Updated tests.

src/librustc/infer/opaque_types/mod.rs

@@ -404,7 +404,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         // For soundness, we need to ensure that every region that is captured by the opaque type
         // but does not explicitly appear in the opaque type outlives the actual (concrete) type.
         // This allows for invariant lifetimes to be captured by opaque types as long as
-        // short-lived lifetimes are not permitted to escape and cause UB. 
+        // short-lived lifetimes are not permitted to escape and cause UB.
         let opaque_substs_regions = opaque_defn.substs.regions().collect();
         let captured_regions = concrete_ty_regions.difference(&opaque_substs_regions);
         for captured_region in captured_regions {

src/test/ui/impl-trait/issue-55608-captures-empty-region.rs

@@ -1,9 +1,9 @@
+// run-pass
+
 // This used to ICE because it creates an `impl Trait` that captures a
 // hidden empty region.
 
-#![feature(conservative_impl_trait)]
-
-fn server() -> impl FilterBase2 { //~ ERROR [E0700]
+fn server() -> impl FilterBase2 {
     segment2(|| { loop { } }).map2(|| "")
 }
 

src/test/ui/impl-trait/region-escape-via-bound-contravariant.rs

@@ -1,4 +1,4 @@
-// In contrast to `region-escape-via-bound-invariant`, in this case we
+// In contrast to `region-escape-via-bound.rs`, in this case we
 // *can* return a value of type `&'x u32`, even though `'x` does not
 // appear in the bounds. This is because `&` is contravariant, and so
 // we are *actually* returning a `&'y u32`.

src/test/ui/impl-trait/region-escape-via-bound.rs

@@ -1,5 +1,7 @@
-// Test that we do not allow the region `'x` to escape in the impl
-// trait **even though** `'y` escapes, which outlives `'x`.
+// run-pass
+
+// Test that we allow the region `'x` to escape in the impl
+// because 'y` escapes, which outlives `'x`.
 //
 // See https://github.com/rust-lang/rust/issues/46541 for more details.
 
@@ -14,10 +16,14 @@ trait Trait<'a> { }
 impl Trait<'b> for Cell<&'a u32> { }
 
 fn foo(x: Cell<&'x u32>) -> impl Trait<'y>
-    //~^ ERROR hidden type for `impl Trait` captures lifetime that does not appear in bounds [E0700]
+    // ^ hidden type for `impl Trait` captures lifetime that does not appear in bounds
+    //   because it outlives the lifetime that *does* appear in the bounds, `'y`
 where 'x: 'y
 {
     x
 }
 
-fn main() { }
+fn main() {
+    let x = 123;
+    let _ = foo(Cell::new(&x));
+}

src/test/ui/impl-trait/region-escape-via-swap.rs

@@ -0,0 +1,29 @@
+// compile-fail
+
+// See https://github.com/rust-lang/rust/pull/57870#issuecomment-457333709 for more details.
+
+trait Swap: Sized {
+    fn swap(self, other: Self);
+}
+
+impl<T> Swap for &mut T {
+    fn swap(self, other: Self) {
+        std::mem::swap(self, other);
+    }
+}
+
+// The hidden relifetimegion `'b` should not be allowed to escape this function, since it may not
+// outlive '`a`, in which case we have a dangling reference.
+fn hide<'a, 'b, T: 'static>(x: &'a mut &'b T) -> impl Swap + 'a {
+//~^ lifetime mismatch [E0623]
+    x
+}
+
+fn dangle() -> &'static [i32; 3] {
+    let mut res = &[4, 5, 6];
+    let x = [1, 2, 3];
+    hide(&mut res).swap(hide(&mut &x));
+    res
+}
+
+fn main() {}

src/test/ui/impl-trait/region-escape-via-swap.stderr

@@ -0,0 +1,12 @@
+error[E0623]: lifetime mismatch
+  --> $DIR/region-escape-via-swap.rs:17:50
+   |
+LL | fn hide<'a, 'b, T: 'static>(x: &'a mut &'b T) -> impl Swap + 'a {
+   |                                        -----     ^^^^^^^^^^^^^^
+   |                                        |         |
+   |                                        |         ...but data from `x` is returned here
+   |                                        this parameter and the return type are declared with different lifetimes...
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0623`.