[WIP] Accessing each build script's OUT_DIR

[namanlp]

This PR is continuation of my work on GSoC Project : Build Script Delegation

What does this PR try to resolve?

Through this PR, I want the user to be able to access each build script's OUT_DIR.

How to test and review this PR?

There is a feature gate multiple-build-scripts that can be passed via cargo-features in Cargo.toml. So, you have to add

cargo-features = ["multiple-build-scripts"]

Preferably on the top of the Cargo.toml and use nightly toolchain to use the feature

Then, you can access the OUT_DIR of given build script by accessing it like a compile time environment variable, for example:

const BUILD1_OUT_DIR: &'static str = env!("BUILD1_OUT_DIR");
const BUILD2_OUT_DIR: &'static str = env!("BUILD2_OUT_DIR");

fn main() {
    println!("{}", BUILD1_OUT_DIR);
    println!("{}", BUILD2_OUT_DIR);
}

[epage]

We'll need to update https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-crates

[namanlp]

In a separate commit maybe? Or should I update the latest commit itself?

[epage]

Unless the commit is ginormous (e.g. adding all of cargo add into cargo), I would generally update docs as part of commit that added the feature, like tests.

[namanlp]

I think only docs are remaining. Everything else should be done.

[epage]

Can we gather this into a HashMap<Name, Vec<Path>> and then iterate through and if Vec<Path> has more than one element, we then error? That would led us report all errors at once and report the full path for each conflicting build script so users can better understand what they need to change

[namanlp]

Sure, I was just following how earlier validate_unique_name functions were written.