Lifetime elision rules in Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of lifetime elision in Rust?

Lifetime elision allows Rust to infer lifetimes in function signatures without explicit annotations, making code easier to read and write.

beginner

How many lifetime elision rules does Rust have?

Rust has three main lifetime elision rules that guide how lifetimes are inferred in function signatures.

intermediate

Explain the first lifetime elision rule.

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

intermediate

What does the second lifetime elision rule state?

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

intermediate

When do you need to explicitly write lifetime annotations in Rust functions?

You must write explicit lifetime annotations when the elision rules do not apply, such as when multiple input lifetimes exist without a self reference and the output lifetime is ambiguous.

How many lifetime elision rules does Rust use to infer lifetimes?

A 5

B 1

C 3

D None

Rust uses three lifetime elision rules to infer lifetimes automatically.

According to the first elision rule, if a function has one input lifetime, what happens to the output lifetime?

A It must be explicitly annotated

B It is ignored

C It gets a static lifetime

D It gets the same lifetime as the input

The output lifetime is assigned the same lifetime as the single input lifetime.

What special input does the second elision rule consider when assigning output lifetimes?

A Any reference

B &self or &mut self

C Static lifetime

D Mutable references only

The second rule applies when one input is &self or &mut self.

When must you write explicit lifetime annotations in Rust functions?

A When elision rules do not clearly assign output lifetimes

B Always, for every function

C Only for functions without inputs

D Never, Rust infers all lifetimes

Explicit annotations are needed when elision rules cannot determine output lifetimes.

Which of these is NOT a lifetime elision rule?

A If multiple input lifetimes exist without self, assign output to the first input

B If one input is &self or &mut self, assign its lifetime to output

C If there is exactly one input lifetime, assign it to output

D If no input lifetimes, output lifetime is static

Option A is not a lifetime elision rule; without a self parameter and with multiple input lifetimes, explicit lifetime annotations are required.

Describe the three lifetime elision rules in Rust and give a simple example for each.

Think about function inputs and outputs and how Rust guesses lifetimes.

Explain when and why you need to write explicit lifetime annotations instead of relying on elision.

Consider functions with multiple references as inputs.