Do not expand environment variables

[svend]

Only expand racer-rust-src-path and racer-cargo-home variables, not
RUST_SRC_PATH and RUST_SRC_PATH environment variables. This make the
behavior of racer the same in Emacs and on the command line with these variables
set. It will also allow RUST_SRC_PATH to be set to an empty string, which
is required with racer installed via nixpkgs, where RUST_SRC_PATH should not be set.

https://github.com/NixOS/nixpkgs/blob/348f938c552dcfdc41e543dd20511b5d0e4e70f6/pkgs/development/tools/rust/racer/rust-src.patch

[coveralls]

Coverage decreased (-0.3%) to 64.247% when pulling 3fea98f on svend:nix into cb74060 on racer-rust:master.

[coveralls]

Coverage decreased (-0.3%) to 64.247% when pulling 3fea98f on svend:nix into cb74060 on racer-rust:master.

[coveralls]

Coverage decreased (-0.3%) to 64.247% when pulling 3fea98f on svend:nix into cb74060 on racer-rust:master.

[coveralls]

Coverage decreased (-0.3%) to 64.247% when pulling 3fea98f on svend:nix into cb74060 on racer-rust:master.

[svend]

Racer installed via nixpkgs is patched to add the correct path to the rust source in the nixpkgs store. With this configuration, RUST_SRC_PATH should not bet set. I couldn't find a way with racer.el to not set RUST_SRC_PATH. With this patch, setting the RUST_SRC_PATH environment variable to an empty string results in racer run with this value, which racer treats the same as if it was not set.

• Nixpkgs: https://github.com/NixOS/nixpkgs
• Nixpkgs racer package: https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/tools/rust/racer/default.nix

[Wilfred]

I think these changes make sense. We've seen a lot of code churn in these settings to accommodate all the different use cases (you're the first to mention Nix I think). We'll probably see even more in future :)