diff --git a/src/comp/front/ast.rs b/src/comp/front/ast.rs
index 4ed513a3e06bd..46f5716861abd 100644
--- a/src/comp/front/ast.rs
+++ b/src/comp/front/ast.rs
@@ -185,7 +185,7 @@ tag expr_ {
     expr_field(@expr, ident, ann);
     expr_index(@expr, @expr, ann);
     expr_path(path, option.t[def], ann);
-    expr_ext(path, vec[@expr], option.t[@expr], ann);
+    expr_ext(path, vec[@expr], option.t[@expr], @expr, ann);
     expr_fail;
     expr_ret(option.t[@expr]);
     expr_put(option.t[@expr]);
diff --git a/src/comp/front/extfmt.rs b/src/comp/front/extfmt.rs
new file mode 100644
index 0000000000000..3f1e5c3039228
--- /dev/null
+++ b/src/comp/front/extfmt.rs
@@ -0,0 +1,568 @@
+/* The 'fmt' extension is modeled on the posix printf system.
+ *
+ * A posix conversion ostensibly looks like this:
+ *
+ * %[parameter][flags][width][.precision][length]type
+ *
+ * Given the different numeric type bestiary we have, we omit the 'length'
+ * parameter and support slightly different conversions for 'type':
+ *
+ * %[parameter][flags][width][.precision]type
+ *
+ * we also only support translating-to-rust a tiny subset of the possible
+ * combinations at the moment.
+ */
+
+import util.common;
+
+import std._str;
+import std._vec;
+import std.option;
+import std.option.none;
+import std.option.some;
+
+export expand_syntax_ext;
+
+tag signedness {
+    signed;
+    unsigned;
+}
+
+tag caseness {
+    case_upper;
+    case_lower;
+}
+
+tag ty {
+    ty_bool;
+    ty_str;
+    ty_char;
+    ty_int(signedness);
+    ty_bits;
+    ty_hex(caseness);
+    // FIXME: More types
+}
+
+tag flag {
+    flag_left_justify;
+    flag_left_zero_pad;
+    flag_left_space_pad;
+    flag_plus_if_positive;
+    flag_alternate;
+}
+
+tag count {
+    count_is(int);
+    count_is_param(int);
+    count_is_next_param;
+    count_implied;
+}
+
+// A formatted conversion from an expression to a string
+type conv = rec(option.t[int] param,
+                vec[flag] flags,
+                count width,
+                count precision,
+                ty ty);
+
+// A fragment of the output sequence
+tag piece {
+    piece_string(str);
+    piece_conv(conv);
+}
+
+// TODO: Need to thread parser through here to handle errors correctly
+fn expand_syntax_ext(vec[@ast.expr] args,
+                     option.t[@ast.expr] body) -> @ast.expr {
+
+    if (_vec.len[@ast.expr](args) == 0u) {
+        log "malformed #fmt call";
+        fail;
+    }
+
+    auto fmt = expr_to_str(args.(0));
+
+    log "Format string:";
+    log fmt;
+
+    auto pieces = parse_fmt_string(fmt);
+    auto args_len = _vec.len[@ast.expr](args);
+    auto fmt_args = _vec.slice[@ast.expr](args, 1u, args_len - 1u);
+    ret pieces_to_expr(pieces, args);
+}
+
+fn expr_to_str(@ast.expr expr) -> str {
+    alt (expr.node) {
+        case (ast.expr_lit(?l, _)) {
+            alt (l.node) {
+                case (ast.lit_str(?s)) {
+                    ret s;
+                }
+            }
+        }
+    }
+    log "malformed #fmt call";
+    fail;
+}
+
+fn parse_fmt_string(str s) -> vec[piece] {
+    let vec[piece] pieces = vec();
+    // FIXME: Should be counting codepoints instead of bytes
+    auto lim = _str.byte_len(s);
+    auto buf = "";
+
+    fn flush_buf(str buf, &vec[piece] pieces) -> str {
+        if (_str.byte_len(buf) > 0u) {
+            auto piece = piece_string(buf);
+            pieces += piece;
+        }
+        ret "";
+    }
+
+    auto i = 0u;
+    while (i < lim) {
+        auto curr = _str.substr(s, i, 1u);
+        if (_str.eq(curr, "%")) {
+            i += 1u;
+            if (i >= lim) {
