Auto merge of #95474 - oli-obk:tait_ub, r=jackh726

Neither require nor imply lifetime bounds on opaque type for well formedness

The actual hidden type can live arbitrarily longer than any individual lifetime and arbitrarily shorter than all but one of the lifetimes.

fixes #86218
fixes #84305

This is a **breaking change** but it is a necessary soundness fix

3 files changed, 64 insertions, 0 deletions

diff --git a/src/test/ui/impl-trait/unactionable_diagnostic.fixed b/src/test/ui/impl-trait/unactionable_diagnostic.fixed
new file mode 100644
index 00000000000..6c2505177fe
--- /dev/null
+++ b/src/test/ui/impl-trait/unactionable_diagnostic.fixed
@@ -0,0 +1,25 @@
+// run-rustfix
+
+pub trait Trait {}
+
+pub struct Foo;
+
+impl Trait for Foo {}
+
+fn foo<'x, P>(
+    _post: P,
+    x: &'x Foo,
+) -> &'x impl Trait {
+    x
+}
+
+pub fn bar<'t, T: 't>(
+    //~^ HELP: consider adding an explicit lifetime bound...
+    post: T,
+    x: &'t Foo,
+) -> &'t impl Trait {
+    foo(post, x)
+    //~^ ERROR: the parameter type `T` may not live long enough
+}
+
+fn main() {}
diff --git a/src/test/ui/impl-trait/unactionable_diagnostic.rs b/src/test/ui/impl-trait/unactionable_diagnostic.rs
new file mode 100644
index 00000000000..bce35cbdd0d
--- /dev/null
+++ b/src/test/ui/impl-trait/unactionable_diagnostic.rs
@@ -0,0 +1,25 @@
+// run-rustfix
+
+pub trait Trait {}
+
+pub struct Foo;
+
+impl Trait for Foo {}
+
+fn foo<'x, P>(
+    _post: P,
+    x: &'x Foo,
+) -> &'x impl Trait {
+    x
+}
+
+pub fn bar<'t, T>(
+    //~^ HELP: consider adding an explicit lifetime bound...
+    post: T,
+    x: &'t Foo,
+) -> &'t impl Trait {
+    foo(post, x)
+    //~^ ERROR: the parameter type `T` may not live long enough
+}
+
+fn main() {}
diff --git a/src/test/ui/impl-trait/unactionable_diagnostic.stderr b/src/test/ui/impl-trait/unactionable_diagnostic.stderr
new file mode 100644
index 00000000000..a32004cda1a
--- /dev/null
+++ b/src/test/ui/impl-trait/unactionable_diagnostic.stderr
@@ -0,0 +1,14 @@
+error[E0309]: the parameter type `T` may not live long enough
+  --> $DIR/unactionable_diagnostic.rs:21:5
+   |
+LL |     foo(post, x)
+   |     ^^^^^^^^^^^^ ...so that the type `T` will meet its required lifetime bounds
+   |
+help: consider adding an explicit lifetime bound...
+   |
+LL | pub fn bar<'t, T: 't>(
+   |                 ++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0309`.