Prevent generators from being movable

compiler/rustc_ast_lowering/src/expr.rs

@@ -715,7 +715,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             body,
             fn_decl_span: self.lower_span(span),
             fn_arg_span: None,
-            movability: Some(hir::Movability::Static),
+            movability: Some(Movability::Movable),
             constness: hir::Constness::NotConst,
         }))
     }

tests/ui/coroutine/self_referential_gen_block.rs

@@ -0,0 +1,17 @@
+// compile-flags: --edition 2024 -Zunstable-options
+#![feature(gen_blocks)]
+//! This test checks that we don't allow self-referential generators
+
+fn main() {
+    let mut x = {
+        let mut x = gen {
+            let y = 42;
+            let z = &y; //~ ERROR: borrow may still be in use when coroutine yields
+            yield 43;
+            panic!("{z}");
+        };
+        x.next();
+        Box::new(x)
+    };
+    x.next();
+}

tests/ui/coroutine/self_referential_gen_block.stderr

@@ -0,0 +1,11 @@
+error[E0626]: borrow may still be in use when coroutine yields
+  --> $DIR/self_referential_gen_block.rs:9:21
+   |
+LL |             let z = &y;
+   |                     ^^
+LL |             yield 43;
+   |             -------- possible yield occurs here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0626`.