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。

1.38版新增std::any::type_name

use std::any::type_name;

fn type_of<T>(_: T) -> &'static str {
    type_name::<T>()
}
fn main() {
    let x = 21;
    let y = 2.5;
    println!("{}", type_of(&y));
    println!("{}", type_of(x));
}

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。

其他一些答案不工作，但我发现typename crate工作。

Create a new project: cargo new test_typename Modify the Cargo.toml [dependencies] typename = "0.1.1" Modify your source code use typename::TypeName; fn main() { assert_eq!(String::type_name(), "std::string::String"); assert_eq!(Vec::<i32>::type_name(), "std::vec::Vec<i32>"); assert_eq!([0, 1, 2].type_name_of(), "[i32; 3]"); let a = 65u8; let b = b'A'; let c = 65; let d = 65i8; let e = 65i32; let f = 65u32; let arr = [1,2,3,4,5]; let first = arr[0]; println!("type of a 65u8 {} is {}", a, a.type_name_of()); println!("type of b b'A' {} is {}", b, b.type_name_of()); println!("type of c 65 {} is {}", c, c.type_name_of()); println!("type of d 65i8 {} is {}", d, d.type_name_of()); println!("type of e 65i32 {} is {}", e, e.type_name_of()); println!("type of f 65u32 {} is {}", f, f.type_name_of()); println!("type of arr {:?} is {}", arr, arr.type_name_of()); println!("type of first {} is {}", first, first.type_name_of()); }

输出结果为:

type of a 65u8  65 is u8
type of b b'A'  65 is u8
type of c 65    65 is i32
type of d 65i8  65 is i8
type of e 65i32 65 is i32
type of f 65u32 65 is u32
type of arr [1, 2, 3, 4, 5] is [i32; 5]
type of first 1 is i32

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。

您还可以使用println中的变量!("{:?}”,var)。如果没有为该类型实现Debug，则可以在编译器的错误消息中看到该类型:

mod some {
    pub struct SomeType;
}

fn main() {
    let unknown_var = some::SomeType;
    println!("{:?}", unknown_var);
}

(游戏围栏)

虽然很脏，但很管用。