TRPL: const and static

Author: steveklabnik

src/doc/trpl/const.md

@@ -1,3 +1,25 @@
 % `const`
 
-Coming soon!
+Rust has a way of defining constants with the `const` keyword:
+
+```rust
+const N: i32 = 5;
+```
+
+Unlike [`let`][let] bindings, you must annotate the type of a `const`.
+
+[let]: variable-bindings.html
+
+Constants live for the entire lifetime of a program, and therefore any
+reference stored in a constant has a [`'static` lifetime][lifetimes]:
+
+```rust
+const NAME: &'static str = "Steve";
+```
+
+[lifetimes]: lifetimes.html
+
+More specifically, constants in Rust have no fixed address in memory. This is
+because they’re effectively inlined to each place that they’re used. References
+to the same constant are not necessarily guaranteed to refer to the same memory
+address for this reason.

src/doc/trpl/static.md

@@ -1,3 +1,47 @@
 % `static`
 
-Coming soon!
+Rust provides a ‘global variable’ sort of facility in static items. They’re
+similar to [constants][const], but static items aren’t inlined upon use.
+
+Here’s an example:
+
+```rust
+static N: i32 = 5;
+```
+
+[const]: const.html
+
+Unlike [`let`][let] bindings, you must annotate the type of a `static`.
+
+[let]: variable-bindings.html
+
+Statics live for the entire lifetime of a program, and therefore any
+reference stored in a constant has a [`’static` lifetime][lifetimes]:
+
+```rust
+static NAME: &'static str = "Steve";
+```
+
+[lifetimes]: lifetimes.html
+
+## Mutability
+
+You can introduce mutability with the `mut` keyword:
+
+```rust
+static mut N: i32 = 5;
+```
+
+Because one thread could be updating `N` while another is reading it, causing
+memory unsafety. As such both accessing and mutating a `static mut` is
+[`unsafe`][unsafe], and so must be done in an `unsafe` block:
+
+```rust
+# static mut N: i32 = 5;
+
+unsafe {
+    N += 1;
+
+    println!("N: {}", N);
+}
+```