+                log "unterminated conversion at end of string";
+                fail;
+            }
+            auto curr2 = _str.substr(s, i, 1u);
+            if (_str.eq(curr2, "%")) {
+                i += 1u;
+            } else {
+                buf = flush_buf(buf, pieces);
+                auto res = parse_conversion(s, i, lim);
+                pieces += res._0;
+                i = res._1;
+            }
+        } else {
+            buf += curr;
+            i += 1u;
+        }
+    }
+    buf = flush_buf(buf, pieces);
+    ret pieces;
+}
+
+fn peek_num(str s, uint i, uint lim) -> option.t[tup(uint, uint)] {
+    if (i >= lim) {
+        ret none[tup(uint, uint)];
+    }
+
+    // FIXME: Presumably s.(i) will return char eventually
+    auto c = s.(i);
+    if (!('0' as u8 <= c && c <= '9' as u8)) {
+        ret option.none[tup(uint, uint)];
+    }
+
+    auto n = (c - ('0' as u8)) as uint;
+    alt (peek_num(s, i + 1u, lim)) {
+        case (none[tup(uint, uint)]) {
+            ret some[tup(uint, uint)](tup(n, i + 1u));
+        }
+        case (some[tup(uint, uint)](?next)) {
+            auto m = next._0;
+            auto j = next._1;
+            ret some[tup(uint, uint)](tup(n * 10u + m, j));
+        }
+    }
+
+}
+
+fn parse_conversion(str s, uint i, uint lim) -> tup(piece, uint) {
+    auto parm = parse_parameter(s, i, lim);
+    auto flags = parse_flags(s, parm._1, lim);
+    auto width = parse_count(s, flags._1, lim);
+    auto prec = parse_precision(s, width._1, lim);
+    auto ty = parse_type(s, prec._1, lim);
+    ret tup(piece_conv(rec(param = parm._0,
+                           flags = flags._0,
+                           width = width._0,
+                           precision = prec._0,
+                           ty = ty._0)),
+            ty._1);
+}
+
+fn parse_parameter(str s, uint i, uint lim) -> tup(option.t[int], uint) {
+    if (i >= lim) {
+        ret tup(none[int], i);
+    }
+
+    auto num = peek_num(s, i, lim);
+    alt (num) {
+        case (none[tup(uint, uint)]) {
+            ret tup(none[int], i);
+        }
+        case (some[tup(uint, uint)](?t)) {
+            auto n = t._0;
+            auto j = t._1;
+            if (j < lim && s.(j) == '$' as u8) {
+                ret tup(some[int](n as int), j + 1u);
+            }
+            else {
+                ret tup(none[int], i);
+            }
+        }
+    }
+}
+
+fn parse_flags(str s, uint i, uint lim) -> tup(vec[flag], uint) {
+    let vec[flag] noflags = vec();
+
+    if (i >= lim) {
+        ret tup(noflags, i);
+    }
+
+    fn more_(flag f, str s, uint i, uint lim) -> tup(vec[flag], uint) {
+        auto next = parse_flags(s, i + 1u, lim);
+        auto rest = next._0;
+        auto j = next._1;
+        let vec[flag] curr = vec(f);
+        ret tup(curr + rest, j);
+    }
+
+    auto more = bind more_(_, s, i, lim);
+
+    auto f = s.(i);
+    if (f == ('-' as u8)) {
+        ret more(flag_left_justify);
+    } else if (f == ('0' as u8)) {
+        ret more(flag_left_zero_pad);
+    } else if (f == (' ' as u8)) {
+        ret more(flag_left_space_pad);
+    } else if (f == ('+' as u8)) {
+        ret more(flag_plus_if_positive);
+    } else if (f == ('#' as u8)) {
+        ret more(flag_alternate);
+    } else {
+        ret tup(noflags, i);
+    }
+}
+
+fn parse_count(str s, uint i, uint lim) -> tup(count, uint) {
+    if (i >= lim) {
+        ret tup(count_implied, i);
+    }
+
+    // FIXME: These inner functions are just to avoid a rustboot
+    // "Unsatisfied precondition constraint" bug with alts nested in ifs
