From c7ea022166d2b6d55c3f69bbf69a31b0d7b053e2 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Mon, 4 Aug 2025 16:24:41 +0000
Subject: [PATCH] Enforce tail call type is related to body return type in
 borrowck

---
 compiler/rustc_borrowck/src/type_check/mod.rs    | 10 +++++++---
 .../ret-ty-borrowck-constraints.rs               | 16 ++++++++++++++++
 .../ret-ty-borrowck-constraints.stderr           | 10 ++++++++++
 3 files changed, 33 insertions(+), 3 deletions(-)
 create mode 100644 tests/ui/explicit-tail-calls/ret-ty-borrowck-constraints.rs
 create mode 100644 tests/ui/explicit-tail-calls/ret-ty-borrowck-constraints.stderr

diff --git a/compiler/rustc_borrowck/src/type_check/mod.rs b/compiler/rustc_borrowck/src/type_check/mod.rs
index f5fedbf95c1c..91973f33bd32 100644
--- a/compiler/rustc_borrowck/src/type_check/mod.rs
+++ b/compiler/rustc_borrowck/src/type_check/mod.rs
@@ -845,9 +845,13 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                     );
                 }
 
-                if let TerminatorKind::Call { destination, target, .. } = term.kind {
-                    self.check_call_dest(term, &sig, destination, target, term_location);
-                }
+                let (destination, target) =
+                    if let TerminatorKind::Call { destination, target, .. } = term.kind {
+                        (destination, target)
+                    } else {
+                        (RETURN_PLACE.into(), Some(BasicBlock::ZERO))
+                    };
+                self.check_call_dest(term, &sig, destination, target, term_location);
 
                 // The ordinary liveness rules will ensure that all
                 // regions in the type of the callee are live here. We
diff --git a/tests/ui/explicit-tail-calls/ret-ty-borrowck-constraints.rs b/tests/ui/explicit-tail-calls/ret-ty-borrowck-constraints.rs
new file mode 100644
index 000000000000..111ae849c0f6
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/ret-ty-borrowck-constraints.rs
@@ -0,0 +1,16 @@
+#![feature(explicit_tail_calls)]
+#![expect(incomplete_features)]
+
+fn link(x: &str) -> &'static str {
+    become passthrough(x);
+    //~^ ERROR lifetime may not live long enough
+}
+
+fn passthrough<T>(t: T) -> T { t }
+
+fn main() {
+    let x = String::from("hello, world");
+    let s = link(&x);
+    drop(x);
+    println!("{s}");
+}
diff --git a/tests/ui/explicit-tail-calls/ret-ty-borrowck-constraints.stderr b/tests/ui/explicit-tail-calls/ret-ty-borrowck-constraints.stderr
new file mode 100644
index 000000000000..26a8e1f0122a
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/ret-ty-borrowck-constraints.stderr
@@ -0,0 +1,10 @@
+error: lifetime may not live long enough
+  --> $DIR/ret-ty-borrowck-constraints.rs:5:5
+   |
+LL | fn link(x: &str) -> &'static str {
+   |            - let's call the lifetime of this reference `'1`
+LL |     become passthrough(x);
+   |     ^^^^^^^^^^^^^^^^^^^^^ returning this value requires that `'1` must outlive `'static`
+
+error: aborting due to 1 previous error
+