Rust doesn't currently enforce pure functions at all; there is no 'pure' keyword. Here is an old mailing list post related to this decision - note that because

Rust doesn't currently enforce pure functions at all; there is no 'pure' keyword. Here is an old mailing list post related to this decision - note that because it is 2 years old and Rust has been in a state of constant flux for most of that time, it may not fully reflect current thinking. https://mail.mozilla.org/pipermail/rust-dev/2013-January/002903.html https://mail.mozilla.org/pipermail/rust-dev/2013-January/002...