Suggest dereferencing on E0277 if appropriate

[estebank]

error[E0277]: the trait bound `syntax_pos::MultiSpan: std::convert::From<&syntax_pos::span_encoding::Span>` is not satisfied
   --> src/librustc_typeck/astconv.rs:726:42
    |
726 | ...                   tcx.sess.delay_span_bug(visitor.0.iter().next().unwrap(), "asdf");
    |                                ^^^^^^^^^^^^^^ the trait `std::convert::From<&syntax_pos::span_encoding::Span>` is not implemented for `syntax_pos::MultiSpan`
    |
    = help: the following implementations were found:
              <syntax_pos::MultiSpan as std::convert::From<std::vec::Vec<syntax_pos::span_encoding::Span>>>
              <syntax_pos::MultiSpan as std::convert::From<syntax_pos::span_encoding::Span>>
    = note: required because of the requirements on the impl of `std::convert::Into<syntax_pos::MultiSpan>` for `&syntax_pos::span_encoding::Span`

Suggest *foo when <&foo as &Foo>, Into<Bar> is expected and Bar: From<Foo> is implemented.

[drmason13]

I would have benefited from this, related forum post: https://users.rust-lang.org/t/methods-that-take-references-to-type-parameter/36088

When writing a method taking &T and comparing to a field T the compiler complains

error[E0277]: can't compare `&T` with `T`
...
help: the trait `std::cmp::PartialEq<T>` is not implemented for `&T`

The suggestion from rustc is:

   = help: consider adding a `where &T: std::cmp::PartialEq<T>` bound

But fellow rustaceans suggested:

do data == &self.data because if you have T: PartialEq<T>, then you will get &T: PartialEq<&T> for free

If the compiler could suggest the latter instead it would be a great help :)

[kornelski]

Referencing would help too:

fn main() {
    takes_ref(Default::default());
}
fn takes_ref(_: &i32) {}

2 |     takes_ref(Default::default());
  |               ^^^^^^^^^^^^^^^^^^ the trait `std::default::Default` is not implemented for `&i32`
  |
  = help: the following implementations were found:
            <i32 as std::default::Default>
  = note: required by `std::default::Default::default`

[estebank]

Current output:

error[E0382]: borrow of moved value: `bar`
  --> src/main.rs:24:27
   |
21 |     let bar = String::from("XYZ");
   |         --- move occurs because `bar` has type `String`, which does not implement the `Copy` trait
22 |
23 |     if foo.matches(bar) {
   |                    --- value moved here
24 |          println!("{:?}", bar);
   |                           ^^^ value borrowed here after move
   |
note: consider changing this parameter type in method `matches` to borrow instead if owning the value isn't necessary
  --> src/main.rs:14:29
   |
14 |     fn matches(&self, data: T) -> bool {
   |        -------              ^ this parameter takes ownership of the value
   |        |
   |        in this method
   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider cloning the value if the performance cost is acceptable
   |
23 |     if foo.matches(bar.clone()) {
   |                       ++++++++

The above "solves" the issue if you apply the suggestion, but if you follow the earlier advice you get the original suggestion:

error[E0277]: can't compare `&T` with `T`
  --> src/main.rs:15:14
   |
15 |         data == self.data
   |              ^^ no implementation for `&T == T`
   |
   = help: the trait `PartialEq<T>` is not implemented for `&T`
help: consider extending the `where` clause, but there might be an alternative better way to express this requirement
   |
8  |     where T: PartialEq, &T: PartialEq<T>
   |                       ~~~~~~~~~~~~~~~~~~

I believe that issue falls under #40660, which we should address.

error[E0277]: the trait bound `&i32: Default` is not satisfied
 --> src/main.rs:2:15
  |
2 |     takes_ref(Default::default());
  |               ^^^^^^^^^^^^^^^^ the trait `Default` is not implemented for `&i32`
  |
help: consider borrowing here
  |
2 |     takes_ref(&Default::default());
  |               +

error[E0308]: mismatched types
 --> src/main.rs:2:15
  |
2 |     takes_ref(&Default::default());
  |     --------- ^^^^^^^^^^^^^^^^^^^ expected `i32`, found `&_`
  |     |
  |     arguments to this function are incorrect
  |
  = note:   expected type `i32`
          found reference `&_`
note: function defined here
 --> src/main.rs:4:4
  |
4 | fn takes_ref(_: i32) {}
  |    ^^^^^^^^^ ------
help: consider removing the borrow
  |
2 -     takes_ref(&Default::default());
2 +     takes_ref(Default::default());

I believe we can close this ticket.