Detect unstable lint docs that dont enable their feature

Author: compiler-errors

Diff for: src/tools/lint-docs/src/lib.rs

@@ -480,26 +480,44 @@ impl<'a> LintExtractor<'a> {
             .map(|msg| msg["rendered"].as_str().expect("rendered field should exist").to_string())
             .collect();
         if matches.is_empty() {
-            // Some lints override their code to something else (E0566).
-            // Try to find something that looks like it could be our lint.
-            let matches: Vec<_> = msgs.iter().filter(|msg|
-                matches!(&msg["rendered"], serde_json::Value::String(s) if s.contains(name)))
-                .map(|msg| msg["rendered"].as_str().expect("rendered field should exist").to_string())
-                .collect();
-            if matches.is_empty() {
+            // Try to detect if an unstable lint forgot to enable a `#![feature(..)]`.
+            if name != "test_unstable_lint" && msgs.iter().any(|msg| {
+                matches!(&msg["code"]["code"], serde_json::Value::String(s) if s=="unknown_lints")
+                    && matches!(&msg["message"], serde_json::Value::String(s) if s.contains(name))
+            }) {
                 let rendered: Vec<&str> =
                     msgs.iter().filter_map(|msg| msg["rendered"].as_str()).collect();
                 let non_json: Vec<&str> =
                     stderr.lines().filter(|line| !line.starts_with('{')).collect();
                 Err(format!(
-                    "did not find lint `{}` in output of example, got:\n{}\n{}",
+                    "lint `{}` is unstable and must be enabled in example. see:\n{}\n{}",
                     name,
+                    rendered.join("\n"),
                     non_json.join("\n"),
-                    rendered.join("\n")
                 )
                 .into())
             } else {
-                Ok(matches.join("\n"))
+                // Some lints override their code to something else (E0566).
+                // Try to find something that looks like it could be our lint.
+                let matches: Vec<_> = msgs.iter().filter(|msg|
+                matches!(&msg["rendered"], serde_json::Value::String(s) if s.contains(name)))
+                .map(|msg| msg["rendered"].as_str().expect("rendered field should exist").to_string())
+                .collect();
+                if matches.is_empty() {
+                    let rendered: Vec<&str> =
+                        msgs.iter().filter_map(|msg| msg["rendered"].as_str()).collect();
+                    let non_json: Vec<&str> =
+                        stderr.lines().filter(|line| !line.starts_with('{')).collect();
+                    Err(format!(
+                        "did not find lint `{}` in output of example, got:\n{}\n{}",
+                        name,
+                        non_json.join("\n"),
+                        rendered.join("\n")
+                    )
+                    .into())
+                } else {
+                    Ok(matches.join("\n"))
+                }
             }
         } else {
             Ok(matches.join("\n"))