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Auto merge of #71896 - spastorino:existential-assoc-types-variance, r=nikomatsakis

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Relate existential associated types with variance Invariant

Fixes #71550 #72315

r? @nikomatsakis

The test case reported in that issue now errors with the following message ...

```
error[E0495]: cannot infer an appropriate lifetime for lifetime parameter 'a in function call due to conflicting requirements
  --> /tmp/test.rs:25:5
   |
25 |     bad(&Bar(PhantomData), x)
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: first, the lifetime cannot outlive the lifetime `'a` as defined on the function body at 24:11...
  --> /tmp/test.rs:24:11
   |
24 | fn extend<'a, T>(x: &'a T) -> &'static T {
   |           ^^
note: ...so that reference does not outlive borrowed content
  --> /tmp/test.rs:25:28
   |
25 |     bad(&Bar(PhantomData), x)
   |                            ^
   = note: but, the lifetime must be valid for the static lifetime...
note: ...so that the types are compatible
  --> /tmp/test.rs:25:9
   |
25 |     bad(&Bar(PhantomData), x)
   |         ^^^^^^^^^^^^^^^^^
   = note: expected  `&'static T`
              found  `&T`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0495`.
```

I could also add that test case if we want to have a weaponized one too.
This commit is contained in:
bors 2020-06-11 04:58:48 +00:00
commit 3ddf48053e
6 changed files with 52 additions and 5 deletions
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4
src/test/ui/issues/issue-20605.stderr
View file

@ -1,10 +1,10 @@
error[E0277]: the size for values of type `dyn std::iter::Iterator<Item = &mut u8>` cannot be known at compilation time
error[E0277]: the size for values of type `dyn std::iter::Iterator<Item = &'a mut u8>` cannot be known at compilation time
--> $DIR/issue-20605.rs:2:17
|
LL | for item in *things { *item = 0 }
| ^^^^^^^ doesn't have a size known at compile-time
|
= help: the trait `std::marker::Sized` is not implemented for `dyn std::iter::Iterator<Item = &mut u8>`
= help: the trait `std::marker::Sized` is not implemented for `dyn std::iter::Iterator<Item = &'a mut u8>`
= note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
= note: required by `std::iter::IntoIterator::into_iter`