Rust 1.45.0 已发布。此版本有两个值得关注的变化：一是修复将大浮点数转换为小整数时出现的 undefined behavior 问题，官方将这种情况称为 unsoundness；二是在 expression, patterns 和 statement 中使用函数式过程宏(procedural macros)的功能已处于稳定阶段。

修复浮点数转换为整数时的 unsoundness

对于此问题，官方的解决方案是为as关键字执行"saturating cast"。下面的例子解释了什么叫 saturating cast。

fn cast(x: f32) -> u8 {
    x as u8
}

fn main() {
    let too_big = 300.0;
    let too_small = -100.0;
    let nan = f32::NAN;

    println!("too_big_casted = {}", cast(too_big));
    println!("too_small_casted = {}", cast(too_small));
    println!("not_a_number_casted = {}", cast(nan));
}

上面的代码打印出

too_big_casted = 255
too_small_casted = 0
not_a_number_casted = 0

或者使用采用 unsafe 方式的新 API 进行转换：

let x: f32 = 1.0;
let y: u8 = unsafe { x.to_int_unchecked() };

稳定在 expression, patterns 和 statement 中使用函数式过程宏的功能

Rust 1.45.0 在三个新地方增加了调用过程宏的功能：

// imagine we have a procedural macro named "mac"

mac!(); // item position, this was what was stable before

// but these three are new:
fn main() {
  let expr = mac!(); // expression position

  match expr {
      mac!() => {} // pattern position
  }

  mac!(); // statement position
}

库变更

在 Rust 1.45.0 中，以下 API 已处于稳定阶段：

• Arc::as_ptr
• BTreeMap::remove_entry
• Rc::as_ptr
• rc::Weak::as_ptr
• rc::Weak::from_raw
• rc::Weak::into_raw
• str::strip_prefix
• str::strip_suffix
• sync::Weak::as_ptr
• sync::Weak::from_raw
• sync::Weak::into_raw
• char::UNICODE_VERSION
• Span::resolved_at
• Span::located_at
• Span::mixed_site
• unix::process::CommandExt::arg0

此外还增加了char with ranges 来迭代代码点(codepoint)：

for ch in 'a'..='z' {
    print!("{}", ch);
}
println!();
// Prints "abcdefghijklmnopqrstuvwxyz"

详情查看 https://blog.rust-lang.org/2020/07/16/Rust-1.45.0.html