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

Summary:
Resolves facebook#6913. As with `new.target`, we can parse the expression, but
we fail to typecheck it. Clients can use a suppression comment to ignore
uses of `import.meta` in an otherwise valid file.

The parse error message on inputs like `import.notMeta` was chosen to be
consistent with the message emitted by Babylon.

Test Plan:
Unit tests added, and all existing tests pass.

wchargin-branch: parse-import-meta

src/parser/expression_parser.ml

@@ -39,6 +39,9 @@ module Expression
   let is_assignable_lhs = Expression.(function
     | _, MetaProperty { MetaProperty.meta = (_, "new"); property = (_, "target") }
       -> false (* #sec-static-semantics-static-semantics-isvalidsimpleassignmenttarget *)
+    | _, MetaProperty { MetaProperty.meta = (_, "import"); property = (_, "meta") }
+      -> false (* #sec-static-semantics-static-semantics-isvalidsimpleassignmenttarget *)
+      (* draft spec: https://tc39.github.io/proposal-import-meta/ *)
 
     | _, Array _
     | _, Identifier _
@@ -193,6 +196,9 @@ module Expression
   and is_lhs = Expression.(function
     | _, MetaProperty { MetaProperty.meta = (_, "new"); property = (_, "target") }
       -> false (* #sec-static-semantics-static-semantics-isvalidsimpleassignmenttarget *)
+    | _, MetaProperty { MetaProperty.meta = (_, "import"); property = (_, "meta") }
+      -> false (* #sec-static-semantics-static-semantics-isvalidsimpleassignmenttarget *)
+      (* draft spec: https://tc39.github.io/proposal-import-meta/ *)
 
     | _, Identifier _
     | _, Member _
@@ -534,11 +540,36 @@ module Expression
       super
 
   and import env = with_loc (fun env ->
+    let start_loc = Peek.loc env in
     Expect.token env T_IMPORT;
-    Expect.token env T_LPAREN;
-    let arg = assignment (with_no_in false env) in
-    Expect.token env T_RPAREN;
-    Expression.Import arg
+    match Peek.token env with
+    | T_LPAREN ->
+      (* "import(...)" syntax *)
+      Eat.token env;
+      let arg = assignment (with_no_in false env) in
+      Expect.token env T_RPAREN;
+      Expression.Import arg
+    | T_PERIOD -> begin
+      (* "import.meta" syntax (no other metaproperties are permitted) *)
+      Eat.token env;
+      match Peek.token env with
+      | T_IDENTIFIER { raw = "meta"; _ } ->
+        Eat.token env;
+        let end_loc = Peek.loc env in
+        Expression.(MetaProperty MetaProperty.({
+          meta = start_loc, "import";
+          property = end_loc, "meta";
+        }))
+      | T_IDENTIFIER _ ->
+        error_at env (start_loc, Parse_error.InvalidImportMetaProperty);
+        Expression.Identifier (start_loc, "import")
+      | _ ->
+        error_unexpected env;
+        Expression.Identifier (start_loc, "import")
+      end
+    | _ ->
+      error_unexpected env;
+      Expression.Identifier (start_loc, "import")
   ) env
 
   and call_cover ?(allow_optional_chain=true) ?(in_optional_chain=false) env start_loc left =

src/parser/parse_error.ml

@@ -35,6 +35,7 @@ type t =
   | InvalidLHSInExponentiation
   | InvalidLHSInForIn
   | InvalidLHSInForOf
+  | InvalidImportMetaProperty
   | ExpectedPatternFoundExpression
   | MultipleDefaultsInSwitch
   | NoCatchOrFinally
@@ -154,6 +155,7 @@ module PP =
       | 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

@@ -100,7 +100,7 @@ module rec Parse : PARSER = struct
     | T_IMPORT ->
         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
     | T_DECLARE when Peek.ith_token ~i:1 env = T_EXPORT ->

tests/import_meta/.flowconfig

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

tests/import_meta/import_meta.exp

@@ -0,0 +1,42 @@
+Error ----------------------------------------------------------------------------------------------- import_meta.js:9:1
+
+Not supported.
+
+   9| import.meta;
+      ^^^^^^^^^^^
+
+
+Error ---------------------------------------------------------------------------------------------- import_meta.js:12:1
+
+Not supported.
+
+   12| import.meta.hello = "world";
+       ^^^^^^^^^^^
+
+
+Error --------------------------------------------------------------------------------------------- import_meta.js:15:14
+
+Not supported.
+
+   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_other_metaproperties.js:4:1
+
+The only valid meta property for import is import.meta
+
+   4| import.atem; // parse error
+      ^^^^^^
+
+
+
+Found 5 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_other_metaproperties.js

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