diff --git a/src/lib/generic_os.rs b/src/lib/generic_os.rs
index 38662e6f95ed..32df07b9c768 100644
--- a/src/lib/generic_os.rs
+++ b/src/lib/generic_os.rs
@@ -1,12 +1,13 @@
-import str::sbuf;
+import istr::sbuf;
 
 
 #[cfg(target_os = "linux")]
 #[cfg(target_os = "macos")]
 fn getenv(n: &istr) -> option::t<istr> {
-    let n = istr::to_estr(n);
-    let s = os::libc::getenv(str::buf(n));
-    ret if s as int == 0 {
+    let s = istr::as_buf(n, { |buf|
+        os::libc::getenv(buf)
+    });
+    ret if unsafe::reinterpret_cast(s) == 0 {
         option::none::<istr>
     } else {
         let s = unsafe::reinterpret_cast(s);
@@ -17,27 +18,30 @@ fn getenv(n: &istr) -> option::t<istr> {
 #[cfg(target_os = "linux")]
 #[cfg(target_os = "macos")]
 fn setenv(n: &istr, v: &istr) {
-    let n = istr::to_estr(n);
-    let v = istr::to_estr(v);
-    let nbuf = str::buf(n);
-    let vbuf = str::buf(v);
-    os::libc::setenv(nbuf, vbuf, 1);
+    // FIXME (868)
+    let _: () = istr::as_buf(n, { |nbuf|
+        // FIXME (868)
+        let _: () = istr::as_buf(v, { |vbuf|
+            os::libc::setenv(nbuf, vbuf, 1);
+        });
+    });
 }
 
 #[cfg(target_os = "win32")]
 fn getenv(n: &istr) -> option::t<istr> {
-    let n = istr::to_estr(n);
-    let nbuf = str::buf(n);
     let nsize = 256u;
     while true {
-        let vstr = str::alloc(nsize - 1u);
-        let vbuf = str::buf(vstr);
-        let res = os::kernel32::GetEnvironmentVariableA(nbuf, vbuf, nsize);
+        let v: [u8] = [];
+        vec::reserve(v, nsize);
+        let res = istr::as_buf(n, { |nbuf|
+            let vbuf = vec::to_ptr(v);
+            os::kernel32::GetEnvironmentVariableA(nbuf, vbuf, nsize)
+        });
         if res == 0u {
             ret option::none;
         } else if res < nsize {
-            let vbuf = unsafe::reinterpret_cast(vbuf);
-            ret option::some(istr::str_from_cstr(vbuf));
+            vec::unsafe::set_len(v, res);
+            ret option::some(istr::unsafe_from_bytes(v));
         } else { nsize = res; }
     }
     fail;
@@ -45,11 +49,12 @@ fn getenv(n: &istr) -> option::t<istr> {
 
 #[cfg(target_os = "win32")]
 fn setenv(n: &istr, v: &istr) {
-    let n = istr::to_estr(n);
-    let v = istr::to_estr(v);
-    let nbuf = str::buf(n);
-    let vbuf = str::buf(v);
-    os::kernel32::SetEnvironmentVariableA(nbuf, vbuf);
+    // FIXME (868)
+    let _: () = istr::as_buf(n, { |nbuf|
+        let _: () = istr::as_buf(v, { |vbuf|
+            os::kernel32::SetEnvironmentVariableA(nbuf, vbuf);
+        });
+    });
 }
 
 // Local Variables:
diff --git a/src/lib/io.rs b/src/lib/io.rs
index 00cc80d1a6b4..50cb1c668c3c 100644
--- a/src/lib/io.rs
+++ b/src/lib/io.rs
@@ -175,10 +175,12 @@ fn stdin() -> reader {
 }
 
 fn file_reader(path: &istr) -> reader {
-    let path = istr::to_estr(path);
-    let mode = "r";
-    let f = os::libc::fopen(str::buf(path), str::buf(mode));
-    if f as uint == 0u { log_err "error opening " + path; fail; }
+    let f = istr::as_buf(path, { |pathbuf|
+        istr::as_buf(~"r", { |modebuf|
+            os::libc::fopen(pathbuf, modebuf)
+        })
+    });
+    if f as uint == 0u { log_err ~"error opening " + path; fail; }
     ret new_reader(FILE_buf_reader(f, option::some(@FILE_res(f))));
 }
 
