Allow extra command line arguments

[FlashSheridan]

Rust newbies (trust me) sometimes need to see what gets passed in as command-line arguments, via env::args or more complicated techniques. Currently std::env::args() just has something like Args { inner: ["~/.cargo/bin/evcxr"] }, even if --opt 2 was passed, and extra arguments stop evcxr from launching. Currently I need to pass in and inspect a bunch of arguments escaped in different ways, so I’d like evcxr just to accept them and put them in std::env::args(), perhaps after an --extra-args argument.

[davidlattimore]

Evcxr has two processes, both of which are the evcxr binary. The parent process is the one that you run. It's what displays the prompt, parses what you type, writes source files to disk, runs cargo etc. Then there's a subprocess, which is where your code get executed. When you run std::env::args, it's looking at the arguments passed to this subprocess.

So I think it would be possible to pass extra arguments to this subprocess, which would, by default, ignore them. If you clone the evcxr repository, then edit evcxr/src/child_process.rs and make a change like the following:

diff --git a/evcxr/src/child_process.rs b/evcxr/src/child_process.rs
index ef15635..1bdaf90 100644
--- a/evcxr/src/child_process.rs
+++ b/evcxr/src/child_process.rs
@@ -37,6 +37,7 @@ impl ChildProcess {
             bail!("Our current binary doesn't call runtime_hook()");
         }
         command
+            .arg("--foo")
             .env(runtime::EVCXR_IS_RUNTIME_VAR, "1")
             .env("RUST_BACKTRACE", "1")
             .stdin(std::process::Stdio::piped())

If you then run cargo run -p evcxr_repl and in the resulting shell do std::env::args(), you get:

Args { inner: ["/home/david/work/evcxr/target/debug/evcxr", "--foo"] }

Hopefully that helps