Emit diagnostic when calling methods on the unit type in method chains

[MaciejWas]

Fixes #104204.

What this PR does: If a method is not found somewhere in a call chain, we check if we called earlier a method with signature (&mut T, ...) -> (). If this is the case then we emit a diagnostic message.

For example given input:

vec![1, 2, 3].into_iter().collect::<Vec<i32>>().sort_by_key(|i| i).sort();

the current output is:

error[E0599]: no method named `sort` found for unit type `()` in the current scope
 --> hello.rs:3:72
  |
3 |     vec![1, 2, 3].into_iter().collect::<Vec<i32>>().sort_by_key(|i| i).sort();
  |                                                                        ^^^^ method not found in `()`

after this PR it will be:

error[E0599]: no method named `sort` found for unit type `()` in the current scope
 --> ./hello.rs:3:72
  |
3 |     vec![1, 2, 3].into_iter().collect::<Vec<i32>>().sort_by_key(|i| i).sort();
  |                                                                        ^^^^ method not found in `()`
  |

note: method `sort_by_key` modifies its receiver in-place, it is not meant to be used in method chains.
 --> ./hello.rs:3:53
  |
3 |     vec![1, 2, 3].into_iter().collect::<Vec<i32>>().sort_by_key(|i| i).sort();
  |                                                     ^^^^^^^^^^^ this call modifies its receiver in-place

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @petrochenkov (or someone else) soon.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

[MaciejWas]

@rustbot label +A-diagnostics

[MaciejWas]

inlined and removed :)
@rustbot review

[petrochenkov]

r=me after squashing commits.
@rustbot author

[MaciejWas]

@bors r=petrochenkov

[bors]

@MaciejWas: 🔑 Insufficient privileges: Not in reviewers

[MaciejWas]

@rustbot review

[albertlarsan68]

@bors r=petrochenkov

[bors]

📌 Commit 6a2a6fe has been approved by petrochenkov

It is now in the queue for this repository.