diff --git a/src/libcore/float.rs b/src/libcore/float.rs
index 098e82f5fad..dd46d30d6ba 100644
--- a/src/libcore/float.rs
+++ b/src/libcore/float.rs
@@ -91,7 +91,7 @@ pub mod consts {
  * * digits - The number of significant digits
  * * exact - Whether to enforce the exact number of significant digits
  */
-pub fn to_str_common(num: float, digits: uint, exact: bool) -> ~str {
+pub pure fn to_str_common(num: float, digits: uint, exact: bool) -> ~str {
     if is_NaN(num) { return ~"NaN"; }
     if num == infinity { return ~"inf"; }
     if num == neg_infinity { return ~"-inf"; }
@@ -125,7 +125,8 @@ pub fn to_str_common(num: float, digits: uint, exact: bool) -> ~str {
         // store the next digit
         frac *= 10.0;
         let digit = frac as uint;
-        fractionalParts.push(digit);
+        // Bleh: not really unsafe.
+        unsafe { fractionalParts.push(digit); }
 
         // calculate the next frac
         frac -= digit as float;
@@ -140,7 +141,8 @@ pub fn to_str_common(num: float, digits: uint, exact: bool) -> ~str {
     // turn digits into string
     // using stack of digits
     while fractionalParts.is_not_empty() {
-        let mut adjusted_digit = carry + fractionalParts.pop();
+        // Bleh; shouldn't need to be unsafe
+        let mut adjusted_digit = carry + unsafe { fractionalParts.pop() };
 
         if adjusted_digit == 10 {
             carry = 1;
@@ -196,7 +198,7 @@ pub fn test_to_str_exact_do_decimal() {
  * * num - The float value
  * * digits - The number of significant digits
  */
-pub fn to_str(num: float, digits: uint) -> ~str {
+pub pure fn to_str(num: float, digits: uint) -> ~str {
     to_str_common(num, digits, false)
 }
 
@@ -361,7 +363,7 @@ pub fn from_str(num: &str) -> Option<float> {
  *
  * `NaN` if both `x` and `pow` are `0u`, otherwise `x^pow`
  */
-pub fn pow_with_uint(base: uint, pow: uint) -> float {
+pub pure fn pow_with_uint(base: uint, pow: uint) -> float {
     if base == 0u {
         if pow == 0u {
             return NaN as float;
diff --git a/src/libcore/int-template.rs b/src/libcore/int-template.rs
index 8cb689fd286..0ea7e1d2944 100644
--- a/src/libcore/int-template.rs
+++ b/src/libcore/int-template.rs
@@ -154,7 +154,7 @@ impl T : FromStr {
 }
 
 /// Convert to a string in a given base
-pub fn to_str(n: T, radix: uint) -> ~str {
+pub pure fn to_str(n: T, radix: uint) -> ~str {
     do to_str_bytes(n, radix) |slice| {
         do vec::as_imm_buf(slice) |p, len| {
             unsafe { str::raw::from_buf_len(p, len) }
@@ -162,7 +162,7 @@ pub fn to_str(n: T, radix: uint) -> ~str {
     }
 }
 
-pub fn to_str_bytes<U>(n: T, radix: uint, f: fn(v: &[u8]) -> U) -> U {
+pub pure fn to_str_bytes<U>(n: T, radix: uint, f: fn(v: &[u8]) -> U) -> U {
     if n < 0 as T {
         uint::to_str_bytes(true, -n as uint, radix, f)
     } else {
@@ -171,7 +171,7 @@ pub fn to_str_bytes<U>(n: T, radix: uint, f: fn(v: &[u8]) -> U) -> U {
 }
 
 /// Convert to a string
-pub fn str(i: T) -> ~str { return to_str(i, 10u); }
+pub pure fn str(i: T) -> ~str { return to_str(i, 10u); }
 
