diff --git a/src/libcore/comm.rs b/src/libcore/comm.rs
index eeed0497c5e..280464e35f5 100644
--- a/src/libcore/comm.rs
+++ b/src/libcore/comm.rs
@@ -180,7 +180,7 @@ fn select2<A: send, B: send>(
         rustrt::rust_port_select(dptr, ports, n_ports, yield)
     }
 
-    let ports = [];
+    let mut ports = [];
     ports += [***p_a, ***p_b];
     let n_ports = 2 as ctypes::size_t;
     let yield = 0u;
diff --git a/src/libcore/either.rs b/src/libcore/either.rs
index 31f43a4c9dc..0daee24650d 100644
--- a/src/libcore/either.rs
+++ b/src/libcore/either.rs
@@ -40,7 +40,7 @@ Function: lefts
 Extracts from a vector of either all the left values.
 */
 fn lefts<T: copy, U>(eithers: [t<T, U>]) -> [T] {
-    let result: [T] = [];
+    let mut result: [T] = [];
     for elt: t<T, U> in eithers {
         alt elt { left(l) { result += [l]; } _ {/* fallthrough */ } }
     }
@@ -53,7 +53,7 @@ Function: rights
 Extracts from a vector of either all the right values
 */
 fn rights<T, U: copy>(eithers: [t<T, U>]) -> [U] {
-    let result: [U] = [];
+    let mut result: [U] = [];
     for elt: t<T, U> in eithers {
         alt elt { right(r) { result += [r]; } _ {/* fallthrough */ } }
     }
@@ -70,8 +70,8 @@ right values.
 */
 fn partition<T: copy, U: copy>(eithers: [t<T, U>])
     -> {lefts: [T], rights: [U]} {
-    let lefts: [T] = [];
-    let rights: [U] = [];
+    let mut lefts: [T] = [];
+    let mut rights: [U] = [];
     for elt: t<T, U> in eithers {
         alt elt { left(l) { lefts += [l]; } right(r) { rights += [r]; } }
     }
diff --git a/src/libcore/extfmt.rs b/src/libcore/extfmt.rs
index 288ae779a2a..106b478e9bf 100644
--- a/src/libcore/extfmt.rs
+++ b/src/libcore/extfmt.rs
@@ -81,9 +81,9 @@ mod ct {
     type error_fn = fn@(str) -> ! ;
 
     fn parse_fmt_string(s: str, error: error_fn) -> [piece] unsafe {
-        let pieces: [piece] = [];
+        let mut pieces: [piece] = [];
         let lim = str::len(s);
-        let buf = "";
+        let mut buf = "";
         fn flush_buf(buf: str, &pieces: [piece]) -> str {
             if str::len(buf) > 0u {
                 let piece = piece_string(buf);
@@ -91,7 +91,7 @@ mod ct {
             }
             ret "";
         }
-        let i = 0u;
+        let mut i = 0u;
         while i < lim {
             let curr = str::slice(s, i, i+1u);
             if str::eq(curr, "%") {
@@ -285,7 +285,7 @@ mod rt {
     fn conv_int(cv: conv, i: int) -> str {
         let radix = 10u;
         let prec = get_int_precision(cv);
-        let s = int_to_str_prec(i, radix, prec);
+        let mut s = int_to_str_prec(i, radix, prec);
         if 0 <= i {
             if have_flag(cv.flags, flag_sign_always) {
                 s = "+" + s;
@@ -335,7 +335,7 @@ mod rt {
               count_is(c) { (float::to_str_exact, c as uint) }
               count_implied { (float::to_str, 6u) }
         };
-        let s = to_str(f, digits);
+        let mut s = to_str(f, digits);
         if 0.0 <= f {
             if have_flag(cv.flags, flag_sign_always) {
                 s = "+" + s;
@@ -389,42 +389,38 @@ mod rt {
     }
     enum pad_mode { pad_signed, pad_unsigned, pad_nozero, }
     fn pad(cv: conv, s: str, mode: pad_mode) -> str unsafe {
-        let uwidth;
-        alt cv.width {
+        let uwidth = alt cv.width {
           count_implied { ret s; }
           count_is(width) {
             // FIXME: Maybe width should be uint
-
-            uwidth = width as uint;
+            width as uint
           }
-        }
+        };
         let strlen = str::char_len(s);
         if uwidth <= strlen { ret s; }
-        let padchar = ' ';
+        let mut padchar = ' ';
         let diff = uwidth - strlen;
         if have_flag(cv.flags, flag_left_justify) {
             let padstr = str_init_elt(diff, padchar);
             ret s + padstr;
         }
-        let might_zero_pad = false;
-        let signed = false;
-        alt mode {
-          pad_nozero {
-            // fallthrough
-
-          }
-          pad_signed { might_zero_pad = true; signed = true; }
-          pad_unsigned { might_zero_pad = true; }
-        }
+        let {might_zero_pad, signed} = alt mode {
+          pad_nozero {   {might_zero_pad:false, signed:false} }
+          pad_signed {   {might_zero_pad:true,  signed:true } }
+          pad_unsigned { {might_zero_pad:true,  signed:false} }
+        };
         fn have_precision(cv: conv) -> bool {
             ret alt cv.precision { count_implied { false } _ { true } };
         }
-        let zero_padding = false;
-        if might_zero_pad && have_flag(cv.flags, flag_left_zero_pad) &&
-               !have_precision(cv) {
-            padchar = '0';
-            zero_padding = true;
-        }
+        let zero_padding = {
+            if might_zero_pad && have_flag(cv.flags, flag_left_zero_pad) &&
+                !have_precision(cv) {
+                padchar = '0';
+                true
+            } else {
+                false
+            }
+        };
         let padstr = str_init_elt(diff, padchar);
         // This is completely heinous. If we have a signed value then
         // potentially rip apart the intermediate result and insert some
diff --git a/src/libcore/float.rs b/src/libcore/float.rs
index 6f23349d2a9..09aa575e003 100644
--- a/src/libcore/float.rs
+++ b/src/libcore/float.rs
@@ -49,14 +49,14 @@ exact - Whether to enforce the exact number of significant digits
 */
 fn to_str_common(num: float, digits: uint, exact: bool) -> str {
     if is_NaN(num) { ret "NaN"; }
-    let (num, accum) = if num < 0.0 { (-num, "-") } else { (num, "") };
+    let mut (num, accum) = if num < 0.0 { (-num, "-") } else { (num, "") };
     let trunc = num as uint;
-    let frac = num - (trunc as float);
+    let mut frac = num - (trunc as float);
     accum += uint::str(trunc);
     if (frac < epsilon && !exact) || digits == 0u { ret accum; }
     accum += ".";
-    let i = digits;
-    let epsilon = 1. / pow_with_uint(10u, i);
+    let mut i = digits;
+    let mut epsilon = 1. / pow_with_uint(10u, i);
     while i > 0u && (frac >= epsilon || exact) {
         frac *= 10.0;
         epsilon *= 10.0;
@@ -132,13 +132,13 @@ Otherwise, some(n) where n is the floating-point
 number represented by [num].
 */
 fn from_str(num: str) -> option<float> {
-   let pos = 0u;                  //Current byte position in the string.
-                                  //Used to walk the string in O(n).
-   let len = str::len(num);  //Length of the string, in bytes.
+   let mut pos = 0u;               //Current byte position in the string.
+                                   //Used to walk the string in O(n).
+   let len = str::len(num);        //Length of the string, in bytes.
 
    if len == 0u { ret none; }
-   let total = 0f;                //Accumulated result
-   let c     = 'z';               //Latest char.
+   let mut total = 0f;             //Accumulated result
+   let mut c     = 'z';            //Latest char.
 
