Rollup merge of #126004 - compiler-errors:captures-soundness-test, r=lcnr

Add another test for hidden types capturing lifetimes that outlive but arent mentioned in substs

Another test to make sure future implementations of https://github.com/rust-lang/rust/pull/116040 don't have any subtle unsoundness 🤔

r? types

tests/ui/impl-trait/alias-liveness/tait-hidden-erased-unsoundness-2.rs

@@ -0,0 +1,38 @@
+// This test should never pass!
+
+#![feature(type_alias_impl_trait)]
+
+pub trait Captures<'a> {}
+impl<T> Captures<'_> for T {}
+
+pub struct MyTy<'a, 'b>(Option<*mut &'a &'b ()>);
+unsafe impl Send for MyTy<'_, 'static> {}
+
+pub mod step1 {
+    use super::*;
+    pub type Step1<'a, 'b: 'a> = impl Sized + Captures<'b> + 'a;
+    pub fn step1<'a, 'b: 'a>() -> Step1<'a, 'b> {
+        MyTy::<'a, 'b>(None)
+    }
+}
+
+pub mod step2 {
+    pub type Step2<'a> = impl Send + 'a;
+
+    // Although `Step2` is WF at the definition site, it's not WF in its
+    // declaration site (above). We check this in `check_opaque_meets_bounds`,
+    // which must remain sound.
+    pub fn step2<'a, 'b: 'a>() -> Step2<'a>
+        where crate::step1::Step1<'a, 'b>: Send
+    {
+        crate::step1::step1::<'a, 'b>()
+        //~^ ERROR hidden type for `Step2<'a>` captures lifetime that does not appear in bounds
+    }
+}
+
+fn step3<'a, 'b>() {
+    fn is_send<T: Send>() {}
+    is_send::<crate::step2::Step2::<'a>>();
+}
+
+fn main() {}

tests/ui/impl-trait/alias-liveness/tait-hidden-erased-unsoundness-2.stderr

@@ -0,0 +1,15 @@
+error[E0700]: hidden type for `Step2<'a>` captures lifetime that does not appear in bounds
+  --> $DIR/tait-hidden-erased-unsoundness-2.rs:28:9
+   |
+LL |     pub type Step2<'a> = impl Send + 'a;
+   |                          -------------- opaque type defined here
+...
+LL |     pub fn step2<'a, 'b: 'a>() -> Step2<'a>
+   |                      -- hidden type `Step1<'a, 'b>` captures the lifetime `'b` as defined here
+...
+LL |         crate::step1::step1::<'a, 'b>()
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0700`.