Diagnostics: compiler should hint about .as_ref() when trying to go from &Option<> to Option<&>.

[vi]

fn main() {
    let a = Some(vec![1,2,3,4]);
    let b : &Option<Vec<u8>> = &a;
    let c : Option<&[u8]>;
    // c = b; // expected enum `std::option::Option`, found reference
    // c = *b; // expected &[u8], found struct `std::vec::Vec`
    // c = (*b).map(|x|x.as_slice()); // cannot move `b` out of borrowed content
    // c = b.map(|x|x.as_slice()); // cannot move `b` out of borrowed content and `x` does not live long enough
    
    // Ask IRC here
    
    c = b.as_ref().map(|x|x.as_slice());

    let _ = c;
}

Compiler message should hint Option::as_ref, to shift user attention from object inside Option to the Option itself.

[estebank]

Similar issue to #43861, which was fixed with a targeted hack in #51100.

The first two cases could probably be handled by extending that code, the last two are lifetime errors so they would need to touch a different part of the compiler.

For future reference, the four cases produce the following output:

error[E0308]: mismatched types
 --> src/main.rs:6:9
  |
6 |     c = b;
  |         ^ expected enum `std::option::Option`, found reference
  |
  = note: expected type `std::option::Option<&[u8]>`
             found type `&std::option::Option<std::vec::Vec<u8>>`

error[E0308]: mismatched types
 --> src/main.rs:7:9
  |
7 |     c = *b;
  |         ^^ expected &[u8], found struct `std::vec::Vec`
  |
  = note: expected type `std::option::Option<&[u8]>`
             found type `std::option::Option<std::vec::Vec<u8>>`

error[E0597]: `x` does not live long enough
 --> src/main.rs:8:21
  |
8 |     c = (*b).map(|x|x.as_slice());
  |                     ^          - `x` dropped here while still borrowed
  |                     |
  |                     borrowed value does not live long enough

error[E0507]: cannot move out of borrowed content
 --> src/main.rs:8:9
  |
8 |     c = (*b).map(|x|x.as_slice());
  |         ^^^^ cannot move out of borrowed content

error[E0597]: `x` does not live long enough
 --> src/main.rs:9:18
  |
9 |     c = b.map(|x|x.as_slice());
  |                  ^          - `x` dropped here while still borrowed
  |                  |
  |                  borrowed value does not live long enough

error[E0507]: cannot move out of borrowed content
 --> src/main.rs:9:9
  |
9 |     c = b.map(|x|x.as_slice());
  |         ^ cannot move out of borrowed content

[estebank]

Current output for 3 and 4:

error[E0515]: cannot return value referencing function parameter `x`
 --> src/main.rs:7:21
  |
7 |     c = (*b).map(|x|x.as_slice()); // cannot move `b` out of borrowed content
  |                     -^^^^^^^^^^^
  |                     |
  |                     returns a value referencing data owned by the current function
  |                     `x` is borrowed here

error[E0507]: cannot move out of `*b` which is behind a shared reference
 --> src/main.rs:7:9
  |
7 |     c = (*b).map(|x|x.as_slice()); // cannot move `b` out of borrowed content
  |         ^^^^
  |         |
  |         move occurs because `*b` has type `std::option::Option<std::vec::Vec<u8>>`, which does not implement the `Copy` trait
  |         help: consider borrowing the `Option`'s content: `(*b).as_ref()`

error[E0515]: cannot return value referencing function parameter `x`
 --> src/main.rs:8:18
  |
8 |     c = b.map(|x|x.as_slice()); // cannot move `b` out of borrowed content and `x` does not live long enough
  |                  -^^^^^^^^^^^
  |                  |
  |                  returns a value referencing data owned by the current function
  |                  `x` is borrowed here

error[E0507]: cannot move out of `*b` which is behind a shared reference
 --> src/main.rs:8:9
  |
8 |     c = b.map(|x|x.as_slice()); // cannot move `b` out of borrowed content and `x` does not live long enough
  |         ^
  |         |
  |         move occurs because `*b` has type `std::option::Option<std::vec::Vec<u8>>`, which does not implement the `Copy` trait
  |         help: consider borrowing the `Option`'s content: `b.as_ref()`

Note the suggestions are accurate and fix the issue.