Drop

Drop trait 只有一个方法：drop，它会在对象离开作用域时自动调用。Drop trait 的主要用途是释放实现该 trait 的实例所拥有的资源。

Box、Vec、String、File 和 Process 是一些实现了 Drop trait 以释放资源的类型示例。你也可以为任何自定义数据类型手动实现 Drop trait。

下面的例子在 drop 函数中添加了一个控制台打印，用于宣告它被调用的时机。

For a more practical example, here's how the Drop trait can be used to automatically clean up temporary files when they're no longer needed:

use std::fs::File;
use std::path::PathBuf;

struct TempFile {
    file: File,
    path: PathBuf,
}

impl TempFile {
    fn new(path: PathBuf) -> std::io::Result<Self> {
        // Note: File::create() will overwrite existing files
        let file = File::create(&path)?;
        
        Ok(Self { file, path })
    }
}

// When TempFile is dropped:
// 1. First, the File will be automatically closed (Drop for File)
// 2. Then our drop implementation will remove the file
impl Drop for TempFile {
    fn drop(&mut self) {
        // Note: File is already closed at this point
        if let Err(e) = std::fs::remove_file(&self.path) {
            eprintln!("Failed to remove temporary file: {}", e);
        }
        println!("> Dropped temporary file: {:?}", self.path);
    }
}

fn main() -> std::io::Result<()> {
    // Create a new scope to demonstrate drop behavior
    {
        let temp = TempFile::new("test.txt".into())?;
        println!("Temporary file created");
        // File will be automatically cleaned up when temp goes out of scope
    }
    println!("End of scope - file should be cleaned up");

    // We can also manually drop if needed
    let temp2 = TempFile::new("another_test.txt".into())?;
    drop(temp2); // Explicitly drop the file
    println!("Manually dropped file");
    
    Ok(())
}