improve diagnostics for non-send Futures that might be solved by introducing temporary variables

[yoshuawuyts]

In http-rs/tide#370 we had a user report that they encountered a Send error similar to the ones discussed in #64130. Thanks to the diagnostics introduced #65345 on nightly we were able to correctly diagnose the root cause of the error.

But for many users this might still not be enough to figure out how to solve this, and we could improve by providing a hint.

Current Diagnostics

   Compiling tide-demo v0.1.0 (/home/anon/Code/importcjj/tide-demo)
error: future cannot be sent between threads safely
  --> src/main.rs:47:27
   |
47 |         app.at("/submit").post(|req: tide::Request<State>| {
   |                           ^^^^ future is not `Send`
   |
   = help: the trait `std::marker::Sync` is not implemented for `(dyn futures_io::if_std::AsyncBufRead + std::marker::Send + 'static)`
note: future is not `Send` as this value is used across an await
  --> src/main.rs:49:29
   |
49 |                 let conn = &req.state().pool.get().await;
   |                             ---^^^^^^^^^^^^^^^^^^^^^^^^^- `req` is later dropped here
   |                             |
   |                             await occurs here, with `req` maybe used later
   |                             has type `&tide::request::Request<State>`

Suggested Diagnostics

@estebank suggested we could add a suggestion here:

help: consider assigning `&req.state().pool` to a temporary variable

Repro

• https://github.com/importcjj/tide-demo/blob/master/src/main.rs

Refs

• non-send futures error at point of use, but could cite point of definition in some cases #64130
• A problem with switching from 0.3 to 0.4 http-rs/tide#370

[tmandry]

Visiting from triage.. assigning @csmoe for implementation and @nikomatsakis to leave some tips.

[csmoe]

reproduced with exsited ui testcase:

use std::any::Any;
use std::future::Future;

struct Client(Box<dyn Any + Send>);

impl Client {
    fn status(&self) -> u16 {
        200
    }
}

async fn get() { }

pub fn foo() -> impl Future + Send {
    //~^ ERROR future cannot be sent between threads safely
    let client = Client(Box::new(true));
    async move {
        match client.status() {
            200 => {
                let _x = get().await;
            },
            _ => (),
        }
    }
}

fn main() {}

  --> $DIR/issue-64130-4-async-move.rs:21:26
   |
LL |         match client.status() {
   |               ------ has type `&Client`
LL |             200 => {
LL |                 let _x = get().await;
   |                          ^^^^^^^^^^^ await occurs here, with `client` maybe used later
...
LL |     }
   |     - `client` is later dropped here
   = note: the return type of a function must have a statically known size

rewrite the match expr as:

let status = client.status();
match status { ... }

will fix this.
The problem is that client does NOT has type &Client actually, but .status() creates a temporary borrow as &Client.