    //The string must start with one of the following characters.
    alt str::char_at(num, 0u) {
@@ -147,7 +147,7 @@ fn from_str(num: str) -> option<float> {
    }
 
    //Determine if first char is '-'/'+'. Set [pos] and [neg] accordingly.
-   let neg = false;               //Sign of the result
+   let mut neg = false;               //Sign of the result
    alt str::char_at(num, 0u) {
       '-' {
           neg = true;
@@ -179,7 +179,7 @@ fn from_str(num: str) -> option<float> {
    }
 
    if c == '.' {//Examine decimal part
-      let decimal = 1f;
+      let mut decimal = 1f;
       while(pos < len) {
          let char_range = str::char_range_at(num, pos);
          c = char_range.ch;
@@ -200,8 +200,8 @@ fn from_str(num: str) -> option<float> {
    }
 
    if (c == 'e') | (c == 'E') {//Examine exponent
-      let exponent = 0u;
-      let neg_exponent = false;
+      let mut exponent = 0u;
+      let mut neg_exponent = false;
       if(pos < len) {
           let char_range = str::char_range_at(num, pos);
           c   = char_range.ch;
@@ -275,9 +275,9 @@ fn pow_with_uint(base: uint, pow: uint) -> float {
       }
        ret 0.;
    }
-   let my_pow     = pow;
-   let total      = 1f;
-   let multiplier = base as float;
+   let mut my_pow     = pow;
+   let mut total      = 1f;
+   let mut multiplier = base as float;
    while (my_pow > 0u) {
      if my_pow % 2u == 1u {
        total = total * multiplier;
diff --git a/src/libcore/future.rs b/src/libcore/future.rs
index fe535ebce5b..43d0e55edf0 100644
--- a/src/libcore/future.rs
+++ b/src/libcore/future.rs
@@ -93,7 +93,7 @@ fn spawn<A:send>(+blk: fn~() -> A) -> future<A> {
 
     "];
 
-    let po = comm::port();
+    let mut po = comm::port();
     let ch = comm::chan(po);
     task::spawn {||
         comm::send(ch, blk())
diff --git a/src/libcore/i16.rs b/src/libcore/i16.rs
index fc6dd5f34de..b3f10a3a979 100644
--- a/src/libcore/i16.rs
+++ b/src/libcore/i16.rs
@@ -26,7 +26,7 @@ pure fn nonnegative(x: i16) -> bool { x >= 0i16 }
 
 #[doc = "Iterate over the range [`lo`..`hi`)"]
 fn range(lo: i16, hi: i16, it: fn(i16)) {
-    let i = lo;
+    let mut i = lo;
     while i < hi { it(i); i += 1i16; }
 }
 
diff --git a/src/libcore/i32.rs b/src/libcore/i32.rs
index 519752682b8..1e151f5d39d 100644
--- a/src/libcore/i32.rs
+++ b/src/libcore/i32.rs
@@ -26,7 +26,7 @@ pure fn nonnegative(x: i32) -> bool { x >= 0i32 }
 
 #[doc = "Iterate over the range [`lo`..`hi`)"]
 fn range(lo: i32, hi: i32, it: fn(i32)) {
-    let i = lo;
+    let mut i = lo;
     while i < hi { it(i); i += 1i32; }
 }
 
diff --git a/src/libcore/i64.rs b/src/libcore/i64.rs
index b8d12df92a1..1eecf56c0b7 100644
--- a/src/libcore/i64.rs
+++ b/src/libcore/i64.rs
@@ -26,7 +26,7 @@ pure fn nonnegative(x: i64) -> bool { x >= 0i64 }
 
 #[doc = "Iterate over the range [`lo`..`hi`)"]
 fn range(lo: i64, hi: i64, it: fn(i64)) {
-    let i = lo;
+    let mut i = lo;
     while i < hi { it(i); i += 1i64; }
 }
 
diff --git a/src/libcore/i8.rs b/src/libcore/i8.rs
index 1d96e0b247b..f88ca0f5892 100644
--- a/src/libcore/i8.rs
+++ b/src/libcore/i8.rs
@@ -26,7 +26,7 @@ pure fn nonnegative(x: i8) -> bool { x >= 0i8 }
 
 #[doc = "Iterate over the range [`lo`..`hi`)"]
 fn range(lo: i8, hi: i8, it: fn(i8)) {
-    let i = lo;
+    let mut i = lo;
     while i < hi { it(i); i += 1i8; }
 }
 
diff --git a/src/libcore/int.rs b/src/libcore/int.rs
index 8567fc7ccc8..8c3b8f195f7 100644
--- a/src/libcore/int.rs
+++ b/src/libcore/int.rs
@@ -90,7 +90,7 @@ Function: range
 Iterate over the range [`lo`..`hi`)
 */
 fn range(lo: int, hi: int, it: fn(int)) {
-    let i = lo;
+    let mut i = lo;
     while i < hi { it(i); i += 1; }
 }
 
@@ -106,15 +106,15 @@ radix - The base of the number
 */
 fn parse_buf(buf: [u8], radix: uint) -> option<int> {
     if vec::len(buf) == 0u { ret none; }
-    let i = vec::len(buf) - 1u;
-    let start = 0u;
-    let power = 1;
+    let mut i = vec::len(buf) - 1u;
+    let mut start = 0u;
+    let mut power = 1;
 
     if buf[0] == ('-' as u8) {
         power = -1;
         start = 1u;
     }
-    let n = 0;
+    let mut n = 0;
     while true {
         alt char::to_digit(buf[i] as char, radix) {
           some(d) { n += (d as int) * power; }
@@ -161,9 +161,9 @@ Returns `base` raised to the power of `exponent`
 fn pow(base: int, exponent: uint) -> int {
     if exponent == 0u { ret 1; } //Not mathemtically true if [base == 0]
     if base     == 0  { ret 0; }
-    let my_pow  = exponent;
-    let acc     = 1;
-    let multiplier = base;
+    let mut my_pow  = exponent;
+    let mut acc     = 1;
+    let mut multiplier = base;
     while(my_pow > 0u) {
       if my_pow % 2u == 1u {
          acc *= multiplier;
diff --git a/src/libcore/iter.rs b/src/libcore/iter.rs
index e9327a26017..7fd98d6e97c 100644
--- a/src/libcore/iter.rs
+++ b/src/libcore/iter.rs
@@ -40,7 +40,7 @@ impl of iterable<char> for str {
 }
 
 fn enumerate<A,IA:iterable<A>>(self: IA, blk: fn(uint, A)) {
-    let i = 0u;
+    let mut i = 0u;
     self.iter {|a|
         blk(i, a);
         i += 1u;
@@ -82,7 +82,7 @@ fn flat_map<A,B,IA:iterable<A>,IB:iterable<B>>(
 }
 
 fn foldl<A,B,IA:iterable<A>>(self: IA, +b0: B, blk: fn(-B, A) -> B) -> B {
-    let b <- b0;
+    let mut b <- b0;
     self.iter {|a|
         b = blk(b, a);
     }
@@ -92,7 +92,7 @@ fn foldl<A,B,IA:iterable<A>>(self: IA, +b0: B, blk: fn(-B, A) -> B) -> B {
 fn foldr<A:copy,B,IA:iterable<A>>(
     self: IA, +b0: B, blk: fn(A, -B) -> B) -> B {
 
