Implement -Zlink-directives=yes/no

`-Zlink-directives=no` will ignored `#[link]` directives while compiling a
crate, so nothing is emitted into the crate's metadata.  The assumption is
that the build system already knows about the crate's native dependencies
and can provide them at link time without these directives.

This is another way to address issue # #70093, which is currently addressed
by `-Zlink-native-libraries` (implemented in #70095). The latter is
implemented at link time, which has the effect of ignoring `#[link]`
in *every* crate. This makes it a very large hammer as it requires all
native dependencies to be known to the build system to be at all usable,
including those in sysroot libraries. I think this means its effectively
unused, and definitely under-used.

Being able to control this on a crate-by-crate basis should make it much
easier to apply when needed.

I'm not sure if we need both mechanisms, but we can decide that later.

6 files changed, 20 insertions, 1 deletions

diff --git a/compiler/rustc_interface/src/tests.rs b/compiler/rustc_interface/src/tests.rs
index bfb1e822a26..f9f4c0ec708 100644
--- a/compiler/rustc_interface/src/tests.rs
+++ b/compiler/rustc_interface/src/tests.rs
@@ -756,6 +756,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(instrument_coverage, Some(InstrumentCoverage::All));
     tracked!(instrument_mcount, true);
     tracked!(instrument_xray, Some(InstrumentXRay::default()));
+    tracked!(link_directives, false);
     tracked!(link_only, true);
     tracked!(llvm_plugins, vec![String::from("plugin_name")]);
     tracked!(location_detail, LocationDetail { file: true, line: false, column: false });
diff --git a/compiler/rustc_metadata/src/native_libs.rs b/compiler/rustc_metadata/src/native_libs.rs
index d823989bb02..91cb35f6724 100644
--- a/compiler/rustc_metadata/src/native_libs.rs
+++ b/compiler/rustc_metadata/src/native_libs.rs
@@ -103,8 +103,13 @@ impl<'tcx> Collector<'tcx> {
         }
 
         // Process all of the #[link(..)]-style arguments
-        let sess = &self.tcx.sess;
+        let sess = self.tcx.sess;
         let features = self.tcx.features();
+
+        if !sess.opts.unstable_opts.link_directives {
+            return;
+        }
+
         for m in self.tcx.hir().attrs(it.hir_id()).iter().filter(|a| a.has_name(sym::link)) {
             let Some(items) = m.meta_item_list() else {
                 continue;
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index 1f3eb8d4832..eb8e54fa0bd 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -1489,6 +1489,8 @@ options! {
         "keep hygiene data after analysis (default: no)"),
     layout_seed: Option<u64> = (None, parse_opt_number, [TRACKED],
         "seed layout randomization"),
+    link_directives: bool = (true, parse_bool, [TRACKED],
+        "honour #[link] directives in crates (default: yes)"),
     link_native_libraries: bool = (true, parse_bool, [UNTRACKED],
         "link native libraries in the linker invocation (default: yes)"),
     link_only: bool = (false, parse_bool, [TRACKED],
diff --git a/tests/rustdoc-ui/z-help.stdout b/tests/rustdoc-ui/z-help.stdout
index 58b2f92d150..55876326e5e 100644
--- a/tests/rustdoc-ui/z-help.stdout
+++ b/tests/rustdoc-ui/z-help.stdout
@@ -81,6 +81,7 @@
          Multiple options can be combined with commas.
     -Z                     keep-hygiene-data=val -- keep hygiene data after analysis (default: no)
     -Z                           layout-seed=val -- seed layout randomization
+    -Z                       link-directives=val -- honour #[link] directives in crates (default: yes)
     -Z                 link-native-libraries=val -- link native libraries in the linker invocation (default: yes)
     -Z                             link-only=val -- link the `.rlink` file generated by `-Z no-link` (default: no)
     -Z                          llvm-plugins=val -- a list LLVM plugins to enable (space separated)
diff --git a/tests/ui/issues/issue-70093/issue-70093-link-directives.rs b/tests/ui/issues/issue-70093/issue-70093-link-directives.rs
new file mode 100644
index 00000000000..83f9b16c408
--- /dev/null
+++ b/tests/ui/issues/issue-70093/issue-70093-link-directives.rs
@@ -0,0 +1,10 @@
+// run-pass
+// compile-flags: -Zlink-directives=no
+// ignore-windows - this will probably only work on unixish systems
+// ignore-fuchsia - missing __libc_start_main for some reason (#84733)
+// ignore-cross-compile - default-linker-libraries=yes doesn't play well with cross compiling
+
+#[link(name = "some-random-non-existent-library", kind = "static")]
+extern "C" {}
+
+fn main() {}
diff --git a/tests/ui/issues/issue-70093.rs b/tests/ui/issues/issue-70093/issue-70093.rs
index 86459dc904a..86459dc904a 100644
--- a/tests/ui/issues/issue-70093.rs
+++ b/tests/ui/issues/issue-70093/issue-70093.rs