Solution

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

{{#include exercise.rs:solution}}

{{#include exercise.rs:tests}}

• Result Return Type: The function signature changes to return Result<i64, DivideByZeroError>. This explicit type signature forces the caller to handle the possibility of failure.
• The ? Operator: We use ? on the recursive calls: eval(*left)?. This cleanly propagates errors. If eval returns Err, the function immediately returns that Err. If it returns Ok(v), v is assigned to left (or right).
• Ok Wrapping: Successful results must be wrapped in Ok(...).
• Handling Division by Zero: We explicitly check for right == 0 and return Err(DivideByZeroError). This replaces the panic in the original code.

• Mention that DivideByZeroError is a unit struct (no fields), which is sufficient here since there's no extra context to provide about the error.
• Discuss how ? makes error handling almost as concise as exceptions, but with explicit control flow.