-    let b <- b0;
+    let mut b <- b0;
     reversed(self) {|a|
         b = blk(a, b);
     }
@@ -119,7 +119,7 @@ fn count<A,IA:iterable<A>>(self: IA, x: A) -> uint {
 }
 
 fn repeat(times: uint, blk: fn()) {
-    let i = 0u;
+    let mut i = 0u;
     while i < times {
         blk();
         i += 1u;
diff --git a/src/libcore/os.rs b/src/libcore/os.rs
index 0258c7b55f6..f6b36f71bde 100644
--- a/src/libcore/os.rs
+++ b/src/libcore/os.rs
@@ -41,7 +41,7 @@ native mod rustrt {
 
 
 fn env() -> [(str,str)] {
-    let pairs = [];
+    let mut pairs = [];
     for p in rustrt::rust_env_pairs() {
         let vs = str::splitn_char(p, '=', 1u);
         assert vec::len(vs) == 2u;
@@ -87,7 +87,7 @@ mod win32 {
     }
 
     fn as_utf16_p<T>(s: str, f: fn(*u16) -> T) -> T {
-        let t = str::to_utf16(s);
+        let mut t = str::to_utf16(s);
         // Null terminate before passing on.
         t += [0u16];
         vec::as_buf(t, f)
@@ -468,13 +468,13 @@ fn list_dir(p: path) -> [str] {
     #[cfg(target_os = "win32")]
     fn star() -> str { "*" }
 
-    let p = p;
+    let mut p = p;
     let pl = str::len(p);
     if pl == 0u || (p[pl - 1u] as char != path::consts::path_sep
                     && p[pl - 1u] as char != path::consts::alt_path_sep) {
         p += path::path_sep();
     }
-    let full_paths: [str] = [];
+    let mut full_paths: [str] = [];
     for filename: str in rustrt::rust_list_files(p + star()) {
         if !str::eq(filename, ".") {
             if !str::eq(filename, "..") {
diff --git a/src/libcore/path.rs b/src/libcore/path.rs
index d5a80c39c7f..d03de47fa95 100644
--- a/src/libcore/path.rs
+++ b/src/libcore/path.rs
@@ -112,8 +112,8 @@ with a single path separator between them.
 */
 
 fn connect(pre: path, post: path) -> path unsafe {
-    let pre_ = pre;
-    let post_ = post;
+    let mut pre_ = pre;
+    let mut post_ = post;
     let sep = consts::path_sep as u8;
     let pre_len  = str::len(pre);
     let post_len = str::len(post);
@@ -246,9 +246,9 @@ fn normalize(p: path) -> path {
             ret [];
         }
 
-        let t = [];
-        let i = vec::len(s);
-        let skip = 0;
+        let mut t = [];
+        let mut i = vec::len(s);
+        let mut skip = 0;
         do {
             i -= 1u;
             if s[i] == ".." {
@@ -261,7 +261,7 @@ fn normalize(p: path) -> path {
                 }
             }
         } while i != 0u;
-        let t = vec::reversed(t);
+        let mut t = vec::reversed(t);
         while skip > 0 {
             t += [".."];
             skip -= 1;
diff --git a/src/libcore/str.rs b/src/libcore/str.rs
index 5760cc8b066..0325e90c18b 100644
--- a/src/libcore/str.rs
+++ b/src/libcore/str.rs
@@ -127,7 +127,7 @@ Convert a byte to a UTF-8 string.  Fails if invalid UTF-8.
 */
 fn from_byte(b: u8) -> str unsafe {
     assert b < 128u8;
-    let v = [b, 0u8];
+    let mut v = [b, 0u8];
     let s: str = ::unsafe::reinterpret_cast(v);
     ::unsafe::leak(v);
     s
@@ -176,7 +176,7 @@ Function: from_char
 Convert a char to a string
 */
 fn from_char(ch: char) -> str {
-    let buf = "";
+    let mut buf = "";
     push_char(buf, ch);
     ret buf;
 }
@@ -187,7 +187,7 @@ Function: from_chars
 Convert a vector of chars to a string
 */
 fn from_chars(chs: [char]) -> str {
-    let buf = "";
+    let mut buf = "";
     reserve(buf, chs.len());
     for ch in chs { push_char(buf, ch); }
     ret buf;
@@ -199,7 +199,7 @@ Function: from_cstr
 Create a Rust string from a null-terminated C string
 */
 fn from_cstr(cstr: sbuf) -> str unsafe {
-    let curr = cstr, i = 0u;
+    let mut curr = cstr, i = 0u;
     while *curr != 0u8 {
         i += 1u;
         curr = ptr::offset(cstr, i);
@@ -213,7 +213,7 @@ Function: from_cstr_len
 Create a Rust string from a C string of the given length
 */
 fn from_cstr_len(cstr: sbuf, len: uint) -> str unsafe {
-    let buf: [u8] = [];
+    let mut buf: [u8] = [];
     vec::reserve(buf, len + 1u);
     vec::as_buf(buf) {|b| ptr::memcpy(b, cstr, len); }
     vec::unsafe::set_len(buf, len);
@@ -231,7 +231,7 @@ Function: concat
 Concatenate a vector of strings
 */
 fn concat(v: [str]) -> str {
-    let s: str = "";
+    let mut s: str = "";
     for ss: str in v { s += ss; }
     ret s;
 }
@@ -242,7 +242,7 @@ Function: connect
 Concatenate a vector of strings, placing a given separator between each
 */
 fn connect(v: [str], sep: str) -> str {
-    let s = "", first = true;
+    let mut s = "", first = true;
     for ss: str in v {
         if first { first = false; } else { s += sep; }
         s += ss;
@@ -350,7 +350,8 @@ Function: chars
 Convert a string to a vector of characters
 */
 fn chars(s: str) -> [char] {
-    let buf = [], i = 0u, len = len(s);
+    let mut buf = [], i = 0u;
+    let len = len(s);
     while i < len {
         let {ch, next} = char_range_at(s, i);
         buf += [ch];
@@ -408,8 +409,9 @@ fn split_char_nonempty(s: str, sep: char) -> [str] {
 fn split_char_inner(s: str, sep: char, count: uint, allow_empty: bool)
     -> [str] unsafe {
     if sep < 128u as char {
-        let result = [], b = sep as u8, l = len(s), done = 0u;
-        let i = 0u, start = 0u;
+        let b = sep as u8, l = len(s);
+        let mut result = [], done = 0u;
+        let mut i = 0u, start = 0u;
         while i < l && done < count {
             if s[i] == b {
                 if allow_empty || start < i {
@@ -458,7 +460,8 @@ fn split_nonempty(s: str, sepfn: fn(char) -> bool) -> [str] {
 
