libsyntax: Implement assert as a macro (called fail_unless! on a transitionary basis to avoid conflicting with the keyword right now). r=brson

src/libsyntax/ext/expand.rs

@@ -300,6 +300,14 @@ fn core_macros() -> ~str {
         )
     )
 
+    macro_rules! fail_unless(
+        ($cond:expr) => {
+            if !$cond {
+                die!(~\"assertion failed: \" + stringify!($cond))
+            }
+        }
+    )
+
     macro_rules! condition (
 
         { $c:ident: $in:ty -> $out:ty; } => {

src/libsyntax/parse/token.rs

@@ -200,13 +200,16 @@ fn to_str(in: @ident_interner, t: Token) -> ~str {
       DOC_COMMENT(s) => *in.get(s),
       EOF => ~"<eof>",
       INTERPOLATED(ref nt) => {
+        match nt {
+            &nt_expr(e) => ::print::pprust::expr_to_str(e, in),
+            _ => {
                 ~"an interpolated " +
                     match (*nt) {
                       nt_item(*) => ~"item",
                       nt_block(*) => ~"block",
                       nt_stmt(*) => ~"statement",
                       nt_pat(*) => ~"pattern",
-              nt_expr(*) => ~"expression",
+                      nt_expr(*) => fail ~"should have been handled above",
                       nt_ty(*) => ~"type",
                       nt_ident(*) => ~"identifier",
                       nt_path(*) => ~"path",
@@ -216,6 +219,8 @@ fn to_str(in: @ident_interner, t: Token) -> ~str {
             }
         }
       }
+    }
+}
 
 pure fn can_begin_expr(t: Token) -> bool {
     match t {

src/test/run-fail/assert-as-macro.rs

@@ -0,0 +1,6 @@
+// error-pattern:assertion failed: 1 == 2
+
+fn main() {
+    fail_unless!(1 == 2);
+}
+