环境变量读取

  U147_SET      var = Ok("hello")   var_os = Some("hello")
  U147_MISSING  var = Err("environment variable not found")   var_os = None
  U147_BAD      var = Err("environment variable was not valid unicode: \"bad\\xFF\\xFE\"")   var_os = Some("bad\xFF\xFE")
  U147_EMPTY    var = Ok("")   var_os = Some("")
  U147_EMPTY 是空字符串不是不存在:Ok(true)
  var_os 比 var 能装下非 UTF-8 的值:Some([98, 97, 100, 255, 254])

  源码:
      1 | use std::{env, process::Command};
      2 | fn main() {
      3 |     unsafe { env::set_var("U147_X", "from-rust"); }
      4 |     let o = Command::new("sh").arg("-c").arg("echo child sees U147_X=$U147_X").output().unwrap(); println!("set_var 之后子进程看到:{}", String::from_utf8_lossy(&o.stdout).trim());
      5 |     unsafe { env::remove_var("U147_X"); }
      6 |     let o = Command::new("sh").arg("-c").arg("echo child sees U147_X=[$U147_X]").output().unwrap(); println!("remove_var 之后子进程看到:{}", String::from_utf8_lossy(&o.stdout).trim());
      7 | }
  编译:成功
  运行(退出码 0):
    set_var 之后子进程看到:child sees U147_X=from-rust
    remove_var 之后子进程看到:child sees U147_X=[]
  源码:
      1 | fn main() { std::env::set_var("A", "1"); }
  编译:失败
    error[E0133]: call to unsafe function `set_var` is unsafe and requires unsafe block
     --> <实验目录>/src/u147c.rs:1:13
      |
    1 | fn main() { std::env::set_var("A", "1"); }
      |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
      |
      = note: consult the function's documentation for information on how to avoid undefined behavior
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0133`.

  编译:成功
  运行(退出码 0):
    child: child
    子进程改完之后,父进程仍然是 Ok("parent")

  环境里放一个非 UTF-8 的变量:vars → 退出码 101,stdout '',stderr ["thread 'main' (7496955) panicked at /rustc/48a229ceaefd4985c50990b14116b6d856af0985/library/std/src/env.rs:168:83:", 'called `Result::unwrap()` on an `Err` value: "bad\\xFF\\xFE"']
  环境里放一个非 UTF-8 的变量:vars_os → 退出码 0,stdout 'vars_os() 共 58 个',stderr []

  编译时设置 A,运行时设置 B:编译期 option_env! = Some("A-at-compile");运行期 env::var = Some("B-at-run")
  编译时没设置,运行时设置 B:编译期 option_env! = None;运行期 env::var = Some("B-at-run")
  env!("U147_REQUIRED") 编译期缺失:['error: environment variable `U147_REQUIRED` not defined at compile time']