Wrong prompt when failed to pattern match boxed enum type

[bachue]

For example:

#[allow(dead_code)]
enum A {
    V1(u8),
    V2(u32),
}

fn main() {
    let a = Some(Box::new(A::V1(1u8)));
    
    if let Some(b) = a.as_ref() {
        if let A::V1(v) = b {
            println!("{:?}", v);
        }
    }
}

The rust 1.40 compiler will failed to compile this code and tell me:

error[E0308]: mismatched types
  --> src/main.rs:11:16
   |
11 |         if let A::V1(v) = b {
   |                ^^^^^^^^   -
   |                |          |
   |                |          this match expression has type `std::boxed::Box<A>`
   |                |          help: consider dereferencing the boxed value: `*b`
   |                expected struct `std::boxed::Box`, found enum `A`
   |
   = note: expected type `std::boxed::Box<A>`
              found type `A`

But unfortunately it's wrong. If I replace b by *b, it will failed again and tell me:

error[E0308]: mismatched types
  --> src/main.rs:11:16
   |
11 |         if let A::V1(v) = *b {
   |                ^^^^^^^^   --
   |                |          |
   |                |          this match expression has type `std::boxed::Box<A>`
   |                |          help: consider dereferencing the boxed value: `**b`
   |                expected struct `std::boxed::Box`, found enum `A`
   |
   = note: expected type `std::boxed::Box<A>`
              found type `A`

Although the second prompt is correct and it works. But I think using b.as_ref() is the better idea.

[estebank]

No suggestion is presented anymore for the first case. We should have a suggestion for **b.

[kadiwa4]

The current output is correct and helpful (nightly-2024-12-22):

error[E0308]: mismatched types
  --> src/main.rs:11:16
   |
11 |         if let A::V1(v) = b {
   |                ^^^^^^^^   - this expression has type `&Box<A>`
   |                |
   |                expected `Box<A>`, found `A`
   |
   = note: expected struct `Box<A>`
                found enum `A`
help: consider dereferencing to access the inner value using the Deref trait
   |
11 |         if let A::V1(v) = &**b {
   |                           ~~~~

For more information about this error, try `rustc --explain E0308`.

A test could be added to tests/ui/suggestions/suggest-deref-in-match-issue-132784.rs.
@rustbot label: +E-needs-test

[compiler-errors]

This was fixed in #132939.