Merge pull request #33 from mgeisler/mgeisler-cfg-fuzzing-repro

Document `fuzzing_repro` in guide.md

src/cargo-fuzz/guide.md

@@ -26,6 +26,15 @@ unsafe = ["project/unsafe"]
 
 Every crate instrumented for fuzzing -- the `fuzz_targets` crate, the project crate, and their entire dependency tree -- is compiled with the `--cfg fuzzing` rustc option. This makes it possible to disable code paths that prevent fuzzing from working, e.g. verification of cryptographic signatures, with a simple `#[cfg(not(fuzzing))]`, and without the need for an externally visible Cargo feature that must be maintained throughout every dependency.
 
+## `#[cfg(fuzzing_repro)]`
+
+When you run `cargo fuzz <fuzz target name> <crash file>`, every crate is compiled with the `--cfg fuzzing_repro` rustc option. This allows you to leave debugging statements in your fuzz targets behind a `#[cfg(fuzzing_repro)]`:
+
+```rust
+#[cfg(fuzzing_repro)]
+eprintln!("Input data: {}", expensive_pretty_print(&data));
+```
+
 ## libFuzzer configuration options
 
 See all the libFuzzer options: