reverse/src/main.rs

//#[inline] 
fn reverse<T>(slice: &mut [T])
{
    match slice {
	[ref mut a, ref mut rest @ .., ref mut b] => {
	    std::mem::swap(a, b);
	    reverse(rest)
	},
	[] | [_] => (),
    }
}

/// This actually works! And is so easily parallelisable with something like rayon, or asyncing by spawning/creating the tail-call into a new task, then either waiting them concurrently or in parallen (spawn or created future from just calling the function without awaiting it)
#[allow(dead_code)]
fn binsearch<'a, V: ?Sized, T: PartialEq<V> + 'a>(slice: &'a [T], find: &V) -> Option<&'a T>
{
    match slice {
	[ref a, pivot @ .., ref b] => {
	    match (a==find, b==find) {
		(true, _) => Some(a),
		(_, true) => Some(b),
		_ => binsearch(pivot, find),
	    }
	},
	[ref a] if a == find => Some(a),
	_ => None,
    }
}

fn collect_input() -> Box<dyn Iterator<Item=String> + 'static>
{
    // TODO: Use non-panicking functions for both reading lines and reading args, just skip invalid lines/args
    
    if std::env::args_os().len() <= 1 {
	use std::io::{
	    self,
	    BufRead,
	};
	// No args, collect stdin lines
	Box::new(io::stdin()
		 .lock()
		 .lines()
		 .filter_map(Result::ok))
    } else {
	// Has arguments, return them
	if cfg!(feature="ignore-invalid-args") {
	    Box::new(std::env::args_os().skip(1).filter_map(|os| os.into_string().ok()))
	} else {
	    Box::new(std::env::args_os().skip(1).map(|os| os.to_string_lossy().into_owned()))
	}
    }
}

#[cfg_attr(feature="ignore-output-errors", inline(always))]
fn handle_fmt_err<T>(res: std::io::Result<T>)
{
    #[cfg(not(feature="ignore-output-errors"))]
    if let Err(e) = res {
	eprintln!("[!] failed to write line: {e}");
    }
    let _ = res;
}

fn main() {
    let mut args: Vec<String> = collect_input().collect();
    reverse(&mut args[..]);
    //eprintln!("{:?}", binsearch(&args[..], "1")); // It works!
    #[cfg(feature="output-lines")]
    {
	#[allow(unused_imports)]
	use std::io::{
	    Write, BufWriter,
	};
	#[cfg(feature="buffer-output")] 
	let mut out = BufWriter::new(std::io::stdout().lock());
	#[cfg(not(feature="buffer-output"))] 
	let mut out = std::io::stdout().lock();
	for x in args.iter() {
	    handle_fmt_err({
		if cfg!(feature="output-quoted") {
		    //XXX: This doesn't flush, right? It shouldn't, but maybe we should test it?
		    writeln!(&mut out, "{:?}", x)
		}
		else  {
		    //writeln!(&mut out, "{}", x)
		    out.write(x.as_bytes())
			.and_then(|_| out.write(b"\n"))
			.map(|_| {})
		}
	    });
	}
	//#[cfg(feature="buffer-output")] 
	handle_fmt_err(out.flush()); //XXX: Do we need to flush when not buffering output? Does it matter since buffering output will be enabled by default and should almost always be enabled?
    }
    #[cfg(not(feature="output-lines"))]
    println!("{:?}", args);
}