Added shadowed hint for overlapping associated types

[TheLazyDutchman]

Previously, when you tried to set an associated type that is shadowed by an associated type in a subtrait, like this:

trait A {
    type X;
}

trait B: A {
    type X; // note: this is legal
}

impl<Y> Clone for Box<dyn B<X=Y, X=Y>> {
    fn clone(&self) -> Self {
        todo!()
    }
}

you got a confusing error message, that says nothing about the shadowing:

error[E0719]: the value of the associated type `X` (from trait `B`) is already specified
 --> test.rs:9:34
  |
9 | impl<Y> Clone for Box<dyn B<X=Y, X=Y>> {
  |                             ---  ^^^ re-bound here
  |                             |
  |                             `X` bound here first

error[E0191]: the value of the associated type `X` (from trait `A`) must be specified
 --> test.rs:9:27
  |
2 |     type X;
  |     ------ `X` defined here
...
9 | impl<Y> Clone for Box<dyn B<X=Y, X=Y>> {
  |                           ^^^^^^^^^^^ help: specify the associated type: `B<X=Y, X=Y, X = Type>`

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0191, E0719.
For more information about an error, try `rustc --explain E0191`.

Now instead, the error shows that the associated type is shadowed, and suggests renaming as a potential fix.

error[E0719]: the value of the associated type `X` in trait `B` is already specified
 --> test.rs:9:34
  |
9 | impl<Y> Clone for Box<dyn B<X=Y, X=Y>> {
  |                             ---  ^^^ re-bound here
  |                             |
  |                             `X` bound here first

error[E0191]: the value of the associated type `X` in `A` must be specified
 --> test.rs:9:27
  |
2 |     type X;
  |     ------ `A::X` defined here
...
6 |     type X; // note: this is legal
  |     ------ `A::X` shadowed here
...
9 | impl<Y> Clone for Box<dyn B<X=Y, X=Y>> {
  |                           ^^^^^^^^^^^ associated type `X` must be specified
  |
help: consider renaming this associated type
 --> test.rs:2:5
  |
2 |     type X;
  |     ^^^^^^
help: consider renaming this associated type
 --> test.rs:6:5
  |
6 |     type X; // note: this is legal
  |     ^^^^^^

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0191, E0719.
For more information about an error, try rustc --explain E0191.

The rename help message is only emitted when the trait is local. This is true both for the supertrait as for the subtrait.

There might be cases where you can use the fully qualified path (for instance, in a where clause), but this PR currently does not deal with that.

fixes #100109

(continues from #117642, because I didn't know renaming the branch would close the PR)

[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.

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[TheLazyDutchman]

After rebasing to try to resolve the conflict I get the following error when running x test tidy: tidy error: could not find exception package supports-hyperlinks`. I do not know why this happened, or how to fix this.

[petrochenkov]

Could you push the rebased version to github?
If CI fails then it's an issue with the rebased PR, if not then it's some issue with the local setup and fresh rebuild may help.

[TheLazyDutchman]

It gives the error too when it runs tidy when pushing, so it happened probably in the rebase

[petrochenkov]

It gives the error too when it runs tidy when pushing

That's a git hook installed by x.py setup, try removing .git/hooks/pre-push in the rust repo to push without running x.py.
I suspect that you committed some submodule changes, and that now prevents x.py from setting that submodule to an up-to-date version.

[TheLazyDutchman]

@rustbot review

[TheLazyDutchman]

@rustbot review

[petrochenkov]

r=me after removing the second commit (but keeping #117661 (comment)).
@rustbot author

[TheLazyDutchman]

@rustbot review

[petrochenkov]

Thanks!
@bors r+

[bors]

📌 Commit 3234418 has been approved by petrochenkov

It is now in the queue for this repository.

[bors]

⌛ Testing commit 3234418 with merge dd6126e...

[bors]

☀️ Test successful - checks-actions
Approved by: petrochenkov
Pushing dd6126e to master...

[rust-timer]

Finished benchmarking commit (dd6126e): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.1%	[-1.1%, -1.1%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.6%	[-3.6%, -3.6%]	1
All ❌✅ (primary)	-	-	0

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 675.083s -> 674.182s (-0.13%)
Artifact size: 314.18 MiB -> 314.16 MiB (-0.01%)