diff --git a/src/librustc/driver/driver.rs b/src/librustc/driver/driver.rs
index b5f9ee68f7c..681f9a4895b 100644
--- a/src/librustc/driver/driver.rs
+++ b/src/librustc/driver/driver.rs
@@ -600,7 +600,7 @@ pub fn pretty_print_input(sess: Session,
                         &crate,
                         source_name(input),
                         rdr as @mut io::Reader,
-                        @mut stdout as @mut io::Writer,
+                        ~stdout as ~io::Writer,
                         annotation,
                         is_expanded);
 }
diff --git a/src/librustc/middle/dataflow.rs b/src/librustc/middle/dataflow.rs
index 6d0c2b497ca..b0ca03181f8 100644
--- a/src/librustc/middle/dataflow.rs
+++ b/src/librustc/middle/dataflow.rs
@@ -347,12 +347,12 @@ impl<O:DataFlowOperator+Clone+'static> DataFlowContext<O> {
         debug!("Dataflow result:");
         debug!("{}", {
             let this = @(*self).clone();
-            this.pretty_print_to(@mut io::stderr() as @mut io::Writer, blk);
+            this.pretty_print_to(~io::stderr() as ~io::Writer, blk);
             ""
         });
     }
 
-    fn pretty_print_to(@self, wr: @mut io::Writer, blk: &ast::Block) {
+    fn pretty_print_to(@self, wr: ~io::Writer, blk: &ast::Block) {
         let mut ps = pprust::rust_printer_annotated(wr,
                                                     self.tcx.sess.intr(),
                                                     self as @pprust::pp_ann);
diff --git a/src/libsyntax/print/pp.rs b/src/libsyntax/print/pp.rs
index ad55d64494b..5afe6266c99 100644
--- a/src/libsyntax/print/pp.rs
+++ b/src/libsyntax/print/pp.rs
@@ -148,7 +148,7 @@ pub struct print_stack_elt {
 
 pub static size_infinity: int = 0xffff;
 
-pub fn mk_printer(out: @mut io::Writer, linewidth: uint) -> Printer {
+pub fn mk_printer(out: ~io::Writer, linewidth: uint) -> Printer {
     // Yes 3, it makes the ring buffers big enough to never
     // fall behind.
     let n: uint = 3 * linewidth;
@@ -255,7 +255,7 @@ pub fn mk_printer(out: @mut io::Writer, linewidth: uint) -> Printer {
  * called 'print'.
  */
 pub struct Printer {
-    out: @mut io::Writer,
+    out: ~io::Writer,
     buf_len: uint,
     margin: int, // width of lines we're constrained to
     space: int, // number of spaces left on line
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index 7cf47cc1fd4..5a529120b07 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -27,6 +27,7 @@ use print::pp::{breaks, consistent, inconsistent, eof};
 use print::pp;
 use print::pprust;
 
+use std::cast;
 use std::char;
 use std::str;
 use std::io;
@@ -86,11 +87,11 @@ pub fn end(s: &mut ps) {
     pp::end(&mut s.s);
 }
 
-pub fn rust_printer(writer: @mut io::Writer, intr: @ident_interner) -> ps {
+pub fn rust_printer(writer: ~io::Writer, intr: @ident_interner) -> ps {
     return rust_printer_annotated(writer, intr, @no_ann::new() as @pp_ann);
 }
 
-pub fn rust_printer_annotated(writer: @mut io::Writer,
+pub fn rust_printer_annotated(writer: ~io::Writer,
                               intr: @ident_interner,
                               ann: @pp_ann)
                               -> ps {
@@ -122,7 +123,7 @@ pub fn print_crate(cm: @CodeMap,
                    crate: &ast::Crate,
                    filename: @str,
                    input: @mut io::Reader,
-                   out: @mut io::Writer,
+                   out: ~io::Writer,
                    ann: @pp_ann,
                    is_expanded: bool) {
     let (cmnts, lits) = comments::gather_comments_and_literals(
@@ -203,26 +204,40 @@ pub fn path_to_str(p: &ast::Path, intr: @ident_interner) -> ~str {
 pub fn fun_to_str(decl: &ast::fn_decl, purity: ast::purity, name: ast::Ident,
                   opt_explicit_self: Option<ast::explicit_self_>,
                   generics: &ast::Generics, intr: @ident_interner) -> ~str {
-    let wr = @mut MemWriter::new();
-    let mut s = rust_printer(wr as @mut io::Writer, intr);
+    let wr = ~MemWriter::new();
+    let mut s = rust_printer(wr as ~io::Writer, intr);
     print_fn(&mut s, decl, Some(purity), AbiSet::Rust(),
              name, generics, opt_explicit_self, ast::inherited);
     end(&mut s); // Close the head box
     end(&mut s); // Close the outer box
     eof(&mut s.s);
-    str::from_utf8_owned(wr.inner_ref().to_owned())
+
+    // XXX(pcwalton): Need checked downcasts.
+    unsafe {
+        let (_, wr): (uint, ~MemWriter) = cast::transmute(s.s.out);
+        let result = str::from_utf8_owned(wr.inner_ref().to_owned());
+        cast::forget(wr);
+        result
+    }
 }
 
 pub fn block_to_str(blk: &ast::Block, intr: @ident_interner) -> ~str {
-    let wr = @mut MemWriter::new();
-    let mut s = rust_printer(wr as @mut io::Writer, intr);
+    let wr = ~MemWriter::new();
+    let mut s = rust_printer(wr as ~io::Writer, intr);
     // containing cbox, will be closed by print-block at }
     cbox(&mut s, indent_unit);
     // head-ibox, will be closed by print-block after {
     ibox(&mut s, 0u);
     print_block(&mut s, blk);
     eof(&mut s.s);
-    str::from_utf8_owned(wr.inner_ref().to_owned())
+
+    // XXX(pcwalton): Need checked downcasts.
+    unsafe {
+        let (_, wr): (uint, ~MemWriter) = cast::transmute(s.s.out);
+        let result = str::from_utf8_owned(wr.inner_ref().to_owned());
+        cast::forget(wr);
+        result
+    }
 }
 
 pub fn meta_item_to_str(mi: &ast::MetaItem, intr: @ident_interner) -> ~str {
@@ -2304,11 +2319,17 @@ pub fn print_string(s: &mut ps, st: &str, style: ast::StrStyle) {
 }
 
 pub fn to_str<T>(t: &T, f: |&mut ps, &T|, intr: @ident_interner) -> ~str {
-    let wr = @mut MemWriter::new();
-    let mut s = rust_printer(wr as @mut io::Writer, intr);
+    let wr = ~MemWriter::new();
+    let mut s = rust_printer(wr as ~io::Writer, intr);
     f(&mut s, t);
     eof(&mut s.s);
-    str::from_utf8_owned(wr.inner_ref().to_owned())
+    // XXX(pcwalton): Need checked downcasts.
+    unsafe {
+        let (_, wr): (uint, ~MemWriter) = cast::transmute(s.s.out);
+        let result = str::from_utf8_owned(wr.inner_ref().to_owned());
+        cast::forget(wr);
+        result
+    }
 }
 
 pub fn next_comment(s: &mut ps) -> Option<comments::cmnt> {