 fn split_inner(s: str, sepfn: fn(cc: char) -> bool, count: uint,
                allow_empty: bool) -> [str] unsafe {
-    let result = [], i = 0u, l = len(s), start = 0u, done = 0u;
+    let l = len(s);
+    let mut result = [], i = 0u, start = 0u, done = 0u;
     while i < l && done < count {
         let {ch, next} = char_range_at(s, i);
         if sepfn(ch) {
@@ -480,7 +483,7 @@ fn split_inner(s: str, sepfn: fn(cc: char) -> bool, count: uint,
 fn iter_matches(s: str, sep: str, f: fn(uint, uint)) {
     let sep_len = len(sep), l = len(s);
     assert sep_len > 0u;
-    let i = 0u, match_start = 0u, match_i = 0u;
+    let mut i = 0u, match_start = 0u, match_i = 0u;
 
     while i < l {
         if s[i] == sep[match_i] {
@@ -505,7 +508,7 @@ fn iter_matches(s: str, sep: str, f: fn(uint, uint)) {
 }
 
 fn iter_between_matches(s: str, sep: str, f: fn(uint, uint)) {
-    let last_end = 0u;
+    let mut last_end = 0u;
     iter_matches(s, sep) {|from, to|
         f(last_end, from);
         last_end = to;
@@ -522,7 +525,7 @@ Note that this has recently been changed.  For example:
 >  assert ["", "XXX", "YYY", ""] == split_str(".XXX.YYY.", ".")
 */
 fn split_str(s: str, sep: str) -> [str] {
-    let result = [];
+    let mut result = [];
     iter_between_matches(s, sep) {|from, to|
         unsafe { result += [unsafe::slice_bytes(s, from, to)]; }
     }
@@ -530,7 +533,7 @@ fn split_str(s: str, sep: str) -> [str] {
 }
 
 fn split_str_nonempty(s: str, sep: str) -> [str] {
-    let result = [];
+    let mut result = [];
     iter_between_matches(s, sep) {|from, to|
         if to > from {
             unsafe { result += [unsafe::slice_bytes(s, from, to)]; }
@@ -555,7 +558,8 @@ separated by LF ('\n') and/or CR LF ('\r\n')
 */
 fn lines_any(s: str) -> [str] {
     vec::map(lines(s), {|s|
-        let l = len(s), cp = s;
+        let l = len(s);
+        let mut cp = s;
         if l > 0u && s[l - 1u] == '\r' as u8 {
             unsafe { unsafe::set_len(cp, l - 1u); }
         }
@@ -607,7 +611,7 @@ Returns:
 The original string with all occurances of `from` replaced with `to`
 */
 fn replace(s: str, from: str, to: str) -> str unsafe {
-    let result = "", first = true;
+    let mut result = "", first = true;
     iter_between_matches(s, from) {|start, end|
         if first { first = false; } else { result += to; }
         unsafe { result += unsafe::slice_bytes(s, start, end); }
@@ -641,7 +645,7 @@ String hash function
 fn hash(&&s: str) -> uint {
     // djb hash.
     // FIXME: replace with murmur.
-    let u: uint = 5381u;
+    let mut u: uint = 5381u;
     for c: u8 in s { u *= 33u; u += c as uint; }
     ret u;
 }
@@ -676,7 +680,7 @@ Function: map
 Apply a function to each character
 */
 fn map(ss: str, ff: fn(char) -> char) -> str {
-    let result = "";
+    let mut result = "";
     reserve(result, len(ss));
     chars_iter(ss) {|cc| str::push_char(result, ff(cc));}
     result
@@ -688,7 +692,7 @@ Function: bytes_iter
 Iterate over the bytes in a string
 */
 fn bytes_iter(ss: str, it: fn(u8)) {
-    let pos = 0u;
+    let mut pos = 0u;
     let len = len(ss);
 
     while (pos < len) {
@@ -703,7 +707,8 @@ Function: chars_iter
 Iterate over the characters in a string
 */
 fn chars_iter(s: str, it: fn(char)) {
-    let pos = 0u, len = len(s);
+    let mut pos = 0u;
+    let len = len(s);
     while (pos < len) {
         let {ch, next} = char_range_at(s, pos);
         pos = next;
@@ -778,7 +783,8 @@ fn find_char_between(s: str, c: char, start: uint, end: uint)
     if c < 128u as char {
         assert start <= end;
         assert end <= len(s);
-        let i = start, b = c as u8;
+        let mut i = start;
+        let b = c as u8;
         while i < end {
             if s[i] == b { ret some(i); }
             i += 1u;
@@ -815,7 +821,8 @@ fn rfind_char_between(s: str, c: char, start: uint, end: uint)
     if c < 128u as char {
         assert start >= end;
         assert start <= len(s);
-        let i = start, b = c as u8;
+        let mut i = start;
+        let b = c as u8;
         while i > end {
             i -= 1u;
             if s[i] == b { ret some(i); }
@@ -851,7 +858,7 @@ fn find_between(s: str, start: uint, end: uint, f: fn(char) -> bool)
     assert start <= end;
     assert end <= len(s);
     assert is_char_boundary(s, start);
-    let i = start;
+    let mut i = start;
     while i < end {
         let {ch, next} = char_range_at(s, i);
         if f(ch) { ret some(i); }
@@ -886,7 +893,7 @@ fn rfind_between(s: str, start: uint, end: uint, f: fn(char) -> bool)
     assert start >= end;
     assert start <= len(s);
     assert is_char_boundary(s, start);
-    let i = start;
+    let mut i = start;
     while i > end {
         let {ch, prev} = char_range_at_reverse(s, i);
         if f(ch) { ret some(prev); }
@@ -897,7 +904,7 @@ fn rfind_between(s: str, start: uint, end: uint, f: fn(char) -> bool)
 
 // Utility used by various searching functions
 fn match_at(haystack: str, needle: str, at: uint) -> bool {
-    let i = at;
+    let mut i = at;
     for c in needle { if haystack[i] != c { ret false; } i += 1u; }
     ret true;
 }
@@ -931,7 +938,8 @@ fn find_str_between(haystack: str, needle: str, start: uint, end:uint)
     if needle_len == 0u { ret some(start); }
     if needle_len > end { ret none; }
 
