Wrong error reporting for missing trait implementations

[flaper87]

Arc's Send and Sync looks like:

#[unsafe_no_drop_flag]
#[stable]
pub struct Arc<T> {
// FIXME #12808: strange name to try to avoid interfering with
// field accesses of the contained type via Deref
_ptr: NonZero<*mut ArcInner<T>>,
}

unsafe impl<T: Sync + Send> Send for Arc<T> { }
unsafe impl<T: Sync + Send> Sync for Arc<T> { }

Therefore, the code below, makes the Arc object neither Send nor Sync.

extern crate alloc;

use std::kinds::Send;
use alloc::arc::Arc;

struct Inner(*mut u8);

unsafe impl Send for Inner {}

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

fn main() {
    let arc = Arc::new(Inner(0 as *mut u8));
    is_send(arc);
}

Unfortunately, the error reported is quite obscure, as it mentions that the is_send function requires T to be Sync which is not true.

test.rs:14:5: 14:12 error: the trait `core::kinds::Sync` is not implemented for the type `*mut u8`
test.rs:14     is_send(arc);
               ^~~~~~~
test.rs:14:5: 14:12 note: the type `*mut u8` must implement `core::kinds::Sync` because it appears within the type `Inner`
test.rs:14     is_send(arc);
               ^~~~~~~
test.rs:14:5: 14:12 note: the trait `core::kinds::Sync` must be implemented because it is required by `is_send`
test.rs:14     is_send(arc);
               ^~~~~~~
error: aborting due to previous error

If we put the Arc in a wrapper type, then the error becomes even more obscure as it reports that Send is not being guaranteed because Sync is not implemented. Although this being true to some extent, it's not helpful at all:

extern crate alloc;

use std::kinds::Send;
use alloc::arc::Arc;

struct Inner(*mut u8);

unsafe impl Send for Inner {}

struct MyWrapper(Arc<Inner>);

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

fn main() {
    let arc = MyWrapper(Arc::new(Inner(0 as *mut u8)));
    is_send(arc);
}

test.rs:16:5: 16:12 error: the trait `core::kinds::Sync` is not implemented for the type `*mut u8`
test.rs:16     is_send(arc);
               ^~~~~~~
test.rs:16:5: 16:12 note: the type `*mut u8` must implement `core::kinds::Sync` because it appears within the type `Inner`
test.rs:16     is_send(arc);
               ^~~~~~~
test.rs:16:5: 16:12 note: the type `Inner` must implement `core::kinds::Sync` because it appears within the type `MyWrapper`
test.rs:16     is_send(arc);
               ^~~~~~~
test.rs:16:5: 16:12 note: the trait `core::kinds::Send` must be implemented because it is required by `is_send`
test.rs:16     is_send(arc);
               ^~~~~~~
error: aborting due to previous error

[mzabaluev]

Might be related: #19950

[steveklabnik]

Triage: updated code:

extern crate alloc;

use std::marker::Send;
use alloc::arc::Arc;

struct Inner(*mut u8);

unsafe impl Send for Inner {}

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

fn main() {
    let arc = Arc::new(Inner(0 as *mut u8));
    is_send(arc);
}

error:

hello.rs:14:5: 14:12 error: the trait `core::marker::Sync` is not implemented for the type `*mut u8` [E0277]
hello.rs:14     is_send(arc);
                ^~~~~~~
hello.rs:14:5: 14:12 help: run `rustc --explain E0277` to see a detailed explanation
hello.rs:14:5: 14:12 note: `*mut u8` cannot be shared between threads safely
hello.rs:14:5: 14:12 note: required because it appears within the type `Inner`
hello.rs:14:5: 14:12 note: required by `is_send`

[Mark-Simulacrum]

The error today indicates that Sync is required because of the requirements on the impl of Send for Arc; I think this solves this issue.

error[E0277]: the trait bound `*mut u8: std::marker::Sync` is not satisfied in `Inner`
  --> test.rs:15:5
   |
15 |     is_send(arc);
   |     ^^^^^^^ `*mut u8` cannot be shared between threads safely
   |
   = help: within `Inner`, the trait `std::marker::Sync` is not implemented for `*mut u8`
   = note: required because it appears within the type `Inner`
   = note: required because of the requirements on the impl of `std::marker::Send` for `alloc::arc::Arc<Inner>`
   = note: required by `is_send`

error: aborting due to previous error(s)