Cannot borrow as mutable more than once at a time in one code – but can in another very similar

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You have fallen into the lifetime-trap. Adding the same lifetime to more references will constrain your program more. Adding more lifetimes and giving each reference the minimal possible lifetime will permit more programs. As @o11c notes, removing the constraints to the 'a lifetime will solve your issue.

impl<'a> Context<'a> {
    fn get_function(&mut self, fun_name: &str) -> Result<Function<'a>, Error> {
        self.program
            .get(fun_name)
            .map(|f| *f)
            .ok_or(Error::FunctionNotFound)
    }

    fn call(&mut self, fun_name: &str) -> Result<(), Error> {
        let fun = try!(self.get_function(fun_name));

        self.call_stack.push(fun);

        Ok(())
    }
}

The reason this works is that Rust inserts new lifetimes, so in the compiler your function’s signatures will look like this:

fn get_function<'b>(&'b mut self, fun_name: &'b str) -> Result<Function<'a>, Error>
fn call<'b>(&'b mut self, fun_name: &'b str) -> Result<(), Error>

Always try to not use any lifetimes and let the compiler be smart. If that fails, don’t spray lifetimes everywhere, think about where you want to pass ownership, and where you want to limit the lifetime of a reference.

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