TRPL: type aliases and unsized types

Author: steveklabnik

src/doc/trpl/type-aliases.md

@@ -1,3 +1,55 @@
 % `type` Aliases
 
-Coming soon
+The `type` keyword lets you declare an alias of another type:
+
+```rust
+type Name = String;
+```
+
+You can then use this type as if it were a real type:
+
+```rust
+type Name = String;
+
+let x: Name = "Hello".to_string();
+```
+
+Note, however, that this is an _alias_, not a new type entirely. In other
+words, because Rust is strongly typed, you’d expect a comparison between two
+different types to fail:
+
+```rust,ignore
+let x: i32 = 5;
+let y: i64 = 5;
+
+if x == y {
+   // ...
+}
+```
+
+this gives
+
+```text
+error: mismatched types:
+ expected `i32`,
+    found `i64`
+(expected i32,
+    found i64) [E0308]
+     if x == y {
+             ^
+```
+
+But, if we had an alias:
+
+```rust
+type Num = i32;
+
+let x: i32 = 5;
+let y: Num = 5;
+
+if x == y {
+   // ...
+}
+```
+
+This compiles without error. `Num` types are the same as `i32`s, in every way.

src/doc/trpl/unsized-types.md

@@ -1,3 +1,34 @@
 % Unsized Types
 
-Coming Soon!
+Most types have a particular size, in bytes, that is knowable at compile time.
+For example, an `i32` is thirty-two bits big, or four bytes. However, there are
+some types which are useful to express, but do not have a defined size. These are
+called ‘unsized’ or ‘dynamically sized’ types. One example is `[T]`. This type
+represents a certain number of `T` in sequence. But we don’t know how many
+there are, so the size is not known.
+
+Rust understands a few of these types, but they have some restrictions. There
+are three:
+
+1. We can only manipulate an instance of an unsized type via a pointer. An
+   `&[T]` works just fine, but a `[T]` doesn’t.
+2. Variables and arguments cannot have dynamically sized types.
+3. Only the last field in a `struct` may have a dynamically sized type; the
+   other fields must not. Enum varants must not have dynamically sized types as
+   data.
+
+# ?Sized
+
+If you want to write a function that accepts a dynamically sized type, you
+can use the special bound, `?Sized`:
+
+```rust
+struct Foo<T: ?Sized> {
+    f: T,
+}
+```
+
+This `?` means that this bound is special: it lets us match more kinds, not
+less. It’s almost like every `T` implicitly has `T: Sized`, and the `?` undoes
+this default.
+