Shebang feature incompatible with upcoming cargo script feature #14146
Description
To launch rust sources files as a script, we can currently use rust-script. And even more powerful, we can combine it with nix's shebang syntax which works with both the old and the new command-line interfaces of nix:
Old:
#!/usr/bin/env nix-shell
//! ```cargo
//! [dependencies]
//! time = "0.1.25"
//! ```
/*
#!nix-shell -i rust-script -p rustc -p rust-script -p cargo
*/
fn main() {
for argument in std::env::args().skip(1) {
println!("{}", argument);
};
println!("{}", std::env::var("HOME").expect(""));
println!("{}", time::now().rfc822z());
}(from https://nixos.wiki/wiki/Nix-shell_shebang#With_dependencies, doc: https://nix.dev/manual/nix/2.18/command-ref/nix-shell#use-as-a--interpreter)
New:
#!/usr/bin/env nix
//! ```cargo
//! [dependencies]
//! time = "0.1.25"
//! ```
/*
#!nix shell nixpkgs#rustc nixpkgs#rust-script nixpkgs#cargo --command rust-script
*/
fn main() {
for argument in std::env::args().skip(1) {
println!("{}", argument);
};
println!("{}", std::env::var("HOME").expect(""));
println!("{}", time::now().rfc822z());
}However, this rust-as-a-script feature is now being integrated directly into the official cargo and the syntax to integrate the content of Cargo.toml is different, which makes it incompatible with nix's shebang:
#!/usr/bin/env nix
---cargo
[dependencies]
time = "0.1.25"
---
/*
#! nix shell github:oxalica/rust-overlay#rust-nightly --command sh -c ``cargo -Zscript $0``
*/
fn main() {
for argument in std::env::args().skip(1) {
println!("{}", argument);
};
println!("{}", std::env::var("HOME").expect(""));
println!("{}", time::now().rfc822z());
}results in:
error: './test.rs' is not a recognised command
This is because nix's multiline shebang can't really be placed anywhere in the file like the documentation suggests:
Line 305 in 76ac375
Nix uses a hardcoded list of characters that should be at the beginning of a line to know if this line can be inserted between multiple nix shebang lines:
Line 284 in de71ceb
An easy solution would be to allow nix shebang lines to be anywhere in the file but I don't know if there's an issue with this or not.
Conversely, cargo could be changed to allow comments to be inserted before the cargo file content. Their syntax RFC had already been merged but the feature has not be stabilized yet: rust-lang/cargo#12207