@@ -278,7 +280,6 @@ obj fd_buf_writer(fd: int, res: option::t<@fd_res>) {
 }
 
 fn file_buf_writer(path: &istr, flags: &[fileflag]) -> buf_writer {
-    let path = istr::to_estr(path);
     let fflags: int =
         os::libc_constants::O_WRONLY() | os::libc_constants::O_BINARY();
     for f: fileflag in flags {
@@ -289,10 +290,11 @@ fn file_buf_writer(path: &istr, flags: &[fileflag]) -> buf_writer {
           none. { }
         }
     }
-    let fd =
-        os::libc::open(str::buf(path), fflags,
+    let fd = istr::as_buf(path, { |pathbuf|
+        os::libc::open(pathbuf, fflags,
                        os::libc_constants::S_IRUSR() |
-                           os::libc_constants::S_IWUSR());
+                           os::libc_constants::S_IWUSR())
+    });
     if fd < 0 {
         log_err "error opening file for writing";
         log_err sys::rustrt::last_os_error();
@@ -365,10 +367,12 @@ fn file_writer(path: &istr, flags: &[fileflag]) -> writer {
 
 // FIXME: fileflags
 fn buffered_file_buf_writer(path: &istr) -> buf_writer {
-    let path = istr::to_estr(path);
-    let mode = "w";
-    let f = os::libc::fopen(str::buf(path), str::buf(mode));
-    if f as uint == 0u { log_err "error opening " + path; fail; }
+    let f = istr::as_buf(path, { |pathbuf|
+        istr::as_buf(~"w", { |modebuf|
+            os::libc::fopen(pathbuf, modebuf)
+        })
+    });
+    if f as uint == 0u { log_err ~"error opening " + path; fail; }
     ret FILE_writer(f, option::some(@FILE_res(f)));
 }
 
diff --git a/src/lib/linux_os.rs b/src/lib/linux_os.rs
index 4c67a01fc56f..8e2e24ffff37 100644
--- a/src/lib/linux_os.rs
+++ b/src/lib/linux_os.rs
@@ -1,6 +1,4 @@
 
-import str::sbuf;
-
 
 // FIXME Somehow merge stuff duplicated here and macosx_os.rs. Made difficult
 // by https://github.com/graydon/rust/issues#issue/268
@@ -9,11 +7,11 @@ native "cdecl" mod libc = "" {
     fn write(fd: int, buf: *u8, count: uint) -> int;
     fn fread(buf: *u8, size: uint, n: uint, f: libc::FILE) -> uint;
     fn fwrite(buf: *u8, size: uint, n: uint, f: libc::FILE) -> uint;
-    fn open(s: sbuf, flags: int, mode: uint) -> int;
+    fn open(s: istr::sbuf, flags: int, mode: uint) -> int;
     fn close(fd: int) -> int;
     type FILE;
-    fn fopen(path: sbuf, mode: sbuf) -> FILE;
-    fn fdopen(fd: int, mode: sbuf) -> FILE;
+    fn fopen(path: istr::sbuf, mode: istr::sbuf) -> FILE;
+    fn fdopen(fd: int, mode: istr::sbuf) -> FILE;
     fn fclose(f: FILE);
     fn fgetc(f: FILE) -> int;
     fn ungetc(c: int, f: FILE);
@@ -21,13 +19,13 @@ native "cdecl" mod libc = "" {
     fn fseek(f: FILE, offset: int, whence: int) -> int;
     fn ftell(f: FILE) -> int;
     type dir;
-    fn opendir(d: sbuf) -> dir;
+    fn opendir(d: istr::sbuf) -> dir;
     fn closedir(d: dir) -> int;
     type dirent;
     fn readdir(d: dir) -> dirent;
-    fn getenv(n: sbuf) -> sbuf;
-    fn setenv(n: sbuf, v: sbuf, overwrite: int) -> int;
-    fn unsetenv(n: sbuf) -> int;
+    fn getenv(n: istr::sbuf) -> istr::sbuf;
+    fn setenv(n: istr::sbuf, v: istr::sbuf, overwrite: int) -> int;
+    fn unsetenv(n: istr::sbuf) -> int;
     fn pipe(buf: *mutable int) -> int;
     fn waitpid(pid: int, status: &mutable int, options: int) -> int;
 }
@@ -64,7 +62,11 @@ fn pipe() -> {in: int, out: int} {
     ret {in: fds.in, out: fds.out};
 }
 