+    fn parse_star_count(str s, uint i, uint lim) -> tup(count, uint) {
+        auto param = parse_parameter(s, i + 1u, lim);
+        auto j = param._1;
+        alt (param._0) {
+            case (none[int]) {
+                ret tup(count_is_next_param, j);
+            }
+            case (some[int](?n)) {
+                ret tup(count_is_param(n), j);
+            }
+        }
+    }
+
+    fn parse_count_(str s, uint i, uint lim) -> tup(count, uint) {
+        auto num = peek_num(s, i, lim);
+        alt (num) {
+            case (none[tup(uint, uint)]) {
+                ret tup(count_implied, i);
+            }
+            case (some[tup(uint, uint)](?num)) {
+                ret tup(count_is(num._0 as int), num._1);
+            }
+        }
+    }
+
+    if (s.(i) == ('*' as u8)) {
+        ret parse_star_count(s, i, lim);
+    } else {
+        ret parse_count_(s, i, lim);
+    }
+}
+
+fn parse_precision(str s, uint i, uint lim) -> tup(count, uint) {
+    if (i >= lim) {
+        ret tup(count_implied, i);
+    }
+
+    if (s.(i) == '.' as u8) {
+        ret parse_count(s, i + 1u, lim);
+    } else {
+        ret tup(count_implied, i);
+    }
+}
+
+fn parse_type(str s, uint i, uint lim) -> tup(ty, uint) {
+    if (i >= lim) {
+        log "missing type in conversion";
+        fail;
+    }
+
+    auto t;
+    auto tstr = _str.substr(s, i, 1u);
+    if (_str.eq(tstr, "b")) {
+        t = ty_bool;
+    } else if (_str.eq(tstr, "s")) {
+        t = ty_str;
+    } else if (_str.eq(tstr, "c")) {
+        t = ty_char;
+    } else if (_str.eq(tstr, "d")
+               || _str.eq(tstr, "i")) {
+        // TODO: Do we really want two signed types here?
+        // How important is it to be printf compatible?
+        t = ty_int(signed);
+    } else if (_str.eq(tstr, "u")) {
+        t = ty_int(unsigned);
+    } else if (_str.eq(tstr, "x")) {
+        t = ty_hex(case_lower);
+    } else if (_str.eq(tstr, "X")) {
+        t = ty_hex(case_upper);
+    } else if (_str.eq(tstr, "t")) {
+        t = ty_bits;
+    } else {
+        // FIXME: This is a hack to avoid 'unsatisfied precondition
+        // constraint' on uninitialized variable t below
+        t = ty_bool;
+        log "unknown type in conversion";
+        fail;
+    }
+
+    ret tup(t, i + 1u);
+}
+
+fn pieces_to_expr(vec[piece] pieces, vec[@ast.expr] args) -> @ast.expr {
+
+    fn make_new_lit(common.span sp, ast.lit_ lit) -> @ast.expr {
+        auto sp_lit = @parser.spanned[ast.lit_](sp, sp, lit);
+        auto expr = ast.expr_lit(sp_lit, ast.ann_none);
+        ret @parser.spanned[ast.expr_](sp, sp, expr);
+    }
+
+    fn make_new_str(common.span sp, str s) -> @ast.expr {
+        auto lit = ast.lit_str(s);
+        ret make_new_lit(sp, lit);
+    }
+
+    fn make_new_uint(common.span sp, uint u) -> @ast.expr {
+        auto lit = ast.lit_uint(u);
+        ret make_new_lit(sp, lit);
+    }
+
+    fn make_add_expr(common.span sp,
+                     @ast.expr lhs, @ast.expr rhs) -> @ast.expr {
+        auto binexpr = ast.expr_binary(ast.add, lhs, rhs, ast.ann_none);
+        ret @parser.spanned[ast.expr_](sp, sp, binexpr);
+    }
+
+    fn make_call(common.span sp, vec[ast.ident] fn_path,
+                 vec[@ast.expr] args) -> @ast.expr {
+        let vec[ast.ident] path_idents = fn_path;
+        let vec[@ast.ty] path_types = vec();
+        auto path = rec(idents = path_idents, types = path_types);
+        auto sp_path = parser.spanned[ast.path_](sp, sp, path);
+        auto pathexpr = ast.expr_path(sp_path, none[ast.def], ast.ann_none);
+        auto sp_pathexpr = @parser.spanned[ast.expr_](sp, sp, pathexpr);
