Add support for mut in the fuzz_target! macro

CHANGELOG.md

@@ -6,6 +6,7 @@ Released YYYY-MM-DD.
 
 * Bindings to `LLVMFuzzerCustomCrossOver` through the `fuzz_crossover` macro.
 * `example_crossover` using both `fuzz_mutator` and `fuzz_crossover` (adapted from @rigtorp)
+* Support for `mut` in the `fuzz_target!` macro.
 
 ### Changed
 

src/lib.rs

@@ -245,14 +245,64 @@ macro_rules! fuzz_target {
         };
     };
 
+    (|mut $bytes:ident| $body:expr) => {
+        const _: () = {
+            /// Auto-generated function
+            #[no_mangle]
+            pub extern "C" fn rust_fuzzer_test_input(bytes: &[u8]) -> i32 {
+                // When `RUST_LIBFUZZER_DEBUG_PATH` is set, write the debug
+                // formatting of the input to that file. This is only intended for
+                // `cargo fuzz`'s use!
+
+                // `RUST_LIBFUZZER_DEBUG_PATH` is set in initialization.
+                if let Some(path) = $crate::RUST_LIBFUZZER_DEBUG_PATH.get() {
+                    use std::io::Write;
+                    let mut file = std::fs::File::create(path)
+                        .expect("failed to create `RUST_LIBFUZZER_DEBUG_PATH` file");
+                    writeln!(&mut file, "{:?}", bytes)
+                        .expect("failed to write to `RUST_LIBFUZZER_DEBUG_PATH` file");
+                    return 0;
+                }
+
+                __libfuzzer_sys_run(bytes);
+                0
+            }
+
+            // Split out the actual fuzzer into a separate function which is
+            // tagged as never being inlined. This ensures that if the fuzzer
+            // panics there's at least one stack frame which is named uniquely
+            // according to this specific fuzzer that this is embedded within.
+            //
+            // Systems like oss-fuzz try to deduplicate crashes and without this
+            // panics in separate fuzzers can accidentally appear the same
+            // because each fuzzer will have a function called
+            // `rust_fuzzer_test_input`. By using a normal Rust function here
+            // it's named something like `the_fuzzer_name::_::__libfuzzer_sys_run` which should
+            // ideally help prevent oss-fuzz from deduplicate fuzz bugs across
+            // distinct targets accidentally.
+            #[inline(never)]
+            fn __libfuzzer_sys_run(mut $bytes: &[u8]) {
+                $body
+            }
+        };
+    };
+
     (|$data:ident: &[u8]| $body:expr) => {
         $crate::fuzz_target!(|$data| $body);
     };
 
+    (|mut $data:ident: &[u8]| $body:expr) => {
+        $crate::fuzz_target!(|mut $data| $body);
+    };
+
     (|$data:ident: $dty:ty| $body:expr) => {
         $crate::fuzz_target!(|$data: $dty| -> () { $body });
     };
 
+    (|mut $data:ident: $dty:ty| $body:expr) => {
+        $crate::fuzz_target!(|mut $data: $dty| -> () { $body });
+    };
+
     (|$data:ident: $dty:ty| -> $rty:ty $body:block) => {
         const _: () = {
             /// Auto-generated function
@@ -306,6 +356,60 @@ macro_rules! fuzz_target {
             }
         };
     };
+
+    (|mut $data:ident: $dty:ty| -> $rty:ty $body:block) => {
+        const _: () = {
+            /// Auto-generated function
+            #[no_mangle]
+            pub extern "C" fn rust_fuzzer_test_input(bytes: &[u8]) -> i32 {
+                use $crate::arbitrary::{Arbitrary, Unstructured};
+
+                // Early exit if we don't have enough bytes for the `Arbitrary`
+                // implementation. This helps the fuzzer avoid exploring all the
+                // different not-enough-input-bytes paths inside the `Arbitrary`
+                // implementation. Additionally, it exits faster, letting the fuzzer
+                // get to longer inputs that actually lead to interesting executions
+                // quicker.
+                if bytes.len() < <$dty as Arbitrary>::size_hint(0).0 {
+                    return -1;
+                }
+
+                let mut u = Unstructured::new(bytes);
+                let data = <$dty as Arbitrary>::arbitrary_take_rest(u);
+
+                // When `RUST_LIBFUZZER_DEBUG_PATH` is set, write the debug
+                // formatting of the input to that file. This is only intended for
+                // `cargo fuzz`'s use!
+
+                // `RUST_LIBFUZZER_DEBUG_PATH` is set in initialization.
+                if let Some(path) = $crate::RUST_LIBFUZZER_DEBUG_PATH.get() {
+                    use std::io::Write;
+                    let mut file = std::fs::File::create(path)
+                        .expect("failed to create `RUST_LIBFUZZER_DEBUG_PATH` file");
+                    (match data {
+                        Ok(data) => writeln!(&mut file, "{:#?}", data),
+                        Err(err) => writeln!(&mut file, "Arbitrary Error: {}", err),
+                    })
+                    .expect("failed to write to `RUST_LIBFUZZER_DEBUG_PATH` file");
+                    return -1;
+                }
+
+                let data = match data {
+                    Ok(d) => d,
+                    Err(_) => return -1,
+                };
+
+                let result = ::libfuzzer_sys::Corpus::from(__libfuzzer_sys_run(data));
+                result.to_libfuzzer_code()
+            }
+
+            // See above for why this is split to a separate function.
+            #[inline(never)]
+            fn __libfuzzer_sys_run(mut $data: $dty) -> $rty {
+                $body
+            }
+        };
+    };
 }
 
 /// Define a custom mutator.