Don't overwrite existing rustdoc args with --document-private-items

Instead, add that flag in addition to any existing flags.

Author: jyn514

src/cargo/ops/cargo_compile.rs

@@ -475,8 +475,13 @@ pub fn create_bcx<'a, 'cfg>(
                 extra_args = Some(args);
             }
 
-            if let Some(args) = extra_args {
-                extra_compiler_args.insert(unit.clone(), args.clone());
+            if let Some(mut args) = extra_args {
+                match extra_compiler_args.get_mut(&unit) {
+                    None => {
+                        extra_compiler_args.insert(unit.clone(), args);
+                    }
+                    Some(existing) => existing.append(&mut args),
+                }
             }
         }
     }

tests/testsuite/rustdoc.rs

@@ -39,6 +39,19 @@ fn rustdoc_args() {
         .run();
 }
 
+#[cargo_test]
+fn rustdoc_binary_args_passed() {
+    let p = project().file("src/main.rs", "").build();
+
+    p.cargo("rustdoc -v")
+        .arg("--")
+        .arg("--theme")
+        .arg("dark")
+        .with_stderr_contains("[RUNNING] `rustdoc [..] --theme dark[..]`")
+        .with_status(101)
+        .run();
+}
+
 #[cargo_test]
 fn rustdoc_foo_with_bar_dependency() {
     let foo = project()