Add coherence leak check test for impls with no higher ranked types

Author: spastorino

tests/ui/coherence/coherence-free-regions.rs

@@ -0,0 +1,12 @@
+// check-pass
+
+trait Trait<'a> {}
+
+impl Trait<'static> for u8 {}
+
+impl Trait<'_> for (u8,) {}
+impl<T> Trait<'_> for (T,) where T: for<'a> Trait<'a> {}
+//~^ WARN conflicting implementations of trait `Trait<'_>` for type `(u8,)`
+//~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+
+fn main() {}

tests/ui/coherence/coherence-free-regions.stderr

@@ -0,0 +1,14 @@
+warning: conflicting implementations of trait `Trait<'_>` for type `(u8,)`
+  --> $DIR/coherence-free-regions.rs:8:1
+   |
+LL | impl Trait<'_> for (u8,) {}
+   | ------------------------ first implementation here
+LL | impl<T> Trait<'_> for (T,) where T: for<'a> Trait<'a> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(u8,)`
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #56105 <https://github.com/rust-lang/rust/issues/56105>
+   = note: `#[warn(coherence_leak_check)]` on by default
+
+warning: 1 warning emitted
+