Some strings can break manifest.nix

[evanjs]

The description from cargo-cache seems to be breaking manifest.nix (when installed via nix-env).

Here is the relevant string:
"Manage cargo cache (\${CARGO_HOME}, ~/.cargo/), print sizes of dirs and remove dirs selectively";

This installs fine, and presents no issues when added to systemPackages on NixOS.

Using nix-env, however, this breaks the manifest file.

Here is the manifest of a user profile with only cargo-cache installed via nix-env.

Note that the description is not escaped in the manifest:
description = "Manage cargo cache (${CARGO_HOME}, ~/.cargo/), print sizes of dirs and remove dirs selectively";

As @cleverca22 mentioned on IRC:

because it just does ${key} = "${value}";

I have seen similar weirdness with e.g. nodePackages.@angular/cli, and wanted to clarify whether this is an issue with the Nix parser or something that is being done improperly with this package.

[thufschmitt]

There's indeed an issue in the way the manifest.nix is generated as it calls nix::printValue which doesn't escape dollars. It's fairly easy to patch this (and the current behavior is certainly a bug), but it might have a bunch of side-effects as printValue is used in a lot of other places (essentially in every place where a Nix expression is dumped like in the output of nix-instantiate or nix eval), so this will be a breaking change for everything that relied on this bug.

I've opened #4012 nontheless to fix this