Add --cfg fuzzing_repro when reproducing a crash

This makes it possible to leave large or expensive debug output in
your fuzzers since it will only be active when you reproduce a crash:

    #[cfg(fuzzing_repro)]
    eprintln!("Expensive debug output: {}", do_something_expensive());

Fixes rust-fuzz#343.

src/options.rs

@@ -126,6 +126,13 @@ pub struct BuildOptions {
     #[arg(long)]
     pub no_cfg_fuzzing: bool,
 
+    #[arg(skip = false)]
+    /// Add the 'cfg(fuzzing-repro)' compilation configuration. This build
+    /// option will be automatically used when running `cargo fuzz run <target>
+    /// <corpus>`. The option will not be shown to the user, which is ensured by
+    /// the `skip` attribute.
+    pub cfg_fuzzing_repro: bool,
+
     #[arg(long)]
     /// Don't build with the `sanitizer-coverage-trace-compares` LLVM argument
     ///
@@ -232,6 +239,7 @@ mod test {
             coverage: false,
             strip_dead_code: false,
             no_cfg_fuzzing: false,
+            cfg_fuzzing_repro: false,
             no_trace_compares: false,
         };
 

src/options/run.rs

@@ -37,6 +37,7 @@ pub struct Run {
 impl RunCommand for Run {
     fn run_command(&mut self) -> Result<()> {
         let project = FuzzProject::new(self.fuzz_dir_wrapper.fuzz_dir.to_owned())?;
+        self.build.cfg_fuzzing_repro = !self.corpus.is_empty();
         project.exec_fuzz(self)
     }
 }

src/project.rs

@@ -177,6 +177,10 @@ impl FuzzProject {
             rustflags.push_str(" --cfg fuzzing");
         }
 
+        if build.cfg_fuzzing_repro {
+            rustflags.push_str(" --cfg fuzzing_repro");
+        }
+
         if !build.strip_dead_code {
             rustflags.push_str(" -Clink-dead-code");
         }

tests/tests/main.rs

@@ -175,6 +175,9 @@ fn run_no_crash() {
                 use libfuzzer_sys::fuzz_target;
 
                 fuzz_target!(|data: &[u8]| {
+                    #[cfg(fuzzing_repro)]
+                    eprintln!("Reproducing a crash");
+
                     run_no_crash::pass_fuzzing(data);
                 });
             "#,
@@ -188,7 +191,10 @@ fn run_no_crash() {
         .arg("--")
         .arg("-runs=1000")
         .assert()
-        .stderr(predicate::str::contains("Done 1000 runs"))
+        .stderr(
+            predicate::str::contains("Done 1000 runs")
+                .and(predicate::str::contains("Reproducing a crash").not()),
+        )
         .success();
 }
 
@@ -429,6 +435,9 @@ fn run_one_input() {
                 use libfuzzer_sys::fuzz_target;
 
                 fuzz_target!(|data: &[u8]| {
+                    #[cfg(fuzzing_repro)]
+                    eprintln!("Reproducing a crash");
+
                     assert!(data.is_empty());
                 });
             "#,
@@ -444,9 +453,11 @@ fn run_one_input() {
         .arg(corpus.join("pass"))
         .assert()
         .stderr(
-            predicate::str::contains("Running 1 inputs 1 time(s) each.").and(
-                predicate::str::contains("Running: fuzz/corpus/run_one/pass"),
-            ),
+            predicate::str::contains("Running 1 inputs 1 time(s) each.")
+                .and(predicate::str::contains(
+                    "Running: fuzz/corpus/run_one/pass",
+                ))
+                .and(predicate::str::contains("Reproducing a crash")),
         )
         .success();
 }