如果你只是想在交互开发过程中知道变量的类型，我强烈建议在你的编辑器或ide中使用rls (rust语言服务器)。然后，您可以简单地永久启用或切换悬停能力，只需将光标放在变量上。一个小对话框将显示关于变量的信息，包括类型。

在稳定rust中有一个@ChrisMorgan答案可以获得近似类型(“float”)，在夜间rust中有一个@ShubhamJain答案可以通过不稳定函数获得精确类型(“f64”)。

现在有一种方法可以得到精确的类型(即在f32和f64之间决定)在稳定的rust:

fn main() {
    let a = 5.;
    let _: () = unsafe { std::mem::transmute(a) };
}

结果

error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
 --> main.rs:3:27
  |
3 |     let _: () = unsafe { std::mem::transmute(a) };
  |                           ^^^^^^^^^^^^^^^^^^^
  |
  = note: source type: `f64` (64 bits)
  = note: target type: `()` (0 bits)

更新

涡轮鱼的变异

fn main() {
    let a = 5.;
    unsafe { std::mem::transmute::<_, ()>(a) }
}

略短，但可读性稍差。

如果你只是想找出一个变量的类型，并愿意在编译时执行，你可能会导致一个错误，并让编译器拾取它。

例如，将变量设置为一个无效的类型:

let mut my_number: () = 32.90;
// let () = x; would work too

error[E0308]: mismatched types
 --> src/main.rs:2:29
  |
2 |     let mut my_number: () = 32.90;
  |                             ^^^^^ expected (), found floating-point number
  |
  = note: expected type `()`
             found type `{float}`

或者调用无效的方法:

let mut my_number = 32.90;
my_number.what_is_this();

error[E0599]: no method named `what_is_this` found for type `{float}` in the current scope
 --> src/main.rs:3:15
  |
3 |     my_number.what_is_this();
  |               ^^^^^^^^^^^^

或访问无效字段:

let mut my_number = 32.90;
my_number.what_is_this

error[E0610]: `{float}` is a primitive type and therefore doesn't have fields
 --> src/main.rs:3:15
  |
3 |     my_number.what_is_this
  |               ^^^^^^^^^^^^

These reveal the type, which in this case is actually not fully resolved. It’s called “floating-point variable” in the first example, and “{float}” in all three examples; this is a partially resolved type which could end up f32 or f64, depending on how you use it. “{float}” is not a legal type name, it’s a placeholder meaning “I’m not completely sure what this is”, but it is a floating-point number. In the case of floating-point variables, if you don't constrain it, it will default to f64¹. (An unqualified integer literal will default to i32.)

参见:

编译器错误消息中的{integer}或{float}是什么?

¹可能仍然有一些让编译器困惑的方法，使它无法在f32和f64之间做出决定;我不确定。它曾经像32.90.eq(&32.90)一样简单，但现在两者都被视为f64，并且可以愉快地进行，所以我不知道。

如果你事先知道所有的类型，你可以使用trait来添加type_of方法:

trait TypeInfo {
    fn type_of(&self) -> &'static str;
}

impl TypeInfo for i32 {
    fn type_of(&self) -> &'static str {
        "i32"
    }
}

impl TypeInfo for i64 {
    fn type_of(&self) -> &'static str {
        "i64"
    }
}

//...

没有复杂或什么都没有，所以尽管有更多的限制，这是唯一的解决方案，让你得到一个字符串，是稳定的。(参见Boiethios的回答)然而，这是非常费力的，并且没有考虑类型参数，所以我们可以……

trait TypeInfo {
    fn type_name() -> String;
    fn type_of(&self) -> String;
}

macro_rules! impl_type_info {
    ($($name:ident$(<$($T:ident),+>)*),*) => {
        $(impl_type_info_single!($name$(<$($T),*>)*);)*
    };
}

macro_rules! mut_if {
    ($name:ident = $value:expr, $($any:expr)+) => (let mut $name = $value;);
    ($name:ident = $value:expr,) => (let $name = $value;);
}

macro_rules! impl_type_info_single {
    ($name:ident$(<$($T:ident),+>)*) => {
        impl$(<$($T: TypeInfo),*>)* TypeInfo for $name$(<$($T),*>)* {
            fn type_name() -> String {
                mut_if!(res = String::from(stringify!($name)), $($($T)*)*);
                $(
                    res.push('<');
                    $(
                        res.push_str(&$T::type_name());
                        res.push(',');
                    )*
                    res.pop();
                    res.push('>');
                )*
                res
            }
            fn type_of(&self) -> String {
                $name$(::<$($T),*>)*::type_name()
            }
        }
    }
}

impl<'a, T: TypeInfo + ?Sized> TypeInfo for &'a T {
    fn type_name() -> String {
        let mut res = String::from("&");
        res.push_str(&T::type_name());
        res
    }
    fn type_of(&self) -> String {
        <&T>::type_name()
    }
}

impl<'a, T: TypeInfo + ?Sized> TypeInfo for &'a mut T {
    fn type_name() -> String {
        let mut res = String::from("&mut ");
        res.push_str(&T::type_name());
        res
    }
    fn type_of(&self) -> String {
        <&mut T>::type_name()
    }
}

macro_rules! type_of {
    ($x:expr) => { (&$x).type_of() };
}

让我们使用它:

impl_type_info!(i32, i64, f32, f64, str, String, Vec<T>, Result<T,S>)

fn main() {
    println!("{}", type_of!(1));
    println!("{}", type_of!(&1));
    println!("{}", type_of!(&&1));
    println!("{}", type_of!(&mut 1));
    println!("{}", type_of!(&&mut 1));
    println!("{}", type_of!(&mut &1));
    println!("{}", type_of!(1.0));
    println!("{}", type_of!("abc"));
    println!("{}", type_of!(&"abc"));
    println!("{}", type_of!(String::from("abc")));
    println!("{}", type_of!(vec![1,2,3]));

    println!("{}", <Result<String,i64>>::type_name());
    println!("{}", <&i32>::type_name());
    println!("{}", <&str>::type_name());
}

输出:

i32
&i32
&&i32
&mut i32
&&mut i32
&mut &i32
f64
&str
&&str
String
Vec<i32>
Result<String,i64>
&i32
&str

生锈的操场

我非常喜欢@Coautose之前的回答，但如果有人只想要没有名称空间的类型名称，例如C而不是a::b::C，这里是一个修改后的宏版本，看起来像预期的那样工作:

macro_rules! ty {
    ($type:ty) => {{
        let result = std::any::type_name::<$type>();
        match result.rsplit_once(':') {
            Some((_, s)) => s,
            None => result,
        }
    }};
}

用法:

debug!("Testing type name: {}", ty!(A));

UPD以下不再工作。检查Shubham的答案以作更正。

检查std::intrinsic::get_tydesc<T>()。它现在处于“实验”状态，但如果您只是对类型系统进行了修改，那么它是OK的。

请看下面的例子:

fn print_type_of<T>(_: &T) -> () {
    let type_name =
        unsafe {
            (*std::intrinsics::get_tydesc::<T>()).name
        };
    println!("{}", type_name);
}

fn main() -> () {
    let mut my_number = 32.90;
    print_type_of(&my_number);       // prints "f64"
    print_type_of(&(vec!(1, 2, 4))); // prints "collections::vec::Vec<int>"
}

这是在内部用来实现著名的{:?}格式化程序。