Add CARGO_PATH environment variable

[dalance]

This PR adds an environment variable CARGO_PATH to specify search path for programs spawned by cargo.

If there is a newer compiler than system compiler on PATH, some rust compilation fail.
This is reported at rust-lang/rust#58394 and #6582.

To resolve the issue, this PR adds CARGO_PATH to specify compiler and linker path.
For example, rust-lang/rust#58394 can be resolved like below.

$ echo $PATH
~/.linuxbrew/bin:~/.cargo/bin:/usr/bin:/bin
$ cargo build
/// failed
$ export CARGO_PATH=~/.cargo/bin:/usr/bin:/bin
$ cargo build
/// success

In this case, cargo without CARGO_PATH calls ~/.linuxbrew/bin/cc as linker, but rustc expects system linker, therefore the compilation fails. Alternatively cargo with CARGO_PATH calls system linker and the compilation succeeds.

[rust-highfive]

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[Eh2406]

@bors r? @alexcrichton

[alexcrichton]

Thanks for the PR! I would personally be hesitant to merge this feature though. This seems like a bit of hack in Cargo to work around something that may not be fully understood? Are there perhaps other ways to solve this problem without resorting to this hammer?

[dalance]

In my understanding about rust-lang/rust#58394, the followings must be system toolchain because official rustc binary is linked to system libc.

• Linker called by cargo
• C compiler called by cc-rs

Because both of them use PATH to search toolchain, I think the issue can be resolved by CARGO_PATH.
Although I can understand this feature is too large.
Currently I have two other ways, but I think both of them are not good.

individual setting

Both of them can be set individually. Linker called by cargo can be set by ~/.cargo/config like below:

[target.x86_64-unknown-linux-gnu]
linker = "/usr/bin/cc"

C compiler called by cc-rs can be set by CC environment variable.

export CC=/usr/bin/cc

Pros

No modification is needed.

Cons

CC environment variable affects other programs like make.
If another crate like cc-rs will be created, another setting will be needed.

wrapper of cargo

A wrapper of cargo can set PATH passed to cargo.
For example, zsh function like below:

function cargo() {
      PATH=~/.cargo/bin:/usr/bin:/bin command cargo $@
}

Pros

No modification is needed.

Cons

This is not a feature of cargo/rustc. So I would be hesitant to announce this solution to other linuxbrew users.

[alexcrichton]

If the goal here is to switch linkers then configuring .cargo/config is the way to do that, but I'm personally still not really understanding what the motivation is to switch out PATH

[dalance]

Linker called by cargo can be specified through .cargo/config, but C compiler/linker used at build.rs is still searched in PATH. So I want to switch PATH.

[alexcrichton]

Ok if this is purely about switching linkers I'm going to close this since that's done with various other configuration that we have in Cargo. Switching rustc's linker is done via .cargo/config, and build scripts should provide their own way of configuring linkers. For crates using cc, for example, there's a variety of env vars to configure.

[dalance]

Ok. I will resolve this issue by cargo wrapper function.
Thanks.

[shijunti19]

How Windows 10 Solves this Problem

error: linker cc not found
|
= note: 系统找不到指定的文件。 (os error 2)

error: aborting due to previous error

[jameshilliard]

Switching rustc's linker is done via .cargo/config, and build scripts should provide their own way of configuring linkers.

This doesn't really work in a number of cases without something like #9322.