Rollup merge of #102971 - est31:tidy_duplicate_lang_features, r=jyn514

tidy: error if a lang feature is already present

If a lang feature gets declared twice, like for example as a result of a mistake during stabilization, emit an error in tidy. Library features already have this logic.

Inspired by a mistake done during `half_open_range_patterns` stabilization: https://github.com/rust-lang/rust/pull/102275/files#r991292215

The PR requires #102883 to be merged before CI turns green because the check is doing its job.

For reviewers, I suggest [turning off whitespace changes](https://github.com/rust-lang/rust/pull/102971/files?w=1) in the diff by adding `?w=1` to the url, as a large part of the diff is just about removing one level of indentation.

src/tools/tidy/src/features.rs

@@ -10,7 +10,7 @@
 //! * Language features in a group are sorted by feature name.
 
 use crate::walk::{filter_dirs, walk, walk_many};
-use std::collections::HashMap;
+use std::collections::hash_map::{Entry, HashMap};
 use std::fmt;
 use std::fs;
 use std::num::NonZeroU32;
@@ -280,13 +280,14 @@ fn test_filen_gate(filen_underscore: &str, features: &mut Features) -> bool {
 }
 
 pub fn collect_lang_features(base_compiler_path: &Path, bad: &mut bool) -> Features {
-    let mut all = collect_lang_features_in(base_compiler_path, "active.rs", bad);
-    all.extend(collect_lang_features_in(base_compiler_path, "accepted.rs", bad));
-    all.extend(collect_lang_features_in(base_compiler_path, "removed.rs", bad));
-    all
+    let mut features = Features::new();
+    collect_lang_features_in(&mut features, base_compiler_path, "active.rs", bad);
+    collect_lang_features_in(&mut features, base_compiler_path, "accepted.rs", bad);
+    collect_lang_features_in(&mut features, base_compiler_path, "removed.rs", bad);
+    features
 }
 
-fn collect_lang_features_in(base: &Path, file: &str, bad: &mut bool) -> Features {
+fn collect_lang_features_in(features: &mut Features, base: &Path, file: &str, bad: &mut bool) {
     let path = base.join("rustc_feature").join("src").join(file);
     let contents = t!(fs::read_to_string(&path));
 
@@ -298,21 +299,19 @@ fn collect_lang_features_in(base: &Path, file: &str, bad: &mut bool) -> Features
     let mut in_feature_group = false;
     let mut prev_names = vec![];
 
-    contents
-        .lines()
-        .zip(1..)
-        .filter_map(|(line, line_number)| {
+    let lines = contents.lines().zip(1..);
+    for (line, line_number) in lines {
         let line = line.trim();
 
         // Within -start and -end, the tracking issue can be omitted.
         match line {
             "// no-tracking-issue-start" => {
                 next_feature_omits_tracking_issue = true;
-                    return None;
+                continue;
             }
             "// no-tracking-issue-end" => {
                 next_feature_omits_tracking_issue = false;
-                    return None;
+                continue;
             }
             _ => {}
         }
@@ -330,11 +329,11 @@ fn collect_lang_features_in(base: &Path, file: &str, bad: &mut bool) -> Features
 
             in_feature_group = true;
             prev_names = vec![];
-                return None;
+            continue;
         } else if line.starts_with(FEATURE_GROUP_END_PREFIX) {
             in_feature_group = false;
             prev_names = vec![];
-                return None;
+            continue;
         }
 
         let mut parts = line.split(',');
@@ -343,7 +342,7 @@ fn collect_lang_features_in(base: &Path, file: &str, bad: &mut bool) -> Features
             Some("incomplete") => Status::Unstable,
             Some("removed") => Status::Removed,
             Some("accepted") => Status::Stable,
-                _ => return None,
+            _ => continue,
         };
         let name = parts.next().unwrap().trim();
 
@@ -377,7 +376,7 @@ fn collect_lang_features_in(base: &Path, file: &str, bad: &mut bool) -> Features
                             name,
                         );
                         // skip any additional checks for this line
-                            return None;
+                        continue;
                     }
                     Err(index) => index,
                 };
@@ -424,9 +423,21 @@ fn collect_lang_features_in(base: &Path, file: &str, bad: &mut bool) -> Features
             let s = issue_str.split('(').nth(1).unwrap().split(')').next().unwrap();
             Some(s.parse().unwrap())
         };
-            Some((name.to_owned(), Feature { level, since, has_gate_test: false, tracking_issue }))
-        })
-        .collect()
+        match features.entry(name.to_owned()) {
+            Entry::Occupied(e) => {
+                tidy_error!(
+                    bad,
+                    "{}:{} feature {name} already specified with status '{}'",
+                    path.display(),
+                    line_number,
+                    e.get().level,
+                );
+            }
+            Entry::Vacant(e) => {
+                e.insert(Feature { level, since, has_gate_test: false, tracking_issue });
+            }
+        }
+    }
 }
 
 fn get_and_check_lib_features(