 // FIXME: Has alignment issues on windows and 32-bit linux (#2609)
 #[test]
diff --git a/src/libcore/path.rs b/src/libcore/path.rs
index 6d023ed1bfb..e5d2397da0d 100644
--- a/src/libcore/path.rs
+++ b/src/libcore/path.rs
@@ -61,7 +61,7 @@ pub pure fn Path(s: &str) -> Path {
 }
 
 impl PosixPath : ToStr {
-    fn to_str() -> ~str {
+    pure fn to_str() -> ~str {
         let mut s = ~"";
         if self.is_absolute {
             s += "/";
@@ -236,7 +236,7 @@ impl PosixPath : GenericPath {
 
 
 impl WindowsPath : ToStr {
-    fn to_str() -> ~str {
+    pure fn to_str() -> ~str {
         let mut s = ~"";
         match self.host {
           Some(ref h) => { s += "\\\\"; s += *h; }
diff --git a/src/libcore/str.rs b/src/libcore/str.rs
index 1256e5b5617..aff4c50cfd2 100644
--- a/src/libcore/str.rs
+++ b/src/libcore/str.rs
@@ -49,7 +49,7 @@ pub pure fn from_byte(b: u8) -> ~str {
 }
 
 /// Appends a character at the end of a string
-pub fn push_char(s: &const ~str, ch: char) {
+pub fn push_char(s: &mut ~str, ch: char) {
     unsafe {
         let code = ch as uint;
         let nb = if code < max_one_b { 1u }
@@ -140,7 +140,7 @@ pub pure fn from_chars(chs: &[char]) -> ~str {
 
 /// Appends a string slice to the back of a string, without overallocating
 #[inline(always)]
-pub fn push_str_no_overallocate(lhs: &const ~str, rhs: &str) {
+pub fn push_str_no_overallocate(lhs: &mut ~str, rhs: &str) {
     unsafe {
         let llen = lhs.len();
         let rlen = rhs.len();
@@ -157,7 +157,7 @@ pub fn push_str_no_overallocate(lhs: &const ~str, rhs: &str) {
 }
 /// Appends a string slice to the back of a string
 #[inline(always)]
-pub fn push_str(lhs: &const ~str, rhs: &str) {
+pub fn push_str(lhs: &mut ~str, rhs: &str) {
     unsafe {
         let llen = lhs.len();
         let rlen = rhs.len();
@@ -214,7 +214,7 @@ Section: Adding to and removing from a string
  *
  * If the string does not contain any characters
  */
-pub fn pop_char(s: &const ~str) -> char {
+pub fn pop_char(s: &mut ~str) -> char {
     let end = len(*s);
     assert end > 0u;
     let {ch, prev} = char_range_at_reverse(*s, end);
@@ -1802,9 +1802,9 @@ pub pure fn as_buf<T>(s: &str, f: fn(*u8, uint) -> T) -> T {
  * * s - A string
  * * n - The number of bytes to reserve space for
  */
-pub fn reserve(s: &const ~str, n: uint) {
+pub fn reserve(s: &mut ~str, n: uint) {
     unsafe {
-        let v: *mut ~[u8] = cast::transmute(copy s);
+        let v: *mut ~[u8] = cast::transmute(s);
         vec::reserve(&mut *v, n + 1);
     }
 }
@@ -1829,7 +1829,7 @@ pub fn reserve(s: &const ~str, n: uint) {
  * * s - A string
  * * n - The number of bytes to reserve space for
  */
-pub fn reserve_at_least(s: &const ~str, n: uint) {
+pub fn reserve_at_least(s: &mut ~str, n: uint) {
     reserve(s, uint::next_power_of_two(n + 1u) - 1u)
 }
 
@@ -1974,7 +1974,7 @@ pub mod raw {
     }
 
     /// Appends a byte to a string. (Not UTF-8 safe).
-    pub unsafe fn push_byte(s: &const ~str, b: u8) {
+    pub unsafe fn push_byte(s: &mut ~str, b: u8) {
         reserve_at_least(s, s.len() + 1);
         do as_buf(*s) |buf, len| {
             let buf: *mut u8 = ::cast::reinterpret_cast(&buf);
@@ -1984,13 +1984,13 @@ pub mod raw {
     }
 