+        auto callexpr = ast.expr_call(sp_pathexpr, args, ast.ann_none);
+        auto sp_callexpr = @parser.spanned[ast.expr_](sp, sp, callexpr);
+        ret sp_callexpr;
+    }
+
+    fn make_new_conv(conv cnv, @ast.expr arg) -> @ast.expr {
+
+        auto unsupported = "conversion not supported in #fmt string";
+
+        alt (cnv.param) {
+            case (option.none[int]) {
+            }
+            case (_) {
+                log unsupported;
+                fail;
+            }
+        }
+
+        if (_vec.len[flag](cnv.flags) != 0u) {
+            log unsupported;
+            fail;
+        }
+
+        alt (cnv.width) {
+            case (count_implied) {
+            }
+            case (_) {
+                log unsupported;
+                fail;
+            }
+        }
+
+        alt (cnv.precision) {
+            case (count_implied) {
+            }
+            case (_) {
+                log unsupported;
+                fail;
+            }
+        }
+
+        alt (cnv.ty) {
+            case (ty_str) {
+                ret arg;
+            }
+            case (ty_int(?sign)) {
+                alt (sign) {
+                    case (signed) {
+                        let vec[str] path = vec("std", "_int", "to_str");
+                        auto radix_expr = make_new_uint(arg.span, 10u);
+                        let vec[@ast.expr] args = vec(arg, radix_expr);
+                        ret make_call(arg.span, path, args);
+                    }
+                    case (unsigned) {
+                        let vec[str] path = vec("std", "_uint", "to_str");
+                        auto radix_expr = make_new_uint(arg.span, 10u);
+                        let vec[@ast.expr] args = vec(arg, radix_expr);
+                        ret make_call(arg.span, path, args);
+                    }
+                }
+            }
+            case (_) {
+                log unsupported;
+                fail;
+            }
+        }
+    }
+
+    fn log_conv(conv c) {
+        alt (c.param) {
+            case (some[int](?p)) {
+                log "param: " + std._int.to_str(p, 10u);
+            }
+            case (_) {
+                log "param: none";
+            }
+        }
+        for (flag f in c.flags) {
+            alt (f) {
+                case (flag_left_justify) {
+                    log "flag: left justify";
+                }
+                case (flag_left_zero_pad) {
+                    log "flag: left zero pad";
+                }
+                case (flag_left_space_pad) {
+                    log "flag: left space pad";
+                }
+                case (flag_plus_if_positive) {
+                    log "flag: plus if positive";
+                }
+                case (flag_alternate) {
+                    log "flag: alternate";
+                }
+            }
+        }
+        alt (c.width) {
+            case (count_is(?i)) {
+                log "width: count is " + std._int.to_str(i, 10u);
+            }
+            case (count_is_param(?i)) {
+                log "width: count is param " + std._int.to_str(i, 10u);
+            }
+            case (count_is_next_param) {
+                log "width: count is next param";
+            }
+            case (count_implied) {
+                log "width: count is implied";
+            }
+        }
+        alt (c.precision) {
+            case (count_is(?i)) {
+                log "prec: count is " + std._int.to_str(i, 10u);
+            }
+            case (count_is_param(?i)) {
+                log "prec: count is param " + std._int.to_str(i, 10u);
