I am in the process of porting some (old) C++ code to rust. The code includes a main class (System), the contains HashMaps and Vectors of various other classes. The original code used pointers. For the most part most of this data structure is "static" -- it is read from a set of files and one section is ever persistently modified, although some of the other structures are updated during processing.

I have most of the data structure implemented and working, but I have hit a snag.

The original C++ code is here:

And my rust port is here:

The build log is here:

type or   Compiling freight_car_forwarder v0.1.0 (/home/heller/RustProjects/freight_car_forwarder)
error: lifetime may not live long enough
   --> src/system.rs:181:9
    |
180 |     pub fn IndustryByIndexMut(&mut self, i: usize) -> Option<&mut Industry> {
    |                               ---------
    |                               |
    |                               let's call the lifetime of this reference `'1`
    |                               has type `&mut system::System<'2>`
181 |         self.industries.get_mut(&i)
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^ associated function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
    |
    = note: requirement occurs because of a mutable reference to `Industry<'_>`
    = note: mutable references are invariant over their type parameter
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance

error: could not compile `freight_car_forwarder` due to previous error

I think I need to specify lifetimes, but I am not sure where.

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