     /// Appends a vector of bytes to a string. (Not UTF-8 safe).
-    unsafe fn push_bytes(s: &const ~str, bytes: &[u8]) {
+    unsafe fn push_bytes(s: &mut ~str, bytes: &[u8]) {
         reserve_at_least(s, s.len() + bytes.len());
         for vec::each(bytes) |byte| { push_byte(s, *byte); }
     }
 
     /// Removes the last byte from a string and returns it. (Not UTF-8 safe).
-    pub unsafe fn pop_byte(s: &const ~str) -> u8 {
+    pub unsafe fn pop_byte(s: &mut ~str) -> u8 {
         let len = len(*s);
         assert (len > 0u);
         let b = s[len - 1u];
@@ -2008,7 +2008,7 @@ pub mod raw {
     }
 
     /// Sets the length of the string and adds the null terminator
-    pub unsafe fn set_len(v: &const ~str, new_len: uint) {
+    pub unsafe fn set_len(v: &mut ~str, new_len: uint) {
         let v: **vec::raw::VecRepr = cast::transmute(copy v);
         let repr: *vec::raw::VecRepr = *v;
         (*repr).unboxed.fill = new_len + 1u;
diff --git a/src/libcore/to_str.rs b/src/libcore/to_str.rs
index a3659937ad4..fb0608906a1 100644
--- a/src/libcore/to_str.rs
+++ b/src/libcore/to_str.rs
@@ -8,80 +8,82 @@ The `ToStr` trait for converting to strings
 #[forbid(deprecated_mode)];
 #[forbid(deprecated_pattern)];
 
-pub trait ToStr { fn to_str() -> ~str; }
+pub trait ToStr { pure fn to_str() -> ~str; }
 
 impl int: ToStr {
-    fn to_str() -> ~str { int::str(self) }
+    pure fn to_str() -> ~str { int::str(self) }
 }
 impl i8: ToStr {
-    fn to_str() -> ~str { i8::str(self) }
+    pure fn to_str() -> ~str { i8::str(self) }
 }
 impl i16: ToStr {
-    fn to_str() -> ~str { i16::str(self) }
+    pure fn to_str() -> ~str { i16::str(self) }
 }
 impl i32: ToStr {
-    fn to_str() -> ~str { i32::str(self) }
+    pure fn to_str() -> ~str { i32::str(self) }
 }
 impl i64: ToStr {
-    fn to_str() -> ~str { i64::str(self) }
+    pure fn to_str() -> ~str { i64::str(self) }
 }
 impl uint: ToStr {
-    fn to_str() -> ~str { uint::str(self) }
+    pure fn to_str() -> ~str { uint::str(self) }
 }
 impl u8: ToStr {
-    fn to_str() -> ~str { u8::str(self) }
+    pure fn to_str() -> ~str { u8::str(self) }
 }
 impl u16: ToStr {
-    fn to_str() -> ~str { u16::str(self) }
+    pure fn to_str() -> ~str { u16::str(self) }
 }
 impl u32: ToStr {
-    fn to_str() -> ~str { u32::str(self) }
+    pure fn to_str() -> ~str { u32::str(self) }
 }
 impl u64: ToStr {
-    fn to_str() -> ~str { u64::str(self) }
+    pure fn to_str() -> ~str { u64::str(self) }
 }
 impl float: ToStr {
-    fn to_str() -> ~str { float::to_str(self, 4u) }
+    pure fn to_str() -> ~str { float::to_str(self, 4u) }
 }
 impl f32: ToStr {
-    fn to_str() -> ~str { float::to_str(self as float, 4u) }
+    pure fn to_str() -> ~str { float::to_str(self as float, 4u) }
 }
 impl f64: ToStr {
-    fn to_str() -> ~str { float::to_str(self as float, 4u) }
+    pure fn to_str() -> ~str { float::to_str(self as float, 4u) }
 }
 impl bool: ToStr {
-    fn to_str() -> ~str { bool::to_str(self) }
+    pure fn to_str() -> ~str { bool::to_str(self) }
 }
 impl (): ToStr {
-    fn to_str() -> ~str { ~"()" }
+    pure fn to_str() -> ~str { ~"()" }
 }
 impl ~str: ToStr {
-    fn to_str() -> ~str { copy self }
+    pure fn to_str() -> ~str { copy self }
 }
 impl &str: ToStr {
-    fn to_str() -> ~str { str::from_slice(self) }
+    pure fn to_str() -> ~str { str::from_slice(self) }
 }
 impl @str: ToStr {
-    fn to_str() -> ~str { str::from_slice(self) }
+    pure fn to_str() -> ~str { str::from_slice(self) }
 }
 
