Resolving Rust Trait Object Lifetime Errors: A Step‑by‑Step Guide

Recently I was writing a Redis data‑validation tool in Rust using the redis::ConnectionLike trait from redis‑rs . To abstract the validation process I needed to store structs as trait objects, which introduced lifetime issues.

Defining the base trait

pub trait Base {
    fn say(&self) -> String;
}

Implementations of Base

pub struct AFromBase {
    content: String,
}

impl Base for AFromBase {
    fn say(&self) -> String {
        self.content.clone()
    }
}

pub struct BFromBase {
    text: String,
}

impl Base for BFromBase {
    fn say(&self) -> String {
        self.text.clone()
    }
}

Attempting to combine two trait objects

I created a struct that holds two Base trait objects and a method that concatenates their say results.

pub struct AddTowBase {
    a: &mut dyn Base,
    b: &mut dyn Base,
}

impl AddTowBase {
    fn add(&self) -> String {
        let result = self.a.say() + &self.b.say();
        result
    }
}

The main function constructs instances of AFromBase and BFromBase, creates an AddTowBase, and prints the concatenated string.

fn main() {
    let mut a = AFromBase { content: "baseA".to_string() };
    let mut b = BFromBase { text: "baseB".to_string() };
    let addtow = AddTowBase { a: &mut a, b: &mut b };
    let r = addtow.add();
    println!("{}", r);
}

Compilation errors

The code fails to compile with errors like:

error[E0106]: missing lifetime specifier
  --> examples/lifetimeinstruct.rs:26:8
   |
26 |     a: &mut dyn Base,
   |        ^ expected named lifetime parameter
   |
   = help: consider introducing a named lifetime parameter

The compiler indicates that the trait objects need explicit lifetime parameters to ensure they live as long as the struct.

Fixing the lifetimes

By adding a lifetime parameter 'a to AddTowBase and to the references, the code compiles:

pub struct AddTowBase<'a> {
    a: &'a mut dyn Base,
    b: &'a mut dyn Base,
}

impl<'a> AddTowBase<'a> {
    fn add(self) -> String {
        let result = self.a.say() + &self.b.say();
        result
    }
}

Conclusion

Rust's lifetimes guarantee memory safety but add mental overhead. Deciding whether to invest effort before release or troubleshoot after deployment is a trade‑off; I prefer catching such issues early to avoid affecting users.