Split stable versus nightly for memory usage tests

**Description**
Add a check for the `nightly` feature flag to change the expected sizes
in different tests.

This seems like a short-term solution because the nightly changes will
eventually get promoted to stable and then this change will need to be
removed.

I found a possibly related [rust-lang issue #104807][rust-issue-104807]
, maybe the fix for this will change the nightly enum sizes back to
their previous values

**Motivation**
CI was breaking on the nightly and miri tests

**Testing Done**
 - `cargo +nightly test --features nightly` passes
 - `cargo +nightly miri test` passes
 - `cargo +nightly test` fails

[rust-issue-104807]: rust-lang/rust#104807

src/nodes/representation.rs

@@ -329,7 +329,7 @@ impl<V> fmt::Debug for ConcreteNodePtr<V> {
 pub struct NodePtr<N: Node>(NonNull<N>);
 
 impl<N: Node> NodePtr<N> {
-    /// Create a safe pointer to a Node_.
+    /// Create a safe pointer to a [`Node`].
     ///
     /// # Safety
     ///

src/nodes/representation/tests.rs

@@ -25,21 +25,39 @@ fn opaque_node_ptr_is_correct() {
 #[test]
 #[cfg(target_pointer_width = "64")]
 fn node_sizes() {
-    assert_eq!(mem::size_of::<Header>(), 32);
+    let expected_header_size = if cfg!(any(miri, feature = "nightly")) {
+        40
+    } else {
+        32
+    };
+
+    assert_eq!(mem::size_of::<Header>(), expected_header_size);
     // key map: 4 * (1 byte) = 4 bytes
     // child map: 4 * (8 bytes (on 64-bit platform)) = 32
     //
     // 4 bytes of padding are inserted after the `keys` field to align the field to
     // an 8 byte boundary.
-    assert_eq!(mem::size_of::<InnerNode4<usize>>(), 72);
+    assert_eq!(
+        mem::size_of::<InnerNode4<usize>>(),
+        expected_header_size + 40
+    );
     // key map: 16 * (1 byte) = 16 bytes
     // child map: 16 * (8 bytes (on 64-bit platform)) = 128
-    assert_eq!(mem::size_of::<InnerNode16<usize>>(), 176);
+    assert_eq!(
+        mem::size_of::<InnerNode16<usize>>(),
+        expected_header_size + 144
+    );
     // key map: 256 * (1 byte) = 256 bytes
     // child map: 48 * (8 bytes (on 64-bit platform)) = 384
-    assert_eq!(mem::size_of::<InnerNode48<usize>>(), 672);
+    assert_eq!(
+        mem::size_of::<InnerNode48<usize>>(),
+        expected_header_size + 640
+    );
     // child & key map: 256 * (8 bytes (on 64-bit platform)) = 2048
-    assert_eq!(mem::size_of::<InnerNode256<usize>>(), 2080);
+    assert_eq!(
+        mem::size_of::<InnerNode256<usize>>(),
+        expected_header_size + 2048
+    );
 
     // Assert that pointer is expected size and has non-null optimization
     assert_eq!(mem::size_of::<Option<OpaqueNodePtr<()>>>(), 8);

src/nodes/visitor/pretty_printer.rs

@@ -7,14 +7,14 @@ use std::{
     io::{self, Write},
 };
 
-/// Settings which custom the output of the [`DotPrinter`] visitor.
+/// Settings which customize the output of the [`DotPrinter`] visitor.
 #[derive(Debug, Clone, PartialEq, Eq, Default)]
 pub struct DotPrinterSettings {
     /// Add node address to output in graphs
     pub display_node_address: bool,
 }
 
-/// A visitor the the radix trie that will print the tree in "dot" notation.
+/// A visitor of the radix trie that will print the tree in "dot" notation.
 ///
 /// See ['DOT Language | Graphviz'](https://graphviz.org/doc/info/lang.html) for
 /// information about syntax and example of the language.

tests/memory_usage_fixed_length_dense.rs

@@ -45,7 +45,11 @@ fn test_memory_usage() {
         dhat::assert_eq!(stats.curr_bytes, 0);
 
         dhat::assert_eq!(stats.max_blocks, 398);
-        dhat::assert_eq!(stats.max_bytes, 17024);
+        if cfg!(any(miri, feature = "nightly")) {
+            dhat::assert_eq!(stats.max_bytes, 18088);
+        } else {
+            dhat::assert_eq!(stats.max_bytes, 17024);
+        }
 
         let num_keys = KEY_LEVEL_WIDTH
             .iter()

tests/memory_usage_large_prefixes.rs

@@ -59,7 +59,11 @@ fn test_memory_usage() {
         dhat::assert_eq!(stats.curr_bytes, 0);
 
         dhat::assert_eq!(stats.max_blocks, 360);
-        dhat::assert_eq!(stats.max_bytes, 17225);
+        if cfg!(any(miri, feature = "nightly")) {
+            dhat::assert_eq!(stats.max_bytes, 17681);
+        } else {
+            dhat::assert_eq!(stats.max_bytes, 17225);
+        }
 
         let num_keys = KEY_LEVEL_WIDTH
             .iter()

tests/memory_usage_skewed.rs

@@ -46,7 +46,11 @@ fn test_memory_usage() {
         dhat::assert_eq!(stats.curr_bytes, 0);
 
         dhat::assert_eq!(stats.max_blocks, 511);
-        dhat::assert_eq!(stats.max_bytes, 25170);
+        if cfg!(any(miri, feature = "nightly")) {
+            dhat::assert_eq!(stats.max_bytes, 27202);
+        } else {
+            dhat::assert_eq!(stats.max_bytes, 25170);
+        }
 
         let mean_blocks_per_key = (stats.max_blocks as f64) / (KEY_LENGTH_LIMIT as f64);
         let mean_bytes_per_key = (stats.max_bytes as f64) / (KEY_LENGTH_LIMIT as f64);