How to load paths dynamically on Cargo.toml files?

help
1

I want to load the path dynamically in Rust for Cargo.toml.

So I have asked chatgpt to try and generate some code for me to do this however I get an error.

So I have a very basic example where in the build.rs file I added this:

// build.rs
fn main()
{
    let blue_flame_path = std::env::var("BLUE_FLAME_PATH")
        .unwrap_or_else(|_| "/home/joe/.local/share/blue_flame".to_string());

    println!("cargo:rustc-env=BLUE_FLAME_PATH={}", blue_flame_path);
}

and in the Cargo.toml file I added this:

[dependencies]
common = {path = "$BLUE_FLAME_PATH/common"}

However I appear to get an error when compiling this.

Not too sure how do I do this dynamically?

2

You cannot create dependencies at build time.

The order of operations in a Cargo build is that first all of your dependencies are built, and only then does any code from your own package get compiled or run. The build script runs before anything else in your package is compiled, but it does not run before your dependencies are analyzed and compiled. Not only that, but also, Cargo determines the entire dependency graph before anything else is done.

If you want to control the path where a dependency is found, you must write a program that generates the Cargo.toml file before the cargo build command is issued.

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3

In Cargo.toml, is there a way to get environment variables inside file paths, for example:

[dependencies]
common = {path = "$HOME/.local/share/blue_flame"}

?

4

The answer from kpreid is clear: cargo doesn't support it. What you can do is

There are many options/tools which are irrelevent to cargo itself, since any script language can achieve your goal.

These days Nickel 1.0, a configuration language written in Rust, is released. If your config is dynamic enough, give it a shot.

And cargo won't support interpolation

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5

And if you want to share some common local dependency, you probably either want a workspace (if there's some relatively small amount of crates depending on it) or want to publish it separately (possibly to some private registry instead of crates.io).

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6

If I understood correctly, it can dynamically generate my Cargo.toml file and it can figure out the $HOME dir?

7

I don't know how nickel reads the environment variables. I memtioned it just because I saw it lately.

For your specific problem, substitute the environment variables in cargo.toml is fairly simple via shell script

echo $HOME | xargs -I '{}' sed -i 's#$HOME#{}#' Cargo.toml
1 Like
8

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