Deref & Deref Coercion

Deref maps smart pointer to inner reference. Deref coercion converts &T to &U when T: Deref<Target=U> at call sites.

Recipe

use std::ops::Deref;
 
struct MyString(String);
impl Deref for MyString {
    type Target = str;
    fn deref(&self) -> &str { &self.0 }
}
 
fn print_len(s: &str) { println!("{}", s.len()); }
 
fn main() {
    let s = MyString("hello".into());
    print_len(&s); // coercion &MyString -> &str
}

When to reach for this: Newtype wrappers and smart pointers acting like inner type.

Working Example

fn main() {
    let s = String::from("rust");
    let len = str_len(&s);
    println!("{len}");
}
 
fn str_len(s: &str) -> usize { s.len() }

What this demonstrates:

• &String coerces to &str via Deref
• Box<T>, Rc<T> similarly coerce when T: Deref
• Automatic at function arguments

Deep Dive

Deref coercion chain limited to avoid infinite recursion. DerefMut for mutable coercion.

Gotchas

• Deref returning wrong lifetime - Compile errors. Fix: Match inner field lifetime.
• Over-Deref in API - Hidden allocations if Deref clones. Fix: impl Deref to inner ref only.
• Deref polymorphism abuse - Not inheritance. Fix: Prefer traits for behavior sharing.
• Ambiguous coercion - Multiple Deref paths rare.
• Manual * deref - *box explicit deref.

Alternatives

FAQs

Related

• Custom Smart Pointers
• Box<T>
• Operator Overloading
• References & Slices

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.