diff --git a/src/bootstrap/src/core/build_steps/compile.rs b/src/bootstrap/src/core/build_steps/compile.rs
index d40a3ea4c88..5bb7dc5b482 100644
--- a/src/bootstrap/src/core/build_steps/compile.rs
+++ b/src/bootstrap/src/core/build_steps/compile.rs
@@ -1130,12 +1130,6 @@ pub fn rustc_cargo_env(
         cargo.env("CFG_DEFAULT_LINKER", s);
     }
 
-    if builder.config.rustc_parallel {
-        // keep in sync with `bootstrap/lib.rs:Build::rustc_features`
-        // `cfg` option for rustc, `features` option for cargo, for conditional compilation
-        cargo.rustflag("--cfg=parallel_compiler");
-        cargo.rustdocflag("--cfg=parallel_compiler");
-    }
     if builder.config.rust_verify_llvm_ir {
         cargo.env("RUSTC_VERIFY_LLVM_IR", "1");
     }
diff --git a/src/bootstrap/src/core/build_steps/tool.rs b/src/bootstrap/src/core/build_steps/tool.rs
index deab3fce54c..1edf65d28b7 100644
--- a/src/bootstrap/src/core/build_steps/tool.rs
+++ b/src/bootstrap/src/core/build_steps/tool.rs
@@ -469,7 +469,7 @@ fn run(self, builder: &Builder<'_>) -> PathBuf {
             features.push("jemalloc".to_string());
         }
 
-        let mut cargo = prepare_tool_cargo(
+        let cargo = prepare_tool_cargo(
             builder,
             build_compiler,
             Mode::ToolRustc,
@@ -480,10 +480,6 @@ fn run(self, builder: &Builder<'_>) -> PathBuf {
             features.as_slice(),
         );
 
-        if builder.config.rustc_parallel {
-            cargo.rustflag("--cfg=parallel_compiler");
-        }
-
         let _guard = builder.msg_tool(
             Mode::ToolRustc,
             "rustdoc",
@@ -732,7 +728,7 @@ fn run(self, builder: &Builder<'_>) -> PathBuf {
         builder.ensure(compile::Std::new(self.compiler, self.compiler.host));
         builder.ensure(compile::Rustc::new(self.compiler, self.target));
 
-        let mut cargo = prepare_tool_cargo(
+        let cargo = prepare_tool_cargo(
             builder,
             self.compiler,
             Mode::ToolRustc,
@@ -743,10 +739,6 @@ fn run(self, builder: &Builder<'_>) -> PathBuf {
             &self.extra_features,
         );
 
-        if builder.config.rustc_parallel {
-            cargo.rustflag("--cfg=parallel_compiler");
-        }
-
         builder.run(&mut cargo.into());
 
         let tool_out = builder
diff --git a/src/bootstrap/src/core/builder.rs b/src/bootstrap/src/core/builder.rs
index c97d8121e0c..4b2ee1d050a 100644
--- a/src/bootstrap/src/core/builder.rs
+++ b/src/bootstrap/src/core/builder.rs
@@ -2100,6 +2100,15 @@ fn cargo(
             rustflags.arg("-Zinline-mir");
         }
 
+        if builder.config.rustc_parallel
+            && matches!(mode, Mode::ToolRustc | Mode::Rustc | Mode::Codegen)
+        {
+            // keep in sync with `bootstrap/lib.rs:Build::rustc_features`
+            // `cfg` option for rustc, `features` option for cargo, for conditional compilation
+            rustflags.arg("--cfg=parallel_compiler");
+            rustdocflags.arg("--cfg=parallel_compiler");
+        }
+
         // set rustc args passed from command line
         let rustc_args =
             self.config.cmd.rustc_args().iter().map(|s| s.to_string()).collect::<Vec<_>>();