From 53c4e0c19ac220085638e856999639fe26cb2a00 Mon Sep 17 00:00:00 2001
From: Jeremy Fitzhardinge <jeremy@goop.org>
Date: Tue, 17 Mar 2020 19:10:56 -0700
Subject: [PATCH] Implement -Zlink-native-libraries

This implements a flag `-Zlink-native-libraries=yes/no`. If set to true/yes, or unspecified, then
native libraries referenced via `#[link]` attributes will be put on the linker line (ie, unchanged
behaviour).

If `-Zlink-native-libraries=no` is specified then rustc will not add the native libraries to the link
line. The assumption is that the outer build system driving the build already knows about the native
libraries and will specify them to the linker directly (for example via `-Clink-arg=`).

Addresses issue #70093
---
 src/librustc_codegen_ssa/back/link.rs | 13 ++++++++++---
 src/librustc_session/options.rs       |  2 ++
 src/test/ui/issues/issue-70093.rs     |  8 ++++++++
 3 files changed, 20 insertions(+), 3 deletions(-)
 create mode 100644 src/test/ui/issues/issue-70093.rs

diff --git a/src/librustc_codegen_ssa/back/link.rs b/src/librustc_codegen_ssa/back/link.rs
index 672b6e4aa4600..613833b7202a7 100644
--- a/src/librustc_codegen_ssa/back/link.rs
+++ b/src/librustc_codegen_ssa/back/link.rs
@@ -1379,10 +1379,17 @@ fn link_args<'a, B: ArchiveBuilder<'a>>(
     // link line. And finally upstream native libraries can't depend on anything
     // in this DAG so far because they're only dylibs and dylibs can only depend
     // on other dylibs (e.g., other native deps).
-    add_local_native_libraries(cmd, sess, codegen_results);
+    //
+    // If -Zlink-native-libraries=false is set, then the assumption is that an
+    // external build system already has the native dependencies defined, and it
+    // will provide them to the linker itself.
+    if sess.opts.debugging_opts.link_native_libraries.unwrap_or(true) {
+        add_local_native_libraries(cmd, sess, codegen_results);
+    }
     add_upstream_rust_crates::<B>(cmd, sess, codegen_results, crate_type, tmpdir);
-    add_upstream_native_libraries(cmd, sess, codegen_results, crate_type);
-
+    if sess.opts.debugging_opts.link_native_libraries.unwrap_or(true) {
+        add_upstream_native_libraries(cmd, sess, codegen_results, crate_type);
+    }
     // Tell the linker what we're doing.
     if crate_type != config::CrateType::Executable {
         cmd.build_dylib(out_filename);
diff --git a/src/librustc_session/options.rs b/src/librustc_session/options.rs
index 72c720d09b0bf..775259911c1dc 100644
--- a/src/librustc_session/options.rs
+++ b/src/librustc_session/options.rs
@@ -959,4 +959,6 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "link the `.rlink` file generated by `-Z no-link`"),
     new_llvm_pass_manager: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "use new LLVM pass manager"),
+    link_native_libraries: Option<bool> = (None, parse_opt_bool, [UNTRACKED],
+        "Link native libraries in the linker invocation."),
 }
diff --git a/src/test/ui/issues/issue-70093.rs b/src/test/ui/issues/issue-70093.rs
new file mode 100644
index 0000000000000..95ab86ebcb1f4
--- /dev/null
+++ b/src/test/ui/issues/issue-70093.rs
@@ -0,0 +1,8 @@
+// run-pass
+// compile-flags: -Zlink-native-libraries=no -Cdefault-linker-libraries=yes
+// ignore-windows - this will probably only work on unixish systems
+
+#[link(name = "some-random-non-existent-library", kind = "static")]
+extern "C" {}
+
+fn main() {}