Use Default visibility for rustc-generated C symbol declarations

Non-default visibilities should only be used for definitions, not
declarations, otherwise linking can fail.

Co-authored-by: Collin Baker <collinbaker@chromium.org>

compiler/rustc_codegen_llvm/src/declare.rs

@@ -84,10 +84,9 @@ pub(crate) fn declare_cfn(
         unnamed: llvm::UnnamedAddr,
         fn_type: &'ll Type,
     ) -> &'ll Value {
-        // Declare C ABI functions with the visibility used by C by default.
+        // Visibility should always be default for declarations, otherwise the linker may report an
-        let visibility = Visibility::from_generic(self.tcx.sess.default_visibility());
+        // error.
-
+        declare_raw_fn(self, name, llvm::CCallConv, unnamed, Visibility::Default, fn_type)
-        declare_raw_fn(self, name, llvm::CCallConv, unnamed, visibility, fn_type)
     }
 
     /// Declare an entry Function

tests/codegen/default-visibility.rs

@@ -31,3 +31,19 @@
 // PROTECTED:    @{{.*}}default_visibility{{.*}}tested_symbol{{.*}} = protected constant
 // INTERPOSABLE: @{{.*}}default_visibility{{.*}}tested_symbol{{.*}} = constant
 // DEFAULT:      @{{.*}}default_visibility{{.*}}tested_symbol{{.*}} = constant
+
+pub fn do_memcmp(left: &[u8], right: &[u8]) -> i32 {
+    left.cmp(right) as i32
+}
+
+// CHECK: define {{.*}} @{{.*}}do_memcmp{{.*}} {
+// CHECK: }
+
+// `do_memcmp` should invoke core::intrinsic::compare_bytes which emits a call
+// to the C symbol `memcmp` (at least on x86_64-unknown-linux-gnu). This symbol
+// should *not* be declared hidden or protected.
+
+// HIDDEN:       declare i32 @memcmp
+// PROTECTED:    declare i32 @memcmp
+// INTERPOSABLE: declare i32 @memcmp
+// DEFAULT:      declare i32 @memcmp