Special syntax when the payload of attribute/extension is a single type expression: [@id : T], [%id : T]. Represented internally as (() : T).

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13918 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02

experimental/frisch/extension_points.txt

@@ -4,16 +4,26 @@ This file describes the changes on the extension_points branch.
 === Attributes
 
 Attributes are "decorations" of the syntax tree which are ignored by
-the type-checker.  An attribute is made of an identifier (written id)
-and a argument, which is an OCaml structure (i.e. a list of structure
-items), written s below.
+the type-checker.  An attribute is made of an identifier (written id below)
+and a payload (written s below).
 
-The identifier can be a lowercase or uppercase identifier (including
-OCaml keywords) or a sequence of such atomic identifiers separated with
-a dots (whitespaces are allowed around the dots).  In the Parsetree,
-the identifier is represented as a single string (without spaces).
+ * The identifier 'id' can be a lowercase or uppercase identifier
+   (including OCaml keywords) or a sequence of such atomic identifiers
+   separated with a dots (whitespaces are allowed around the dots).
+   In the Parsetree, the identifier is represented as a single string
+   (without spaces).
+
+ * The payload 's' is an OCaml structure (i.e. a list of structure items).
+   Note that a payload can be empty or reduced to a single expression
+   (both cases are valid structures).  To support attributes whose
+   payload needs to be a type expression, the special syntax:
+
+      : T
+
+   (where T is a type expression) is also accepted as a valid payload,
+   and is interpreted internally as the structure made of the single
+   expression (() : T).
 
-Note: a structure can be empty, or reduced to a single expression.
 
 Attributes on expressions, type expressions, module expressions, module type expressions,
 patterns, class expressions, class type expressions:

parsing/parser.mly

@@ -2003,6 +2003,9 @@ item_extension:
   LBRACKETPERCENTPERCENT attr_id ext_arg RBRACKET { ($2, $3) }
 ;
 ext_arg:
-  structure { $1 }
+    structure { $1 }
+  | COLON core_type {
+      [ mkstrexp (ghexp (Pexp_constraint (ghunit (), $2))) [] ]
+  }
 ;
 %%