Allow shortcut for extension on semicolons

Similarly to what is done for keywords. The syntax is:

  <expr> ;%foo <expr>

and is a shortcut for: [%foo <expr> ; <expr>]

The rationale is to have the equivalent of the normal semicolon when
writing ml code using a monadic let%foo

Changes

@@ -6,6 +6,8 @@ OCaml 4.04.0:
 Language features:
 - GPR#301: local exception declarations "let exception ... in"
   (Alain Frisch)
+- GPR#508: Allow shortcut for extension on semicolons: ;%foo
+  (Jeremie Dimino)
 
 Tools:
 - GPR#452: Make the output of ocamldep is more stable

parsing/parser.mly

@@ -1305,6 +1305,10 @@ seq_expr:
   | expr        %prec below_SEMI  { $1 }
   | expr SEMI                     { reloc_exp $1 }
   | expr SEMI seq_expr            { mkexp(Pexp_sequence($1, $3)) }
+  | expr SEMI PERCENT attr_id seq_expr
+      { let seq = mkexp(Pexp_sequence ($1, $5)) in
+        let payload = PStr [mkstrexp seq []] in
+        mkexp (Pexp_extension ($4, payload)) }
 ;
 labeled_simple_pattern:
     QUESTION LPAREN label_let_pattern opt_default RPAREN

testsuite/tests/parsing/shortcut_ext_attr.ml

@@ -11,6 +11,7 @@ let () =
   (if%foo[@foo] () then () else ()) ;
   while%foo[@foo] () do () done ;
   for%foo[@foo] x = () to () do () done ;
+  () ;%foo () ;
   assert%foo[@foo] true ;
   lazy%foo[@foo] x ;
   object%foo[@foo] end ;