Fixed rust incompatibility with crates.io

[fdoyon]

Contributing to Linuxbrew:

To contribute a new formula or a new version of an existing formula, please submit your pull request to Homebrew rather than to Linuxbrew. Patches to fix issues that you have reproduced on both Linuxbrew and Homebrew should be sent to Homebrew. Please send your pull request to Linuxbrew if you are in doubt.

Patches to fix issues particular to Linux should not affect the behaviour of the formula on Mac. Use if OS.mac? and if OS.linux? as necessary to preserve the existing behaviour on Mac. For example, to add a new dependency that is not necessary on Mac:

depends_on "zlib" unless OS.mac?

All Submissions:

You can erase any parts of this template not applicable to your Pull Request.

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you included a log of the failed build without your patch using brew gist-logs <formula>?
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  brew audit --strict <formula> (where <formula> is the name of the formula you're submitting)?
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Changes to Linuxbrew's Core:

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• Have you written new tests for your core changes, as applicable? Here's an example if you'd like one.
• Have you successfully ran brew tests with your changes locally?

Rust cannot build from source without bootstrapping with a snapshot Cargo.
The snapshot file in cargo 0.8.0 is out of date, resulting in a crash at install time.
Using the latest snapshot from rust-lang/cargo@c8fcec3#diff-7cb61adf6593bc4207784377efe8fe29