Merge pull request #408 from Enselic/unbounded-lifetime-example

Expand unbounded lifetime example code and improve wording

src/unbounded-lifetimes.md

@@ -1,13 +1,13 @@
 # Unbounded Lifetimes
 
 Unsafe code can often end up producing references or lifetimes out of thin air.
-Such lifetimes come into the world as *unbounded*. The most common source of this
-is dereferencing a raw pointer, which produces a reference with an unbounded lifetime.
-Such a lifetime becomes as big as context demands. This is in fact more powerful
-than simply becoming `'static`, because for instance `&'static &'a T`
-will fail to typecheck, but the unbound lifetime will perfectly mold into
-`&'a &'a T` as needed. However for most intents and purposes, such an unbounded
-lifetime can be regarded as `'static`.
+Such lifetimes come into the world as *unbounded*. The most common source of
+this is taking a reference to a dereferenced raw pointer, which produces a
+reference with an unbounded lifetime. Such a lifetime becomes as big as context
+demands. This is in fact more powerful than simply becoming `'static`, because
+for instance `&'static &'a T` will fail to typecheck, but the unbound lifetime
+will perfectly mold into `&'a &'a T` as needed. However for most intents and
+purposes, such an unbounded lifetime can be regarded as `'static`.
 
 Almost no reference is `'static`, so this is probably wrong. `transmute` and
 `transmute_copy` are the two other primary offenders. One should endeavor to
@@ -17,17 +17,25 @@ boundaries.
 Given a function, any output lifetimes that don't derive from inputs are
 unbounded. For instance:
 
-<!-- ignore: simplified code -->
-```rust,ignore
-fn get_str<'a>() -> &'a str;
+<!-- no_run: This example exhibits undefined behavior. -->
+```rust,no_run
+fn get_str<'a>(s: *const String) -> &'a str {
+    unsafe { &*s }
+}
+
+fn main() {
+    let soon_dropped = String::from("hello");
+    let dangling = get_str(&soon_dropped);
+    drop(soon_dropped);
+    println!("Invalid str: {}", dangling); // Invalid str: gӚ_`
+}
 ```
 
-will produce an `&str` with an unbounded lifetime. The easiest way to avoid
-unbounded lifetimes is to use lifetime elision at the function boundary.
-If an output lifetime is elided, then it *must* be bounded by an input lifetime.
-Of course it might be bounded by the *wrong* lifetime, but this will usually
-just cause a compiler error, rather than allow memory safety to be trivially
-violated.
+The easiest way to avoid unbounded lifetimes is to use lifetime elision at the
+function boundary. If an output lifetime is elided, then it *must* be bounded by
+an input lifetime. Of course it might be bounded by the *wrong* lifetime, but
+this will usually just cause a compiler error, rather than allow memory safety
+to be trivially violated.
 
 Within a function, bounding lifetimes is more error-prone. The safest and easiest
 way to bound a lifetime is to return it from a function with a bound lifetime.