+            }
+            case (count_is_next_param) {
+                log "prec: count is next param";
+            }
+            case (count_implied) {
+                log "prec: count is implied";
+            }
+        }
+        alt (c.ty) {
+            case (ty_bool) {
+                log "type: bool";
+            }
+            case (ty_str) {
+                log "type: str";
+            }
+            case (ty_char) {
+                log "type: char";
+            }
+            case (ty_int(?s)) {
+                alt (s) {
+                    case (signed) {
+                        log "type: signed";
+                    }
+                    case (unsigned) {
+                        log "type: unsigned";
+                    }
+                }
+            }
+            case (ty_bits) {
+                log "type: bits";
+            }
+            case (ty_hex(?cs)) {
+                alt (cs) {
+                    case (case_upper) {
+                        log "type: uhex";
+                    }
+                    case (case_lower) {
+                        log "type: lhex";
+                    }
+                }
+            }
+        }
+    }
+
+    auto sp = args.(0).span;
+    auto n = 0u;
+    auto tmp_expr = make_new_str(sp, "");
+
+    for (piece p in pieces) {
+        alt (p) {
+            case (piece_string(?s)) {
+                auto s_expr = make_new_str(sp, s);
+                tmp_expr = make_add_expr(sp, tmp_expr, s_expr);
+            }
+            case (piece_conv(?conv)) {
+                if (n >= _vec.len[@ast.expr](args)) {
+                    log "too many conversions in #fmt string";
+                    fail;
+                }
+
+                // TODO: Remove debug logging
+                log "Building conversion:";
+                log_conv(conv);
+
+                n += 1u;
+                auto arg_expr = args.(n);
+                auto c_expr = make_new_conv(conv, arg_expr);
+                tmp_expr = make_add_expr(sp, tmp_expr, c_expr);
+            }
+        }
+    }
+
+    // TODO: Remove this debug logging
+    log "dumping expanded ast:";
+    log pretty.print_expr(tmp_expr);
+    ret tmp_expr;
+}
+
+//
+// Local Variables:
+// mode: rust
+// fill-column: 78;
+// indent-tabs-mode: nil
+// c-basic-offset: 4
+// buffer-file-coding-system: utf-8-unix
+// compile-command: "make -k -C ../.. 2>&1 | sed -e 's/\\/x\\//x:\\//g'";
+// End:
+//
diff --git a/src/comp/front/parser.rs b/src/comp/front/parser.rs
index f747c084b5bdc..50545a2f9b09d 100644
--- a/src/comp/front/parser.rs
+++ b/src/comp/front/parser.rs
@@ -656,7 +656,8 @@ impure fn parse_bottom_expr(parser p) -> @ast.expr {
                                            some(token.COMMA),
                                            pf, p);
             hi = es.span;
-            ex = ast.expr_ext(pth, es.node, none[@ast.expr], ast.ann_none);
+            ex = expand_syntax_ext(p, es.span, pth, es.node,
+                                   none[@ast.expr]);
         }
 
         case (token.FAIL) {
@@ -736,6 +737,34 @@ impure fn parse_bottom_expr(parser p) -> @ast.expr {
     ret @spanned(lo, hi, ex);
 }
 
+/*
+ * FIXME: This is a crude approximation of the syntax-extension system,
+ * for purposes of prototyping and/or hard-wiring any extensions we
+ * wish to use while bootstrapping. The eventual aim is to permit
+ * loading rust crates to process extensions, but this will likely
+ * require a rust-based frontend, or an ocaml-FFI-based connection to
+ * rust crates. At the moment we have neither.
