rustc: Allow cdylibs to link against dylibs

Previously, rustc mandated that cdylibs could only link against rlibs as
dependencies (not dylibs).
This commit disables that restriction and tests that it works in a
simple case.

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
+}