Call the original function in the replaced function

[mazong1123]

Got several requests for this. The scenario is in the fake function, user wants to call original function under certain condition. This requires injectorpp to have the ability to temporarily recover the original function.

[regerj]

Context

When trying to override std::process::Command::spawn() to avoid running external binaries on the system using injectorpp, the fake! function provided to will_execute() must return a std::io::Result<std::process::Child>. The only way to create a std::process::Child is via std::process::Command::spawn() because it has private members and no factory methods. However, you cannot call the "real" std::process::Command::spawn() inside of the fake!d function.

Example

I want to replace the echo hello world call with a true call instead. In practice this would be a more system modifying call being replaced.

This will fail because it will match the first call at the bottom, call the fake function, and upon encountering the second call to true it will panic with the message: Fake function called with unexpected arguments.

let mut injector = InjectorPP::new();
injector
    .when_called(injectorpp::func!(
        fn (std::process::Command::spawn)(&mut std::process::Command) -> std::io::Result<std::process::Child>))
    .will_execute(injectorpp::fake!(
        func_type: fn(cmd: &mut std::process::Command) -> std::io::Result<std::process::Child>,
        when:
            cmd.get_program() == "echo" &&
            cmd.get_args().collect::<Vec<&OsStr>>() == ["hello world"],
        returns: std::process::Command::new("true").spawn(),
        times: 1
    ));

let result = std::process::Command::new("echo")
    .arg("hello world")
    .spawn();

assert!(result.is_ok());

It would be great to somehow be able to invoke the original function in the overriding function in order to be able to return objects that cannot be constructed aside from the overridden method.