Add documentation about mutable statics to rust.md

Author: alexcrichton

doc/rust.md

@@ -1124,6 +1124,41 @@ static bits_n_strings: BitsNStrings<'static> = BitsNStrings {
 };
 ~~~~
 
+#### Mutable statics
+
+If a static item is declared with the ```mut``` keyword, then it is allowed to
+be modified by the program. One of Rust's goals is to make concurrency bugs hard
+to run into, and this is obviously a very large source of race conditions or
+other bugs. For this reason, an ```unsafe``` block is required when either
+reading or writing a mutable static variable. Care should be taken to ensure
+that modifications to a mutable static are safe with respect to other tasks
+running in the same process.
+
+Mutable statics are still very useful, however. They can be used with C
+libraries and can also be bound from C libraries (in an ```extern``` block).
+
+~~~
+# fn atomic_add(_: &mut uint, _: uint) -> uint { 2 }
+
+static mut LEVELS: uint = 0;
+
+// This violates the idea of no shared state, and this doesn't internally
+// protect against races, so this function is `unsafe`
+unsafe fn bump_levels_unsafe1() -> uint {
+    let ret = LEVELS;
+    LEVELS += 1;
+    return ret;
+}
+
+// Assuming that we have an atomic_add function which returns the old value,
+// this function is "safe" but the meaning of the return value may not be what
+// callers expect, so it's still marked as `unsafe`
+unsafe fn bump_levels_unsafe2() -> uint {
+    return atomic_add(&mut LEVELS, 1);
+}
+
+~~~
+
 ### Traits
 
 A _trait_ describes a set of method types.