+ */
+
+impure fn expand_syntax_ext(parser p, ast.span sp,
+                     &ast.path path, vec[@ast.expr] args,
+                     option.t[@ast.expr] body) -> ast.expr_ {
+
+    check (_vec.len[ast.ident](path.node.idents) > 0u);
+    auto extname = path.node.idents.(0);
+    if (_str.eq(extname, "fmt")) {
+        auto expanded = extfmt.expand_syntax_ext(args, body);
+        auto newexpr = ast.expr_ext(path, args, body,
+                                    expanded,
+                                    ast.ann_none);
+
+        ret newexpr;
+    } else {
+        p.err("unknown syntax extension");
+        fail;
+    }
+}
+
 impure fn extend_expr_by_ident(parser p, span lo, span hi,
                                @ast.expr e, ast.ident i) -> @ast.expr {
     auto e_ = e.node;
diff --git a/src/comp/front/pretty.rs b/src/comp/front/pretty.rs
new file mode 100644
index 0000000000000..267763e39972c
--- /dev/null
+++ b/src/comp/front/pretty.rs
@@ -0,0 +1,85 @@
+import std._int;
+import std._str;
+import std._uint;
+import std._vec;
+
+export print_expr;
+
+fn unknown() -> str {
+    ret "<unknown ast node>";
+}
+
+fn print_expr(@ast.expr expr) -> str {
+    alt (expr.node) {
+        case (ast.expr_lit(?lit, _)) {
+            ret print_lit(lit);
+        }
+        case (ast.expr_binary(?op, ?lhs, ?rhs, _)) {
+            ret print_expr_binary(op, lhs, rhs);
+        }
+        case (ast.expr_call(?path, ?args, _)) {
+            ret print_expr_call(path, args);
+        }
+        case (ast.expr_path(?path, _, _)) {
+            ret print_path(path);
+        }
+        case (_) {
+            ret unknown();
+        }
+    }
+}
+
+fn print_lit(@ast.lit lit) -> str {
+    alt (lit.node) {
+        case (ast.lit_str(?s)) {
+            ret "\"" + s + "\"";
+        }
+        case (ast.lit_int(?i)) {
+            ret _int.to_str(i, 10u);
+        }
+        case (ast.lit_uint(?u)) {
+            ret _uint.to_str(u, 10u);
+        }
+        case (_) {
+            ret unknown();
+        }
+    }
+}
+
+fn print_expr_binary(ast.binop op, @ast.expr lhs, @ast.expr rhs) -> str {
+    alt (op) {
+        case (ast.add) {
+            auto l = print_expr(lhs);
+            auto r = print_expr(rhs);
+            ret l + " + " + r;
+        }
+    }
+}
+
+fn print_expr_call(@ast.expr path_expr, vec[@ast.expr] args) -> str {
+    auto s = print_expr(path_expr);
+
+    s += "(";
+    fn print_expr_ref(&@ast.expr e) -> str { ret print_expr(e); }
+    auto mapfn = print_expr_ref;
+    auto argstrs = _vec.map[@ast.expr, str](mapfn, args);
+    s += _str.connect(argstrs, ", ");
+    s += ")";
+
+    ret s;
+}
+
+fn print_path(ast.path path) -> str {
+    ret _str.connect(path.node.idents, ".");
+}
+
+//
+// Local Variables:
+// mode: rust
+// fill-column: 78;
+// indent-tabs-mode: nil
+// c-basic-offset: 4
+// buffer-file-coding-system: utf-8-unix
+// compile-command: "make -k -C ../.. 2>&1 | sed -e 's/\\/x\\//x:\\//g'";
+// End:
+//
diff --git a/src/comp/middle/fold.rs b/src/comp/middle/fold.rs
index 0978307025294..c7041b263542b 100644
--- a/src/comp/middle/fold.rs
