3 files changed, 68 insertions, 0 deletions
diff --git a/src/tools/tidy/src/lib.rs b/src/tools/tidy/src/lib.rs
index e8a12d56335..28aa80225b1 100644
--- a/src/tools/tidy/src/lib.rs
+++ b/src/tools/tidy/src/lib.rs
@@ -83,6 +83,7 @@ pub mod pal;
 pub mod rustdoc_css_themes;
 pub mod rustdoc_gui_tests;
 pub mod rustdoc_js;
+pub mod rustdoc_json;
 pub mod rustdoc_templates;
 pub mod style;
 pub mod target_policy;
diff --git a/src/tools/tidy/src/main.rs b/src/tools/tidy/src/main.rs
index 776f1bde2eb..420260a97a0 100644
--- a/src/tools/tidy/src/main.rs
+++ b/src/tools/tidy/src/main.rs
@@ -110,6 +110,7 @@ fn main() {
         check!(rustdoc_css_themes, &librustdoc_path);
         check!(rustdoc_templates, &librustdoc_path);
         check!(rustdoc_js, &librustdoc_path, &tools_path, &src_path);
+        check!(rustdoc_json);
         check!(known_bug, &crashes_path);
         check!(unknown_revision, &tests_path);
 
diff --git a/src/tools/tidy/src/rustdoc_json.rs b/src/tools/tidy/src/rustdoc_json.rs
new file mode 100644
index 00000000000..a98c4f4cc3e
--- /dev/null
+++ b/src/tools/tidy/src/rustdoc_json.rs
@@ -0,0 +1,66 @@
+//! Tidy check to ensure that `FORMAT_VERSION` was correctly updated if `rustdoc-json-types` was
+//! updated as well.
+
+use std::process::Command;
+
+fn git_diff(base_commit: &str, extra_arg: &str) -> Option<String> {
+    let output = Command::new("git").arg("diff").arg(base_commit).arg(extra_arg).output().ok()?;
+    Some(String::from_utf8_lossy(&output.stdout).into())
+}
+
+pub fn check(bad: &mut bool) {
+    let Ok(base_commit) = std::env::var("BASE_COMMIT") else {
+        // Not in CI so nothing we can check here.
+        println!("not checking rustdoc JSON `FORMAT_VERSION` update");
+        return;
+    };
+
+    // First we check that `src/rustdoc-json-types` was modified.
+    match git_diff(&base_commit, "--name-status") {
+        Some(output) => {
+            if !output
+                .lines()
+                .any(|line| line.starts_with("M") && line.contains("src/rustdoc-json-types"))
+            {
+                // `rustdoc-json-types` was not modified so nothing more to check here.
+                return;
+            }
+        }
+        None => {
+            *bad = true;
+            eprintln!("Failed to run `git diff`");
+            return;
+        }
+    }
+    // Then we check that if `FORMAT_VERSION` was updated, the `Latest feature:` was also updated.
+    match git_diff(&base_commit, "src/rustdoc-json-types") {
+        Some(output) => {
+            let mut format_version_updated = false;
+            let mut latest_feature_comment_updated = false;
+            for line in output.lines() {
+                if line.starts_with("+pub const FORMAT_VERSION: u32 =") {
+                    format_version_updated = true;
+                } else if line.starts_with("+// Latest feature:") {
+                    latest_feature_comment_updated = true;
+                }
+            }
+            if format_version_updated != latest_feature_comment_updated {
+                *bad = true;
+                if latest_feature_comment_updated {
+                    eprintln!(
+                        "`Latest feature` comment was updated whereas `FORMAT_VERSION` wasn't"
+                    );
+                } else {
+                    eprintln!(
+                        "`Latest feature` comment was not updated whereas `FORMAT_VERSION` was"
+                    );
+                }
+            }
+        }
+        None => {
+            *bad = true;
+            eprintln!("Failed to run `git diff`");
+            return;
+        }
+    }
+}