Auto merge of #8449 - jyn514:rustdoc, r=ehuss

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

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

Closes https://github.com/rust-lang/cargo/issues/8445

src/cargo/ops/cargo_compile.rs

@@ -476,7 +476,10 @@ pub fn create_bcx<'a, 'cfg>(
             }
 
             if let Some(args) = extra_args {
-                extra_compiler_args.insert(unit.clone(), args.clone());
+                extra_compiler_args
+                    .entry(unit.clone())
+                    .or_default()
+                    .extend(args);
             }
         }
     }

tests/testsuite/rustdoc.rs

@@ -39,6 +39,17 @@ 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("--markdown-no-toc")
+        .with_stderr_contains("[RUNNING] `rustdoc [..] --markdown-no-toc[..]`")
+        .run();
+}
+
 #[cargo_test]
 fn rustdoc_foo_with_bar_dependency() {
     let foo = project()