Rollup merge of rust-lang#139151 - mejrs:underscore_to_dash, r=onur-ozkan

tidy: properly check for orphaned unstable_book pages

This also recommends using underscores - something that took me a little bit too long to figure out.

Note: this PR deletes the page `src/doc/unstable-book/src/library-features/c-variadic.md`. The page `src/doc/unstable-book/src/lang-features/c-variadic.md` remains.

Author: matthiaskrgr

src/doc/unstable-book/src/library-features/c-variadic.md

@@ -72,6 +72,19 @@ fn collect_unstable_book_lib_features_section_file_names(base_src_path: &Path) -
     collect_unstable_book_section_file_names(&unstable_book_lib_features_path(base_src_path))
 }
 
+/// Would switching underscores for dashes work?
+fn maybe_suggest_dashes(names: &BTreeSet<String>, feature_name: &str, bad: &mut bool) {
+    let with_dashes = feature_name.replace('_', "-");
+    if names.contains(&with_dashes) {
+        tidy_error!(
+            bad,
+            "the file `{}.md` contains underscores; use dashes instead: `{}.md`",
+            feature_name,
+            with_dashes,
+        );
+    }
+}
+
 pub fn check(path: &Path, features: CollectedFeatures, bad: &mut bool) {
     let lang_features = features.lang;
     let lib_features = features
@@ -93,14 +106,13 @@ pub fn check(path: &Path, features: CollectedFeatures, bad: &mut bool) {
     // Check for Unstable Book sections that don't have a corresponding unstable feature
     for feature_name in &unstable_book_lib_features_section_file_names - &unstable_lib_feature_names
     {
-        if !unstable_lang_feature_names.contains(&feature_name) {
-            tidy_error!(
-                bad,
-                "The Unstable Book has a 'library feature' section '{}' which doesn't \
+        tidy_error!(
+            bad,
+            "The Unstable Book has a 'library feature' section '{}' which doesn't \
                          correspond to an unstable library feature",
-                feature_name
-            );
-        }
+            feature_name
+        );
+        maybe_suggest_dashes(&unstable_lib_feature_names, &feature_name, bad);
     }
 
     // Check for Unstable Book sections that don't have a corresponding unstable feature.
@@ -112,7 +124,8 @@ pub fn check(path: &Path, features: CollectedFeatures, bad: &mut bool) {
             "The Unstable Book has a 'language feature' section '{}' which doesn't \
                      correspond to an unstable language feature",
             feature_name
-        )
+        );
+        maybe_suggest_dashes(&unstable_lang_feature_names, &feature_name, bad);
     }
 
     // List unstable features that don't have Unstable Book sections.