Improve style guide regarding how to use Result<_, ()>.

STYLE.md

@@ -9,19 +9,44 @@ there are some differences and *ring* adds additional guidelines.
 
 ## Error checking
 
-Use `Result<T, ()>` as the return type for functions that may fail. In general,
-functions do not report error codes for a variety of reasons; when they fail,
-they only report that they fail. If a function only needs to return a boolean
-indicator that it succeeded or failed, use `Result<(), ()>` as the return type.
-Never use `Option<T>` or `bool` or other types as return values to indicate
-failure. If an external function (e.g. part of the Rust standard library)
-returns `Option<T>` to indicate failure, use `ok_or(())` to map it to
-`Result<T, ()>`.
+Use `Result<T, ()>` as the return type for functions that may fail. Never use
+`Option<T>` or `bool` or other types as return values to indicate failure.
+`Result` is used because it is annotated `#[must_use]`, so the Rust compiler
+will not let callers silently ignore the return value of functions that return
+`Result`s.
+
+*ring* functions generally do not report error codes for a variety of reasons;
+when they fail, they only report that they fail. If a function only needs to
+return a boolean indicator that it succeeded or failed, use `Result<(), ()>` as
+the return type. 
+
+If an external function (e.g. part of the Rust standard library) returns
+`Option<T>` to indicate failure, use `ok_or(())` to map it to `Result<T, ()>`.
+When the last statement `x` in a function is already the same `Result<T, ()>`
+type that the function returns, just make that statement the return expression;
+that is, write `x`, not `let result = try!(x); Ok(result)`.
 
 Use the early-return-on-failure pattern by wrapping calls to functions that may
 fail with `try!()`. Do not use `Result::or_else`, `Result::and`, etc. to chain
 together strings of potentially-failing operations.
 
+```rust
+// The return type is of the form `Result<_, ()>`, not `Option<_>` or something
+// else.
+fn good_example(x: u32, y: u32) -> Result<u32, ()> {
+    // * `ok_or` is used to map `Option<u32>` to `Result<u32, ()>` here.
+    // * `try!` is used to return early on failure.
+    let sum = try!(x.checked_add(y).ok_or(()));
+
+    // Early return is used.
+    try!(foo(sum));
+
+    // `try!()` isn't used when the last statement is already of the form
+    // `Result<_, ()>`.
+    bar(sum)
+}
+```
+
 ## Arrays and Slices
 
 When creating a slice from the start of a indexable value, use `x[..n]`, not