Rollup merge of rust-lang#70111 - Mark-Simulacrum:btree-no-shared, r=cuviper

BTreeMap: remove shared root

This replaces the shared root with `Option`s in the BTreeMap code, and then slightly cleans up the node manipulation code taking advantage of the removal of the shared root. I expect that further simplification is possible, but wanted to get this posted for initial review.

Note that `BTreeMap::new()` continues to not allocate.

Benchmarks seem within the margin of error/unaffected, as expected for an entirely predictable branch.

```
 name                                 alloc-bench-a ns/iter  alloc-bench-b ns/iter  diff ns/iter  diff %  speedup
 btree::map::iter_mut_20              20                     21                                1   5.00%   x 0.95
 btree::set::clone_100                1,360                  1,439                            79   5.81%   x 0.95
 btree::set::clone_100_and_into_iter  1,319                  1,434                           115   8.72%   x 0.92
 btree::set::clone_10k                143,515                150,991                       7,476   5.21%   x 0.95
 btree::set::clone_10k_and_clear      142,792                152,916                      10,124   7.09%   x 0.93
 btree::set::clone_10k_and_into_iter  146,019                154,561                       8,542   5.85%   x 0.94
```

Author: Dylan-DPC

src/etc/gdb_rust_pretty_printing.py

@@ -370,12 +370,25 @@ def to_string(self):
                 ("(len: %i)" % self.__val.get_wrapped_value()['map']['length']))
 
     def children(self):
-        root = self.__val.get_wrapped_value()['map']['root']
-        node_ptr = root['node']
-        i = 0
-        for child in children_of_node(node_ptr, root['height'], False):
-            yield (str(i), child)
-            i = i + 1
+        if self.__val.get_wrapped_value()['map']['length'] > 0:
+            root = self.__val.get_wrapped_value()['map']['root']
+            # get at `Option` innards
+            root = GdbValue(root).get_child_at_index(0).get_wrapped_value()
+            # pull out the `Some` variant of the enum
+            try:
+                root = root['Some']
+            except:
+                # Do nothing, we just want to pull out Some if it exists.
+                # If it didn't, then it seems at least on some versions of gdb
+                # we don't actually need to do the above line, so just skip it.
+                pass
+            # And now the value of the Some variant
+            root = GdbValue(root).get_child_at_index(0).get_wrapped_value()
+            node_ptr = root['node']
+            i = 0
+            for child in children_of_node(node_ptr, root['height'], False):
+                yield (str(i), child)
+                i = i + 1
 
 
 class RustStdBTreeMapPrinter(object):
@@ -391,13 +404,26 @@ def to_string(self):
                 ("(len: %i)" % self.__val.get_wrapped_value()['length']))
 
     def children(self):
-        root = self.__val.get_wrapped_value()['root']
-        node_ptr = root['node']
-        i = 0
-        for child in children_of_node(node_ptr, root['height'], True):
-            yield (str(i), child[0])
-            yield (str(i), child[1])
-            i = i + 1
+        if self.__val.get_wrapped_value()['length'] > 0:
+            root = self.__val.get_wrapped_value()['root']
+            # get at `Option` innards
+            root = GdbValue(root).get_child_at_index(0).get_wrapped_value()
+            # pull out the `Some` variant of the enum
+            try:
+                root = root['Some']
+            except:
+                # Do nothing, we just want to pull out Some if it exists.
+                # If it didn't, then it seems at least on some versions of gdb
+                # we don't actually need to do the above line, so just skip it.
+                pass
+            # And now the value of the Some variant
+            root = GdbValue(root).get_child_at_index(0).get_wrapped_value()
+            node_ptr = root['node']
+            i = 0
+            for child in children_of_node(node_ptr, root['height'], True):
+                yield (str(i), child[0])
+                yield (str(i), child[1])
+                i = i + 1
 
 
 class RustStdStringPrinter(object):