Give a better error when an impl block implicitly requires `Sized`

[jyn514]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=277b0605705776743255b4aa99055334

use core::fmt::Display;

struct S<T: ?Sized> {
    inner: Box<T>,
}

impl<T> S<T> {
    fn foo(&self) {}
}

fn main() {
    let x: S<dyn Display> = S { inner: Box::new(0) as _ };
    x.foo();
}

The current output is:

error[E0599]: the method `foo` exists for struct `S<dyn std::fmt::Display>`, but its trait bounds were not satisfied
   --> src/main.rs:12:7
    |
2   |   struct S<T: ?Sized> {
    |   ------------------- method `foo` not found for this
...
12  |       x.foo();
    |         ^^^ method cannot be called on `S<dyn std::fmt::Display>` due to unsatisfied trait bounds
    |
    = note: the following trait bounds were not satisfied:
            `dyn std::fmt::Display: Sized`

Ideally the output should look like:

error[E0599]: the method `foo` exists for struct `S<dyn std::fmt::Display>`, but its trait bounds were not satisfied
   --> src/main.rs:12:7
    |
2   |   struct S<T: ?Sized> {
    |   ------------------- method `foo` not found for this
...
12  |       x.foo();
    |         ^^^ method cannot be called on `S<dyn std::fmt::Display>` due to unsatisfied trait bounds
    |
    = note: the following trait bounds were not satisfied:
            `dyn std::fmt::Display: Sized`
note: implicitly required by the following `impl` block:
 7 | impl<T> S<T> {
    = help: consider relaxing the implicit `Sized` restriction
 7 | impl<T: ?Sized> S<T> {
           ++++++++

This already exists for free functions; not sure why it's broken above. https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=4c7a6a69f9678c4ea5b410431e201c8c

[WaffleLapkin]

So, I've tried to fix this, but got stuck fairly quickly.

This already exists for free functions; not sure why it's broken above.

The reason is that handling of errors for methods and free functions is different, for method calls the error reporting happens in rustc_typeck::check::method::suggest::FnCtxt::report_method_error, while for free functions it happens in rustc_trait_selection::traits::error_reporting::InferCtxt::report_selection_error. Note that calling S::foo(&x) in @jyn514's example also makes the error nicer.

The problem with fixing this is that both functions are fairly long (around 1k lines IIRC) and do a lot of stuff. Ideally we would unify their logic, but that will be a big refactor.

Linking a zulip thread, just in case: [zulip thread].