-    let i = start, e = end - needle_len;
+    let mut i = start;
+    let e = end - needle_len;
     while i <= e {
         if match_at(haystack, needle, i) { ret some(i); }
         i += 1u;
@@ -995,7 +1003,7 @@ Function: is_ascii
 Determines if a string contains only ASCII characters
 */
 fn is_ascii(s: str) -> bool {
-    let i: uint = len(s);
+    let mut i: uint = len(s);
     while i > 0u { i -= 1u; if !u8::is_ascii(s[i]) { ret false; } }
     ret true;
 }
@@ -1048,10 +1056,10 @@ Function: is_utf8
 Determines if a vector of bytes contains valid UTF-8
 */
 fn is_utf8(v: [const u8]) -> bool {
-    let i = 0u;
+    let mut i = 0u;
     let total = vec::len::<u8>(v);
     while i < total {
-        let chsize = utf8_char_width(v[i]);
+        let mut chsize = utf8_char_width(v[i]);
         if chsize == 0u { ret false; }
         if i + chsize > total { ret false; }
         i += 1u;
@@ -1067,7 +1075,7 @@ fn is_utf8(v: [const u8]) -> bool {
 
 fn is_utf16(v: [const u16]) -> bool {
     let len = v.len();
-    let i = 0u;
+    let mut i = 0u;
     while (i < len) {
         let u = v[i];
 
@@ -1085,12 +1093,11 @@ fn is_utf16(v: [const u16]) -> bool {
     ret true;
 }
 
-
 fn to_utf16(s: str) -> [u16] {
-    let u = [];
+    let mut u = [];
     chars_iter(s) {|cch|
         // Arithmetic with u32 literals is easier on the eyes than chars.
-        let ch = cch as u32;
+        let mut ch = cch as u32;
 
         if (ch & 0xFFFF_u32) == ch {
             // The BMP falls through (assuming non-surrogate, as it should)
@@ -1110,9 +1117,9 @@ fn to_utf16(s: str) -> [u16] {
 
 fn utf16_chars(v: [const u16], f: fn(char)) {
     let len = v.len();
-    let i = 0u;
+    let mut i = 0u;
     while (i < len && v[i] != 0u16) {
-        let u = v[i];
+        let mut u = v[i];
 
         if  u <= 0xD7FF_u16 || u >= 0xE000_u16 {
             f(u as char);
@@ -1122,7 +1129,7 @@ fn utf16_chars(v: [const u16], f: fn(char)) {
             let u2 = v[i+1u];
             assert u >= 0xD800_u16 && u <= 0xDBFF_u16;
             assert u2 >= 0xDC00_u16 && u2 <= 0xDFFF_u16;
-            let c = (u - 0xD800_u16) as char;
+            let mut c = (u - 0xD800_u16) as char;
             c = c << 10;
             c |= (u2 - 0xDC00_u16) as char;
             c |= 0x1_0000_u32 as char;
@@ -1134,7 +1141,7 @@ fn utf16_chars(v: [const u16], f: fn(char)) {
 
 
 fn from_utf16(v: [const u16]) -> str {
-    let buf = "";
+    let mut buf = "";
     reserve(buf, v.len());
     utf16_chars(v) {|ch| push_char(buf, ch); }
     ret buf;
@@ -1157,7 +1164,7 @@ Returns:
 fn count_chars(s: str, start: uint, end: uint) -> uint {
     assert is_char_boundary(s, start);
     assert is_char_boundary(s, end);
-    let i = start, len = 0u;
+    let mut i = start, len = 0u;
     while i < end {
         let {next, _} = char_range_at(s, i);
         len += 1u;
@@ -1172,7 +1179,8 @@ fn count_chars(s: str, start: uint, end: uint) -> uint {
 // `start`.
 fn count_bytes(s: str, start: uint, n: uint) -> uint {
     assert is_char_boundary(s, start);
-    let end = start, cnt = n, l = len(s);
+    let mut end = start, cnt = n;
+    let l = len(s);
     while cnt > 0u {
         assert end < l;
         let {next, _} = char_range_at(s, end);
@@ -1260,9 +1268,9 @@ fn char_range_at(s: str, i: uint) -> {ch: char, next: uint} {
     let w = utf8_char_width(b0);
     assert (w != 0u);
     if w == 1u { ret {ch: b0 as char, next: i + 1u}; }
-    let val = 0u;
+    let mut val = 0u;
     let end = i + w;
-    let i = i + 1u;
+    let mut i = i + 1u;
     while i < end {
         let byte = s[i];
         assert (byte & 192u8 == tag_cont_u8);
@@ -1289,7 +1297,7 @@ fn char_at(s: str, i: uint) -> char { ret char_range_at(s, i).ch; }
 // Given a byte position and a str, return the previous char and its position
 // This function can be used to iterate over a unicode string in reverse.
 fn char_range_at_reverse(ss: str, start: uint) -> {ch: char, prev: uint} {
-    let prev = start;
+    let mut prev = start;
 
     // while there is a previous byte == 10......
     while prev > 0u && ss[prev - 1u] & 192u8 == tag_cont_u8 {
@@ -1327,7 +1335,7 @@ Safety note:
  */
 fn all_between(s: str, start: uint, end: uint, it: fn(char) -> bool) -> bool {
     assert is_char_boundary(s, start);
-    let i = start;
+    let mut i = start;
     while i < end {
         let {ch, next} = char_range_at(s, i);
         if !it(ch) { ret false; }
@@ -1366,7 +1374,7 @@ Example:
 
 */
 fn as_bytes<T>(s: str, f: fn([u8]) -> T) -> T unsafe {
-    let v: [u8] = ::unsafe::reinterpret_cast(s);
+    let mut v: [u8] = ::unsafe::reinterpret_cast(s);
     let r = f(v);
     ::unsafe::leak(v);
     r
@@ -1421,7 +1429,7 @@ mod unsafe {
    // Converts a vector of bytes to a string. Does not verify that the
    // vector contains valid UTF-8.
    unsafe fn from_bytes(v: [const u8]) -> str unsafe {
-       let vcopy: [u8] = v + [0u8];
+       let mut vcopy: [u8] = v + [0u8];
        let scopy: str = ::unsafe::reinterpret_cast(vcopy);
        ::unsafe::leak(vcopy);
        ret scopy;
@@ -1448,7 +1456,7 @@ mod unsafe {
        assert (begin <= end);
        assert (end <= len(s));
 
-       let v = as_bytes(s) { |v| vec::slice(v, begin, end) };
+       let mut v = as_bytes(s) { |v| vec::slice(v, begin, end) };
        v += [0u8];
        let s: str = ::unsafe::reinterpret_cast(v);
        ::unsafe::leak(v);
diff --git a/src/libcore/task.rs b/src/libcore/task.rs
index 97821e3660a..12a02411715 100644
--- a/src/libcore/task.rs
+++ b/src/libcore/task.rs
@@ -309,7 +309,7 @@ fn future_result(builder: task_builder) -> future::future<task_result> {
 fn future_task(builder: task_builder) -> future::future<task> {
     #[doc = "Get a future representing the handle to the new task"];
 
