rust - Expected bound lifetime parameter, found concrete lifetime

Question

I can't figure out the lifetime parameters for this code. Everything I try usually results in a compiler error: "Expected bound lifetime parameter 'a, found concrete lifetime" or something like "consider using an explicit lifetime parameter as shown" (and the example shown doesn't help) or "method not compatible with trait".

Request, Response, and Action are simplified versions to keep this example minimal.

struct Request {
    data: String,
}
struct Response<'a> {
    data: &'a str,
}

pub enum Action<'a> {
    Next(Response<'a>),
    Done,
}

pub trait Handler: Send + Sync {
    fn handle<'a>(&self, req: Request, res: Response<'a>) -> Action<'a>;
}

impl<'a, T> Handler for T
where
    T: Send + Sync + Fn(Request, Response<'a>) -> Action<'a>,
{
    fn handle(&self, req: Request, res: Response<'a>) -> Action<'a> {
        (*self)(req, res)
    }
}

fn main() {
    println!("running");
}

Rust Playground

Answer 1

Your trait function definition is this:

fn handle<'a>(&self, req: Request, res: Response<'a>) -> Action<'a>;

Note that 'a is specified by the caller and can be anything and is not necessarily tied to self in any way.

Your trait implementation definition is this:

fn handle(&self, req: Request, res: Response<'a>) -> Action<'a>;

'a is not here specified by the caller, but is instead tied to the type you are implementing the trait for. Thus the trait implementation does not match the trait definition.

Here is what you need:

trait Handler: Send + Sync {
    fn handle<'a>(&self, req: Request, res: Response<'a>) -> Action<'a>;
}

impl<T> Handler for T
where
    T: Send + Sync + for<'a> Fn(Request, Response<'a>) -> Action<'a>,
{
    fn handle<'a>(&self, req: Request, res: Response<'a>) -> Action<'a> {
        (*self)(req, res)
    }
}

The key point is the change in the T bound: for<'a> Fn(Request, Response<'a>) -> Action<'a>. This means: “given an arbitrary lifetime parameter 'a, T must satisfy Fn(Request, Response<'a>) -> Action<'a>; or, “T must, for all 'a, satisfy Fn(Request, Response<'a>) -> Action<'a>.