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@@ -61,6 +61,22 @@ In general, we use modules from only the Rust standard library
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whenever possible. We will review including external crates on a
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case-by-case basis.
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+If a crate only contains traits meant for compatibility between Rust
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+crates, such as [the digest crate](https://crates.io/crates/digest) or
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+[the failure crate](https://crates.io/crates/failure), it is very likely
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+permissible to add it as a dependency. However, a brief review should
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+be conducted as to the usefulness of implementing external traits
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+(i.e. how widespread is the usage, how many other crates either
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+implement the traits or have trait bounds based upon them), as well as
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+the stability of the traits (i.e. if the trait is going to change, we'll
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+potentially have to re-do all our implementations of it).
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+
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+For large external libraries, especially which implement features which
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+would be labour-intensive to reproduce/maintain ourselves, such as
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+cryptographic or mathematical/statistics libraries, only crates which
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+have stabilised to 1.0.0 should be considered, however, again, we may
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+make exceptions on a case-by-case basis.
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+
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Currently, Tor requires that you use the latest stable Rust version. At
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some point in the future, we will freeze on a given stable Rust version,
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to ensure backward compatibility with stable distributions that ship it.
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