auto merge of #15593 : steveklabnik/rust/string_guide, r=kballard

I decided to change it up a little today and hack out the beginning of the String guide. Strings are different enough in Rust that I think they deserve a specific guide, especially for those who are used to managed languages.

I decided to start with Strings because they get asked about a lot in IRC, and also based on discussions like this one on reddit: http://www.reddit.com/r/rust/comments/2ac390/generic_string_literals/

I blatantly stole bits from our other documentation on Strings. It's a little sparse at current, but I wanted to start somewhere.

I am not exactly sure what should go in "Best Practices," and would like the feedback from the team on this. Specifically due to comments like this one: http://www.reddit.com/r/rust/comments/2ac390/generic_string_literals/citmxb5

src/doc/guide-strings.md

@@ -0,0 +1,129 @@
+% The Strings Guide
+
+# Strings
+
+Strings are an important concept to master in any programming language. If you
+come from a managed language background, you may be surprised at the complexity
+of string handling in a systems programming language. Efficient access and
+allocation of memory for a dynamically sized structure involves a lot of
+details. Luckily, Rust has lots of tools to help us here.
+
+A **string** is a sequence of unicode scalar values encoded as a stream of
+UTF-8 bytes. All strings are guaranteed to be validly-encoded UTF-8 sequences.
+Additionally, strings are not null-terminated and can contain null bytes.
+
+Rust has two main types of strings: `&str` and `String`.
+
+## &str
+
+The first kind is a `&str`. This is pronounced a 'string slice.' String literals
+are of the type `&str`:
+
+```{rust}
+let string = "Hello there.";
+```
+
+Like any Rust type, string slices have an associated lifetime. A string literal
+is a `&'static str`.  A string slice can be written without an explicit
+lifetime in many cases, such as in function arguments. In these cases the
+lifetime will be inferred:
+
+```{rust}
+fn takes_slice(slice: &str) {
+    println!("Got: {}", slice);
+}
+```
+
+Like vector slices, string slices are simply a pointer plus a length. This
+means that they're a 'view' into an already-allocated string, such as a
+`&'static str` or a `String`.
+
+## String
+
+A `String` is a heap-allocated string. This string is growable, and is also
+guaranteed to be UTF-8.
+
+```{rust}
+let mut s = "Hello".to_string();
+println!("{}", s);
+
+s.push_str(", world.");
+println!("{}", s);
+```
+
+You can coerce a `String` into a `&str` with the `as_slice()` method:
+
+```{rust}
+fn takes_slice(slice: &str) {
+    println!("Got: {}", slice);
+}
+
+fn main() {
+    let s = "Hello".to_string();
+    takes_slice(s.as_slice());
+}
+```
+
+You can also get a `&str` from a stack-allocated array of bytes:
+
+```{rust}
+use std::str;
+
+let x: &[u8] = &[b'a', b'b'];
+let stack_str: &str = str::from_utf8(x).unwrap();
+```
+
+## Best Practices
+
+### `String` vs. `&str`
+
+In general, you should prefer `String` when you need ownership, and `&str` when
+you just need to borrow a string. This is very similar to using `Vec<T>` vs. `&[T]`,
+and `T` vs `&T` in general.
+
+This means starting off with this:
+
+```{rust,ignore}
+fn foo(s: &str) {
+```
+
+and only moving to this:
+
+```{rust,ignore}
+fn foo(s: String) {
+```
+
+If you have good reason. It's not polite to hold on to ownership you don't
+need, and it can make your lifetimes more complex. Furthermore, you can pass
+either kind of string into `foo` by using `.as_slice()` on any `String` you
+need to pass in, so the `&str` version is more flexible.
+
+### Comparisons
+
+To compare a String to a constant string, prefer `as_slice()`...
+
+```{rust}
+fn compare(string: String) {
+    if string.as_slice() == "Hello" {
+        println!("yes");
+    }
+}
+```
+
+... over `to_string()`:
+
+```{rust}
+fn compare(string: String) {
+    if string == "Hello".to_string() {
+        println!("yes");
+    }
+}
+```
+
+Converting a `String` to a `&str` is cheap, but converting the `&str` to a
+`String` involves an allocation.
+
+## Other Documentation
+
+* [the `&str` API documentation](/std/str/index.html)
+* [the `String` API documentation](std/string/index.html)

src/libcollections/str.rs

@@ -55,10 +55,10 @@ fn main() {
 
 # Representation
 
-Rust's string type, `str`, is a sequence of unicode codepoints encoded as a
+Rust's string type, `str`, is a sequence of unicode scalar values encoded as a
-stream of UTF-8 bytes. All safely-created strings are guaranteed to be validly
+stream of UTF-8 bytes. All strings are guaranteed to be validly encoded UTF-8
-encoded UTF-8 sequences. Additionally, strings are not null-terminated
+sequences. Additionally, strings are not null-terminated and can contain null
-and can contain null codepoints.
+bytes.
 
 The actual representation of strings have direct mappings to vectors: `&str`
 is the same as `&[u8]`.