fn longest_explicit<'a>(x: &'a str, y: &'a str) -> &'a str {
if x.len() > y.len() {
x
} else {
y
}
}

If you’ve read The Rust Programming Language (a.k.a. The Rust Book) or implemented a struct, you’ve likely come across lifetime annotations — or even used them yourself.

This topic builds directly on the concept of lifetimes, so if you’re not familiar with them, it’s worth reviewing the fundamentals before diving deeper.

In the early days of Rust, functions that dealt with references often required explicit and verbose lifetime annotations. For example:

fn longest_explicit<'a>(x: &'a str, y: &'a str) -> &'a str {
if x.len() > y.len() {
x
} else {
y
}
}

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While correct, this can be overly verbose. Rust’s designers observed that most use cases followed consistent patterns. By codifying these patterns, they introduced lifetime elision rules, which allow the compiler to infer lifetimes in common cases — eliminating the need to annotate them manually in ~87% of real-world scenarios.

These rules are not guidelines for programmers to follow, but internal heuristics the compiler uses to infer lifetimes when they’re omitted.

What Is Lifetime Elision?

Lifetime elision is a feature that lets you omit explicit lifetime annotations in function signatures when certain rules are met. This makes code cleaner and easier to read, while still retaining all the safety guarantees of Rust’s borrow checker.

Why Lifetime Elision Matters

Reduces verbosity.

Improves code readability.

Lets you focus on logic instead of boilerplate annotations.

Helps streamline common borrowing patterns.

The Lifetime Elision Rules

Rust’s compiler uses four primary rules to determine when lifetimes can be elided:

Rule 1:

Each elided lifetime in an input position becomes a distinct lifetime parameter.

When you elide lifetimes in function parameters (e.g., use &str instead of &'a str), Rust treats each input reference as having a unique, separate lifetime.

Example:

fn example1(x: &str, y: &str) {
println!("First string: {}", x);
println!("Second string: {}", y);
}

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Rust interprets this as:

fn example1<'a, 'b>(x: &'a str, y: &'b str) {
println!("First string: {}", x);
println!("Second string: {}", y);
}

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This works fine because the function doesn’t return a reference. Each parameter’s lifetime is independent, and no output lifetime needs to be inferred.

Rule 2:

If there is exactly one input lifetime, that lifetime is assigned to all elided output lifetimes.

If a function takes only one reference as input and returns a reference (with elided lifetimes), Rust infers that the returned reference must live as long as the input.

Example:

fn example2(x: &str) -> &str {
x
}

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Rust interprets this as:

fn example2<'a>(x: &'a str) -> &'a str {
x
}

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Because there is exactly one input lifetime, Rust can confidently assign it to the output.

Rule 3:

If there are multiple input lifetimes, but one of them is &self or &mut self, the lifetime of self is assigned to all elided output lifetimes.

This rule applies specifically to methods defined in impl blocks. When you return a reference from a method and use &self or &mut self as a parameter, the returned reference is assumed to have the same lifetime as self.

Example:

struct MyStruct<'a> {
data: &'a str,
}

impl<'a> MyStruct<'a> {
fn get_data_slice(&self, start_index: usize) -> &str {
&self.data[start_index..]
}
}

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Rust interprets this method as:

fn get_data_slice<'b>(&'b self, start_index: usize) -> &'b str {
&self.data[start_index..]
}

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This rule works because the returned reference is derived from self, so tying the output lifetime to self is safe and intuitive.

Rule 4:

In all other cases, you must explicitly annotate the output lifetime.

If none of the above rules apply, and you try to return a reference without explicitly declaring its lifetime, Rust will raise a compile-time error.

Example (Invalid Code):

rust
CopyEdit
fn example4(x: &str, y: &str) -> &str {
if x.len() > y.len() {
x
} else {
y
}
}

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Compiler Error:

// error[E0106]: missing lifetime specifier
// --> src/main.rs:1:34
//  |
// 1 | fn example4(x: &str, y: &str) -> &str {
//  |                                  ^^^^ expected named lifetime parameter
// help: consider introducing a named lifetime parameter
//  |
// 1 | fn example4<'a>(x: &'a str, y: &'a str) -> &'a str {

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Fixed Version:

fn example4_fixed<'a>(x: &'a str, y: &'a str) -> &'a str {
if x.len() > y.len() {
x
} else {
y
}
}

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Here, we explicitly tie all references to the same lifetime ’a, making the borrowing relationships clear to the compiler.

Summary

Lifetime elision rules in Rust were introduced to simplify common patterns of borrowing. They remove the need for repetitive annotations in the majority of cases, improving both readability and developer experience.

However, as your Rust codebase becomes more complex—with functions handling multiple borrowed values—the elision rules may no longer suffice. In such cases, explicit lifetime annotations become essential to maintain clarity and safety.

Understanding these rules is not just academic; it’s a practical skill. As you write more idiomatic Rust code, you’ll begin to intuitively know when lifetimes can be elided and when they must be specified.

Further Reading

You use lifetime elision almost daily when writing idiomatic Rust, even if you’re not thinking about it. Now that you understand how and why it works, you can better interpret compiler messages and write clearer, safer APIs.

While lifetime elision handles ~87% of use cases, more complex borrowing scenarios will require explicit annotations. Understanding the rules gives you the confidence to know when to annotate lifetimes manually and when Rust can take care of it for you.

If you have any questions or comments, feel free to reach out to me on my LinkedIn

Lifetime Elision RFC

The Rustonomicon: Lifetime Elision

Rust Book: Lifetime Elision

Final Thoughts

Have a great one!!!

Author: Ugochukwu Chizaram

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