如何在Rust中匹配数据类型？

我正在尝试匹配结构的通用字段的数据类型并做出相应的反应.我的总体想法是这样的(代码不编译):

struct Foo {
    bar: T,
}

fn main() {
    let x = Foo:: {
        bar: "world".to_string(),
    };

    match x.bar {
        String => println!("It's a string!"),
        u32 => println!("It's a u32!"),
        _ => println!("Something else"),
    };

    println!("end of program!");
}

来自的错误消息x:

warning: unreachable pattern
  --> src/main.rs:12:9
   |
11 |         String => println!("It's a string!"),
   |         ------ matches any value
12 |         u32 => println!("It's a u32!"),
   |         ^^^ unreachable pattern
   |
   = note: `#[warn(unreachable_patterns)]` on by default

warning: unreachable pattern
  --> src/main.rs:13:9
   |
11 |         String => println!("It's a string!"),
   |         ------ matches any value
12 |         u32 => println!("It's a u32!"),
13 |         _ => println!("Something else"),
   |         ^ unreachable pattern

warning: unused variable: `String`
  --> src/main.rs:11:9
   |
11 |         String => println!("It's a string!"),
   |         ^^^^^^ help: consider prefixing with an underscore: `_String`
   |
   = note: `#[warn(unused_variables)]` on by default

warning: unused variable: `u32`
  --> src/main.rs:12:9
   |
12 |         u32 => println!("It's a u32!"),
   |         ^^^ help: consider prefixing with an underscore: `_u32`

warning: variable `String` should have a snake case name
  --> src/main.rs:11:9
   |
11 |         String => println!("It's a string!"),
   |         ^^^^^^ help: convert the identifier to snake case: `string`
   |
   = note: `#[warn(non_snake_case)]` on by default

trait PrintMe {
    fn print_me(&self);
}

impl PrintMe for String {
    fn print_me(&self) { println!("I am a string"); }
}

struct Foo {
    bar: T
}

fn main() {
    // ...
    x.bar.print_me();
}

fn print_me(x: &Foo) {
    if TypeId::of::() == TypeId::of::() {
        println!("I am a string");
    } else // ...
}