Auto merge of #108121 - aliemjay:resolve-var-region, r=lcnr

always resolve to universal regions if possible

`RegionConstraintCollector::opportunistic_resolve_var`, which is used in canonicalization and projection logic, doesn't resolve the region var to an equal universal region. So if we have equated `'static == '1 == '2`, it doesn't resolve `'1` or `'2` to `'static`. Now it does!

Addresses review comment https://github.com/rust-lang/rust/pull/107376#discussion_r1093233687.

r? `@lcnr`

5 files changed, 19 insertions, 25 deletions

diff --git a/tests/rustdoc/normalize-assoc-item.rs b/tests/rustdoc/normalize-assoc-item.rs
index af7b2f955fd..c6fd5e1101e 100644
--- a/tests/rustdoc/normalize-assoc-item.rs
+++ b/tests/rustdoc/normalize-assoc-item.rs
@@ -63,12 +63,12 @@ impl<'a> Lifetimes<'a> for usize {
     type Y = &'a isize;
 }
 
-// @has 'normalize_assoc_item/fn.g.html' '//pre[@class="rust item-decl"]' "pub fn g() -> &isize"
+// @has 'normalize_assoc_item/fn.g.html' '//pre[@class="rust item-decl"]' "pub fn g() -> &'static isize"
 pub fn g() -> <usize as Lifetimes<'static>>::Y {
     &0
 }
 
-// @has 'normalize_assoc_item/constant.A.html' '//pre[@class="rust item-decl"]' "pub const A: &isize"
+// @has 'normalize_assoc_item/constant.A.html' '//pre[@class="rust item-decl"]' "pub const A: &'static isize"
 pub const A: <usize as Lifetimes<'static>>::Y = &0;
 
 // test cross-crate re-exports
diff --git a/tests/ui/traits/inductive-overflow/lifetime.rs b/tests/ui/traits/inductive-overflow/lifetime.rs
index bf536d21cf9..2f3c90dcece 100644
--- a/tests/ui/traits/inductive-overflow/lifetime.rs
+++ b/tests/ui/traits/inductive-overflow/lifetime.rs
@@ -15,9 +15,9 @@ impl<'a> Y for C<'a> {
 struct C<'a>(&'a ());
 struct X<T: Y>(T::P);
 
-impl<T: NotAuto> NotAuto for Box<T> {} //~ NOTE: required
+impl<T: NotAuto> NotAuto for Box<T> {}
+impl<T: Y> NotAuto for X<T> where T::P: NotAuto {} //~ NOTE: required
 //~^ NOTE unsatisfied trait bound introduced here
-impl<T: Y> NotAuto for X<T> where T::P: NotAuto {}
 impl<'a> NotAuto for C<'a> {}
 
