Make UniqueArc invariant for soundness

3 files changed, 43 insertions, 1 deletions

diff --git a/tests/ui/variance/variance-uniquearc.rs b/tests/ui/variance/variance-uniquearc.rs
new file mode 100644
index 00000000000..03589512388
--- /dev/null
+++ b/tests/ui/variance/variance-uniquearc.rs
@@ -0,0 +1,27 @@
+// regression test of https://github.com/rust-lang/rust/pull/133572#issuecomment-2543007164
+// see also the test for UniqueRc` in variance-uniquerc.rs
+//
+// inline comments explain how this code *would* compile if UniqueArc was still covariant
+
+#![feature(unique_rc_arc)]
+
+use std::sync::UniqueArc;
+
+fn extend_lifetime<'a, 'b>(x: &'a str) -> &'b str {
+    let r = UniqueArc::new(""); // UniqueArc<&'static str>
+    let w = UniqueArc::downgrade(&r); // Weak<&'static str>
+    let mut r = r; // [IF COVARIANT]: ==>> UniqueArc<&'a str>
+    *r = x; // assign the &'a str
+    let _r = UniqueArc::into_arc(r); // Arc<&'a str>, but we only care to activate the weak
+    let r = w.upgrade().unwrap(); // Arc<&'static str>
+    *r // &'static str, coerces to &'b str
+    //~^ ERROR lifetime may not live long enough
+}
+
+fn main() {
+    let s = String::from("Hello World!");
+    let r = extend_lifetime(&s);
+    println!("{r}");
+    drop(s);
+    println!("{r}");
+}
diff --git a/tests/ui/variance/variance-uniquearc.stderr b/tests/ui/variance/variance-uniquearc.stderr
new file mode 100644
index 00000000000..55076dae732
--- /dev/null
+++ b/tests/ui/variance/variance-uniquearc.stderr
@@ -0,0 +1,15 @@
+error: lifetime may not live long enough
+  --> $DIR/variance-uniquearc.rs:17:5
+   |
+LL | fn extend_lifetime<'a, 'b>(x: &'a str) -> &'b str {
+   |                    --  -- lifetime `'b` defined here
+   |                    |
+   |                    lifetime `'a` defined here
+...
+LL |     *r // &'static str, coerces to &'b str
+   |     ^^ function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
+   |
+   = help: consider adding the following bound: `'a: 'b`
+
+error: aborting due to 1 previous error
+
diff --git a/tests/ui/variance/variance-uniquerc.rs b/tests/ui/variance/variance-uniquerc.rs
index 0c395ab06ea..2e9738f66dc 100644
--- a/tests/ui/variance/variance-uniquerc.rs
+++ b/tests/ui/variance/variance-uniquerc.rs
@@ -1,5 +1,5 @@
 // regression test of https://github.com/rust-lang/rust/pull/133572#issuecomment-2543007164
-// we should also test UniqueArc once implemented
+// see also the test for UniqueArc in variance-uniquearc.rs
 //
 // inline comments explain how this code *would* compile if UniqueRc was still covariant