Confusing error when impl is available only on RefCell contents

[iximeow]

Some Rust (playground):

use std::cell::RefCell;

struct WorkDoer<'f, F> {
    f: &'f F,
}

impl<'f> WorkDoer<'f, i32> {
    fn do_work(&self) {}
}

fn main() {
    let f = RefCell::new(1i32);
    let work = WorkDoer { f: &f };
    work.do_work();
}

yields an error about no method do_work being found:

error[E0599]: no method named `do_work` found for struct `WorkDoer<'_, std::cell::RefCell<i32>>` in the current scope
  --> src/main.rs:14:10
   |
3  | struct WorkDoer<'f, F> {
   | ---------------------- method `do_work` not found for this
...
14 |     work.do_work();
   |          ^^^^^^^ method not found in `WorkDoer<'_, std::cell::RefCell<i32>>`

where do_work does exist, but on a different type. Specifically, WorkDoer<'_, i32>.

In the above example, WorkDoer should be made from a borrow of the RefCell, like WorkDoer { f: &*f.borrow() }, which rustc is entirely happy to build. It would be fantastic if rustc could suggest that the borrow would make a do_work available.

Related messages

There's (what I think to be) a similar variation of this with a more generic impl:

use std::cell::RefCell;

struct WorkDoer<'f, F> {
    f: &'f F,
}

impl<'f, I: std::ops::Add> WorkDoer<'f, I> {
    fn do_work(&self) {}
}

fn main() {
    let f = RefCell::new(1i32);
    let work = WorkDoer { f: &f };
    work.do_work();
}

yielding a more informative error:

error[E0599]: no method named `do_work` found for struct `WorkDoer<'_, std::cell::RefCell<i32>>` in the current scope
   --> src/main.rs:14:10
    |
3   |   struct WorkDoer<'f, F> {
    |   ---------------------- method `do_work` not found for this
...
14  |       work.do_work();
    |            ^^^^^^^ method not found in `WorkDoer<'_, std::cell::RefCell<i32>>`
    |
    = note: the method `do_work` exists but the following trait bounds were not satisfied:
            `std::cell::RefCell<i32>: std::ops::Add`

but if the constraints become much more complex than I: std::ops::Add it can be very confusing to read through mismatched types only to realize you've forgotten to borrow the RefCell.

If rustc could observe that the inner type satisfies the constraints and makes do_work available, then suggest that change, it'd be kind of incredibly good and cool.

(this is a minimization of https://twitter.com/iximeow/status/1297651292148359170 which @estebank also asked me to file an issue about :) the aforementioned "try borrowing the RefCell contents" suggestion would be very handy here.)

[estebank]

Current output:

error[E0599]: no method named `do_work` found for struct `WorkDoer<'_, RefCell<i32>>` in the current scope
  --> src/main.rs:14:10
   |
3  | struct WorkDoer<'f, F> {
   | ---------------------- method `do_work` not found for this struct
...
14 |     work.do_work();
   |          ^^^^^^^ method not found in `WorkDoer<'_, RefCell<i32>>`
   |
   = note: the method was found for
           - `WorkDoer<'f, i32>`

error[E0599]: the method `do_work` exists for struct `WorkDoer<'_, RefCell<i32>>`, but its trait bounds were not satisfied
  --> src/main.rs:14:10
   |
3  | struct WorkDoer<'f, F> {
   | ---------------------- method `do_work` not found for this struct
...
14 |     work.do_work();
   |          ^^^^^^^ method cannot be called on `WorkDoer<'_, RefCell<i32>>` due to unsatisfied trait bounds
  --> /rustc/8ede3aae28fe6e4d52b38157d7bfe0d3bceef225/library/core/src/cell.rs:678:1
   |
   = note: doesn't satisfy `RefCell<i32>: Add`
   |
note: trait bound `RefCell<i32>: Add` was not satisfied
  --> src/main.rs:7:13
   |
7  | impl<'f, I: std::ops::Add> WorkDoer<'f, I> {
   |             ^^^^^^^^^^^^^  ---------------
   |             |
   |             unsatisfied trait bound introduced here

[estebank]

CC #71505