Explain that we do need to have 'static now

Author: carols10cents

src/ch13-03-improving-our-io-project.md

@@ -89,6 +89,16 @@ signature of the `Config::new` function so the parameter `args` has the type
 `args` and we’ll be mutating `args` by iterating over it, we can add the `mut`
 keyword into the specification of the `args` parameter to make it mutable.
 
+We also needed to specify that the string slice error type can now only have
+the `'static` lifetime. Because we’re only ever returning string literals, this
+was true before. However, when we had a reference in the parameters, there was
+the possibility that the reference in the return type could have had the same
+lifetime as the reference in the parameters. The rules that we discussed in the
+[“Lifetime Elision”][lifetime-elision] section of Chapter 10 applied, and we
+weren’t required to annotate the lifetime of `&str`. With the change to `args`,
+the lifetime elision rules no longer apply, and we must specify the `'static`
+lifetime.
+
 #### Using `Iterator` Trait Methods Instead of Indexing
 
 Next, we’ll fix the body of `Config::new`. The standard library documentation
@@ -165,3 +175,5 @@ the iterator must pass.
 But are the two implementations truly equivalent? The intuitive assumption
 might be that the more low-level loop will be faster. Let’s talk about
 performance.
+
+[lifetime-elision]: ch10-03-lifetime-syntax.html#lifetime-elision