Missing trait error for type (used as a type parameter) containing a trait object can have a broader span than expected

[Imberflur]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=7a574fab349d3be4d4574b12df5a9932

struct TypeCombiner<A, B>(A, B);

trait MyTrait {
    type Assoc;
}

trait OtherTrait {}

struct A;
struct B;
struct C {
    a: A,
    b: B,
}

impl MyTrait for A {
    type Assoc = ();
}

// missing implementation for B

impl MyTrait for C {
    // span for error output of missing MyTrait for the `Box<T>` type is taken
    // from the whole `TypeCombiner<...>` when `T` is a trait object 
    type Assoc = TypeCombiner<
        <A as MyTrait>::Assoc,
        <Box<dyn OtherTrait> as MyTrait>::Assoc,
    >;
}

The current output is:

error[[E0277]](https://doc.rust-lang.org/stable/error-index.html#E0277): the trait bound `Box<(dyn OtherTrait + 'static)>: MyTrait` is not satisfied
  --> src/lib.rs:25:18
   |
25 |       type Assoc = TypeCombiner<
   |  __________________^
26 | |         <A as MyTrait>::Assoc,
27 | |         <Box<dyn OtherTrait> as MyTrait>::Assoc,
28 | |     >;
   | |_____^ the trait `MyTrait` is not implemented for `Box<(dyn OtherTrait + 'static)>`
   |
   = help: the following other types implement trait `MyTrait`:
             A
             C

Ideally the output should look like:

error[[E0277]](https://doc.rust-lang.org/stable/error-index.html#E0277): the trait bound `Box<(dyn OtherTrait + 'static)>: MyTrait` is not satisfied
  --> src/lib.rs:27:9
   |
27 |         <Box<dyn OtherTrait> as MyTrait>::Assoc,
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `MyTrait` is not implemented for `Box<(dyn OtherTrait + 'static)>: MyTrait`
   |
   = help: the following other types implement trait `MyTrait`:
             A
             C

Note that replacing dyn OtherTrait with a non trait object type like u8 yields the desired error output. My first guess would be that this is related to expanding dyn OtherTrait to (dyn OtherTrait + 'static) (but this is just a random guess).

In my case, this causes error output in custom derive macro generated code to not point back to the original fields. Normally, the proc macro can give each type parameter the span info for the field it is derived from and then error output will show that field.

[fmease]

The imprecise span isn't specific to trait objects per se but generally occurs with failed trait obligations/goals for constituents of more complex types. Cf:

struct TypeCombiner<A, B>(A, B);

trait MyTrait { type Assoc; }
trait Bound {}
impl<T: Bound> MyTrait for T { type Assoc = (); }

impl MyTrait for () {
    type Assoc = TypeCombiner<
        (),
        <i32 as MyTrait>::Assoc,
    >;
}

error[E0277]: the trait bound `i32: Bound` is not satisfied
  --> src/lib.rs:14:18
   |
14 |       type Assoc = TypeCombiner<
   |  __________________^
15 | |         (),
16 | |         <i32 as MyTrait>::Assoc,
17 | |     >;
   | |_____^ the trait `Bound` is not implemented for `i32`, which is required by `i32: MyTrait`
   |
help: this trait has no implementations, consider adding one
  --> src/lib.rs:7:1
   |
7  | trait Bound {}
   | ^^^^^^^^^^^
note: required for `i32` to implement `MyTrait`
  --> src/lib.rs:9:16
   |
9  | impl<T: Bound> MyTrait for T {
   |         -----  ^^^^^^^     ^
   |         |
   |         unsatisfied trait bound introduced here

error[E0277]: the trait bound `i32: Bound` is not satisfied
  --> src/lib.rs:16:9
   |
16 |         <i32 as MyTrait>::Assoc,
   |         ^^^^^^^^^^^^^^^^^^^^^^^ the trait `Bound` is not implemented for `i32`, which is required by `i32: MyTrait`
   |
help: this trait has no implementations, consider adding one
  --> src/lib.rs:7:1
   |
7  | trait Bound {}
   | ^^^^^^^^^^^
note: required for `i32` to implement `MyTrait`
  --> src/lib.rs:9:16
   |
9  | impl<T: Bound> MyTrait for T {
   |         -----  ^^^^^^^     ^
   |         |
   |         unsatisfied trait bound introduced here

However, as you can see in this non-trait-object case the compiler also manages to properly highlight the constituent <i32 as MyTrait>::Assoc of TypeCombiner<…, …> while also emitting a duplicate diagnostic with the imprecise span probably caused by a failed WellFormed(…) obligation/goal for the containing type.