Rust expected type parameter, found struct

我一直在用Rust 来混淆特性，我遇到了一个问题.以下是一些代码:

struct Foo;

trait Bar {}

impl Bar for Foo {}

fn bar<B: Bar>(bar: B) {}

fn barr<B: Bar>() {
    bar(Foo); // 1. THIS WILL WORK
    let foo: B = Foo; // 2. THIS WILL NOT WORK
    let foo_vec: Vec<B> = vec![Foo]; // 3. THIS WILL NOT WORK
}

这会产生以下错误:

error[E0308]: mismatched types
  --> src/main.rs:11:18
   |
11 |     let foo: B = Foo; // 2. THIS WILL NOT WORK
   |                  ^^^ expected type parameter, found struct `Foo`
   |
   = note: expected type `B`
              found type `Foo`

error[E0308]: mismatched types
  --> src/main.rs:12:32
   |
12 |     let foo_vec: Vec<B> = vec![Foo]; // 3. THIS WILL NOT WORK
   |                                ^^^ expected type parameter, found struct `Foo`
   |
   = note: expected type `_`
              found type `Foo`

为什么#2&amp#3.工作？我怎样才能让编译器知道Foo实际上有Bar impl呢？

另一个例子:

struct Foo<B: Bar> {
    bar: Option<B>,
}

struct Foo2;

trait Bar {}

impl<B: Bar> Bar for Foo<B> {}

impl Bar for Foo2 {}

fn bar<B: Bar>(bar: B) {}

fn circle_vec<B: Bar>() {
    bar(Foo2); // 1. WORKS
    Foo { bar: Some(Foo { bar: None }) }; // 2. WILL NOT WORK
}

这将给我带来以下错误:

error[E0282]: type annotations needed
  --> src/main.rs:17:21
   |
17 |     Foo { bar: Some(Foo { bar: None }) }; // 2. WILL NOT WORK
   |                     ^^^ cannot infer type for `B`