diff --git a/src/bootstrap/tool.rs b/src/bootstrap/tool.rs
index d30532ef3c6..3c9a154da9a 100644
--- a/src/bootstrap/tool.rs
+++ b/src/bootstrap/tool.rs
@@ -595,7 +595,7 @@ impl Step for Rustdoc {
             features.push("jemalloc".to_string());
         }
 
-        let cargo = prepare_tool_cargo(
+        let mut cargo = prepare_tool_cargo(
             builder,
             build_compiler,
             Mode::ToolRustc,
@@ -606,6 +606,10 @@ impl Step for Rustdoc {
             features.as_slice(),
         );
 
+        if builder.config.rustc_parallel {
+            cargo.rustflag("--cfg=parallel_compiler");
+        }
+
         let msg = tooling_output(
             Mode::ToolRustc,
             "rustdoc",