Returning mutable reference when self is mutable, otherwise reference

Question

I have the following snippet of Rust:

pub fn scope(&mut self) -> &mut HashMap<String, Type> {
    let idx = self.vars.len() - 1;
    &mut self.vars[idx]
}

I've realized that I have some contexts where I would like to use this function with a non-mutable version of the function e.g.:

pub fn scope(&self) -> &HashMap<String, Type> {
    let idx = self.vars.len() - 1;
    &self.vars[idx]
}

There's only 3 muts removed between the two functions. Can I somehow turn these into one function that derives the mutability of my returned reference depending on the mutability of self? Is there perhaps some kind of trait I can make use of or similar?

On a side note, `self.vars.iter_mut().rev().next().unwrap()` is a one-liner for what you're doing there, which does no indexing. :) — E_net4 - Krabbe mit Hüten, Jun 13 '17 at 11:01

score 6 · Accepted Answer · answered Jun 13 '17 at 10:57

6

You cannot have a single function to achieve that. However, it is actually conventional (and recommended by the API guidelines) to have _mut variants of the same function for that purpose:

pub fn scope(&self) -> &HashMap<String, Type> {
    let idx = self.vars.len() - 1;
    &self.vars[idx]
}

pub fn scope_mut(&mut self) -> &mut HashMap<String, Type> {
    let idx = self.vars.len() - 1;
    &mut self.vars[idx]
}

answered Jun 13 '17 at 10:57

E_net4 - Krabbe mit Hüten

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Thanks a heap, I'll go with that then! – lfxgroove Jun 13 '17 at 11:14

Returning mutable reference when self is mutable, otherwise reference

1 Answers1