Add regression test for rustdoc output format

tests/run-make/rustdoc-doctest-output-format/file.rs

@@ -0,0 +1,3 @@
+//! ```
+//! let x = 12;
+//! ```

tests/run-make/rustdoc-doctest-output-format/rmake.rs

@@ -0,0 +1,83 @@
+//! Regression test to ensure that the output format is respected for doctests.
+//!
+//! Regression test for <https://github.com/rust-lang/rust/issues/144798>.
+
+//@ ignore-cross-compile
+
+use run_make_support::{rustdoc, serde_json};
+
+fn run_test(edition: &str, format: Option<&str>) -> String {
+    let mut r = rustdoc();
+    r.input("file.rs").edition(edition).arg("--test");
+    if let Some(format) = format {
+        r.args(&[
+            "--test-args",
+            "-Zunstable-options",
+            "--test-args",
+            "--format",
+            "--test-args",
+            format,
+        ]);
+    }
+    r.run().stdout_utf8()
+}
+
+fn check_json_output(edition: &str, expected_reports: usize) {
+    let out = run_test(edition, Some("json"));
+    let mut found_report = 0;
+    for (line_nb, line) in out.lines().enumerate() {
+        match serde_json::from_str::<serde_json::Value>(&line) {
+            Ok(value) => {
+                if value.get("type") == Some(&serde_json::json!("report")) {
+                    found_report += 1;
+                }
+            }
+            Err(error) => panic!(
+                "failed for {edition} edition (json format) at line {}: non-JSON value: {error}\n\
+                ====== output ======\n{out}",
+                line_nb + 1,
+            ),
+        }
+    }
+    if found_report != expected_reports {
+        panic!(
+            "failed for {edition} edition (json format): expected {expected_reports} doctest \
+             time `report`, found {found_report}\n====== output ======\n{out}",
+        );
+    }
+}
+
+fn check_non_json_output(edition: &str, expected_reports: usize) {
+    let out = run_test(edition, None);
+    let mut found_report = 0;
+    for (line_nb, line) in out.lines().enumerate() {
+        if line.starts_with('{') && serde_json::from_str::<serde_json::Value>(&line).is_ok() {
+            panic!(
+                "failed for {edition} edition: unexpected json at line {}: `{line}`\n\
+                 ====== output ======\n{out}",
+                line_nb + 1
+            );
+        }
+        if line.starts_with("all doctests ran in")
+            && line.contains("; merged doctests compilation took ")
+        {
+            found_report += 1;
+        }
+    }
+    if found_report != expected_reports {
+        panic!(
+            "failed for {edition} edition: expected {expected_reports} doctest time `report`, \
+             found {found_report}\n====== output ======\n{out}",
+        );
+    }
+}
+
+fn main() {
+    // Only the merged doctests generate the "times report".
+    check_json_output("2021", 0);
+    check_json_output("2024", 1);
+
+    // Only the merged doctests generate the "times report".
+    check_non_json_output("2021", 0);
+    check_non_json_output("2024", 1);
+}