 impl<A: ToStr Copy, B: ToStr Copy> (A, B): ToStr {
-    fn to_str() -> ~str {
+    pure fn to_str() -> ~str {
         let (a, b) = self;
         ~"(" + a.to_str() + ~", " + b.to_str() + ~")"
     }
 }
 impl<A: ToStr Copy, B: ToStr Copy, C: ToStr Copy> (A, B, C): ToStr {
-    fn to_str() -> ~str {
+    pure fn to_str() -> ~str {
         let (a, b, c) = self;
         ~"(" + a.to_str() + ~", " + b.to_str() + ~", " + c.to_str() + ~")"
     }
 }
 
 impl<A: ToStr> ~[A]: ToStr {
-    fn to_str() -> ~str {
+    pure fn to_str() -> ~str unsafe {
+        // Bleh -- not really unsafe
+        // push_str and push_char
         let mut acc = ~"[", first = true;
-        for vec::each(self) |elt| {
+        for vec::each(self) |elt| unsafe {
             if first { first = false; }
             else { str::push_str(&mut acc, ~", "); }
             str::push_str(&mut acc, elt.to_str());
@@ -92,10 +94,10 @@ impl<A: ToStr> ~[A]: ToStr {
 }
 
 impl<A: ToStr> @A: ToStr {
-    fn to_str() -> ~str { ~"@" + (*self).to_str() }
+    pure fn to_str() -> ~str { ~"@" + (*self).to_str() }
 }
 impl<A: ToStr> ~A: ToStr {
-    fn to_str() -> ~str { ~"~" + (*self).to_str() }
+    pure fn to_str() -> ~str { ~"~" + (*self).to_str() }
 }
 
 #[cfg(test)]
diff --git a/src/libcore/uint-template.rs b/src/libcore/uint-template.rs
index 7d0421c0730..00dd9be76db 100644
--- a/src/libcore/uint-template.rs
+++ b/src/libcore/uint-template.rs
@@ -232,7 +232,7 @@ pub pure fn to_str_bytes<U>(neg: bool, num: T, radix: uint,
 }
 
 /// Convert to a string
-pub fn str(i: T) -> ~str { return to_str(i, 10u); }
+pub pure fn str(i: T) -> ~str { return to_str(i, 10u); }
 
 #[test]
 pub fn test_to_str() {
diff --git a/src/libstd/json.rs b/src/libstd/json.rs
index fa7c0286dc1..bb331240b76 100644
--- a/src/libstd/json.rs
+++ b/src/libstd/json.rs
@@ -51,7 +51,7 @@ fn escape_str(s: &str) -> ~str {
 
 fn spaces(n: uint) -> ~str {
     let mut ss = ~"";
-    for n.times { str::push_str(&ss, " "); }
+    for n.times { str::push_str(&mut ss, " "); }
     return ss;
 }
 
@@ -302,7 +302,8 @@ pub fn to_writer(wr: io::Writer, json: &Json) {
 }
 
 /// Serializes a json value into a string
-pub fn to_str(json: &Json) -> ~str {
+pub pure fn to_str(json: &Json) -> ~str unsafe {
+    // ugh, should be safe
     io::with_str_writer(|wr| to_writer(wr, json))
 }
 
