Suggest changing `-> T<U>` to a `-> T<&U>` when trying to return a `T<&U>`

[DropDemBits]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=a33eca3ff60c0fa5b2f73dc3e34c362f

fn bing<'a>() -> Option<&'a ()> {
    None
}

struct Thing;

impl Thing {
    fn bang(&self) -> Option<()> {
        bing()
    }
}

The current output is:

error[E0308]: mismatched types
 --> src/lib.rs:9:9
  |
8 |     fn bang(&self) -> Option<()> {
  |                       ---------- expected `Option<()>` because of return type
9 |         bing()
  |         ^^^^^^ expected `()`, found `&()`
  |
  = note: expected enum `Option<()>`
             found enum `Option<&()>`

Ideally, there should be a suggestion to add a & to the inner type

  = help: try adding a `&` to the inner type:
  |
8 |     fn bang(&self) -> Option<&()> {
  |                              +

[compiler-errors]

Hm, this suggestion in particular is very specific to T itself, and also the signature of the function (i.e. it's only meaningful to suggest a reference with an elided lifetime in the return type -- like &T -- if there's a lifetime that matches in the inputs...).

This also changes the meaning of the function. It's equally likely, I think, that the user could've forgotten an Option::copied method call in the case of the example you provided.

[DropDemBits]

Yeah, I probably reduced the example too much when making the issue.
This snippet is a closer approximation of the original code:

use std::collections::HashMap;

struct Def;

struct DefThings {
    things: HashMap<u32, Vec<Def>>
}

impl DefThings {
    fn get_thing(&self, id: u32) -> Option<Vec<Def>> {
        self.things.get(&id)
    }
}

Though this also falls under the same reasoning as the previous example (an Option::cloned could've been forgotten).

---

I was definitely confused initially because I'd first seen the error text in vscode (and thus from r-a), and the presentation of the error there is less than ideal:

mismatched types
expected enum `std::option::Option<std::vec::Vec<Def>>`
   found enum `std::option::Option<&std::vec::Vec<Def>>` rustc(E0308)
lib.rs(11, 37): expected `std::option::Option<std::vec::Vec<Def>>` because of return type