Rollup merge of #102556 - WaffleLapkin:implied_by_btree_new, r=Mark-Simulacrum

Make `feature(const_btree_len)` implied by `feature(const_btree_new)`

...this should fix code that used the old feature that was changed in #102197

cc ```@davidtwco``` it seems like tidy doesn't check `implied_by`, should it?

Author: Dylan-DPC

library/alloc/src/collections/btree/map.rs

@@ -2392,7 +2392,11 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// ```
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_btree_len", issue = "71835")]
+    #[rustc_const_unstable(
+        feature = "const_btree_len",
+        issue = "71835",
+        implied_by = "const_btree_new"
+    )]
     pub const fn len(&self) -> usize {
         self.length
     }
@@ -2413,7 +2417,11 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// ```
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_btree_len", issue = "71835")]
+    #[rustc_const_unstable(
+        feature = "const_btree_len",
+        issue = "71835",
+        implied_by = "const_btree_new"
+    )]
     pub const fn is_empty(&self) -> bool {
         self.len() == 0
     }

library/alloc/src/collections/btree/set.rs

@@ -1174,7 +1174,11 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     /// ```
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_btree_len", issue = "71835")]
+    #[rustc_const_unstable(
+        feature = "const_btree_len",
+        issue = "71835",
+        implied_by = "const_btree_new"
+    )]
     pub const fn len(&self) -> usize {
         self.map.len()
     }
@@ -1193,7 +1197,11 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     /// ```
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_btree_len", issue = "71835")]
+    #[rustc_const_unstable(
+        feature = "const_btree_len",
+        issue = "71835",
+        implied_by = "const_btree_new"
+    )]
     pub const fn is_empty(&self) -> bool {
         self.len() == 0
     }

src/tools/tidy/src/features.rs

@@ -538,7 +538,9 @@ fn map_lib_features(
                 becoming_feature = None;
                 if line.contains("rustc_const_unstable(") {
                     // `const fn` features are handled specially.
-                    let feature_name = match find_attr_val(line, "feature") {
+                    let feature_name = match find_attr_val(line, "feature").or_else(|| {
+                        iter_lines.peek().and_then(|next| find_attr_val(next.1, "feature"))
+                    }) {
                         Some(name) => name,
                         None => err!("malformed stability attribute: missing `feature` key"),
                     };