有这么一个函数:

pub fn new<B>() -> Result<Terminal<B>> {
    let backend = CrosstermBackend::new(io::stdout());
    Terminal::new(backend)
}

其中CrosstermBackend的大致定义:

pub struct CrosstermBackend<W: Write>;
impl<W> Backend for CrosstermBackend<W>
where W: Write {}

Terminal的定义:

pub struct Terminal<B>
where B: Backend {}

报错如下:

error[E0308]: mismatched types
   --> src/term.rs:13:19
    |
11  | pub fn new<B: Backend>() -> Result<Terminal<B>> {
    |            - this type parameter
12  |     let backend = CrosstermBackend::new(io::stdout());
13  |     Terminal::new(backend)
    |     ------------- ^^^^^^^ expected type parameter `B`, found struct `CrosstermBackend`
    |     |
    |     arguments to this function are incorrect
    |
    = note: expected type parameter `B`
                       found struct `CrosstermBackend<Stdout>`
note: associated function defined here
   --> /home/jedsek/.cargo/registry/src/mirrors.tuna.tsinghua.edu.cn-df7c3c540f42cdbd/tui-0.19.0/src/terminal.rs:175:12
    |
175 |     pub fn new(backend: B) -> io::Result<Terminal<B>> {
    |            ^^^

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

但是换成impl后就可以了:

pub fn new() -> Result<Terminal<impl Backend>> {
    let backend = CrosstermBackend::new(io::stdout());
    Terminal::new(backend)
}

这是为什么?