Suggest dereferencing boxed enum when matching

[estebank]

Given the following code:

enum E {
 A(usize),
 B(usize),
}

fn main() {
 let x = Box::new(E::A(3));
 let y = match x {
 E::A(a) | E::B(a) => a,
 };
}

the compiler outputs:

error[E0308]: mismatched types
 --> src/main.rs:9:9
 |
8 | let y = match x {
 | - this match expression has type `std::boxed::Box<E>`
9 | E::A(a) | E::B(a) => a,
 | ^^^^^^^ expected struct `std::boxed::Box`, found enum `E`
 |
 = note: expected type `std::boxed::Box<E>`
 found type `E`

error[E0308]: mismatched types
 --> src/main.rs:9:19
 |
8 | let y = match x {
 | - this match expression has type `std::boxed::Box<E>`
9 | E::A(a) | E::B(a) => a,
 | ^^^^^^^ expected struct `std::boxed::Box`, found enum `E`
 |
 = note: expected type `std::boxed::Box<E>`
 found type `E`

Adding changing line 8 to let y = match *x { makes the code compile correctly (partly thanks to match ergonomics, I believe).

The error message would ideally be closer to

error[E0308]: mismatched types
 --> src/main.rs:9:9
 |
8 | let y = match x {
 | -
 | |
 | this match expression has type `std::boxed::Box<E>`
 | help: dereference the boxed value: `*x`
9 | E::A(a) | E::B(a) => a,
 | ^^^^^^^ ^^^^^^^ expected struct `std::boxed::Box`, found enum `E`
 | |
 | expected struct `std::boxed::Box`, found enum `E`
 |
 = note: expected type `std::boxed::Box<E>`
 found type `E`

Adding the suggestion should be relatively easy, condensing the multiple alternative match arm discriminants with mismatched type errors into one might be more involved.

---

@nikomatsakis should we consider extending the match ergonomics to also account for Box?

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Case taken from an old reddit post from a newcomer to the language.

[davidtwco]

Submitted #57783 for the suggestion - this doesn't condense duplicate errors from the match arms.