Add unstable feature flags

compiler/rustc_metadata/src/dependency_format.rs

@@ -91,9 +91,22 @@ fn calculate_type(tcx: TyCtxt<'_>, ty: CrateType) -> DependencyList {
         //
         // Treat cdylibs and staticlibs similarly. If `-C prefer-dynamic` is set,
         // the caller may be code-size conscious, but without it, it makes sense
-        // to statically link a cdylib or staticlib.
+        // to statically link a cdylib or staticlib. For staticlibs we use
-        CrateType::Dylib | CrateType::Cdylib | CrateType::Staticlib => {
+        // `-Z staticlib-prefer-dynamic` for now. This may be merged into
-            if sess.opts.cg.prefer_dynamic { Linkage::Dynamic } else { Linkage::Static }
+        // `-C prefer-dynamic` in the future.
+        CrateType::Dylib | CrateType::Cdylib => {
+            if sess.opts.cg.prefer_dynamic {
+                Linkage::Dynamic
+            } else {
+                Linkage::Static
+            }
+        }
+        CrateType::Staticlib => {
+            if sess.opts.unstable_opts.staticlib_prefer_dynamic {
+                Linkage::Dynamic
+            } else {
+                Linkage::Static
+            }
         }
 
         // If the global prefer_dynamic switch is turned off, or the final
@@ -123,9 +136,10 @@ fn calculate_type(tcx: TyCtxt<'_>, ty: CrateType) -> DependencyList {
 
             // Static executables must have all static dependencies.
             // If any are not found, generate some nice pretty errors.
-            if ty == CrateType::Executable
+            if (ty == CrateType::Staticlib && !sess.opts.unstable_opts.staticlib_allow_rdylib_deps)
+                || (ty == CrateType::Executable
                     && sess.crt_static(Some(ty))
-                && !sess.target.crt_static_allows_dylibs
+                    && !sess.target.crt_static_allows_dylibs)
             {
                 for &cnum in tcx.crates(()).iter() {
                     if tcx.dep_kind(cnum).macros_only() {

compiler/rustc_session/src/options.rs

@@ -1709,6 +1709,10 @@ options! {
     #[rustc_lint_opt_deny_field_access("use `Session::stack_protector` instead of this field")]
     stack_protector: StackProtector = (StackProtector::None, parse_stack_protector, [TRACKED],
         "control stack smash protection strategy (`rustc --print stack-protector-strategies` for details)"),
+    staticlib_allow_rdylib_deps: bool = (false, parse_bool, [TRACKED],
+        "allow staticlibs to have rust dylib dependencies"),
+    staticlib_prefer_dynamic: bool = (false, parse_bool, [TRACKED],
+        "prefer dynamic linking to static linking for staticlibs (default: no)"),
     strict_init_checks: bool = (false, parse_bool, [TRACKED],
         "control if mem::uninitialized and mem::zeroed panic on more UB"),
     strip: Strip = (Strip::None, parse_strip, [UNTRACKED],

tests/run-make/staticlib-dylib-linkage/Makefile

@@ -2,7 +2,9 @@ include ../tools.mk
 
 all:
 	$(RUSTC) -C prefer-dynamic bar.rs
-	$(RUSTC) foo.rs --crate-type staticlib --print native-static-libs 2>&1 | grep 'note: native-static-libs: ' | sed 's/note: native-static-libs: \(.*\)/\1/' > $(TMPDIR)/libs.txt
+	$(RUSTC) foo.rs --crate-type staticlib --print native-static-libs \
+		-Z staticlib-allow-rdylib-deps 2>&1 | grep 'note: native-static-libs: ' \
+		| sed 's/note: native-static-libs: \(.*\)/\1/' > $(TMPDIR)/libs.txt
 	cat $(TMPDIR)/libs.txt
 
 ifdef IS_MSVC