+++ b/src/comp/middle/fold.rs
@@ -154,6 +154,12 @@ type ast_fold[ENV] =
          &option.t[def] d,
          ann a) -> @expr)                         fold_expr_path,
 
+     (fn(&ENV e, &span sp,
+         &path p, vec[@expr] args,
+         option.t[@expr] body,
+         @expr expanded,
+         ann a) -> @expr)                         fold_expr_ext,
+
      (fn(&ENV e, &span sp) -> @expr)              fold_expr_fail,
 
      (fn(&ENV e, &span sp,
@@ -644,6 +650,14 @@ fn fold_expr[ENV](&ENV env, ast_fold[ENV] fld, &@expr e) -> @expr {
             ret fld.fold_expr_path(env_, e.span, p_, r, t);
         }
 
+        case (ast.expr_ext(?p, ?args, ?body, ?expanded, ?t)) {
+            // Only fold the expanded expression, not the
+            // expressions involved in syntax extension
+            auto exp = fold_expr(env_, fld, expanded);
+            ret fld.fold_expr_ext(env_, e.span, p, args, body,
+                                  exp, t);
+        }
+
         case (ast.expr_fail) {
             ret fld.fold_expr_fail(env_, e.span);
         }
@@ -1166,6 +1180,14 @@ fn identity_fold_expr_path[ENV](&ENV env, &span sp,
     ret @respan(sp, ast.expr_path(p, d, a));
 }
 
+fn identity_fold_expr_ext[ENV](&ENV env, &span sp,
+                               &path p, vec[@expr] args,
+                               option.t[@expr] body,
+                               @expr expanded,
+                               ann a) -> @expr {
+    ret @respan(sp, ast.expr_ext(p, args, body, expanded, a));
+}
+
 fn identity_fold_expr_fail[ENV](&ENV env, &span sp) -> @expr {
     ret @respan(sp, ast.expr_fail);
 }
@@ -1447,6 +1469,7 @@ fn new_identity_fold[ENV]() -> ast_fold[ENV] {
          fold_expr_field  = bind identity_fold_expr_field[ENV](_,_,_,_,_),
          fold_expr_index  = bind identity_fold_expr_index[ENV](_,_,_,_,_),
          fold_expr_path   = bind identity_fold_expr_path[ENV](_,_,_,_,_),
+         fold_expr_ext    = bind identity_fold_expr_ext[ENV](_,_,_,_,_,_,_),
          fold_expr_fail   = bind identity_fold_expr_fail[ENV](_,_),
          fold_expr_ret    = bind identity_fold_expr_ret[ENV](_,_,_),
          fold_expr_put    = bind identity_fold_expr_put[ENV](_,_,_),
diff --git a/src/comp/middle/trans.rs b/src/comp/middle/trans.rs
index e961acb4e9191..2594590a4b91a 100644
--- a/src/comp/middle/trans.rs
+++ b/src/comp/middle/trans.rs
@@ -3693,6 +3693,10 @@ fn trans_expr(@block_ctxt cx, @ast.expr e) -> result {
             ret trans_rec(cx, args, base, ann);
         }
 
+        case (ast.expr_ext(_, _, _, ?expanded, _)) {
+            ret trans_expr(cx, expanded);
+        }
+
         case (ast.expr_fail) {
             ret trans_fail(cx, e.span, "explicit failure");
         }
diff --git a/src/comp/middle/ty.rs b/src/comp/middle/ty.rs
index 8b9ef6d146f7f..478f54d1ab4f9 100644
--- a/src/comp/middle/ty.rs
+++ b/src/comp/middle/ty.rs
@@ -749,6 +749,7 @@ fn expr_ty(@ast.expr expr) -> @t {
         case (ast.expr_field(_, _, ?ann))     { ret ann_to_type(ann); }
         case (ast.expr_index(_, _, ?ann))     { ret ann_to_type(ann); }
