E0119 can overwrite E0277 leading to confusing error message

[CAD97]

Code:

trait Trait {}

struct Unsized {
    tail: [()],
    marker: (),
}

impl<T: Sized> Trait for T {}
impl Trait for Unsized {}

error:

error[E0119]: conflicting implementations of trait `Trait` for type `Unsized`:
  --> src/lib.rs:10:1
   |
9  | impl<T: Sized> Trait for T {}
   | -------------------------- first implementation here
10 | impl Trait for Unsized {}
   | ^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Unsized`

Without impl Trait for Unsized:

error[E0277]: the size for values of type `[()]` cannot be known at compilation time
 --> src/lib.rs:4:5
  |
4 |     tail: [()],
  |     ^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `[()]`
  = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
  = note: only the last field of a struct may have a dynamically sized type

The better behavior would be to

• Make sure that the E0277 is emitted even in the case of other errors.
• When finding an E0277 struct definition, treat it as if it were ?Sized through the rest of the pipeline.

[estebank]

Current output:

error[E0277]: the size for values of type `[()]` cannot be known at compilation time
 --> src/lib.rs:4:11
  |
4 |     tail: [()],
  |           ^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `[()]`
  = note: only the last field of a struct may have a dynamically sized type
  = help: change the field's type to have a statically known size
help: borrowed types always have a statically known size
  |
4 |     tail: &[()],
  |           +
help: the `Box` type always has a statically known size and allocates its contents in the heap
  |
4 |     tail: Box<[()]>,
  |           ++++    +

error[E0119]: conflicting implementations of trait `Trait` for type `Unsized`
 --> src/lib.rs:9:1
  |
8 | impl<T: Sized> Trait for T {}
  | -------------------------- first implementation here
9 | impl Trait for Unsized {}
  | ^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Unsized`

Some errors have detailed explanations: E0119, E0277.
For more information about an error, try `rustc --explain E0119`.

Both errors are emitted (and the E0277 now also has additional context).

When finding an E0277 struct definition, treat it as if it were ?Sized through the rest of the pipeline.

I don't think we do this yet.