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);
}

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[namanlp]

r? @epage

[epage]

This should probably be on unstable.md

[epage]

Doesn't is need a feature gate?

[epage]

Suggested change

	err_msg += &format!(
	"\nBuild Scripts : {} have the same file stem, that is {}",
	paths.join(", "),
	stem
	);
	write!(&mut err_msg, " for stem `{stem}`: {}", paths.join(", "))?;

• generally better to use write! to append formatted text
• duplicating the error message
• needs indentation to set apart from the rest of the message

[epage]

Shouldn't this test be added before the validation?