Add test for -znostart-stop-gc in LLD

Author: Kobzol

tests/run-make/rust-lld-encapsulation-symbol/dep.rs

@@ -0,0 +1,40 @@
+#![crate_type = "rlib"]
+
+// This file contains a very minimalistic reproduction of what the
+// linkme (https://github.com/dtolnay/linkme) crate does.
+// This linker behavior fails with LLD without -znostart-stop-gc.
+
+type Item = fn(u32);
+
+pub struct DistributedSlice {
+    section_start: *const Item,
+    section_stop: *const Item,
+}
+
+unsafe impl Sync for DistributedSlice {}
+
+impl DistributedSlice {
+    pub fn get_slice(&self) -> &'static [Item] {
+        let stride = std::mem::size_of::<Item>();
+        let start = self.section_start;
+        let stop = self.section_stop;
+        let byte_offset = stop as usize - start as usize;
+        let len = byte_offset / stride;
+        unsafe { std::slice::from_raw_parts(start, len) }
+    }
+}
+
+pub static EXTERN_FNS: DistributedSlice = {
+    unsafe extern "Rust" {
+        // The __start_ and __stop_ section names are magical for the linker,
+        // It will put link sections named EXTERNFNS in-between them.
+        #[link_name = "__start_EXTERNFNS"]
+        static SECTION_START: Item;
+        #[link_name = "__stop_EXTERNFNS"]
+        static SECTION_STOP: Item;
+    }
+    DistributedSlice {
+        section_start: &raw const SECTION_START,
+        section_stop: &raw const SECTION_STOP
+    }
+};

tests/run-make/rust-lld-encapsulation-symbol/foo.rs

@@ -0,0 +1,19 @@
+extern crate dep;
+
+use dep::EXTERN_FNS;
+
+// Add a function to the distributed slice contained in
+// the EXTERNFNS link section.
+#[used]
+#[unsafe(link_section = "EXTERNFNS")]
+pub static EXTERN_FN_LOCAL: fn(u32) = extern_fn;
+
+fn extern_fn(x: u32) {
+    println!("{x}");
+}
+
+fn main() {
+    for func in EXTERN_FNS.get_slice() {
+        func(42);
+    }
+}

tests/run-make/rust-lld-encapsulation-symbol/rmake.rs

@@ -0,0 +1,16 @@
+// This test checks that when certain link sections are not garbage
+// collected by LLD. The compiler needs to use the -znostart-stop-gc flag for LLD,
+// otherwise this test will fail.
+// The test replicates the behavior of the linkme crate.
+//
+//@ needs-rust-lld
+//@ only-x86_64-unknown-linux-gnu
+
+use run_make_support::{run, rustc};
+
+fn main() {
+    rustc().input("dep.rs").run();
+    rustc().input("foo.rs").run();
+
+    run("foo").assert_stdout_equals("42\n");
+}