Rust 是否有等效于 C++ 的 decltype() 来获取表达式的类型

不，Rust无法获取任意表达式的类型.typeof is保留关键字，以便将来可能允许:

fn main() {
    let a: i32 = 42;
    let b: typeof(a) = a;
}

error[E0516]: `typeof` is a reserved keyword but unimplemented
 --> src/main.rs:3:12
  |
3 |     let b: typeof(a) = a;
  |            ^^^^^^^^^ reserved keyword

有一些RFC建议添加它.

另见:

• How do I match the type of an expression in a Rust macro?
• Is it possible to access the type of a struct member for function signatures or declarations?
• .type` for getting concrete type of a binding — issue #2704

对于你的具体情况，我会使用特征:

use std::ops::RangeTo;

trait Mask {
    fn mask(self, range: RangeTo<usize>) -> Self;
}

impl Mask for u8 {
    #[inline]
    fn mask(self, range: RangeTo<usize>) -> Self {
        // Feel free to make this your more complicated bitwise logic
        let mut m = 0;
        for _ in 0..range.end {
            m <<= 1;
            m |= 1;
        }
        self & m
    }
}


fn main() {
    let bitmap: u8 = 0b_1111_1111;
    let masked_bitmap = bitmap.mask(..5);
    println!("{:#010b}", masked_bitmap);
}

但是，您可以使用宏创建implement the trait:

macro_rules! impl_mask {
    ($($typ:ty),*) => {
        $(
            impl Mask for $typ {
                #[inline]
                fn mask(self, range: RangeTo<usize>) -> Self {
                    let mut m = 0;
                    for _ in 0..range.end {
                        m <<= 1;
                        m |= 1;
                    }
                    self & m
                }
            }
        )*
    };
}

impl_mask!(u8, u16, u32, u64, u128);