diff --git a/library/core/src/lib.rs b/library/core/src/lib.rs
index 4408b5a3d2088..f41eeae9cc1fa 100644
--- a/library/core/src/lib.rs
+++ b/library/core/src/lib.rs
@@ -142,6 +142,7 @@
 #![feature(link_llvm_intrinsics)]
 #![feature(llvm_asm)]
 #![feature(min_specialization)]
+#![feature(mixed_integer_ops)]
 #![cfg_attr(not(bootstrap), feature(must_not_suspend))]
 #![feature(negative_impls)]
 #![feature(never_type)]
diff --git a/library/core/src/num/int_macros.rs b/library/core/src/num/int_macros.rs
index daef5c98967cc..5bdb6ed91761f 100644
--- a/library/core/src/num/int_macros.rs
+++ b/library/core/src/num/int_macros.rs
@@ -433,6 +433,28 @@ macro_rules! int_impl {
             unsafe { intrinsics::unchecked_add(self, rhs) }
         }
 
+        /// Checked addition with an unsigned integer. Computes `self + rhs`,
+        /// returning `None` if overflow occurred.
+        ///
+        /// # Examples
+        ///
+        /// Basic usage:
+        ///
+        /// ```
+        /// # #![feature(mixed_integer_ops)]
+        #[doc = concat!("assert_eq!(1", stringify!($SelfT), ".checked_add_unsigned(2), Some(3));")]
+        #[doc = concat!("assert_eq!((", stringify!($SelfT), "::MAX - 2).checked_add_unsigned(3), None);")]
+        /// ```
+        #[unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[rustc_const_unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[must_use = "this returns the result of the operation, \
+                      without modifying the original"]
+        #[inline]
+        pub const fn checked_add_unsigned(self, rhs: $UnsignedT) -> Option<Self> {
+            let (a, b) = self.overflowing_add_unsigned(rhs);
+            if unlikely!(b) {None} else {Some(a)}
+        }
+
         /// Checked integer subtraction. Computes `self - rhs`, returning `None` if
         /// overflow occurred.
         ///
@@ -479,6 +501,28 @@ macro_rules! int_impl {
             unsafe { intrinsics::unchecked_sub(self, rhs) }
         }
 
+        /// Checked subtraction with an unsigned integer. Computes `self - rhs`,
+        /// returning `None` if overflow occurred.
+        ///
+        /// # Examples
+        ///
+        /// Basic usage:
+        ///
+        /// ```
+        /// # #![feature(mixed_integer_ops)]
+        #[doc = concat!("assert_eq!(1", stringify!($SelfT), ".checked_sub_unsigned(2), Some(-1));")]
+        #[doc = concat!("assert_eq!((", stringify!($SelfT), "::MIN + 2).checked_sub_unsigned(3), None);")]
+        /// ```
+        #[unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[rustc_const_unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[must_use = "this returns the result of the operation, \
+                      without modifying the original"]
+        #[inline]
+        pub const fn checked_sub_unsigned(self, rhs: $UnsignedT) -> Option<Self> {
+            let (a, b) = self.overflowing_sub_unsigned(rhs);
+            if unlikely!(b) {None} else {Some(a)}
+        }
+
         /// Checked integer multiplication. Computes `self * rhs`, returning `None` if
         /// overflow occurred.
         ///
@@ -822,6 +866,32 @@ macro_rules! int_impl {
             intrinsics::saturating_add(self, rhs)
         }
 
+        /// Saturating addition with an unsigned integer. Computes `self + rhs`,
+        /// saturating at the numeric bounds instead of overflowing.
+        ///
+        /// # Examples
+        ///
+        /// Basic usage:
+        ///
+        /// ```
+        /// # #![feature(mixed_integer_ops)]
+        #[doc = concat!("assert_eq!(1", stringify!($SelfT), ".saturating_add_unsigned(2), 3);")]
+        #[doc = concat!("assert_eq!(", stringify!($SelfT), "::MAX.saturating_add_unsigned(100), ", stringify!($SelfT), "::MAX);")]
+        /// ```
+        #[unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[rustc_const_unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[must_use = "this returns the result of the operation, \
+                      without modifying the original"]
+        #[inline]
+        pub const fn saturating_add_unsigned(self, rhs: $UnsignedT) -> Self {
+            // Overflow can only happen at the upper bound
+            // We cannot use `unwrap_or` here because it is not `const`
+            match self.checked_add_unsigned(rhs) {
+                Some(x) => x,
+                None => Self::MAX,
+            }
+        }
+
         /// Saturating integer subtraction. Computes `self - rhs`, saturating at the
         /// numeric bounds instead of overflowing.
         ///
@@ -843,6 +913,32 @@ macro_rules! int_impl {
             intrinsics::saturating_sub(self, rhs)
         }
 
