Auto merge of #1327 - db48x:fuzzing-extra-file, r=emilio

Add an option to extend the compiled code after each reducing step

This let me add a test function which called a method in the reduced
code, preventing creduce from eliminating everything entirely.

Author: bors-servo

csmith-fuzzing/predicate.py

@@ -83,6 +83,10 @@
     action="store_false",
     dest="rustc",
     help="Do not attempt to compile the emitted bindings with `rustc`.")
+reducing.add_argument(
+    "--extra-compile-file",
+    type=str,
+    help="Append the content of this extra file to the end of the emitted bindings just before compiling it.")
 reducing.add_argument(
     "--expect-compile-fail",
     action="store_true",
@@ -221,6 +225,10 @@ def run_bindgen(args, bindings):
                 raise ExitOne()
 
 def run_rustc(args, bindings, test_exe):
+    if args.extra_compile_file:
+        with open(bindings, mode="a") as outfile:
+            with open(args.extra_compile_file, mode="r") as infile:
+                outfile.write(infile.read())
     child = run(
         ["rustc", "--crate-type", "lib", "--test", "-o", test_exe, bindings],
         stdout=subprocess.PIPE,