Improve the fmt output of the instantiate fuzz target (#4804)

Add an Arbitrary instance for the input to the instantiate fuzz target, so that cargo fuzz fmt instantiate <file> produces more meaningful output.

fuzz/fuzz_targets/instantiate.rs

@@ -1,31 +1,33 @@
 #![no_main]
 
-use libfuzzer_sys::arbitrary::{Result, Unstructured};
+use libfuzzer_sys::arbitrary::{Arbitrary, Result, Unstructured};
 use libfuzzer_sys::fuzz_target;
-use wasmtime_fuzzing::oracles::Timeout;
+use wasmtime_fuzzing::generators::Config;
-use wasmtime_fuzzing::{generators, oracles};
+use wasmtime_fuzzing::oracles::{instantiate, Timeout};
+use wasmtime_fuzzing::wasm_smith::Module;
 
-fuzz_target!(|data: &[u8]| {
+#[derive(Debug)]
-    // errors in `run` have to do with not enough input in `data`, which we
+struct InstantiateInput {
-    // ignore here since it doesn't affect how we'd like to fuzz.
+    config: Config,
-    drop(run(data));
+    timeout: Timeout,
-});
+    module: Module,
+}
 
-fn run(data: &[u8]) -> Result<()> {
+impl<'a> Arbitrary<'a> for InstantiateInput {
-    let mut u = Unstructured::new(data);
+    fn arbitrary(u: &mut Unstructured<'a>) -> Result<Self> {
-    let mut config: generators::Config = u.arbitrary()?;
+        let mut config: Config = u.arbitrary()?;
 
         // Pick either fuel, duration-based, or module-based timeout. Note that the
         // module-based timeout is implemented with wasm-smith's
         // `ensure_termination` option.
         let timeout = if u.arbitrary()? {
-        config.generate_timeout(&mut u)?
+            config.generate_timeout(u)?
         } else {
             Timeout::None
         };
 
         let module = config.generate(
-        &mut u,
+            u,
             if let Timeout::None = timeout {
                 Some(1000)
             } else {
@@ -33,6 +35,14 @@ fn run(data: &[u8]) -> Result<()> {
             },
         )?;
 
-    oracles::instantiate(&module.to_bytes(), true, &config, timeout);
+        Ok(InstantiateInput {
-    Ok(())
+            config,
+            timeout,
+            module,
+        })
+    }
 }
+
+fuzz_target!(|data: InstantiateInput| {
+    instantiate(&data.module.to_bytes(), true, &data.config, data.timeout);
+});