non-send futures error at point of use, but could cite point of definition in some cases

[nikomatsakis]

Consider this example (playground):

#![feature(async_await)]

use std::sync::Mutex;

fn is_send<T: Send>(t: T) {
    
}

async fn foo() {
  bar(&Mutex::new(22)).await;
}

async fn bar(x: &Mutex<u32>) {
  let g = x.lock().unwrap();
  baz().await;
}

async fn baz() {
    
}

fn main() {
    is_send(foo());
} 

This program is in error because bar() is holding a mutex guard (which is not Send) live across the baz().await call. The error however is rather opaque:

error[E0277]: `std::sync::MutexGuard<'_, u32>` cannot be sent between threads safely
  --> src/main.rs:23:5
   |
23 |     is_send(foo());
   |     ^^^^^^^ `std::sync::MutexGuard<'_, u32>` cannot be sent between threads safely
   |
   = help: within `impl std::future::Future`, the trait `std::marker::Send` is not implemented for `std::sync::MutexGuard<'_, u32>`
   = note: required because it appears within the type `for<'r, 's> {&'r std::sync::Mutex<u32>, std::sync::MutexGuard<'s, u32>, impl std::future::Future, ()}`
   = note: required because it appears within the type `[static generator@src/main.rs:13:30: 16:2 x:&std::sync::Mutex<u32> for<'r, 's> {&'r std::sync::Mutex<u32>, std::sync::MutexGuard<'s, u32>, impl std::future::Future, ()}]`
   = note: required because it appears within the type `std::future::GenFuture<[static generator@src/main.rs:13:30: 16:2 x:&std::sync::Mutex<u32> for<'r, 's> {&'r std::sync::Mutex<u32>, std::sync::MutexGuard<'s, u32>, impl std::future::Future, ()}]>`
   = note: required because it appears within the type `impl std::future::Future`
   = note: required because it appears within the type `impl std::future::Future`
   = note: required because it appears within the type `for<'r> {impl std::future::Future, ()}`
   = note: required because it appears within the type `[static generator@src/main.rs:9:16: 11:2 for<'r> {impl std::future::Future, ()}]`
   = note: required because it appears within the type `std::future::GenFuture<[static generator@src/main.rs:9:16: 11:2 for<'r> {impl std::future::Future, ()}]>`
   = note: required because it appears within the type `impl std::future::Future`
   = note: required because it appears within the type `impl std::future::Future`
note: required by `is_send`
  --> src/main.rs:5:1
   |
5  | fn is_send<T: Send>(t: T) {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^

When discussing this error, @cramertj suggested that we might look for patterns like this and try to offer a customized error. They sketched out

error[E0277]: `foo()` cannot be sent between threads
note: the `is_send` function requires `T: Send`:
  --> src/main.rs:5:1
   |
5  | fn is_send<T: Send>(t: T) {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^
but in this invocation, the `foo()` is not `Send`
  --> src/main.rs:23:5
   |
23 |     is_send(foo());
   |             ^^^^^
because it is of type `impl Future<Output = ()>`
which contains an `std::sync::MutexGuard<'_, u32>`
note: pass `--full-error-type-info` for more information

Something along these lines could be a big improvement!

[nikomatsakis]

I'm abusing the blocking tag here to try and keep this on the shortlist of "things we might try to do before beta". Assigning to myself to leave some mentoring notes.

[estebank]

CC #42855

[nikomatsakis]

I laid out something thinking in this Zulip thread

[davidtwco]

@rustbot claim