114 lines
3.9 KiB
Markdown
114 lines
3.9 KiB
Markdown
# panic!
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Rust 中最简单的错误处理方式就是使用 `panic`。它会打印出一条错误信息并打印出栈调用情况，最终结束当前线程:
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- 若 panic 发生在 `main` 线程，那程序会随之退出
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- 如果是在生成的( spawn )子线程中发生 panic, 那么当前的线程会结束，但是程序依然会继续运行
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1. 🌟🌟
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```rust,editable
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// 填空
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fn drink(beverage: &str) {
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if beverage == "lemonade" {
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println!("Success!");
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// 实现下面的代码
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__
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}
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println!("Exercise Failed if printing out this line!");
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}
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fn main() {
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drink(__);
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println!("Exercise Failed if printing out this line!");
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}
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```
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## 常见的 panic
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2. 🌟🌟
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```rust,editable
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// 修复所有的 panic，让代码工作
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fn main() {
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assert_eq!("abc".as_bytes(), [96, 97, 98]);
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let v = vec![1, 2, 3];
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let ele = v[3];
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let ele = v.get(3).unwrap();
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// 大部分时候编译器是可以帮我们提前发现溢出错误，并阻止编译通过。但是也有一些时候，这种溢出问题直到运行期才会出现
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let v = production_rate_per_hour(2);
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divide(15, 0);
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println!("Success!")
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}
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fn divide(x:u8, y:u8) {
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println!("{}", x / y)
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}
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fn production_rate_per_hour(speed: u8) -> f64 {
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let cph: u8 = 221;
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match speed {
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1..=4 => (speed * cph) as f64,
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5..=8 => (speed * cph) as f64 * 0.9,
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9..=10 => (speed * cph) as f64 * 0.77,
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_ => 0 as f64,
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}
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}
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pub fn working_items_per_minute(speed: u8) -> u32 {
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(production_rate_per_hour(speed) / 60 as f64) as u32
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}
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```
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### 详细的栈调用信息
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默认情况下，栈调用只会展示最基本的信息:
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```shell
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thread 'main' panicked at 'index out of bounds: the len is 3 but the index is 99', src/main.rs:4:5
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note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
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```
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但是有时候，我们还希望获取更详细的信息:
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3. 🌟
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```shell
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## 填空以打印全部的调用栈
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## 提示: 你可以在之前的默认 panic 信息中找到相关线索
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$ __ cargo run
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thread 'main' panicked at 'assertion failed: `(left == right)`
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left: `[97, 98, 99]`,
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right: `[96, 97, 98]`', src/main.rs:3:5
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stack backtrace:
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0: rust_begin_unwind
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at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/std/src/panicking.rs:498:5
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1: core::panicking::panic_fmt
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at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/core/src/panicking.rs:116:14
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2: core::panicking::assert_failed_inner
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3: core::panicking::assert_failed
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at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/core/src/panicking.rs:154:5
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4: study_cargo::main
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at ./src/main.rs:3:5
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5: core::ops::function::FnOnce::call_once
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at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/core/src/ops/function.rs:227:5
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note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
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```
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### `unwinding` 和 `abort`
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当出现 `panic!` 时，程序提供了两种方式来处理终止流程：**栈展开**和**直接终止**。
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其中，默认的方式就是 `栈展开`，这意味着 Rust 会回溯栈上数据和函数调用，因此也意味着更多的善后工作，好处是可以给出充分的报错信息和栈调用信息，便于事后的问题复盘。`直接终止`，顾名思义，不清理数据就直接退出程序，善后工作交与操作系统来负责。
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对于绝大多数用户，使用默认选择是最好的，但是当你关心最终编译出的二进制可执行文件大小时，那么可以尝试去使用直接终止的方式，例如下面的配置修改 `Cargo.toml` 文件，实现在 [`release`](../first-try/cargo.md#手动编译和运行项目) 模式下遇到 `panic` 直接终止：
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```rust
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[profile.release]
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panic = 'abort'
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```
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> 你可以在[这里](https://github.com/sunface/rust-by-practice/blob/master/solutions/result-panic/panic.md)找到答案(在 solutions 路径下)
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