-    let po = comm::port();
+    let mut po = comm::port();
     let ch = comm::chan(po);
     add_wrapper(builder) {|body|
         fn~() {
@@ -349,7 +349,7 @@ fn run_listener<A:send>(-builder: task_builder,
 
     run(builder) {||
         let po = comm::port();
-        let ch = comm::chan(po);
+        let mut ch = comm::chan(po);
         comm::send(setup_ch, ch);
         f(po);
     }
@@ -421,7 +421,7 @@ fn spawn_sched(mode: sched_mode, +f: fn~()) {
 
     ")];
 
-    let builder = mk_task_builder();
+    let mut builder = mk_task_builder();
     set_opts(builder, {
         sched: some({
             mode: mode,
@@ -448,7 +448,7 @@ fn try<T:send>(+f: fn~() -> T) -> result::t<T,()> {
 
     let po = comm::port();
     let ch = comm::chan(po);
-    let builder = mk_task_builder();
+    let mut builder = mk_task_builder();
     unsupervise(builder);
     let result = future_result(builder);
     run(builder) {||
@@ -467,7 +467,7 @@ fn yield() {
     #[doc = "Yield control to the task scheduler"];
 
     let task_ = rustrt::rust_get_task();
-    let killed = false;
+    let mut killed = false;
     rusti::task_yield(task_, killed);
     if killed && !failing() {
         fail "killed";
@@ -499,7 +499,7 @@ type rust_closure = ctypes::void;
 
 fn spawn_raw(opts: task_opts, +f: fn~()) unsafe {
 
-    let f = if opts.supervise {
+    let mut f = if opts.supervise {
         f
     } else {
         // FIXME: The runtime supervision API is weird here because it
@@ -924,4 +924,4 @@ fn test_avoid_copying_the_body_unsupervise() {
             f();
         }
     }
-}
\ No newline at end of file
+}
diff --git a/src/libcore/to_str.rs b/src/libcore/to_str.rs
index e361fc5e514..96ed71eae7e 100644
--- a/src/libcore/to_str.rs
+++ b/src/libcore/to_str.rs
@@ -37,7 +37,7 @@ impl <A: to_str copy, B: to_str copy, C: to_str copy> of to_str for (A, B, C){
 
 impl <A: to_str> of to_str for [A] {
     fn to_str() -> str {
-        let acc = "[", first = true;
+        let mut acc = "[", first = true;
         for elt in self {
             if first { first = false; }
             else { acc += ", "; }
diff --git a/src/libcore/u16.rs b/src/libcore/u16.rs
index b038f525201..2d69a523e66 100644
--- a/src/libcore/u16.rs
+++ b/src/libcore/u16.rs
@@ -26,7 +26,7 @@ pure fn nonnegative(x: u16) -> bool { x >= 0u16 }
 
 #[doc = "Iterate over the range [`lo`..`hi`)"]
 fn range(lo: u16, hi: u16, it: fn(u16)) {
-    let i = lo;
+    let mut i = lo;
     while i < hi { it(i); i += 1u16; }
 }
 
diff --git a/src/libcore/u32.rs b/src/libcore/u32.rs
index 2fb393b6577..267f3666554 100644
--- a/src/libcore/u32.rs
+++ b/src/libcore/u32.rs
@@ -60,7 +60,7 @@ Function: range
 Iterate over the range [`lo`..`hi`)
 */
 fn range(lo: u32, hi: u32, it: fn(u32)) {
-    let i = lo;
+    let mut i = lo;
     while i < hi { it(i); i += 1u32; }
 }
 
diff --git a/src/libcore/u64.rs b/src/libcore/u64.rs
index 77b658f0662..1e6b02ef1b4 100644
--- a/src/libcore/u64.rs
+++ b/src/libcore/u64.rs
@@ -60,7 +60,7 @@ Function: range
 Iterate over the range [`lo`..`hi`)
 */
 fn range(lo: u64, hi: u64, it: fn(u64)) {
-    let i = lo;
+    let mut i = lo;
     while i < hi { it(i); i += 1u64; }
 }
 
@@ -98,9 +98,9 @@ fn to_str(n: u64, radix: uint) -> str {
 
     if n == 0u64 { ret "0"; }
 
-    let s = "";
+    let mut s = "";
 
-    let n = n;
+    let mut n = n;
     while n > 0u64 { s = digit(n % r64) + s; n /= r64; }
     ret s;
 }
@@ -119,8 +119,8 @@ Parse a string as an unsigned integer.
 */
 fn from_str(buf: str, radix: u64) -> option<u64> {
     if str::len(buf) == 0u { ret none; }
-    let i = str::len(buf) - 1u;
-    let power = 1u64, n = 0u64;
+    let mut i = str::len(buf) - 1u;
+    let mut power = 1u64, n = 0u64;
     while true {
         alt char::to_digit(buf[i] as char, radix as uint) {
           some(d) { n += d as u64 * power; }
diff --git a/src/libcore/u8.rs b/src/libcore/u8.rs
index 03eead6b763..eb433f4e9b3 100644
--- a/src/libcore/u8.rs
+++ b/src/libcore/u8.rs
@@ -60,7 +60,7 @@ Function: range
 Iterate over the range [`lo`..`hi`)
 */
 fn range(lo: u8, hi: u8, it: fn(u8)) {
-    let i = lo;
+    let mut i = lo;
     while i < hi { it(i); i += 1u8; }
 }
 
diff --git a/src/libcore/uint.rs b/src/libcore/uint.rs
index d337746f532..69981d2e980 100644
--- a/src/libcore/uint.rs
+++ b/src/libcore/uint.rs
@@ -133,7 +133,7 @@ Iterate over the range [`lo`..`hi`)
 */
 #[inline(always)]
 fn range(lo: uint, hi: uint, it: fn(uint)) {
-    let i = lo;
+    let mut i = lo;
     while i < hi { it(i); i += 1u; }
 }
 
