Rust HRTB 是相同的，但编译器说一种类型比另一种更通用

following code带有完整的类型注释:

fn enter<'a, F, R>(x: &'a i32, func: F) -> R
where
    F: for<'b> FnOnce(&'b i32) -> R,
{
    func(x)
}

fn identity<'a>(x: &'a i32) -> &'a i32 {
    x
}

fn main() {
    let x = &42;
    enter(x, identity);
}

编译器在编译时报告错误:

error[E0308]: mismatched types
  --> src/main.rs:14:5
   |
14 |     enter(x, identity);
   |     ^^^^^^^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected trait `for<'b> FnOnce<(&'b i32,)>`
              found trait `for<'a> FnOnce<(&'a i32,)>`
note: the lifetime requirement is introduced here
  --> src/main.rs:3:35
   |
3  |     F: for<'b> FnOnce(&'b i32) -> R,
   |                                   ^

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` due to previous error

我的理解是，性状界限for<'b> FnOnce(&'b i32) -> R比identity的签名for<'a> fn(&'a i32) -> &'a i32更一般，所以它应该通过.

然而，通用-> R似乎是停止编译的罪魁祸首，但我不知道为什么.

我注意到有类似的问题，但他们遇到了错误，因为他们的类型没有完全注释，编译器错误地推断了类型.

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