Prepare for some extra :decl(...) possibilities.

These will cover things that today require code generation or are not
available at NQP level, but probably should be. It will allow Perl 6's
custom lexpad type to be eliminated in favor of the shared NQP one, as
well as simplifying code generation for certain constructs and giving
us a freer hand in how we compile time. Mostly, this just documents
the new things.

Author: jnthn

docs/qast.markdown

@@ -318,8 +318,35 @@ For example, the following block takes two positional parameters.
         ...
     )
 
+Parameter declarations can also be given:
+
+* :default(...) - a QAST tree that produces a default value for the parameter
+  if it is not passed. This makes it optional.
+* :slurpy(1) - specifies that the parameter is slurpy. Use :named(1) also for
+  a named slurpy parameter.
+
+Finally, there are a couple of other values of decl that work with lexicals.
+
+* static - means that the lexical should be given the value specified in the
+  :value(...) argument. Useful for things that wish to install symbols in the
+  lexical scope with the intention they'll not be mutated (for example, a type
+  declaration may be installed using this). No attempt is made to ensure you
+  do not re-bind such a symbol, but do not do this; runtimes are free to turn
+  lookups of static lexical symbols into direct references to the symbol.
+* contvar - means that the lexical should be initialized to a clone of the
+  :value(...) argument. Presumably, this represents some kind of container
+  type. There are no restrictions on re-binding.
+* statevar - same as for contvar, except the container created will be used
+  for all given closure clones. To be clear, cloning a code ref doesn't bring
+  state variables along. On the initial call, containers are formed in the way
+  that contvar forms them: by cloning the :value(...) argument.
+
 ## QAST::VarWithFallback
 
+In the context of a bind, or with native types, this is exactly the same as a
+QAST::Var. For fetches of object types, if a null is produced, the QAST tree
+in :fallback(...) will be run and the value that it evaluates to produced
+instead.
 
 ## QAST::BVal
 A QAST::Block can only appear once in the QAST tree. So what if you want to

src/QAST/Var.nqp

@@ -3,7 +3,7 @@ class QAST::Var is QAST::Node {
     has str $!scope;
     has str $!decl;
     has int $!slurpy;
-    has $!default;
+    has $!default_or_value;
 
     method name(*@value) {
         $!name := @value[0] if @value;
@@ -18,7 +18,8 @@ class QAST::Var is QAST::Node {
         !nqp::isnull_s($!decl) ?? $!decl !! ""
     }
     method slurpy(*@value)  { $!slurpy := @value[0] if @value; $!slurpy }
-    method default(*@value) { $!default := @value[0] if @value; $!default }
+    method default(*@value) { $!default_or_value := @value[0] if @value; $!default_or_value }
+    method value(*@value) { $!default_or_value := @value[0] if @value; $!default_or_value }
 
     method substitute_inline_placeholders(@fillers) {
         self