diff --git a/src/tools/compiletest/src/runtest.rs b/src/tools/compiletest/src/runtest.rs
index ff275df9b31c0..c85864631a0a3 100644
--- a/src/tools/compiletest/src/runtest.rs
+++ b/src/tools/compiletest/src/runtest.rs
@@ -1000,8 +1000,15 @@ impl<'test> TestCx<'test> {
 
     /// `root_out_dir` and `root_testpaths` refer to the parameters of the actual test being run.
     /// Auxiliaries, no matter how deep, have the same root_out_dir and root_testpaths.
-    fn document(&self, root_out_dir: &Utf8Path, root_testpaths: &TestPaths) -> ProcRes {
+    fn document(
+        &self,
+        root_out_dir: &Utf8Path,
+        root_testpaths: &TestPaths,
+        kind: DocKind,
+    ) -> ProcRes {
         if self.props.build_aux_docs {
+            assert_eq!(kind, DocKind::Html, "build-aux-docs doesn't make sense for rustdoc json");
+
             for rel_ab in &self.props.aux.builds {
                 let aux_testpaths = self.compute_aux_test_paths(root_testpaths, rel_ab);
                 let props_for_aux =
@@ -1018,7 +1025,7 @@ impl<'test> TestCx<'test> {
                 create_dir_all(aux_cx.output_base_dir()).unwrap();
                 // use root_testpaths here, because aux-builds should have the
                 // same --out-dir and auxiliary directory.
-                let auxres = aux_cx.document(&root_out_dir, root_testpaths);
+                let auxres = aux_cx.document(&root_out_dir, root_testpaths, kind);
                 if !auxres.status.success() {
                     return auxres;
                 }
@@ -1063,8 +1070,11 @@ impl<'test> TestCx<'test> {
             .args(&self.props.compile_flags)
             .args(&self.props.doc_flags);
 
-        if self.config.mode == TestMode::RustdocJson {
-            rustdoc.arg("--output-format").arg("json").arg("-Zunstable-options");
+        match kind {
+            DocKind::Html => {}
+            DocKind::Json => {
+                rustdoc.arg("--output-format").arg("json").arg("-Zunstable-options");
+            }
         }
 
         if let Some(ref linker) = self.config.target_linker {
@@ -2203,7 +2213,7 @@ impl<'test> TestCx<'test> {
         let aux_dir = new_rustdoc.aux_output_dir();
         new_rustdoc.build_all_auxiliary(&new_rustdoc.testpaths, &aux_dir, &mut rustc);
 
-        let proc_res = new_rustdoc.document(&compare_dir, &new_rustdoc.testpaths);
+        let proc_res = new_rustdoc.document(&compare_dir, &new_rustdoc.testpaths, DocKind::Html);
         if !proc_res.status.success() {
             writeln!(self.stderr, "failed to run nightly rustdoc");
             return;
@@ -3121,6 +3131,12 @@ enum CompareOutcome {
     Differed,
 }
 
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+enum DocKind {
+    Html,
+    Json,
+}
+
 impl CompareOutcome {
     fn should_error(&self) -> bool {
         matches!(self, CompareOutcome::Differed)
diff --git a/src/tools/compiletest/src/runtest/js_doc.rs b/src/tools/compiletest/src/runtest/js_doc.rs
index fd53f01ca1746..93b05617e6f8c 100644
--- a/src/tools/compiletest/src/runtest/js_doc.rs
+++ b/src/tools/compiletest/src/runtest/js_doc.rs
@@ -1,13 +1,13 @@
 use std::process::Command;
 
-use super::TestCx;
+use super::{DocKind, TestCx};
 
 impl TestCx<'_> {
     pub(super) fn run_rustdoc_js_test(&self) {
         if let Some(nodejs) = &self.config.nodejs {
             let out_dir = self.output_base_dir();
 
-            self.document(&out_dir, &self.testpaths);
+            self.document(&out_dir, &self.testpaths, DocKind::Html);
 
             let file_stem = self.testpaths.file.file_stem().expect("no file stem");
             let res = self.run_command_to_procres(
diff --git a/src/tools/compiletest/src/runtest/rustdoc.rs b/src/tools/compiletest/src/runtest/rustdoc.rs
index 4a143aa5824e7..32b1823961bea 100644
--- a/src/tools/compiletest/src/runtest/rustdoc.rs
+++ b/src/tools/compiletest/src/runtest/rustdoc.rs
@@ -1,6 +1,6 @@
 use std::process::Command;
 
-use super::{TestCx, remove_and_create_dir_all};
+use super::{DocKind, TestCx, remove_and_create_dir_all};
 
 impl TestCx<'_> {
     pub(super) fn run_rustdoc_test(&self) {
@@ -11,7 +11,7 @@ impl TestCx<'_> {
             panic!("failed to remove and recreate output directory `{out_dir}`: {e}")
         });
 
-        let proc_res = self.document(&out_dir, &self.testpaths);
+        let proc_res = self.document(&out_dir, &self.testpaths, DocKind::Html);
         if !proc_res.status.success() {
             self.fatal_proc_rec("rustdoc failed!", &proc_res);
         }
diff --git a/src/tools/compiletest/src/runtest/rustdoc_json.rs b/src/tools/compiletest/src/runtest/rustdoc_json.rs
index b8da6e2ac5280..dd7ebe9efaeef 100644
--- a/src/tools/compiletest/src/runtest/rustdoc_json.rs
+++ b/src/tools/compiletest/src/runtest/rustdoc_json.rs
@@ -1,6 +1,6 @@
 use std::process::Command;
 
-use super::{TestCx, remove_and_create_dir_all};
+use super::{DocKind, TestCx, remove_and_create_dir_all};
 
 impl TestCx<'_> {
     pub(super) fn run_rustdoc_json_test(&self) {
@@ -13,7 +13,7 @@ impl TestCx<'_> {
             panic!("failed to remove and recreate output directory `{out_dir}`: {e}")
         });
 
-        let proc_res = self.document(&out_dir, &self.testpaths);
+        let proc_res = self.document(&out_dir, &self.testpaths, DocKind::Json);
         if !proc_res.status.success() {
             self.fatal_proc_rec("rustdoc failed!", &proc_res);
         }