From 4950c503768fcebce6f9ab9dbaac2a7da30b35ba Mon Sep 17 00:00:00 2001
From: Brad Dunbar <dunbarb2@gmail.com>
Date: Sat, 11 May 2024 13:41:50 -0400
Subject: [PATCH] Offset from (#705)

---
 src/bytes.rs     | 25 ++++---------------------
 src/bytes_mut.rs | 19 +------------------
 src/lib.rs       | 15 +++++++++++++++
 3 files changed, 20 insertions(+), 39 deletions(-)

diff --git a/src/bytes.rs b/src/bytes.rs
index b4359b08d..e23d9a81f 100644
--- a/src/bytes.rs
+++ b/src/bytes.rs
@@ -15,7 +15,7 @@ use crate::buf::IntoIter;
 #[allow(unused)]
 use crate::loom::sync::atomic::AtomicMut;
 use crate::loom::sync::atomic::{AtomicPtr, AtomicUsize, Ordering};
-use crate::{Buf, BytesMut};
+use crate::{offset_from, Buf, BytesMut};
 
 /// A cheaply cloneable and sliceable chunk of contiguous memory.
 ///
@@ -1037,7 +1037,7 @@ unsafe fn promotable_to_vec(
 
         let buf = f(shared);
 
-        let cap = (ptr as usize - buf as usize) + len;
+        let cap = offset_from(ptr, buf) + len;
 
         // Copy back buffer
         ptr::copy(ptr, buf, len);
@@ -1150,7 +1150,7 @@ unsafe fn promotable_is_unique(data: &AtomicPtr<()>) -> bool {
 }
 
 unsafe fn free_boxed_slice(buf: *mut u8, offset: *const u8, len: usize) {
-    let cap = (offset as usize - buf as usize) + len;
+    let cap = offset_from(offset, buf) + len;
     dealloc(buf, Layout::from_size_align(cap, 1).unwrap())
 }
 
@@ -1312,7 +1312,7 @@ unsafe fn shallow_clone_vec(
     // vector.
     let shared = Box::new(Shared {
         buf,
-        cap: (offset as usize - buf as usize) + len,
+        cap: offset_from(offset, buf) + len,
         // Initialize refcount to 2. One for this reference, and one
         // for the new clone that will be returned from
         // `shallow_clone`.
@@ -1422,23 +1422,6 @@ where
     new_addr as *mut u8
 }
 
-/// Precondition: dst >= original
-///
-/// The following line is equivalent to:
-///
-/// ```rust,ignore
-/// self.ptr.as_ptr().offset_from(ptr) as usize;
-/// ```
-///
-/// But due to min rust is 1.39 and it is only stabilized
-/// in 1.47, we cannot use it.
-#[inline]
-fn offset_from(dst: *const u8, original: *const u8) -> usize {
-    debug_assert!(dst >= original);
-
-    dst as usize - original as usize
-}
-
 // compile-fails
 
 /// ```compile_fail
diff --git a/src/bytes_mut.rs b/src/bytes_mut.rs
index 569f8be63..537f01ad3 100644
--- a/src/bytes_mut.rs
+++ b/src/bytes_mut.rs
@@ -17,7 +17,7 @@ use crate::bytes::Vtable;
 #[allow(unused)]
 use crate::loom::sync::atomic::AtomicMut;
 use crate::loom::sync::atomic::{AtomicPtr, AtomicUsize, Ordering};
-use crate::{Buf, BufMut, Bytes};
+use crate::{offset_from, Buf, BufMut, Bytes};
 
 /// A unique reference to a contiguous slice of memory.
 ///
@@ -1683,23 +1683,6 @@ fn invalid_ptr<T>(addr: usize) -> *mut T {
     ptr.cast::<T>()
 }
 
-/// Precondition: dst >= original
-///
-/// The following line is equivalent to:
-///
-/// ```rust,ignore
-/// self.ptr.as_ptr().offset_from(ptr) as usize;
-/// ```
-///
-/// But due to min rust is 1.39 and it is only stabilized
-/// in 1.47, we cannot use it.
-#[inline]
-fn offset_from(dst: *mut u8, original: *mut u8) -> usize {
-    debug_assert!(dst >= original);
-
-    dst as usize - original as usize
-}
-
 unsafe fn rebuild_vec(ptr: *mut u8, mut len: usize, mut cap: usize, off: usize) -> Vec<u8> {
     let ptr = ptr.sub(off);
     len += off;
diff --git a/src/lib.rs b/src/lib.rs
index 4dd118007..7ddd2205b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -148,3 +148,18 @@ fn panic_does_not_fit(size: usize, nbytes: usize) -> ! {
         size, nbytes
     );
 }
+
+/// Precondition: dst >= original
+///
+/// The following line is equivalent to:
+///
+/// ```rust,ignore
+/// self.ptr.as_ptr().offset_from(ptr) as usize;
+/// ```
+///
+/// But due to min rust is 1.39 and it is only stabilized
+/// in 1.47, we cannot use it.
+#[inline]
+fn offset_from(dst: *const u8, original: *const u8) -> usize {
+    dst as usize - original as usize
+}