auto merge of #12998 : huonw/rust/log_syntax, r=alexcrichton

syntax: allow `trace_macros!` and `log_syntax!` in item position.

Previously

    trace_macros!(true)
    fn main() {}

would complain about `trace_macros` being an expression macro in item
position. This is a pointless limitation, because the macro is purely
compile-time, with no runtime effect. (And similarly for log_syntax.)

This also changes the behaviour of `trace_macros!` very slightly, it
used to be equivalent to

    macro_rules! trace_macros {
        (true $($_x: tt)*) => { true };
        (false $($_x: tt)*) => { false }
    }

I.e. you could invoke it with arbitrary trailing arguments, which were
ignored. It is changed to accept only exactly `true` or `false` (with no
trailing arguments) and expands to `()`.

src/libsyntax/ext/base.rs

@@ -119,13 +119,31 @@ impl MacResult {
     pub fn raw_dummy_expr(sp: codemap::Span) -> @ast::Expr {
         @ast::Expr {
             id: ast::DUMMY_NODE_ID,
-            node: ast::ExprTup(Vec::new()),
-            span: sp
+            node: ast::ExprLit(@codemap::respan(sp, ast::LitNil)),
+            span: sp,
         }
     }
     pub fn dummy_expr(sp: codemap::Span) -> MacResult {
         MRExpr(MacResult::raw_dummy_expr(sp))
     }
+    pub fn dummy_any(sp: codemap::Span) -> MacResult {
+        MRAny(~DummyMacResult { sp: sp })
+    }
+}
+struct DummyMacResult {
+    sp: codemap::Span
+}
+impl AnyMacro for DummyMacResult {
+    fn make_expr(&self) -> @ast::Expr {
+        MacResult::raw_dummy_expr(self.sp)
+    }
+    fn make_items(&self) -> SmallVector<@ast::Item> {
+        SmallVector::zero()
+    }
+    fn make_stmt(&self) -> @ast::Stmt {
+        @codemap::respan(self.sp,
+                         ast::StmtExpr(MacResult::raw_dummy_expr(self.sp), ast::DUMMY_NODE_ID))
+    }
 }
 
 /// An enum representing the different kinds of syntax extensions.