“expected type parameter, found struct”

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You have two different problems so I guess I’ll write two different answers.

In your first code sample, 2 and 3 don’t work because B is an input type parameter; it is the caller of barr that decides what B is. However, you are trying to force it to be Foo.

Let’s suppose we have another implementation of Bar:

struct Quux;

impl Bar for Quux {}

And let’s suppose we call barr like this:

barr::<Quux>()

barr would essentially be compiled as this:

fn barr() {
    bar(Foo);
    let foo: Quux = Foo;
    let foo_vec: Vec<Quux> = vec![Foo];
}

Foo and Quux are not compatible, and Vec<Foo> and Vec<Quux> are not compatible either.

If you’re trying to create a vector of arbitrary Bar objects, you need to use Bar in a non-generic way. As trait types are unsized, you can’t store them directly in a Vec, so you must use Vec<Box<Bar>>, Vec<&Bar> or some other type that wraps a pointer.

fn barr() {
    bar(Foo);
    let foo: Box<Bar> = Box::new(Foo);
    let foo_vec: Vec<Box<Bar>> = vec![Box::new(Foo) as Box<Bar>];
}

In your second code sample, the error is that None has the type Option<T>, and the compiler is unable to infer an appropriate type for T. We can explicitly specify T like this:

fn circle_vec<B: Bar>() {
    bar(Foo2);
    Foo {
        bar: Some(Foo { bar: None::<Foo2> }),
    };
}

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