refactor(allocator): do not make Allocator::bump field public (#12936)

Follow-on after #12555.

Avoid making `bump` field of `Allocator` public when `track_allocations` feature is enabled, by moving the field offset calculations to next to `Allocator`'s definition.

Author: overlookmotel

crates/oxc_allocator/src/alloc.rs

@@ -100,16 +100,10 @@ impl Alloc for Bump {
         // This will go away when we add a custom allocator to oxc.
         #[cfg(all(feature = "track_allocations", not(feature = "disable_track_allocations")))]
         unsafe {
-            use crate::Allocator;
-            use std::{
-                mem::offset_of,
-                ptr,
-                sync::atomic::{AtomicUsize, Ordering},
-            };
-            #[expect(clippy::cast_possible_wrap)]
-            const OFFSET: isize = (offset_of!(Allocator, num_alloc) as isize)
-                - (offset_of!(Allocator, bump) as isize);
-            let num_alloc_ptr = ptr::from_ref(self).byte_offset(OFFSET).cast::<AtomicUsize>();
+            use crate::allocator::NUM_ALLOC_FIELD_OFFSET;
+            use std::sync::atomic::{AtomicUsize, Ordering};
+            let num_alloc_ptr =
+                std::ptr::from_ref(self).byte_offset(NUM_ALLOC_FIELD_OFFSET).cast::<AtomicUsize>();
             let num_alloc = num_alloc_ptr.as_ref().unwrap_unchecked();
             num_alloc.fetch_add(1, Ordering::SeqCst);
         }
@@ -157,16 +151,11 @@ impl Alloc for Bump {
         // This will go away when we add a custom allocator to oxc.
         #[cfg(all(feature = "track_allocations", not(feature = "disable_track_allocations")))]
         unsafe {
-            use crate::Allocator;
-            use std::{
-                mem::offset_of,
-                ptr,
-                sync::atomic::{AtomicUsize, Ordering},
-            };
-            #[expect(clippy::cast_possible_wrap)]
-            const OFFSET: isize = (offset_of!(Allocator, num_realloc) as isize)
-                - (offset_of!(Allocator, bump) as isize);
-            let num_realloc_ptr = ptr::from_ref(self).byte_offset(OFFSET).cast::<AtomicUsize>();
+            use crate::allocator::NUM_REALLOC_FIELD_OFFSET;
+            use std::sync::atomic::{AtomicUsize, Ordering};
+            let num_realloc_ptr = std::ptr::from_ref(self)
+                .byte_offset(NUM_REALLOC_FIELD_OFFSET)
+                .cast::<AtomicUsize>();
             let num_realloc = num_realloc_ptr.as_ref().unwrap_unchecked();
             num_realloc.fetch_add(1, Ordering::SeqCst);
         }

crates/oxc_allocator/src/allocator.rs

@@ -4,6 +4,9 @@ use std::{
     slice, str,
 };
 
+#[cfg(all(feature = "track_allocations", not(feature = "disable_track_allocations")))]
+use std::mem::offset_of;
+
 use bumpalo::Bump;
 
 use oxc_data_structures::assert_unchecked;
@@ -214,12 +217,7 @@ use oxc_data_structures::assert_unchecked;
 /// [`HashMap::new_in`]: crate::HashMap::new_in
 #[derive(Default)]
 pub struct Allocator {
-    #[cfg(not(all(feature = "track_allocations", not(feature = "disable_track_allocations"))))]
     bump: Bump,
-    // NOTE: We need to expose `bump` publicly here for calculating its field offset in memory.
-    #[cfg(all(feature = "track_allocations", not(feature = "disable_track_allocations")))]
-    #[doc(hidden)]
-    pub bump: Bump,
     /// Used to track the total number of allocations made in this allocator when the `track_allocations` feature is enabled.
     #[cfg(all(feature = "track_allocations", not(feature = "disable_track_allocations")))]
     #[doc(hidden)]
@@ -230,6 +228,16 @@ pub struct Allocator {
     pub num_realloc: std::sync::atomic::AtomicUsize,
 }
 
+// Consts used in `Alloc` trait for allocation tracking
+#[cfg(all(feature = "track_allocations", not(feature = "disable_track_allocations")))]
+#[expect(clippy::cast_possible_wrap)]
+pub const NUM_ALLOC_FIELD_OFFSET: isize =
+    (offset_of!(Allocator, num_alloc) as isize) - (offset_of!(Allocator, bump) as isize);
+#[cfg(all(feature = "track_allocations", not(feature = "disable_track_allocations")))]
+#[expect(clippy::cast_possible_wrap)]
+pub const NUM_REALLOC_FIELD_OFFSET: isize =
+    (offset_of!(Allocator, num_realloc) as isize) - (offset_of!(Allocator, bump) as isize);
+
 impl Allocator {
     /// Create a new [`Allocator`] with no initial capacity.
     ///