提供给 with_context 的闭包是惰性求值的，这也是你使用 with_context 而不是 code>context 与您选择懒惰评估任何事情的原因相同:它很少发生并且计算成本很高.一旦满足这些条件，with_context 就比 context 更可取.注释伪示例:

The closure provided to with_context is evaluated lazily, and the reason you'd use with_context over context is the same reason you'd choose to lazily evaluate anything: it rarely happens and it's expensive to compute. Once those conditions are satisfied then with_context becomes preferable over context. Commented pseudo-example:

fn calculate_expensive_context() -> Result<()> {
    // really expensive
    std::thread::sleep(std::time::Duration::from_secs(1));
    todo!()
}

// eagerly evaluated expensive context
// this function ALWAYS takes 1+ seconds to execute
// consistently terrible performance
fn failable_operation_eager_context(some_struct: Struct) -> Result<()> {
    some_struct
        .some_failable_action()
        .context(calculate_expensive_context())
}

// lazily evaluated expensive context
// function returns instantly, only takes 1+ seconds on failure
// great performance for average case, only terrible performance on error cases
fn failable_operation_lazy_context(some_struct: Struct) -> Result<()> {
    some_struct
        .some_failable_action()
        .with_context(|| calculate_expensive_context())
}