@@ -546,14 +547,14 @@ priv impl Parser {
 
             if (escape) {
                 match self.ch {
-                  '"' => str::push_char(&res, '"'),
-                  '\\' => str::push_char(&res, '\\'),
-                  '/' => str::push_char(&res, '/'),
-                  'b' => str::push_char(&res, '\x08'),
-                  'f' => str::push_char(&res, '\x0c'),
-                  'n' => str::push_char(&res, '\n'),
-                  'r' => str::push_char(&res, '\r'),
-                  't' => str::push_char(&res, '\t'),
+                  '"' => str::push_char(&mut res, '"'),
+                  '\\' => str::push_char(&mut res, '\\'),
+                  '/' => str::push_char(&mut res, '/'),
+                  'b' => str::push_char(&mut res, '\x08'),
+                  'f' => str::push_char(&mut res, '\x0c'),
+                  'n' => str::push_char(&mut res, '\n'),
+                  'r' => str::push_char(&mut res, '\r'),
+                  't' => str::push_char(&mut res, '\t'),
                   'u' => {
                       // Parse \u1234.
                       let mut i = 0u;
@@ -582,7 +583,7 @@ priv impl Parser {
                             ~"invalid \\u escape (not four digits)");
                       }
 
-                      str::push_char(&res, n as char);
+                      str::push_char(&mut res, n as char);
                   }
                   _ => return self.error(~"invalid escape")
                 }
@@ -594,7 +595,7 @@ priv impl Parser {
                     self.bump();
                     return Ok(res);
                 }
-                str::push_char(&res, self.ch);
+                str::push_char(&mut res, self.ch);
             }
         }
 
@@ -1166,11 +1167,11 @@ impl <A: ToJson> Option<A>: ToJson {
 }
 
 impl Json: to_str::ToStr {
-    fn to_str() -> ~str { to_str(&self) }
+    pure fn to_str() -> ~str { to_str(&self) }
 }
 
 impl Error: to_str::ToStr {
-    fn to_str() -> ~str {
+    pure fn to_str() -> ~str {
         fmt!("%u:%u: %s", self.line, self.col, *self.msg)
     }
 }
diff --git a/src/libstd/map.rs b/src/libstd/map.rs
index 765d40339d3..9f78f98fa31 100644
--- a/src/libstd/map.rs
+++ b/src/libstd/map.rs
@@ -341,7 +341,8 @@ pub mod chained {
             wr.write_str(~" }");
         }
 
-        fn to_str() -> ~str {
+        pure fn to_str() -> ~str unsafe {
+            // Meh -- this should be safe
             do io::with_str_writer |wr| { self.to_writer(wr) }
         }
     }
diff --git a/src/libstd/net_url.rs b/src/libstd/net_url.rs
index 0ab4d89f363..c3fd3383979 100644
--- a/src/libstd/net_url.rs
+++ b/src/libstd/net_url.rs
@@ -94,7 +94,8 @@ pub fn encode(s: &str) -> ~str {
  *
  * This function is compliant with RFC 3986.
  */
-pub fn encode_component(s: &str) -> ~str {
+
+fn encode_component(s: &str) -> ~str {
     encode_inner(s, false)
 }
 
@@ -297,7 +298,7 @@ fn userinfo_from_str(uinfo: &str) -> UserInfo {
     return UserInfo(user, pass);
 }
 
-fn userinfo_to_str(userinfo: UserInfo) -> ~str {
+pure fn userinfo_to_str(userinfo: UserInfo) -> ~str {
     if option::is_some(&userinfo.pass) {
         return str::concat(~[copy userinfo.user, ~":",
                           option::unwrap(copy userinfo.pass),
@@ -325,11 +326,15 @@ fn query_from_str(rawquery: &str) -> Query {
     return query;
 }
 