 fn is_send<S: NotAuto>() {}
@@ -28,6 +28,4 @@ fn main() {
     // Should only be a few notes.
     is_send::<X<C<'static>>>();
     //~^ ERROR overflow evaluating
-    //~| 3 redundant requirements hidden
-    //~| required for
 }
diff --git a/tests/ui/traits/inductive-overflow/lifetime.stderr b/tests/ui/traits/inductive-overflow/lifetime.stderr
index 357e59991a3..7ab2864a8cf 100644
--- a/tests/ui/traits/inductive-overflow/lifetime.stderr
+++ b/tests/ui/traits/inductive-overflow/lifetime.stderr
@@ -1,18 +1,14 @@
-error[E0275]: overflow evaluating the requirement `X<C<'_>>: NotAuto`
+error[E0275]: overflow evaluating the requirement `Box<X<C<'static>>>: NotAuto`
   --> $DIR/lifetime.rs:29:5
    |
 LL |     is_send::<X<C<'static>>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^
    |
-note: required for `Box<X<C<'_>>>` to implement `NotAuto`
-  --> $DIR/lifetime.rs:18:18
+note: required for `X<C<'static>>` to implement `NotAuto`
+  --> $DIR/lifetime.rs:19:12
    |
-LL | impl<T: NotAuto> NotAuto for Box<T> {}
-   |         -------  ^^^^^^^     ^^^^^^
-   |         |
-   |         unsatisfied trait bound introduced here
-   = note: 3 redundant requirements hidden
-   = note: required for `X<C<'static>>` to implement `NotAuto`
+LL | impl<T: Y> NotAuto for X<T> where T::P: NotAuto {}
+   |            ^^^^^^^     ^^^^             ------- unsatisfied trait bound introduced here
 note: required by a bound in `is_send`
   --> $DIR/lifetime.rs:23:15
    |
diff --git a/tests/ui/wf/hir-wf-check-erase-regions.rs b/tests/ui/wf/hir-wf-check-erase-regions.rs
index 2b4b480df0a..3855f2c35c1 100644
--- a/tests/ui/wf/hir-wf-check-erase-regions.rs
+++ b/tests/ui/wf/hir-wf-check-erase-regions.rs
@@ -4,10 +4,10 @@
 pub struct Table<T, const N: usize>([Option<T>; N]);
 
 impl<'a, T, const N: usize> IntoIterator for &'a Table<T, N> {
-    type IntoIter = std::iter::Flatten<std::slice::Iter<'a, T>>; //~ ERROR `&T` is not an iterator
+    type IntoIter = std::iter::Flatten<std::slice::Iter<'a, T>>; //~ ERROR `&'a T` is not an iterator
     type Item = &'a T;
 
-    fn into_iter(self) -> Self::IntoIter { //~ ERROR `&T` is not an iterator
+    fn into_iter(self) -> Self::IntoIter { //~ ERROR `&'a T` is not an iterator
         unimplemented!()
     }
 }
diff --git a/tests/ui/wf/hir-wf-check-erase-regions.stderr b/tests/ui/wf/hir-wf-check-erase-regions.stderr
index 7bc19dd2e21..2843983c716 100644
--- a/tests/ui/wf/hir-wf-check-erase-regions.stderr
+++ b/tests/ui/wf/hir-wf-check-erase-regions.stderr
@@ -1,24 +1,24 @@
-error[E0277]: `&T` is not an iterator
+error[E0277]: `&'a T` is not an iterator
   --> $DIR/hir-wf-check-erase-regions.rs:7:21
    |
 LL |     type IntoIter = std::iter::Flatten<std::slice::Iter<'a, T>>;
-   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `&T` is not an iterator
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `&'a T` is not an iterator
    |
-   = help: the trait `Iterator` is not implemented for `&T`
+   = help: the trait `Iterator` is not implemented for `&'a T`
    = help: the trait `Iterator` is implemented for `&mut I`
-   = note: required for `&T` to implement `IntoIterator`
+   = note: required for `&'a T` to implement `IntoIterator`
 note: required by a bound in `Flatten`
   --> $SRC_DIR/core/src/iter/adapters/flatten.rs:LL:COL
 
-error[E0277]: `&T` is not an iterator
+error[E0277]: `&'a T` is not an iterator
   --> $DIR/hir-wf-check-erase-regions.rs:10:27
    |
 LL |     fn into_iter(self) -> Self::IntoIter {
-   |                           ^^^^^^^^^^^^^^ `&T` is not an iterator
+   |                           ^^^^^^^^^^^^^^ `&'a T` is not an iterator
    |
-   = help: the trait `Iterator` is not implemented for `&T`
+   = help: the trait `Iterator` is not implemented for `&'a T`
    = help: the trait `Iterator` is implemented for `&mut I`
-   = note: required for `&T` to implement `IntoIterator`
+   = note: required for `&'a T` to implement `IntoIterator`
 note: required by a bound in `Flatten`
   --> $SRC_DIR/core/src/iter/adapters/flatten.rs:LL:COL