Add test for garbage collection of encapsulation symbols

Author: Kobzol

tests/ui/no-gc-encapsulation-symbols.rs

@@ -0,0 +1,25 @@
+// This test checks that encapsulation symbols are not garbage collected by the linker.
+// LLD will remove them by default, so this test checks that we pass `-znostart-stop-gc` to LLD
+// to avoid that behavior. Without that flag, the test should fail.
+// This test is inspired by the behavior of the linkme crate.
+//
+//@ build-pass
+//@ only-x86_64
+
+unsafe extern "Rust" {
+    // The __start_ section name is magical for the linker,
+    // It will put link sections named EXTERNFNS after it.
+    #[link_name = "__start_EXTERNFNS"]
+    static SECTION_START: fn();
+}
+
+#[used]
+#[unsafe(link_section = "EXTERNFNS")]
+static EXTERN_FN_LOCAL: fn() = extern_fn;
+
+fn extern_fn() {}
+
+fn main() {
+    // We need to reference the SECTION_START symbol to avoid it being garbage collected
+    let slice = unsafe { SECTION_START };
+}