-fn fd_FILE(fd: int) -> libc::FILE { ret libc::fdopen(fd, str::buf("r")); }
+fn fd_FILE(fd: int) -> libc::FILE {
+    ret istr::as_buf(~"r", { |modebuf|
+        libc::fdopen(fd, modebuf)
+    });
+}
 
 fn waitpid(pid: int) -> int {
     let status = 0;
diff --git a/src/lib/macos_os.rs b/src/lib/macos_os.rs
index 2ac38c35aa89..7f2a1df5541f 100644
--- a/src/lib/macos_os.rs
+++ b/src/lib/macos_os.rs
@@ -1,16 +1,14 @@
 
-import str::sbuf;
-
 native "cdecl" mod libc = "" {
     fn read(fd: int, buf: *u8, count: uint) -> int;
     fn write(fd: int, buf: *u8, count: uint) -> int;
     fn fread(buf: *u8, size: uint, n: uint, f: libc::FILE) -> uint;
     fn fwrite(buf: *u8, size: uint, n: uint, f: libc::FILE) -> uint;
-    fn open(s: sbuf, flags: int, mode: uint) -> int;
+    fn open(s: istr::sbuf, flags: int, mode: uint) -> int;
     fn close(fd: int) -> int;
     type FILE;
-    fn fopen(path: sbuf, mode: sbuf) -> FILE;
-    fn fdopen(fd: int, mode: sbuf) -> FILE;
+    fn fopen(path: istr::sbuf, mode: istr::sbuf) -> FILE;
+    fn fdopen(fd: int, mode: istr::sbuf) -> FILE;
     fn fclose(f: FILE);
     fn fgetc(f: FILE) -> int;
     fn ungetc(c: int, f: FILE);
@@ -18,13 +16,13 @@ native "cdecl" mod libc = "" {
     fn fseek(f: FILE, offset: int, whence: int) -> int;
     fn ftell(f: FILE) -> int;
     type dir;
-    fn opendir(d: sbuf) -> dir;
+    fn opendir(d: istr::sbuf) -> dir;
     fn closedir(d: dir) -> int;
     type dirent;
     fn readdir(d: dir) -> dirent;
-    fn getenv(n: sbuf) -> sbuf;
-    fn setenv(n: sbuf, v: sbuf, overwrite: int) -> int;
-    fn unsetenv(n: sbuf) -> int;
+    fn getenv(n: istr::sbuf) -> istr::sbuf;
+    fn setenv(n: istr::sbuf, v: istr::sbuf, overwrite: int) -> int;
+    fn unsetenv(n: istr::sbuf) -> int;
     fn pipe(buf: *mutable int) -> int;
     fn waitpid(pid: int, status: &mutable int, options: int) -> int;
 }
@@ -61,7 +59,11 @@ fn pipe() -> {in: int, out: int} {
     ret {in: fds.in, out: fds.out};
 }
 
-fn fd_FILE(fd: int) -> libc::FILE { ret libc::fdopen(fd, str::buf("r")); }
+fn fd_FILE(fd: int) -> libc::FILE {
+    ret istr::as_buf(~"r", { |modebuf|
+        libc::fdopen(fd, modebuf)
+    });
+}
 
 fn waitpid(pid: int) -> int {
     let status = 0;
diff --git a/src/lib/run_program.rs b/src/lib/run_program.rs
index f8dfa56d950e..4fc546b07b16 100644
--- a/src/lib/run_program.rs
+++ b/src/lib/run_program.rs
@@ -1,5 +1,5 @@
 
-import str::sbuf;
+import istr::sbuf;
 
 export program;
 export run_program;
@@ -12,19 +12,21 @@ native "rust" mod rustrt {
        int;
 }
 