         case (ast.expr_path(_, _, ?ann))      { ret ann_to_type(ann); }
+        case (ast.expr_ext(_, _, _, _, ?ann)) { ret ann_to_type(ann); }
 
         case (ast.expr_fail)                  { ret plain_ty(ty_nil); }
         case (ast.expr_log(_))                { ret plain_ty(ty_nil); }
diff --git a/src/comp/middle/typeck.rs b/src/comp/middle/typeck.rs
index ce1c59ec01f78..f8f7fc72719c2 100644
--- a/src/comp/middle/typeck.rs
+++ b/src/comp/middle/typeck.rs
@@ -1156,6 +1156,11 @@ fn demand_expr_full(&@fn_ctxt fcx, @ty.t expected, @ast.expr e,
                                  ann_to_type(ann), adk);
             e_1 = ast.expr_path(pth, d, ast.ann_type(t));
         }
+        case (ast.expr_ext(?p, ?args, ?body, ?expanded, ?ann)) {
+            auto t = demand_full(fcx, e.span, expected,
+                                 ann_to_type(ann), adk);
+            e_1 = ast.expr_ext(p, args, body, expanded, ast.ann_type(t));
+        }
         case (ast.expr_fail) { e_1 = e.node; }
         case (ast.expr_log(_)) { e_1 = e.node; }
         case (ast.expr_ret(_)) { e_1 = e.node; }
@@ -1508,6 +1513,14 @@ fn check_expr(&@fn_ctxt fcx, @ast.expr expr) -> @ast.expr {
                                                       ast.ann_type(t)));
         }
 
+        case (ast.expr_ext(?p, ?args, ?body, ?expanded, _)) {
+            auto exp_ = check_expr(fcx, expanded);
+            auto t = expr_ty(exp_);
+            ret @fold.respan[ast.expr_](expr.span,
+                                        ast.expr_ext(p, args, body, exp_,
+                                                     ast.ann_type(t)));
+        }
+
         case (ast.expr_fail) {
             ret expr;
         }
diff --git a/src/comp/rustc.rc b/src/comp/rustc.rc
index bc4aaa52c1aef..43a0411768085 100644
--- a/src/comp/rustc.rc
+++ b/src/comp/rustc.rc
@@ -5,8 +5,10 @@ use std;
 
 mod front {
     mod ast;
+    mod extfmt;
     mod lexer;
     mod parser;
+    mod pretty;
     mod token;
     mod eval;
 }
diff --git a/src/rt/memory_region.cpp b/src/rt/memory_region.cpp
index 7ede3761e9a3e..fb19620f8b951 100644
--- a/src/rt/memory_region.cpp
+++ b/src/rt/memory_region.cpp
@@ -1,7 +1,7 @@
 #include "rust_internal.h"
 #include "memory_region.h"
 
-// #define TRACK_ALLOCATIONS
+#define TRACK_ALLOCATIONS
 
 memory_region::memory_region(rust_srv *srv, bool synchronized) :
     _srv(srv), _parent(NULL), _live_allocations(0),
diff --git a/src/test/run-pass/syntax-extension-fmt.rs b/src/test/run-pass/syntax-extension-fmt.rs
index 65e7647ee8b50..ebb09f96ba2c9 100644
--- a/src/test/run-pass/syntax-extension-fmt.rs
+++ b/src/test/run-pass/syntax-extension-fmt.rs
@@ -1,5 +1,16 @@
 use std;
+import std._str;
+
+fn test(str actual, str expected) {
+  log actual;
+  log expected;
+  check (_str.eq(actual, expected));
+}
+
 fn main() {
-  auto s = #fmt("hello %d friends and %s things", 10, "formatted");
-  log s;
+  test(#fmt("hello %d friends and %s things", 10, "formatted"),
+    "hello 10 friends and formatted things");
+  test(#fmt("d: %d", 1), "d: 1");
+  test(#fmt("i: %i", 2), "i: 2");
+  test(#fmt("s: %s", "test"), "s: test");
 }