diff --git a/src/comp/front/config.rs b/src/comp/front/config.rs
index 4aa1c07be57c..526bf20ebb76 100644
--- a/src/comp/front/config.rs
+++ b/src/comp/front/config.rs
@@ -77,7 +77,7 @@ fn fold_block(cfg: &ast::crate_cfg, b: &ast::blk_, fld: fold::ast_fold) ->
     let filtered_stmts = vec::filter_map(filter, b.stmts);
     ret {stmts: vec::map(fld.fold_stmt, filtered_stmts),
          expr: option::map(fld.fold_expr, b.expr),
-         id: b.id};
+         id: b.id, rules: b.rules};
 }
 
 fn item_in_cfg(cfg: &ast::crate_cfg, item: &@ast::item) -> bool {
diff --git a/src/comp/front/test.rs b/src/comp/front/test.rs
index 8e0b71caf848..d76e26064a7f 100644
--- a/src/comp/front/test.rs
+++ b/src/comp/front/test.rs
@@ -4,7 +4,8 @@ import std::option;
 import std::vec;
 import syntax::ast;
 import syntax::ast_util;
-import syntax::ast_util::dummy_sp;
+import syntax::ast_util::*;
+//import syntax::ast_util::dummy_sp;
 import syntax::fold;
 import syntax::print::pprust;
 import front::attr;
@@ -189,8 +190,8 @@ fn mk_tests(cx: &test_ctxt) -> @ast::item {
     // The vector of test_descs for this crate
     let test_descs = mk_test_desc_vec(cx);
 
-    let body_: ast::blk_ =
-        {stmts: [], expr: option::some(test_descs), id: cx.next_node_id()};
+    let body_: ast::blk_ = checked_blk([], option::some(test_descs),
+                                       cx.next_node_id());
     let body = nospan(body_);
 
     let fn_ = {decl: decl, proto: proto, body: body};
@@ -305,10 +306,8 @@ fn mk_main(cx: &test_ctxt) -> @ast::item {
 
     let test_main_call_expr = mk_test_main_call(cx);
 
-    let body_: ast::blk_ =
-        {stmts: [],
-         expr: option::some(test_main_call_expr),
-         id: cx.next_node_id()};
+    let body_: ast::blk_ = checked_blk([], option::some(test_main_call_expr),
+                                       cx.next_node_id());
     let body = {node: body_, span: dummy_sp()};
 
     let fn_ = {decl: decl, proto: proto, body: body};
diff --git a/src/comp/middle/typeck.rs b/src/comp/middle/typeck.rs
index ad199ba5cb3d..7519e2541f58 100644
--- a/src/comp/middle/typeck.rs
+++ b/src/comp/middle/typeck.rs
@@ -2081,7 +2081,12 @@ fn check_expr_with_unifier(fcx: &@fn_ctxt, expr: &@ast::expr, unify: &unifier,
         check_fn(fcx.ccx, f, id, some(fcx));
       }
       ast::expr_block(b) {
-        bot = check_block(fcx, b);
+        // If this is an unchecked block, turn off purity-checking
+        let fcx_for_block = alt b.node.rules {
+          ast::unchecked. { @{ purity: ast::impure_fn with *fcx } }
+          _               { fcx }
+        };
+        bot = check_block(fcx_for_block, b);
         let typ =
             alt b.node.expr {
               some(expr) { expr_ty(tcx, expr) }
diff --git a/src/comp/syntax/ast.rs b/src/comp/syntax/ast.rs
index 4caddb5e25bd..d44baebb6f3d 100644
--- a/src/comp/syntax/ast.rs
+++ b/src/comp/syntax/ast.rs
@@ -82,7 +82,8 @@ tag meta_item_ {
 
 type blk = spanned<blk_>;
 
-type blk_ = {stmts: [@stmt], expr: option::t<@expr>, id: node_id};
+type blk_ = {stmts: [@stmt], expr: option::t<@expr>,
+    id: node_id, rules: check_mode};
 
 type pat = {id: node_id, node: pat_, span: span};
 
@@ -223,6 +224,15 @@ tag expr_ {
     expr_uniq(@expr);
 }
 
+/*
+// Says whether this is a block the user marked as
+// "unchecked"
+tag blk_sort {
+    blk_unchecked; // declared as "exception to effect-checking rules"
+    blk_checked; // all typing rules apply
+}
+*/
+
 type mac = spanned<mac_>;
 
 tag mac_ {
diff --git a/src/comp/syntax/ast_util.rs b/src/comp/syntax/ast_util.rs
index 48bdfc47b68c..517e252dd9cc 100644
--- a/src/comp/syntax/ast_util.rs
+++ b/src/comp/syntax/ast_util.rs
@@ -184,10 +184,14 @@ fn eq_ty(a: &@ty, b: &@ty) -> bool { ret std::box::ptr_eq(a, b); }
 fn hash_ty(t: &@ty) -> uint { ret t.span.lo << 16u + t.span.hi; }
 
 fn block_from_expr(e: @expr) -> blk {
-    let blk_ = {stmts: [], expr: option::some::<@expr>(e), id: e.id};
+    let blk_ = checked_blk([], option::some::<@expr>(e), e.id);
     ret {node: blk_, span: e.span};
 }
 
