Forbid newlines in string expressions

Partial port of projectfluent/fluent.js#135 affecting
the tooling parser.

Author: stasm

fluent/syntax/errors.py

@@ -46,4 +46,6 @@ def get_error_message(code, args):
         return 'Attributes of public messages cannot be used as selectors'
     if code == 'E0019':
         return 'Attributes of private messages cannot be used as placeables'
+    if code == 'E0020':
+        return 'Unterminated string expression'
     return code

fluent/syntax/parser.py

@@ -582,10 +582,13 @@ def get_string(self, ps):
 
         ps.expect_char('"')
 
-        ch = ps.take_char(lambda x: x != '"')
+        ch = ps.take_char(lambda x: x != '"' and x != '\n')
         while ch:
             val += ch
-            ch = ps.take_char(lambda x: x != '"')
+            ch = ps.take_char(lambda x: x != '"' and x != '\n')
+
+        if ps.current_is('\n'):
+            raise ParseError('E0020')
 
         ps.next()
 

tests/syntax/fixtures_behavior/multiline_string.ftl

@@ -1,3 +1,4 @@
 key = { BUILTIN(key: "
  text
     ") }
+//~ ERROR E0020, pos 22