+        /// Saturating subtraction with an unsigned integer. Computes `self - rhs`,
+        /// saturating at the numeric bounds instead of overflowing.
+        ///
+        /// # Examples
+        ///
+        /// Basic usage:
+        ///
+        /// ```
+        /// # #![feature(mixed_integer_ops)]
+        #[doc = concat!("assert_eq!(100", stringify!($SelfT), ".saturating_sub_unsigned(127), -27);")]
+        #[doc = concat!("assert_eq!(", stringify!($SelfT), "::MIN.saturating_sub_unsigned(100), ", stringify!($SelfT), "::MIN);")]
+        /// ```
+        #[unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[rustc_const_unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[must_use = "this returns the result of the operation, \
+                      without modifying the original"]
+        #[inline]
+        pub const fn saturating_sub_unsigned(self, rhs: $UnsignedT) -> Self {
+            // Overflow can only happen at the lower bound
+            // We cannot use `unwrap_or` here because it is not `const`
+            match self.checked_sub_unsigned(rhs) {
+                Some(x) => x,
+                None => Self::MIN,
+            }
+        }
+
         /// Saturating integer negation. Computes `-self`, returning `MAX` if `self == MIN`
         /// instead of overflowing.
         ///
@@ -998,6 +1094,27 @@ macro_rules! int_impl {
             intrinsics::wrapping_add(self, rhs)
         }
 
+        /// Wrapping (modular) addition with an unsigned integer. Computes
+        /// `self + rhs`, wrapping around at the boundary of the type.
+        ///
+        /// # Examples
+        ///
+        /// Basic usage:
+        ///
+        /// ```
+        /// # #![feature(mixed_integer_ops)]
+        #[doc = concat!("assert_eq!(100", stringify!($SelfT), ".wrapping_add_unsigned(27), 127);")]
+        #[doc = concat!("assert_eq!(", stringify!($SelfT), "::MAX.wrapping_add_unsigned(2), ", stringify!($SelfT), "::MIN + 1);")]
+        /// ```
+        #[unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[rustc_const_unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[must_use = "this returns the result of the operation, \
+                      without modifying the original"]
+        #[inline(always)]
+        pub const fn wrapping_add_unsigned(self, rhs: $UnsignedT) -> Self {
+            self.wrapping_add(rhs as Self)
+        }
+
         /// Wrapping (modular) subtraction. Computes `self - rhs`, wrapping around at the
         /// boundary of the type.
         ///
@@ -1018,6 +1135,27 @@ macro_rules! int_impl {
             intrinsics::wrapping_sub(self, rhs)
         }
 
+        /// Wrapping (modular) subtraction with an unsigned integer. Computes
+        /// `self - rhs`, wrapping around at the boundary of the type.
+        ///
+        /// # Examples
+        ///
+        /// Basic usage:
+        ///
+        /// ```
+        /// # #![feature(mixed_integer_ops)]
+        #[doc = concat!("assert_eq!(0", stringify!($SelfT), ".wrapping_sub_unsigned(127), -127);")]
+        #[doc = concat!("assert_eq!((-2", stringify!($SelfT), ").wrapping_sub_unsigned(", stringify!($UnsignedT), "::MAX), -1);")]
+        /// ```
+        #[unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[rustc_const_unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[must_use = "this returns the result of the operation, \
+                      without modifying the original"]
+        #[inline(always)]
+        pub const fn wrapping_sub_unsigned(self, rhs: $UnsignedT) -> Self {
+            self.wrapping_sub(rhs as Self)
+        }
+
         /// Wrapping (modular) multiplication. Computes `self * rhs`, wrapping around at
         /// the boundary of the type.
         ///
@@ -1368,6 +1506,33 @@ macro_rules! int_impl {
             (sum as $SelfT, carry)
         }
 
+        /// Calculates `self` + `rhs` with an unsigned `rhs`
+        ///
+        /// Returns a tuple of the addition along with a boolean indicating
+        /// whether an arithmetic overflow would occur. If an overflow would
+        /// have occurred then the wrapped value is returned.
+        ///
+        /// # Examples
+        ///
+        /// Basic usage:
+        ///
+        /// ```
+        /// # #![feature(mixed_integer_ops)]
+        #[doc = concat!("assert_eq!(1", stringify!($SelfT), ".overflowing_add_unsigned(2), (3, false));")]
+        #[doc = concat!("assert_eq!((", stringify!($SelfT), "::MIN).overflowing_add_unsigned(", stringify!($UnsignedT), "::MAX), (", stringify!($SelfT), "::MAX, false));")]
+        #[doc = concat!("assert_eq!((", stringify!($SelfT), "::MAX - 2).overflowing_add_unsigned(3), (", stringify!($SelfT), "::MIN, true));")]
+        /// ```
+        #[unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[rustc_const_unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[must_use = "this returns the result of the operation, \
+                      without modifying the original"]
+        #[inline]
+        pub const fn overflowing_add_unsigned(self, rhs: $UnsignedT) -> (Self, bool) {
+            let rhs = rhs as Self;
+            let (res, overflowed) = self.overflowing_add(rhs);
+            (res, overflowed ^ (rhs < 0))
+        }
+
         /// Calculates `self` - `rhs`
         ///
         /// Returns a tuple of the subtraction along with a boolean indicating whether an arithmetic overflow
@@ -1419,6 +1584,33 @@ macro_rules! int_impl {
             (sum as $SelfT, borrow)
         }
 
