Rollup merge of #22937 - MDCox:str-docs, r=steveklabnik

 Fixes #22902
r? @steveklabnik

diff --combined src/libcollections/str.rs
index 7ea5b3fe9c05b525afc58a5026aab4d1422a4584,7941283287e2c0a3101d96e26a467248643a526b..19c085df2c43d8f5f2bae6a08c09b91742f8bc0c
--- 1/src/libcollections/str.rs
--- 2/src/libcollections/str.rs
+++ b/src/libcollections/str.rs
@@@ -10,12 -10,15 +10,15 @@@
  //
  // ignore-lexer-test FIXME #15679
  
- //! Unicode string manipulation (the `str` type).
+ //! Unicode string manipulation (the [`str`](../primitive.str.html) type).
  //!
- //! Rust's `str` type is one of the core primitive types of the language. `&str` is the borrowed
- //! string type. This type of string can only be created from other strings, unless it is a static
- //! string (see below). As the word "borrowed" implies, this type of string is owned elsewhere, and
- //! this string cannot be moved out of.
+ //! Rust's [`str`](../primitive.str.html) type is one of the core primitive types of the
+ //! language. `&str` is the borrowed string type. This type of string can only be created
+ //! from other strings, unless it is a `&'static str` (see below). It is not possible to
+ //! move out of borrowed strings because they are owned elsewhere.
+ //!
+ //! Basic operations are implemented directly by the compiler, but more advanced operations are
+ //! defined on the [`StrExt`](trait.StrExt.html) trait.
  //!
  //! # Examples
  //!
@@@ -383,7 -386,7 +386,7 @@@ macro_rules! utf8_first_byte 
  
  // return the value of $ch updated with continuation byte $byte
  macro_rules! utf8_acc_cont_byte {
 -    ($ch:expr, $byte:expr) => (($ch << 6) | ($byte & 63u8) as u32)
 +    ($ch:expr, $byte:expr) => (($ch << 6) | ($byte & 63) as u32)
  }
  
  #[stable(feature = "rust1", since = "1.0.0")]
@@@ -2300,8 -2303,8 +2303,8 @@@ mod tests 
  
      #[test]
      fn test_chars_decoding() {
 -        let mut bytes = [0u8; 4];
 -        for c in (0u32..0x110000).filter_map(|c| ::core::char::from_u32(c)) {
 +        let mut bytes = [0; 4];
 +        for c in (0..0x110000).filter_map(|c| ::core::char::from_u32(c)) {
              let len = c.encode_utf8(&mut bytes).unwrap_or(0);
              let s = ::core::str::from_utf8(&bytes[..len]).unwrap();
              if Some(c) != s.chars().next() {
@@@ -2312,8 -2315,8 +2315,8 @@@
  
      #[test]
      fn test_chars_rev_decoding() {
 -        let mut bytes = [0u8; 4];
 -        for c in (0u32..0x110000).filter_map(|c| ::core::char::from_u32(c)) {
 +        let mut bytes = [0; 4];
 +        for c in (0..0x110000).filter_map(|c| ::core::char::from_u32(c)) {
              let len = c.encode_utf8(&mut bytes).unwrap_or(0);
              let s = ::core::str::from_utf8(&bytes[..len]).unwrap();
              if Some(c) != s.chars().rev().next() {