RogerBW's Blog

A Shebang For Rust 23 November 2020

Rust is a compiled language. How can I run a program directly from the shell?

The answer is Unix abuse, and a program that is valid in both Rust and shell.

What I want the program to do from the shell is to compile itself, run that compiled version, then erase the compiled version and exit. And the directives to do that can't interfere with the actual Rust.

Fortunately Rust comments start with a double slash, and # indicates a macro directive which will be ignored if not understood. So at the top of the program:

#! /bin/sh
//usr/bin/env rustc --test $0 -o ${0}x && ./${0}x; rm -f ${0}x ; exit

The shell script finds and runs the Rust compiler, in test mode, against itself, generating an executable (its own filename, as it might be foo.rs, with an "x" on the end on the basis that that's easy to work out). If that was successful, it runs the executable. Then it removes it (using -f to avoid errors in case compilation wasn't successful) and stops before the shell starts trying to interpret Rust code. All of this is regarded by rustc as irrelevance.

I don't usually play the programming games like quines, Perl golf, and such, but this one has some actual use: I can run foo.rs, and get back either compilation errors or the output of the program, just as I do for the other languages in the weekly challenge.

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