add opaque struct

1 files changed, 7 insertions, 1 deletions

diff --git a/language/type-definition.rst b/language/type-definition.rst
index 327fb4d..b596d0a 100644
--- a/language/type-definition.rst
+++ b/language/type-definition.rst
@@ -5,7 +5,8 @@ Defining Types
 
     Crowbar has three different kinds of user-defined types.
 
-.. crowbar:element:: StructDefinition <- 'struct' identifier '{' VariableDeclaration+ '}'
+.. crowbar:element:: StructDefinition <- NormalStructDefinition / OpaqueStructDefinition
+                     NormalStructDefinition <- 'struct' identifier '{' VariableDeclaration+ '}'
 
     A ``struct`` defines a composite type with several members.
 
@@ -13,6 +14,11 @@ Defining Types
 
         define struct layout in memory
 
+.. crowbar:element:: OpaqueStructDefinition <- 'opaque' 'struct' identifier ';'
+
+    An *opaque struct* is a struct whose name is part of an API boundary but whose contents are not.
+    Its size is left unspecified, and it can only be used as the target of a pointer.
+
 .. crowbar:element:: EnumDefinition <- 'enum' identifier '{' EnumMember (',' EnumMember)* ','? '}'
                      EnumMember <- identifier ('=' Expression)?