+fn checked_blk(stmts1: [@stmt], expr1: option::t<@expr>, id1: node_id)
+    -> blk_ {
+    ret {stmts: stmts1, expr: expr1, id: id1, rules: checked};
+}
 
 fn obj_field_from_anon_obj_field(f: &anon_obj_field) -> obj_field {
     ret {mut: f.mut, ty: f.ty, ident: f.ident, id: f.id};
diff --git a/src/comp/syntax/fold.rs b/src/comp/syntax/fold.rs
index a91b2f31814b..770ed307e141 100644
--- a/src/comp/syntax/fold.rs
+++ b/src/comp/syntax/fold.rs
@@ -255,7 +255,8 @@ fn noop_fold_method(m: &method_, fld: ast_fold) -> method_ {
 fn noop_fold_block(b: &blk_, fld: ast_fold) -> blk_ {
     ret {stmts: vec::map(fld.fold_stmt, b.stmts),
          expr: option::map(fld.fold_expr, b.expr),
-         id: b.id};
+         id: b.id,
+         rules: b.rules};
 }
 
 fn noop_fold_stmt(s: &stmt_, fld: ast_fold) -> stmt_ {
diff --git a/src/comp/syntax/parse/parser.rs b/src/comp/syntax/parse/parser.rs
index f7329bd3b3b8..729338b4b302 100644
--- a/src/comp/syntax/parse/parser.rs
+++ b/src/comp/syntax/parse/parser.rs
@@ -837,7 +837,7 @@ fn parse_bottom_expr(p: &parser) -> @ast::expr {
         } else if p.peek() == token::BINOP(token::OR) {
             ret parse_fn_block_expr(p);
         } else {
-            let blk = parse_block_tail(p, lo);
+            let blk = parse_block_tail(p, lo, ast::checked);
             ret mk_expr(p, blk.span.lo, blk.span.hi, ast::expr_block(blk));
         }
     } else if eat_word(p, "if") {
@@ -860,6 +860,10 @@ fn parse_bottom_expr(p: &parser) -> @ast::expr {
         ret parse_fn_expr(p, ast::proto_block);
     } else if eat_word(p, "lambda") {
         ret parse_fn_expr(p, ast::proto_closure);
+    } else if eat_word(p, "unchecked") {
+        expect(p, token::LBRACE);
+        let blk = parse_block_tail(p, lo, ast::unchecked);
+        ret mk_expr(p, blk.span.lo, blk.span.hi, ast::expr_block(blk));
     } else if p.peek() == token::LBRACKET {
         p.bump();
         let mut = parse_mutability(p);
@@ -876,7 +880,7 @@ fn parse_bottom_expr(p: &parser) -> @ast::expr {
         ret mk_mac_expr(p, lo, p.get_hi_pos(), ast::mac_embed_type(ty))
     } else if p.peek() == token::POUND_LBRACE {
         p.bump();
-        let blk = ast::mac_embed_block(parse_block_tail(p, lo));
+        let blk = ast::mac_embed_block(parse_block_tail(p, lo, ast::checked));
         ret mk_mac_expr(p, lo, p.get_hi_pos(), blk);
     } else if p.peek() == token::ELLIPSIS {
         p.bump();
@@ -1309,7 +1313,7 @@ fn parse_fn_expr(p: &parser, proto: ast::proto) -> @ast::expr {
 fn parse_fn_block_expr(p: &parser) -> @ast::expr {
     let lo = p.get_last_lo_pos();
     let decl = parse_fn_block_decl(p);
-    let body = parse_block_tail(p, lo);
+    let body = parse_block_tail(p, lo, ast::checked);
     let _fn = {decl: decl, proto: ast::proto_block, body: body};
     ret mk_expr(p, lo, body.span.hi, ast::expr_fn(_fn));
 }
@@ -1664,12 +1668,20 @@ fn stmt_ends_with_semi(stmt: &ast::stmt) -> bool {
 
 fn parse_block(p: &parser) -> ast::blk {
     let lo = p.get_lo_pos();
-    expect(p, token::LBRACE);
-    be parse_block_tail(p, lo);
+    if eat_word(p, "unchecked") {
+        be parse_block_tail(p, lo, ast::unchecked);
+    }
+    else {
+        expect(p, token::LBRACE);
+        be parse_block_tail(p, lo, ast::checked);
+    }
 }
 
+// Precondition: already parsed the '{' or '#{'
+// I guess that also means "already parsed the 'impure'" if
+// necessary, and this should take a qualifier.
 // some blocks start with "#{"...
-fn parse_block_tail(p: &parser, lo: uint) -> ast::blk {
+fn parse_block_tail(p: &parser, lo: uint, s: ast::check_mode) -> ast::blk {
     let stmts: [@ast::stmt] = [];
     let expr: option::t<@ast::expr> = none;
     while p.peek() != token::RBRACE {
@@ -1710,7 +1722,7 @@ fn parse_block_tail(p: &parser, lo: uint) -> ast::blk {
     }
     let hi = p.get_hi_pos();
     p.bump();
-    let bloc = {stmts: stmts, expr: expr, id: p.get_id()};
+    let bloc = {stmts: stmts, expr: expr, id: p.get_id(), rules: s};
     ret spanned(lo, hi, bloc);
 }
 
diff --git a/src/comp/syntax/print/pprust.rs b/src/comp/syntax/print/pprust.rs
index 051c114002b5..52ee1eaeacc1 100644
--- a/src/comp/syntax/print/pprust.rs
+++ b/src/comp/syntax/print/pprust.rs
@@ -579,6 +579,11 @@ tag embed_type { block_macro; block_block_fn; block_normal; }
 
 fn print_possibly_embedded_block(s: &ps, blk: &ast::blk, embedded: embed_type,
                                  indented: uint) {
+    alt blk.node.rules {
+      ast::unchecked. { word(s.s, "unchecked"); }
+      _ {}
+    }
+
     maybe_print_comment(s, blk.span.lo);
     let ann_node = node_block(s, blk);
     s.ann.pre(ann_node);