-pub fn query_to_str(query: Query) -> ~str {
+pub pure fn query_to_str(query: Query) -> ~str {
     let mut strvec = ~[];
     for query.each |kv| {
         let (k, v) = copy *kv;
-        strvec += ~[#fmt("%s=%s", encode_component(k), encode_component(v))];
+        // This is really safe...
+        unsafe {
+          strvec += ~[#fmt("%s=%s",
+                           encode_component(k), encode_component(v))];
+        }
     };
     return str::connect(strvec, ~"&");
 }
@@ -672,7 +677,7 @@ impl Url : FromStr {
  * result in just "http://somehost.com".
  *
  */
-pub fn to_str(url: Url) -> ~str {
+pub pure fn to_str(url: Url) -> ~str {
     let user = if url.user.is_some() {
       userinfo_to_str(option::unwrap(copy url.user))
     } else {
@@ -688,7 +693,8 @@ pub fn to_str(url: Url) -> ~str {
     } else {
         str::concat(~[~"?", query_to_str(url.query)])
     };
-    let fragment = if url.fragment.is_some() {
+    // ugh, this really is safe
+    let fragment = if url.fragment.is_some() unsafe {
         str::concat(~[~"#", encode_component(
             option::unwrap(copy url.fragment))])
     } else {
@@ -704,7 +710,7 @@ pub fn to_str(url: Url) -> ~str {
 }
 
 impl Url: to_str::ToStr {
-    pub fn to_str() -> ~str {
+    pub pure fn to_str() -> ~str {
         to_str(self)
     }
 }
diff --git a/src/libsyntax/ext/pipes/proto.rs b/src/libsyntax/ext/pipes/proto.rs
index 6d58d209fcf..a501df4c32d 100644
--- a/src/libsyntax/ext/pipes/proto.rs
+++ b/src/libsyntax/ext/pipes/proto.rs
@@ -18,7 +18,7 @@ impl direction : cmp::Eq {
 }
 
 impl direction: ToStr {
-    fn to_str() -> ~str {
+    pure fn to_str() -> ~str {
         match self {
           send => ~"Send",
           recv => ~"Recv"
diff --git a/src/rustc/middle/liveness.rs b/src/rustc/middle/liveness.rs
index 12d63cdacbf..3ae0727de60 100644
--- a/src/rustc/middle/liveness.rs
+++ b/src/rustc/middle/liveness.rs
@@ -195,11 +195,11 @@ fn check_crate(tcx: ty::ctxt,
 }
 
 impl LiveNode: to_str::ToStr {
-    fn to_str() -> ~str { fmt!("ln(%u)", *self) }
+    pure fn to_str() -> ~str { fmt!("ln(%u)", *self) }
 }
 
 impl Variable: to_str::ToStr {
-    fn to_str() -> ~str { fmt!("v(%u)", *self) }
+    pure fn to_str() -> ~str { fmt!("v(%u)", *self) }
 }
 
 // ______________________________________________________________________
diff --git a/src/test/auxiliary/cci_class_cast.rs b/src/test/auxiliary/cci_class_cast.rs
index 288fe66dd20..ca2af387587 100644
--- a/src/test/auxiliary/cci_class_cast.rs
+++ b/src/test/auxiliary/cci_class_cast.rs
@@ -13,7 +13,7 @@ struct cat {
 }
 
     impl cat : ToStr {
-        fn to_str() -> ~str { self.name }
+       pure fn to_str() -> ~str { self.name }
     }
 
     priv impl cat {
diff --git a/src/test/run-pass/class-separate-impl.rs b/src/test/run-pass/class-separate-impl.rs
index c7517ab1b3d..8a51eb33857 100644
--- a/src/test/run-pass/class-separate-impl.rs
+++ b/src/test/run-pass/class-separate-impl.rs
@@ -45,7 +45,7 @@ fn cat(in_x : uint, in_y : int, in_name: ~str) -> cat {
 }
 
 impl cat: ToStr {
-  fn to_str() -> ~str { self.name }
+  pure fn to_str() -> ~str { self.name }
 }
 
 fn print_out<T: ToStr>(thing: T, expected: ~str) {
diff --git a/src/test/run-pass/issue-2904.rs b/src/test/run-pass/issue-2904.rs
index 6d771dc7386..4bb2c561422 100644
--- a/src/test/run-pass/issue-2904.rs
+++ b/src/test/run-pass/issue-2904.rs
@@ -16,7 +16,7 @@ enum square {
 }
 
 impl square: to_str::ToStr {
-    fn to_str() -> ~str {
+    pure fn to_str() -> ~str {
         match self {
           bot => { ~"R" }
           wall => { ~"#" }