Rollup merge of #80352 - ssomers:btree_test_diagnostics, r=Mark-Simulacrum

BTreeMap: make test cases more explicit on failure

r? `@Mark-Simulacrum`

Author: Dylan-DPC

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

@@ -115,7 +115,7 @@ impl<K, V> BTreeMap<K, V> {
 
 impl<'a, K: 'a, V: 'a> NodeRef<marker::Immut<'a>, K, V, marker::LeafOrInternal> {
     fn assert_min_len(self, min_len: usize) {
-        assert!(self.len() >= min_len, "{} < {}", self.len(), min_len);
+        assert!(self.len() >= min_len, "node len {} < {}", self.len(), min_len);
         if let node::ForceResult::Internal(node) = self.force() {
             for idx in 0..=node.len() {
                 let edge = unsafe { Handle::new_edge(node, idx) };

library/alloc/src/collections/btree/node/tests.rs

@@ -30,11 +30,15 @@ impl<'a, K: 'a, V: 'a> NodeRef<marker::Immut<'a>, K, V, marker::LeafOrInternal>
                 let depth = self.height();
                 let indent = "  ".repeat(depth);
                 result += &format!("\n{}", indent);
-                for idx in 0..leaf.len() {
-                    if idx > 0 {
-                        result += ", ";
+                if leaf.len() == 0 {
+                    result += "(empty node)";
+                } else {
+                    for idx in 0..leaf.len() {
+                        if idx > 0 {
+                            result += ", ";
+                        }
+                        result += &format!("{:?}", unsafe { leaf.key_at(idx) });
                     }
-                    result += &format!("{:?}", unsafe { leaf.key_at(idx) });
                 }
             }
             navigate::Position::Internal(_) => {}