Show the location of a type usage for trait bound failures

[oli-cosmian]

cc @estebank

The following diagnostic doesn't tell the user anything about where the type that causes the failure is coming from. I understand that we can't report a correct source site, but we could create a heuristic:

Instead of having a note: without an associated span, we could check where (if at all) the type is used in the function and report those spans as potential sources of the error. Though I'm not sure what would be needed to fiddle this information through to the diagnostic reporting site.

fn foo(i: i32) -> impl std::future::Future<Output = ()> + Send {
    async move {
        boo(format!("{}", i).as_str()).await;
    }
}

async fn boo(_: &str) {
    
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0277]: `*mut (dyn std::ops::Fn() + 'static)` cannot be shared between threads safely
 --> src/lib.rs:1:19
  |
1 | fn foo(i: i32) -> impl std::future::Future<Output = ()> + Send {
  |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `*mut (dyn std::ops::Fn() + 'static)` cannot be shared between threads safely
  |
  = help: within `core::fmt::Void`, the trait `std::marker::Sync` is not implemented for `*mut (dyn std::ops::Fn() + 'static)`
  = note: required because it appears within the type `std::marker::PhantomData<*mut (dyn std::ops::Fn() + 'static)>`
  = note: required because it appears within the type `core::fmt::Void`
  = note: required because of the requirements on the impl of `std::marker::Send` for `&core::fmt::Void`
  = note: required because it appears within the type `std::fmt::ArgumentV1<'_>`
  = note: required because it appears within the type `[std::fmt::ArgumentV1<'_>; 1]`
  = note: required because it appears within the type `for<'r, 's, 't0, 't1, 't2, 't3, 't4, 't5, 't6, 't7, 't8, 't9, 't10, 't11, 't12, 't13, 't14, 't15, 't16, 't17, 't18> {for<'_> fn(&str) -> impl std::future::Future {boo}, for<'t19> fn(std::fmt::Arguments<'t19>) -> std::string::String {std::fmt::format}, fn(&'r [&'r str], &'r [std::fmt::ArgumentV1<'r>]) -> std::fmt::Arguments<'r> {std::fmt::Arguments::<'r>::new_v1}, &'s str, &'t0 str, [&'t1 str; 1], &'t2 [&'t3 str], &'t4 [&'t5 str; 1], i32, &'t6 i32, (&'t7 i32,), [std::fmt::ArgumentV1<'t8>; 1], &'t9 [std::fmt::ArgumentV1<'t10>], &'t11 [std::fmt::ArgumentV1<'t12>; 1], std::fmt::Arguments<'t13>, &'t14 std::string::String, std::string::String, &'t15 str, &'t16 str, impl std::future::Future, impl std::future::Future, ()}`
  = note: required because it appears within the type `[static generator@src/lib.rs:2:16: 4:6 i:i32 for<'r, 's, 't0, 't1, 't2, 't3, 't4, 't5, 't6, 't7, 't8, 't9, 't10, 't11, 't12, 't13, 't14, 't15, 't16, 't17, 't18> {for<'_> fn(&str) -> impl std::future::Future {boo}, for<'t19> fn(std::fmt::Arguments<'t19>) -> std::string::String {std::fmt::format}, fn(&'r [&'r str], &'r [std::fmt::ArgumentV1<'r>]) -> std::fmt::Arguments<'r> {std::fmt::Arguments::<'r>::new_v1}, &'s str, &'t0 str, [&'t1 str; 1], &'t2 [&'t3 str], &'t4 [&'t5 str; 1], i32, &'t6 i32, (&'t7 i32,), [std::fmt::ArgumentV1<'t8>; 1], &'t9 [std::fmt::ArgumentV1<'t10>], &'t11 [std::fmt::ArgumentV1<'t12>; 1], std::fmt::Arguments<'t13>, &'t14 std::string::String, std::string::String, &'t15 str, &'t16 str, impl std::future::Future, impl std::future::Future, ()}]`
  = note: required because it appears within the type `std::future::GenFuture<[static generator@src/lib.rs:2:16: 4:6 i:i32 for<'r, 's, 't0, 't1, 't2, 't3, 't4, 't5, 't6, 't7, 't8, 't9, 't10, 't11, 't12, 't13, 't14, 't15, 't16, 't17, 't18> {for<'_> fn(&str) -> impl std::future::Future {boo}, for<'t19> fn(std::fmt::Arguments<'t19>) -> std::string::String {std::fmt::format}, fn(&'r [&'r str], &'r [std::fmt::ArgumentV1<'r>]) -> std::fmt::Arguments<'r> {std::fmt::Arguments::<'r>::new_v1}, &'s str, &'t0 str, [&'t1 str; 1], &'t2 [&'t3 str], &'t4 [&'t5 str; 1], i32, &'t6 i32, (&'t7 i32,), [std::fmt::ArgumentV1<'t8>; 1], &'t9 [std::fmt::ArgumentV1<'t10>], &'t11 [std::fmt::ArgumentV1<'t12>; 1], std::fmt::Arguments<'t13>, &'t14 std::string::String, std::string::String, &'t15 str, &'t16 str, impl std::future::Future, impl std::future::Future, ()}]>`
  = note: required because it appears within the type `impl std::future::Future`
  = note: the return type of a function must have a statically known size

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground`.

To learn more, run the command again with --verbose.

[nikomatsakis]

This is possibly addressed by @davidtwco's PR #65345

[davidtwco]

Assigning to myself, should get handled by #65345 but currently isn't (presumably due to this comment).

@rustbot claim

[estebank]

The code no longer fails since 1.41, an alternative example should be devised.

[workingjubilee]

improving diagnostics of actually-compiling code is non-actionable. closing!