Remove invalid assertion back::link::from add_upstream_rust_crates().

src/librustc_codegen_llvm/back/link.rs

@@ -1396,10 +1396,6 @@ fn add_upstream_rust_crates(cmd: &mut dyn Linker,
 
     // Same thing as above, but for dynamic crates instead of static crates.
     fn add_dynamic_crate(cmd: &mut dyn Linker, sess: &Session, cratepath: &Path) {
-        // If we're performing LTO, then it should have been previously required
-        // that all upstream rust dependencies were available in an rlib format.
-        assert!(!are_upstream_rust_objects_already_included(sess));
-
         // Just need to tell the linker about where the library lives and
         // what its name is
         let parent = cratepath.parent();

src/test/run-make-fulldeps/lto-dylib-dep/Makefile

@@ -0,0 +1,10 @@
+-include ../tools.mk
+
+# Test that we don't run into an assertion when using a Rust dylib dependency
+# while compiling with full LTO.
+# See https://github.com/rust-lang/rust/issues/59137
+
+all:
+	$(RUSTC) a_dylib.rs --crate-type=dylib -C prefer-dynamic
+	$(RUSTC) main.rs -C lto
+	$(call RUN,main)

src/test/run-make-fulldeps/lto-dylib-dep/a_dylib.rs

@@ -0,0 +1,4 @@
+
+pub fn foo() {
+    println!("bar");
+}

src/test/run-make-fulldeps/lto-dylib-dep/main.rs

@@ -0,0 +1,6 @@
+
+extern crate a_dylib;
+
+fn main() {
+    a_dylib::foo();
+}