
+      let meta = Flow_ast_utils.ident_of_source (start_loc, "import") in
+      match Peek.token env with
+      | T_IDENTIFIER { raw = "meta"; _ } ->
+        let property = Parse.identifier env in
         let trailing = Eat.trailing_comments env in
-        Expression.Import
+        let end_loc = fst property in
+        let meta_property = let open Expression.MetaProperty in Expression.MetaProperty
           {
-            Expression.Import.argument;
+            meta;
+            property;
             comments = Flow_ast_utils.mk_comments_opt ~leading ~trailing ();
-          })
-      env
+          } in
+        (Loc.btwn start_loc end_loc, meta_property)
+      | T_IDENTIFIER _ ->
+        error_at env (start_loc, Parse_error.InvalidImportMetaProperty);
+        Eat.token env;
+        (start_loc, Expression.Identifier meta)
+      | _ ->
+        error_unexpected ~expected:"identifier for `import` metaproperty" env;
+        Eat.token env;
+        (start_loc, Expression.Identifier meta)
+    ) else (
+      (* "import(...)" syntax *)
+      let leading_arg = Peek.comments env in
+      Expect.token env T_LPAREN;
+      let argument = add_comments (assignment (with_no_in false env)) ~leading:leading_arg in
+      let end_loc = Peek.loc env in
+      Expect.token env T_RPAREN;
+      let trailing = Eat.trailing_comments env in
+      let import = Expression.Import
+        {
+          Expression.Import.argument;
+          comments = Flow_ast_utils.mk_comments_opt ~leading ~trailing ();
+        } in
+      (Loc.btwn start_loc end_loc, import)
+    )
 
   and call_cover ?(allow_optional_chain = true) ?(in_optional_chain = false) env start_loc left =
     let left = member_cover ~allow_optional_chain ~in_optional_chain env start_loc left in

src/parser/parse_error.ml

@@ -69,6 +69,7 @@ type t =
   | InvalidLHSInExponentiation
   | InvalidLHSInForIn
   | InvalidLHSInForOf
+  | InvalidImportMetaProperty
   | ExpectedPatternFoundExpression
   | MultipleDefaultsInSwitch
   | NoCatchOrFinally
@@ -276,6 +277,7 @@ module PP = struct
     | InvalidLHSInExponentiation -> "Invalid left-hand side in exponentiation expression"
     | InvalidLHSInForIn -> "Invalid left-hand side in for-in"
     | InvalidLHSInForOf -> "Invalid left-hand side in for-of"
+    | InvalidImportMetaProperty ->  "The only valid meta property for import is import.meta"
     | ExpectedPatternFoundExpression ->
       "Expected an object pattern, array pattern, or an identifier but "
       ^ "found an expression instead"

src/parser/parser_flow.ml

@@ -134,7 +134,7 @@ module rec Parse : PARSER = struct
       error_on_decorators env decorators;
       let statement =
         match Peek.ith_token ~i:1 env with
-        | T_LPAREN -> Statement.expression env
+        | T_LPAREN | T_PERIOD -> Statement.expression env
         | _ -> Statement.import_declaration env
       in
       statement

tests/import_meta/.flowconfig

@@ -0,0 +1,3 @@
+[options]
+module.system=haste
+no_flowlib=false

tests/import_meta/import_meta.exp

@@ -0,0 +1,50 @@
+Error ----------------------------------------------------------------------------------------------- import_meta.js:9:1
+
+Not supported. [unsupported-syntax]
+
+   9| import.meta;
+      ^^^^^^^^^^^
+
+
+Error ---------------------------------------------------------------------------------------------- import_meta.js:12:1
+
+Not supported. [unsupported-syntax]
+
+   12| import.meta.hello = "world";
+       ^^^^^^^^^^^
+
+
+Error --------------------------------------------------------------------------------------------- import_meta.js:15:14
+
+Not supported. [unsupported-syntax]
+
+   15| const meta = import.meta;
+                    ^^^^^^^^^^^
+
+
+Error ---------------------------------------------------------------------------------- no_assign_to_import_meta.js:4:1
+
+Invalid left-hand side in assignment
+
+   4| import.meta = {wat: "wat"}; // parse error
+      ^^^^^^^^^^^
+
+
+Error ------------------------------------------------------------------------- no_nonidentifier_after_import_dot.js:4:8
+
+Unexpected token `while`, expected identifier for `import` metaproperty
+
+   4| import.while; // parse error
+             ^^^^^
+
+
+Error ----------------------------------------------------------------------------------- no_other_metaproperties.js:4:1
+
+The only valid meta property for import is import.meta
+
+   4| import.atem; // parse error
+      ^^^^^^
+
+
+
+Found 6 errors

tests/import_meta/import_meta.js

@@ -0,0 +1,22 @@
+// @flow
+
+// NOTE: The following tests all currently yield a "Not supported."
+// error because Flow does not process MetaProperty expressions at all.
+// The comments above each test case describe what the behavior _should_
+// be once typechecking is implemented.
+
+// You can access `import.meta` at the root of the module.
+import.meta;
+
+// You can write to `import.meta` at the root of the module.
+import.meta.hello = "world";
+
+// You can use `import.meta` in a normal expression context.
+const meta = import.meta;
+
+// All properties are `mixed`.
+(meta.whatever: mixed);
+(meta.whatnever: number); // error
+
+// Properties are writable.
+meta.writeable = "definitely";

tests/import_meta/no_assign_to_import_meta.js

@@ -0,0 +1,4 @@
+// @flow
+
+// You can't assign to it.
+import.meta = {wat: "wat"}; // parse error

tests/import_meta/no_nonidentifier_after_import_dot.js

@@ -0,0 +1,4 @@
+// @flow
+
+// If you have "import.SOMETHING", that "SOMETHING" must be an identifier.
+import.while; // parse error

tests/import_meta/no_other_metaproperties.js

@@ -0,0 +1,4 @@
+// @flow
+
+// You can't access any other metaproperties on `import`.
+import.atem; // parse error