rustc: Allow cdylibs to link against dylibs

src/librustc_metadata/dependency_format.rs

@@ -83,14 +83,17 @@ fn calculate_type(tcx: TyCtxt<'_>, ty: config::CrateType) -> DependencyList {
     }
 
     let preferred_linkage = match ty {
-        // cdylibs must have all static dependencies.
-        config::CrateType::Cdylib => Linkage::Static,
-
         // Generating a dylib without `-C prefer-dynamic` means that we're going
         // to try to eagerly statically link all dependencies. This is normally
         // done for end-product dylibs, not intermediate products.
-        config::CrateType::Dylib if !sess.opts.cg.prefer_dynamic => Linkage::Static,
-        config::CrateType::Dylib => Linkage::Dynamic,
+        //
+        // Treat cdylibs 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.
+        config::CrateType::Dylib | config::CrateType::Cdylib if !sess.opts.cg.prefer_dynamic => {
+            Linkage::Static
+        }
+        config::CrateType::Dylib | config::CrateType::Cdylib => Linkage::Dynamic,
 
         // If the global prefer_dynamic switch is turned off, or the final
         // executable will be statically linked, prefer static crate linkage.
@@ -122,10 +125,9 @@ fn calculate_type(tcx: TyCtxt<'_>, ty: config::CrateType) -> DependencyList {
             return v;
         }
 
-        // Staticlibs, cdylibs, and static executables must have all static
-        // dependencies. If any are not found, generate some nice pretty errors.
-        if ty == config::CrateType::Cdylib
-            || ty == config::CrateType::Staticlib
+        // Staticlibs and static executables must have all static dependencies.
+        // If any are not found, generate some nice pretty errors.
+        if ty == config::CrateType::Staticlib
             || (ty == config::CrateType::Executable
                 && sess.crt_static()
                 && !sess.target.target.options.crt_static_allows_dylibs)

src/test/run-make-fulldeps/cdylib-dylib-linkage/Makefile

@@ -0,0 +1,27 @@
+include ../tools.mk
+
+TARGET_SYSROOT := $(shell $(RUSTC) --print sysroot)/lib/rustlib/$(TARGET)/lib
+
+ifdef IS_MSVC
+LIBSTD := $(wildcard $(TARGET_SYSROOT)/libstd-*.dll.lib)
+else
+LIBSTD := $(wildcard $(TARGET_SYSROOT)/$(call DYLIB_GLOB,std))
+STD := $(basename $(patsubst lib%,%, $(notdir $(LIBSTD))))
+endif
+
+all: $(call RUN_BINFILE,foo)
+	$(call RUN,foo)
+
+ifdef IS_MSVC
+CLIBS := $(TMPDIR)/foo.dll.lib $(TMPDIR)/bar.dll.lib $(LIBSTD)
+$(call RUN_BINFILE,foo): $(call DYLIB,foo)
+	$(CC) $(CFLAGS) foo.c $(CLIBS) $(call OUT_EXE,foo)
+else
+CLIBS := -lfoo -lbar -l$(STD) -L $(TMPDIR) -L $(TARGET_SYSROOT)
+$(call RUN_BINFILE,foo): $(call DYLIB,foo)
+	$(CC) $(CFLAGS) foo.c $(CLIBS) -o $(call RUN_BINFILE,foo)
+endif
+
+$(call DYLIB,foo):
+	$(RUSTC) -C prefer-dynamic bar.rs
+	$(RUSTC) foo.rs

src/test/run-make-fulldeps/cdylib-dylib-linkage/bar.rs

@@ -0,0 +1,5 @@
+#![crate_type = "dylib"]
+
+pub fn bar() {
+    println!("hello!");
+}

src/test/run-make-fulldeps/cdylib-dylib-linkage/foo.c

@@ -0,0 +1,10 @@
+#include <assert.h>
+
+extern void foo();
+extern unsigned bar(unsigned a, unsigned b);
+
+int main() {
+  foo();
+  assert(bar(1, 2) == 3);
+  return 0;
+}

src/test/run-make-fulldeps/cdylib-dylib-linkage/foo.rs

@@ -0,0 +1,13 @@
+#![crate_type = "cdylib"]
+
+extern crate bar;
+
+#[no_mangle]
+pub extern fn foo() {
+    bar::bar();
+}
+
+#[no_mangle]
+pub extern fn bar(a: u32, b: u32) -> u32 {
+    a + b
+}