Improve error message when moving out of a &-reference

[nwin]

It would be nice if the following error message

test.rs:7:9: 7:13 error: cannot move out of dereference of `&`-pointer
test.rs:7         self.option.map(|x| x)
                  ^~~~
error: aborting due to previous error

would give a hint why exactly something was moved in the following code

struct Type {
    option: Option<Vec<u8>>
}

impl Type {
    fn method(&self) -> Option<Vec<u8>> {
        self.option.map(|x| x)
    }
}

fn main() {
    let _ = Type { option: None }.method();
}

Ideally "map takes option by value" or something like that.

[estebank]

Also reported in #25507 (comment)

[estebank]

Possible final output:

error[E0507]: cannot move out of borrowed content
 --> src/main.rs:7:9
6 |    fn method(&self) -> Option<Vec<u8>> {
  |              ----- `self` is borrowed here
7 |        self.option.map(|x| x)
  |        ^^^^^^^^^^^ --- `map` consumes `self.option` here, moving it
  |        |
  |        cannot move out of borrowed content

[estebank]

Output with #61147 applied:

error[E0507]: cannot move out of borrowed content
 --> file2.rs:7:9
  |
7 |         self.option.map(|x| x)
  |         ^^^^^^^^^^^
  |         |
  |         cannot move out of borrowed content
  |         help: consider borrowing the `Option`'s content: `self.option.as_ref()`

If applying the suggestion, we'll emit

error[E0308]: mismatched types
 --> file2.rs:7:9
  |
6 |     fn method(&self) -> Option<Vec<u8>> {
  |                         --------------- expected `std::option::Option<std::vec::Vec<u8>>` because of return type
7 |         self.option.as_ref().map(|x| x)
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `std::vec::Vec`, found reference
  |
  = note: expected type `std::option::Option<std::vec::Vec<u8>>`
             found type `std::option::Option<&std::vec::Vec<u8>>`

[estebank]

Current output:

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

[rylev]

Triage: no change (playground).

I believe the original suggestion of pointing to map as taking self and thus trying to move out of a &self is the right way to go to close this issue.

[TimJentzsch]

Current output with 1.65.0-nightly (2022-08-10 29e4a9ee0253cd39e552)

struct Type {
    option: Option<Vec<u8>>
}

impl Type {
    fn method(&self) -> Option<Vec<u8>> {
        self.option.as_ref().map(|x| x)
    }
}

fn main() {
    let _ = Type { option: None }.method();
}

error[[E0308]](https://doc.rust-lang.org/nightly/error-index.html#E0308): mismatched types
 --> src/main.rs:7:9
  |
6 |     fn method(&self) -> Option<Vec<u8>> {
  |                         --------------- expected `Option<Vec<u8>>` because of return type
7 |         self.option.as_ref().map(|x| x)
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `Vec`, found `&Vec<u8>`
  |
  = note: expected enum `Option<Vec<u8>>`
             found enum `Option<&Vec<u8>>`

[estebank]

Current output

error[E0308]: mismatched types
 --> src/main.rs:7:9
  |
6 |     fn method(&self) -> Option<Vec<u8>> {
  |                         --------------- expected `Option<Vec<u8>>` because of return type
7 |         self.option.as_ref().map(|x| x)
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `Vec`, found `&Vec<u8>`
  |
  = note: expected enum `Option<Vec<u8>>`
             found enum `Option<&Vec<u8>>`
help: use `Option::cloned` to clone the value inside the `Option`
  |
7 |         self.option.as_ref().map(|x| x).cloned()
  |                                        +++++++++

error[E0507]: cannot move out of `self.option` which is behind a shared reference
 --> src/main.rs:7:9
  |
7 |         self.option.map(|x| x)
  |         ^^^^^^^^^^^ ---------- `self.option` moved due to this method call
  |         |
  |         help: consider calling `.as_ref()` or `.as_mut()` to borrow the type's contents
  |         move occurs because `self.option` has type `Option<Vec<u8>>`, which does not implement the `Copy` trait
  |
note: `Option::<T>::map` takes ownership of the receiver `self`, which moves `self.option`
 --> /rustc/ee0412d1ef81efcfabe7f66cd21476ca85d618b1/library/core/src/option.rs:963:28
help: you can `clone` the value and consume it, but this might not be your desired behavior
  |
7 |         self.option.clone().map(|x| x)
  |                     ++++++++