-fn arg_vec(prog: str, args: &[str]) -> [sbuf] {
-    let argptrs = [str::buf(prog)];
-    for arg: str in args { argptrs += [str::buf(arg)]; }
-    argptrs += [0 as sbuf];
+fn arg_vec(prog: &istr, args: &[@istr]) -> [sbuf] {
+    let argptrs = istr::as_buf(prog, { |buf| [buf] });
+    for arg in args {
+        argptrs += istr::as_buf(*arg, { |buf| [buf] });
+    }
+    argptrs += [unsafe::reinterpret_cast(0)];
     ret argptrs;
 }
 
 fn spawn_process(prog: &istr, args: &[istr], in_fd: int, out_fd: int,
                  err_fd: int) -> int {
-    let prog = istr::to_estr(prog);
-    let args = istr::to_estrs(args);
-    // Note: we have to hold on to this vector reference while we hold a
-    // pointer to its buffer
+    // Note: we have to hold on to these vector references while we hold a
+    // pointer to their buffers
+    let prog = prog;
+    let args = vec::map({ |&arg| @arg }, args);
     let argv = arg_vec(prog, args);
     let pid =
         rustrt::rust_run_program(vec::unsafe::to_ptr(argv),
diff --git a/src/lib/str.rs b/src/lib/str.rs
index 47366e4a5cb6..4c1ae9c49ff4 100644
--- a/src/lib/str.rs
+++ b/src/lib/str.rs
@@ -1,7 +1,7 @@
 
 import rustrt::sbuf;
 import uint::le;
-export sbuf;
+// export sbuf;
 // export rustrt;
 // export eq;
 // export lteq;
@@ -10,7 +10,7 @@ export sbuf;
 // export is_ascii;
 export alloc;
 // export byte_len;
-export buf;
+// export buf;
 // export bytes;
 // export unsafe_from_byte;
 // export str_from_cstr;
diff --git a/src/lib/win32_os.rs b/src/lib/win32_os.rs
index f7c3d4570ddf..b01f748d109b 100644
--- a/src/lib/win32_os.rs
+++ b/src/lib/win32_os.rs
@@ -1,16 +1,14 @@
 
-import str::sbuf;
-
 native "cdecl" mod libc = "" {
     fn read(fd: int, buf: *u8, count: uint) -> int;
     fn write(fd: int, buf: *u8, count: uint) -> int;
     fn fread(buf: *u8, size: uint, n: uint, f: libc::FILE) -> uint;
     fn fwrite(buf: *u8, size: uint, n: uint, f: libc::FILE) -> uint;
-    fn open(s: sbuf, flags: int, mode: uint) -> int = "_open";
+    fn open(s: istr::sbuf, flags: int, mode: uint) -> int = "_open";
     fn close(fd: int) -> int = "_close";
     type FILE;
-    fn fopen(path: sbuf, mode: sbuf) -> FILE;
-    fn _fdopen(fd: int, mode: sbuf) -> FILE;
+    fn fopen(path: istr::sbuf, mode: istr::sbuf) -> FILE;
+    fn _fdopen(fd: int, mode: istr::sbuf) -> FILE;
     fn fclose(f: FILE);
     fn fgetc(f: FILE) -> int;
     fn ungetc(c: int, f: FILE);
@@ -42,8 +40,9 @@ mod libc_constants {
 }
 
 native "x86stdcall" mod kernel32 {
-    fn GetEnvironmentVariableA(n: sbuf, v: sbuf, nsize: uint) -> uint;
-    fn SetEnvironmentVariableA(n: sbuf, v: sbuf) -> int;
+    fn GetEnvironmentVariableA(n: istr::sbuf, v: istr::sbuf,
+                               nsize: uint) -> uint;
+    fn SetEnvironmentVariableA(n: istr::sbuf, v: istr::sbuf) -> int;
 }
 
 fn exec_suffix() -> istr { ret ~".exe"; }
@@ -69,7 +68,11 @@ fn pipe() -> {in: int, out: int} {
     ret {in: fds.in, out: fds.out};
 }
 
-fn fd_FILE(fd: int) -> libc::FILE { ret libc::_fdopen(fd, str::buf("r")); }
+fn fd_FILE(fd: int) -> libc::FILE {
+    ret istr::as_buf(~"r", { |modebuf|
+        libc::_fdopen(fd, modebuf)
+    });
+}
 
 native "rust" mod rustrt {
     fn rust_process_wait(handle: int) -> int;
diff --git a/src/test/compile-fail/native-type-mismatch.rs b/src/test/compile-fail/native-type-mismatch.rs
index ddb6375e36d8..d2c28ae47f18 100644
--- a/src/test/compile-fail/native-type-mismatch.rs
+++ b/src/test/compile-fail/native-type-mismatch.rs
@@ -1,4 +1,4 @@
-// error-pattern:expected native but found native
+// error-pattern:expected *Mb but found native
 use std;
 
 fn main() {