Respect top stage when documenting Cargo

Author: Kobzol

src/bootstrap/src/core/build_steps/doc.rs

@@ -1005,7 +1005,7 @@ macro_rules! tool_doc {
                     }
                     Mode::ToolTarget => {
                         // target tools should be documented with the in-tree compiler
-                        prepare_doc_compiler(run.builder, run.builder.host_target, 2)
+                        prepare_doc_compiler(run.builder, run.builder.host_target, run.builder.top_stage)
                     }
                     _ => {
                         panic!("Unexpected tool mode for documenting: {:?}", $mode);

src/bootstrap/src/core/builder/tests.rs

@@ -2463,12 +2463,25 @@ mod snapshot {
     }
 
     #[test]
-    fn doc_cargo() {
+    fn doc_cargo_stage_1() {
         let ctx = TestCtx::new();
         insta::assert_snapshot!(
             ctx.config("doc")
                 .path("cargo")
                 .render_steps(), @r"
+        [build] rustdoc 0 <host>
+        [doc] rustc 0 <host> -> Cargo 1 <host>
+        ");
+    }
+
+    #[test]
+    fn doc_cargo_stage_2() {
+        let ctx = TestCtx::new();
+        insta::assert_snapshot!(
+            ctx.config("doc")
+                .path("cargo")
+                .stage(2)
+                .render_steps(), @r"
         [build] llvm <host>
         [build] rustc 0 <host> -> rustc 1 <host>
         [build] rustc 1 <host> -> std 1 <host>