@@ -154,7 +154,7 @@ Returns:
 that is if `it` returned `false` at any point.
 */
 fn loop(lo: uint, hi: uint, it: fn(uint) -> bool) -> bool {
-    let i = lo;
+    let mut i = lo;
     while i < hi {
         if (!it(i)) { ret false; }
         i += 1u;
@@ -169,8 +169,8 @@ Returns the smallest power of 2 greater than or equal to `n`
 */
 fn next_power_of_two(n: uint) -> uint {
     let halfbits: uint = sys::size_of::<uint>() * 4u;
-    let tmp: uint = n - 1u;
-    let shift: uint = 1u;
+    let mut tmp: uint = n - 1u;
+    let mut shift: uint = 1u;
     while shift <= halfbits { tmp |= tmp >> shift; shift <<= 1u; }
     ret tmp + 1u;
 }
@@ -191,9 +191,9 @@ buf must not be empty
 */
 fn parse_buf(buf: [u8], radix: uint) -> option<uint> {
     if vec::len(buf) == 0u { ret none; }
-    let i = vec::len(buf) - 1u;
-    let power = 1u;
-    let n = 0u;
+    let mut i = vec::len(buf) - 1u;
+    let mut power = 1u;
+    let mut n = 0u;
     while true {
         alt char::to_digit(buf[i] as char, radix) {
           some(d) { n += d * power; }
@@ -219,7 +219,7 @@ Function: to_str
 Convert to a string in a given base
 */
 fn to_str(num: uint, radix: uint) -> str {
-    let n = num;
+    let mut n = num;
     assert (0u < radix && radix <= 16u);
     fn digit(n: uint) -> char {
         ret alt n {
@@ -243,13 +243,13 @@ fn to_str(num: uint, radix: uint) -> str {
             };
     }
     if n == 0u { ret "0"; }
-    let s: str = "";
+    let mut s: str = "";
     while n != 0u {
         s += str::from_byte(digit(n % radix) as u8);
         n /= radix;
     }
-    let s1: str = "";
-    let len: uint = str::len(s);
+    let mut s1: str = "";
+    let mut len: uint = str::len(s);
     while len != 0u { len -= 1u; s1 += str::from_byte(s[len]); }
     ret s1;
 }
diff --git a/src/libcore/vec.rs b/src/libcore/vec.rs
index 0baf5a34751..310232dec9b 100644
--- a/src/libcore/vec.rs
+++ b/src/libcore/vec.rs
@@ -90,9 +90,9 @@ Creates an immutable vector of size `n_elts` and initializes the elements
 to the value returned by the function `op`.
 */
 fn init_fn<T>(n_elts: uint, op: init_op<T>) -> [T] {
-    let v = [];
+    let mut v = [];
     reserve(v, n_elts);
-    let i: uint = 0u;
+    let mut i: uint = 0u;
     while i < n_elts { v += [op(i)]; i += 1u; }
     ret v;
 }
@@ -106,9 +106,9 @@ Creates an immutable vector of size `n_elts` and initializes the elements
 to the value `t`.
 */
 fn init_elt<T: copy>(n_elts: uint, t: T) -> [T] {
-    let v = [];
+    let mut v = [];
     reserve(v, n_elts);
-    let i: uint = 0u;
+    let mut i: uint = 0u;
     while i < n_elts { v += [t]; i += 1u; }
     ret v;
 }
@@ -217,9 +217,9 @@ Returns a copy of the elements from [`start`..`end`) from `v`.
 fn slice<T: copy>(v: [const T], start: uint, end: uint) -> [T] {
     assert (start <= end);
     assert (end <= len(v));
-    let result = [];
+    let mut result = [];
     reserve(result, end - start);
-    let i = start;
+    let mut i = start;
     while i < end { result += [v[i]]; i += 1u; }
     ret result;
 }
@@ -233,8 +233,8 @@ fn split<T: copy>(v: [const T], f: fn(T) -> bool) -> [[T]] {
     let ln = len(v);
     if (ln == 0u) { ret [] }
 
-    let start = 0u;
-    let result = [];
+    let mut start = 0u;
+    let mut result = [];
     while start < ln {
         alt position_from(v, start, ln, f) {
           none { break }
@@ -258,9 +258,9 @@ fn splitn<T: copy>(v: [const T], n: uint, f: fn(T) -> bool) -> [[T]] {
     let ln = len(v);
     if (ln == 0u) { ret [] }
 
-    let start = 0u;
-    let count = n;
-    let result = [];
+    let mut start = 0u;
+    let mut count = n;
+    let mut result = [];
     while start < ln && count > 0u {
         alt position_from(v, start, ln, f) {
           none { break }
@@ -286,8 +286,8 @@ fn rsplit<T: copy>(v: [const T], f: fn(T) -> bool) -> [[T]] {
     let ln = len(v);
     if (ln == 0u) { ret [] }
 
-    let end = ln;
-    let result = [];
+    let mut end = ln;
+    let mut result = [];
     while end > 0u {
         alt rposition_from(v, 0u, end, f) {
           none { break }
@@ -311,9 +311,9 @@ fn rsplitn<T: copy>(v: [const T], n: uint, f: fn(T) -> bool) -> [[T]] {
     let ln = len(v);
     if (ln == 0u) { ret [] }
 
-    let end = ln;
-    let count = n;
-    let result = [];
+    let mut end = ln;
+    let mut count = n;
+    let mut result = [];
     while end > 0u && count > 0u {
         alt rposition_from(v, 0u, end, f) {
           none { break }
@@ -385,7 +385,7 @@ initval - The value for the new elements
 */
 fn grow<T: copy>(&v: [const T], n: uint, initval: T) {
     reserve(v, next_power_of_two(len(v) + n));
-    let i: uint = 0u;
+    let mut i: uint = 0u;
     while i < n { v += [initval]; i += 1u; }
 }
 
@@ -405,7 +405,7 @@ init_op - A function to call to retreive each appended element's value
 */
 fn grow_fn<T>(&v: [const T], n: uint, op: init_op<T>) {
     reserve(v, next_power_of_two(len(v) + n));
-    let i: uint = 0u;
+    let mut i: uint = 0u;
     while i < n { v += [op(i)]; i += 1u; }
 }
 
