Confusing errors with missing let in if let

[camelid]

I thought this code would provide one of the nice suggestions that @estebank implemented in #75931, but it doesn't. (By the way, making cache a reference to Cache will give an ICE because of #77218.)

pub struct Cache {
    data: Vec<i32>,
}

pub fn list_data(cache: Cache, key: &usize) {
    for reference in vec![1, 2, 3] {
        if /*let*/ Some(reference) = cache.data.get(*key) {
            unimplemented!()
        }
    }
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
 --> src/lib.rs:7:38
  |
7 |         if /*let*/ Some(reference) = cache.data.get(*key) {
  |                                      ^^^^^^^^^^^^^^^^^^^^ expected integer, found `&i32`
  |
  = note: expected enum `Option<{integer}>`
             found enum `Option<&i32>`

error[E0308]: mismatched types
 --> src/lib.rs:7:20
  |
7 |         if /*let*/ Some(reference) = cache.data.get(*key) {
  |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `bool`, found `()`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground`

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

[estebank]

Reopening as per #77283 (comment)

[estebank]

This is fixed in beta now:

error[[E0425]](https://doc.rust-lang.org/nightly/error-index.html#E0425): cannot find value `foo` in this scope
 --> src/main.rs:2:18
  |
2 |     if Some(3) = foo {}
  |                  ^^^ not found in this scope

error[[E0070]](https://doc.rust-lang.org/nightly/error-index.html#E0070): invalid left-hand side of assignment
 --> src/main.rs:2:16
  |
2 |     if Some(3) = foo {}
  |             -  ^
  |             |
  |             cannot assign to this expression

error[[E0308]](https://doc.rust-lang.org/nightly/error-index.html#E0308): mismatched types
 --> src/main.rs:2:8
  |
2 |     if Some(3) = foo {}
  |        ^^^^^^^^^^^^^ expected `bool`, found `()`
  |
help: consider adding `let`
  |
2 |     if let Some(3) = foo {}
  |        +++

error[[E0308]](https://doc.rust-lang.org/nightly/error-index.html#E0308): mismatched types
 --> src/lib.rs:7:25
  |
6 |     for reference in vec![1, 2, 3] {
  |         --------- expected due to the type of this binding
7 |         if /*let*/ Some(reference) = cache.data.get(*key) {
  |                         ^^^^^^^^^ expected integer, found `&i32`
  |
help: consider dereferencing the borrow
  |
7 |         if /*let*/ Some(*reference) = cache.data.get(*key) {
  |                         +

error[[E0308]](https://doc.rust-lang.org/nightly/error-index.html#E0308): mismatched types
 --> src/lib.rs:7:20
  |
7 |         if /*let*/ Some(reference) = cache.data.get(*key) {
  |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `bool`, found `()`
  |
help: consider adding `let`
  |
7 |         if /*let*/ let Some(reference) = cache.data.get(*key) {
  |                    +++

It is still quite verbose (too many errors), but we lead people in the right direction.