introducing let-syntax

The let-syntax expander is different in that it doesn't apply
a mark to its token trees before expansion. This is used
for macro_rules, and it's because macro_rules is essentially
MTWT's let-syntax. You don't want to mark before expand sees
let-syntax, because there's no "after" syntax to mark again.

In some sense, the cleaner approach might be to introduce a new
AST node that macro_rules expands into; this would make it clearer
that the expansion of a macro is distinct from the addition of a
new macro binding.

This should work for now, though...

1 files changed, 3 insertions, 1 deletions

diff --git a/src/librustc/plugin/registry.rs b/src/librustc/plugin/registry.rs
index 587bedd502e..2581ba51c2e 100644
--- a/src/librustc/plugin/registry.rs
+++ b/src/librustc/plugin/registry.rs
@@ -13,7 +13,7 @@
 use lint::LintPassObject;
 
 use syntax::ext::base::{SyntaxExtension, NamedSyntaxExtension, NormalTT};
-use syntax::ext::base::{IdentTT, ItemDecorator, ItemModifier, BasicMacroExpander};
+use syntax::ext::base::{IdentTT, LetSyntaxTT, ItemDecorator, ItemModifier, BasicMacroExpander};
 use syntax::ext::base::{MacroExpanderFn};
 use syntax::codemap::Span;
 use syntax::parse::token;
@@ -57,6 +57,8 @@ impl Registry {
             IdentTT(ext, _) => IdentTT(ext, Some(self.krate_span)),
             ItemDecorator(ext) => ItemDecorator(ext),
             ItemModifier(ext) => ItemModifier(ext),
+            // there's probably a nicer way to signal this:
+            LetSyntaxTT(_, _) => fail!("can't register a new LetSyntax!"),
         }));
     }