@@ -433,7 +433,7 @@ Function: map
 Apply a function to each element of a vector and return the results
 */
 fn map<T, U>(v: [T], f: fn(T) -> U) -> [U] {
-    let result = [];
+    let mut result = [];
     reserve(result, len(v));
     for elem: T in v { result += [f(elem)]; }
     ret result;
@@ -448,8 +448,8 @@ fn map2<T: copy, U: copy, V>(v0: [const T], v1: [const U],
                              f: fn(T, U) -> V) -> [V] {
     let v0_len = len(v0);
     if v0_len != len(v1) { fail; }
-    let u: [V] = [];
-    let i = 0u;
+    let mut u: [V] = [];
+    let mut i = 0u;
     while i < v0_len { u += [f(copy v0[i], copy v1[i])]; i += 1u; }
     ret u;
 }
@@ -464,7 +464,7 @@ the resulting vector.
 */
 fn filter_map<T: copy, U: copy>(v: [const T], f: fn(T) -> option<U>)
     -> [U] {
-    let result = [];
+    let mut result = [];
     for elem: T in v {
         alt f(copy elem) {
           none {/* no-op */ }
@@ -484,7 +484,7 @@ Apply function `f` to each element of `v` and return a vector containing
 only those elements for which `f` returned true.
 */
 fn filter<T: copy>(v: [T], f: fn(T) -> bool) -> [T] {
-    let result = [];
+    let mut result = [];
     for elem: T in v {
         if f(elem) { result += [elem]; }
     }
@@ -498,7 +498,7 @@ Concatenate a vector of vectors. Flattens a vector of vectors of T into
 a single vector of T.
 */
 fn concat<T: copy>(v: [const [const T]]) -> [T] {
-    let new: [T] = [];
+    let mut new: [T] = [];
     for inner: [T] in v { new += inner; }
     ret new;
 }
@@ -509,8 +509,8 @@ Function: connect
 Concatenate a vector of vectors, placing a given separator between each
 */
 fn connect<T: copy>(v: [const [const T]], sep: T) -> [T] {
-    let new: [T] = [];
-    let first = true;
+    let mut new: [T] = [];
+    let mut first = true;
     for inner: [T] in v {
         if first { first = false; } else { push(new, sep); }
         new += inner;
@@ -524,7 +524,7 @@ Function: foldl
 Reduce a vector from left to right
 */
 fn foldl<T: copy, U>(z: T, v: [const U], p: fn(T, U) -> T) -> T {
-    let accum = z;
+    let mut accum = z;
     iter(v) { |elt|
         accum = p(accum, elt);
     }
@@ -537,7 +537,7 @@ Function: foldr
 Reduce a vector from right to left
 */
 fn foldr<T, U: copy>(v: [const T], z: U, p: fn(T, U) -> U) -> U {
-    let accum = z;
+    let mut accum = z;
     riter(v) { |elt|
         accum = p(elt, accum);
     }
@@ -566,7 +566,7 @@ If the vectors contains no elements then false is returned.
 fn any2<T, U>(v0: [const T], v1: [U], f: fn(T, U) -> bool) -> bool {
     let v0_len = len(v0);
     let v1_len = len(v1);
-    let i = 0u;
+    let mut i = 0u;
     while i < v0_len && i < v1_len {
         if f(v0[i], v1[i]) { ret true; };
         i += 1u;
@@ -596,7 +596,7 @@ If the vectors are not the same size then false is returned.
 fn all2<T, U>(v0: [const T], v1: [const U], f: fn(T, U) -> bool) -> bool {
     let v0_len = len(v0);
     if v0_len != len(v1) { ret false; }
-    let i = 0u;
+    let mut i = 0u;
     while i < v0_len { if !f(v0[i], v1[i]) { ret false; }; i += 1u; }
     ret true;
 }
@@ -617,7 +617,7 @@ Function: count
 Returns the number of elements that are equal to a given value
 */
 fn count<T>(v: [const T], x: T) -> uint {
-    let cnt = 0u;
+    let mut cnt = 0u;
     for elt: T in v { if x == elt { cnt += 1u; } }
     ret cnt;
 }
@@ -716,7 +716,7 @@ fn position_from<T>(v: [const T], start: uint, end: uint,
                     f: fn(T) -> bool) -> option<uint> {
     assert start <= end;
     assert end <= len(v);
-    let i = start;
+    let mut i = start;
     while i < end { if f(v[i]) { ret some::<uint>(i); } i += 1u; }
     ret none;
 }
@@ -761,7 +761,7 @@ fn rposition_from<T>(v: [const T], start: uint, end: uint,
                      f: fn(T) -> bool) -> option<uint> {
     assert start <= end;
     assert end <= len(v);
-    let i = end;
+    let mut i = end;
     while i > start {
         if f(v[i - 1u]) { ret some::<uint>(i - 1u); }
         i -= 1u;
@@ -784,7 +784,7 @@ and the i-th element of the second vector contains the second element
 of the i-th tuple of the input vector.
 */
 fn unzip<T: copy, U: copy>(v: [const (T, U)]) -> ([T], [U]) {
-    let as = [], bs = [];
+    let mut as = [], bs = [];
     for (a, b) in v { as += [a]; bs += [b]; }
     ret (as, bs);
 }
@@ -802,8 +802,9 @@ Preconditions:
 <same_length> (v, u)
 */
 fn zip<T: copy, U: copy>(v: [const T], u: [const U]) -> [(T, U)] {
-    let zipped = [];
-    let sz = len(v), i = 0u;
+    let mut zipped = [];
+    let sz = len(v);
+    let mut i = 0u;
     assert sz == len(u);
     while i < sz { zipped += [(v[i], u[i])]; i += 1u; }
     ret zipped;
@@ -829,7 +830,7 @@ Function: reverse
 Reverse the order of elements in a vector, in place
 */
 fn reverse<T>(v: [mutable T]) {
-    let i: uint = 0u;
+    let mut i: uint = 0u;
     let ln = len::<T>(v);
     while i < ln / 2u { v[i] <-> v[ln - i - 1u]; i += 1u; }
 }
@@ -841,8 +842,8 @@ Function: reversed
 Returns a vector with the order of elements reversed
 */
 fn reversed<T: copy>(v: [const T]) -> [T] {
-    let rs: [T] = [];
-    let i = len::<T>(v);
+    let mut rs: [T] = [];
+    let mut i = len::<T>(v);
     if i == 0u { ret rs; } else { i -= 1u; }
     while i != 0u { rs += [v[i]]; i -= 1u; }
     rs += [v[0]];
@@ -857,8 +858,8 @@ Returns a vector containing a range of chars
 */
 fn enum_chars(start: u8, end: u8) -> [char] {
     assert start < end;
-    let i = start;
-    let r = [];
+    let mut i = start;
+    let mut r = [];
     while i <= end { r += [i as char]; i += 1u as u8; }
     ret r;
 }
@@ -871,8 +872,8 @@ Returns a vector containing a range of uints
 */
 fn enum_uints(start: uint, end: uint) -> [uint] {
     assert start < end;
-    let i = start;
-    let r = [];
+    let mut i = start;
+    let mut r = [];
     while i <= end { r += [i]; i += 1u; }
     ret r;
 }
@@ -907,7 +908,7 @@ Iterates over two vectors in parallel
 */
 #[inline]
 fn iter2<U, T>(v: [ U], v2: [const T], f: fn(U, T)) {
-    let i = 0;
+    let mut i = 0;
     for elt in v { f(elt, v2[i]); i += 1; }
 }
 
@@ -921,7 +922,8 @@ element's value and index.
 */
 #[inline(always)]
 fn iteri<T>(v: [const T], f: fn(uint, T)) {
-    let i = 0u, l = len(v);
+    let mut i = 0u;
+    let l = len(v);
     while i < l { f(i, v[i]); i += 1u; }
 }
 
@@ -947,7 +949,7 @@ Iterates over vector `v` and, for each element, calls function `f` with the
 element's value and index.
 */
 fn riteri<T>(v: [const T], f: fn(uint, T)) {
-    let i = len(v);
+    let mut i = len(v);
     while 0u < i {
         i -= 1u;
         f(i, v[i]);
@@ -969,7 +971,7 @@ fn permute<T: copy>(v: [T], put: fn([T])) {
   if ln == 0u {
     put([]);
   } else {
-    let i = 0u;
+    let mut i = 0u;
     while i < ln {
       let elt = v[i];
       let rest = slice(v, 0u, i) + slice(v, i+1u, ln);
@@ -980,7 +982,7 @@ fn permute<T: copy>(v: [T], put: fn([T])) {
 }
 
 fn windowed <TT: copy> (nn: uint, xx: [const TT]) -> [[TT]] {
-   let ww = [];
+   let mut ww = [];
 
    assert 1u <= nn;
 
@@ -1153,7 +1155,7 @@ mod u8 {
         // djb hash.
         // FIXME: replace with murmur.
 
-        let u: uint = 5381u;
+        let mut u: uint = 5381u;
         vec::iter(s, { |c| u *= 33u; u += c as uint; });
         ret u;
     }