Solution

rust,editable

# // Copyright 2023 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
{{#include exercise.rs:solution}}

This solution demonstrates a few key Rust features:

mut arguments: The n argument is declared as mut n. This makes the local variable n mutable within the function scope. It does not affect the caller's value, as integers are Copy types passed by value.
if expressions: Rust's if is an expression, meaning it produces a value. We assign the result of the if/else block directly to n. This is more concise than writing n = ... inside each branch.
Implicit return: The function ends with len (without a semicolon), which is automatically returned.

Note that n must be strictly greater than 0 for the Collatz sequence to be valid. The function signature takes i32, but the problem description implies positive integers. A more robust implementation might use u32 or return an Option or Result to handle invalid inputs (0 or negative numbers), but panic or infinite loops are potential outcomes here if n <= 0.
The overflow is a potential issue if n grows too large, similar to the Fibonacci exercise.

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