Add changes suggested from code review.

Docs: make it explicit that we're ignoring the return value of the
function under test. Add comments from libfuzzer explaining why one
might want to keep inputs out of the corpus.

Convert From<Corpus> to i32 to a pub fn to_libfuzzer_code() that is
impl on Corpus to avoid accidental conversion.

src/lib.rs

@@ -32,9 +32,13 @@ impl From<()> for Corpus {
     }
 }
 
-impl From<Corpus> for i32 {
-    fn from(value: Corpus) -> i32 {
-        match value {
+impl Corpus {
+    #[doc(hidden)]
+    /// Convert this Corpus result into the [integer codes used by
+    /// `libFuzzer`](https://llvm.org/docs/LibFuzzer.html#rejecting-unwanted-inputs).
+    /// This is -1 for reject, 0 for keep.
+    pub fn to_libfuzzer_code(self) -> i32 {
+        match self {
             Corpus::Keep => 0,
             Corpus::Reject => -1,
         }
@@ -118,8 +122,17 @@ pub fn initialize(_argc: *const isize, _argv: *const *const *const u8) -> isize
 ///
 /// ## Rejecting Inputs
 ///
-/// To indicate whether an input should be kept in or rejected from the corpus,
-/// return a [Corpus] value from your fuzz target. For example:
+/// It may be desirable to reject some inputs, i.e. to not add them to the
+/// corpus.
+///
+/// For example, when fuzzing an API consisting of parsing and other logic,
+/// one may want to allow only those inputs into the corpus that parse
+/// successfully. To indicate whether an input should be kept in or rejected
+/// from the corpus, return either [Corpus::Keep] or [Corpus::Reject] from your
+/// fuzz target. The default behavior (e.g. if `()` is returned) is to keep the
+/// input in the corpus.
+/// 
+/// For example:
 ///
 /// ```no_run
 /// #![no_main]
@@ -134,7 +147,7 @@ pub fn initialize(_argc: *const isize, _argv: *const *const *const u8) -> isize
 ///
 ///     let key = parts[0];
 ///     let value = parts[1];
-///     my_crate::parse(key, value);
+///     let _result: Result<_, _> = my_crate::parse(key, value);
 ///     Corpus::Keep
 /// );
 /// # mod my_crate { pub fn parse(_key: &str, _value: &str) -> Result<(), ()> { unimplemented!() } }
@@ -281,8 +294,8 @@ macro_rules! fuzz_target {
                     Err(_) => return -1,
                 };
 
-                let result: i32 = ::libfuzzer_sys::Corpus::from(run(data)).into();
-                result
+                let result = ::libfuzzer_sys::Corpus::from(run(data));
+                result.to_libfuzzer_code()
             }
 
             // See above for why this is split to a separate function.