+        /// Calculates `self` - `rhs` with an unsigned `rhs`
+        ///
+        /// Returns a tuple of the subtraction along with a boolean indicating
+        /// whether an arithmetic overflow would occur. If an overflow would
+        /// have occurred then the wrapped value is returned.
+        ///
+        /// # Examples
+        ///
+        /// Basic usage:
+        ///
+        /// ```
+        /// # #![feature(mixed_integer_ops)]
+        #[doc = concat!("assert_eq!(1", stringify!($SelfT), ".overflowing_sub_unsigned(2), (-1, false));")]
+        #[doc = concat!("assert_eq!((", stringify!($SelfT), "::MAX).overflowing_sub_unsigned(", stringify!($UnsignedT), "::MAX), (", stringify!($SelfT), "::MIN, false));")]
+        #[doc = concat!("assert_eq!((", stringify!($SelfT), "::MIN + 2).overflowing_sub_unsigned(3), (", stringify!($SelfT), "::MAX, true));")]
+        /// ```
+        #[unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[rustc_const_unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[must_use = "this returns the result of the operation, \
+                      without modifying the original"]
+        #[inline]
+        pub const fn overflowing_sub_unsigned(self, rhs: $UnsignedT) -> (Self, bool) {
+            let rhs = rhs as Self;
+            let (res, overflowed) = self.overflowing_sub(rhs);
+            (res, overflowed ^ (rhs < 0))
+        }
+
         /// Calculates the multiplication of `self` and `rhs`.
         ///
         /// Returns a tuple of the multiplication along with a boolean indicating whether an arithmetic overflow
diff --git a/library/core/src/num/mod.rs b/library/core/src/num/mod.rs
index 59b68cbe9c0ce..8966a9c11d2b4 100644
--- a/library/core/src/num/mod.rs
+++ b/library/core/src/num/mod.rs
@@ -245,7 +245,7 @@ const ASCII_CASE_MASK: u8 = 0b0010_0000;
 #[lang = "u8"]
 impl u8 {
     widening_impl! { u8, u16, 8 }
-    uint_impl! { u8, u8, 8, 255, 2, "0x82", "0xa", "0x12", "0x12", "0x48", "[0x12]",
+    uint_impl! { u8, u8, i8, 8, 255, 2, "0x82", "0xa", "0x12", "0x12", "0x48", "[0x12]",
     "[0x12]", "", "" }
 
     /// Checks if the value is within the ASCII range.
@@ -779,21 +779,21 @@ impl u8 {
 #[lang = "u16"]
 impl u16 {
     widening_impl! { u16, u32, 16 }
-    uint_impl! { u16, u16, 16, 65535, 4, "0xa003", "0x3a", "0x1234", "0x3412", "0x2c48",
+    uint_impl! { u16, u16, i16, 16, 65535, 4, "0xa003", "0x3a", "0x1234", "0x3412", "0x2c48",
     "[0x34, 0x12]", "[0x12, 0x34]", "", "" }
 }
 
 #[lang = "u32"]
 impl u32 {
     widening_impl! { u32, u64, 32 }
-    uint_impl! { u32, u32, 32, 4294967295, 8, "0x10000b3", "0xb301", "0x12345678",
+    uint_impl! { u32, u32, i32, 32, 4294967295, 8, "0x10000b3", "0xb301", "0x12345678",
     "0x78563412", "0x1e6a2c48", "[0x78, 0x56, 0x34, 0x12]", "[0x12, 0x34, 0x56, 0x78]", "", "" }
 }
 
 #[lang = "u64"]
 impl u64 {
     widening_impl! { u64, u128, 64 }
-    uint_impl! { u64, u64, 64, 18446744073709551615, 12, "0xaa00000000006e1", "0x6e10aa",
+    uint_impl! { u64, u64, i64, 64, 18446744073709551615, 12, "0xaa00000000006e1", "0x6e10aa",
     "0x1234567890123456", "0x5634129078563412", "0x6a2c48091e6a2c48",
     "[0x56, 0x34, 0x12, 0x90, 0x78, 0x56, 0x34, 0x12]",
     "[0x12, 0x34, 0x56, 0x78, 0x90, 0x12, 0x34, 0x56]",
@@ -802,7 +802,7 @@ impl u64 {
 
 #[lang = "u128"]
 impl u128 {
-    uint_impl! { u128, u128, 128, 340282366920938463463374607431768211455, 16,
+    uint_impl! { u128, u128, i128, 128, 340282366920938463463374607431768211455, 16,
     "0x13f40000000000000000000000004f76", "0x4f7613f4", "0x12345678901234567890123456789012",
     "0x12907856341290785634129078563412", "0x48091e6a2c48091e6a2c48091e6a2c48",
     "[0x12, 0x90, 0x78, 0x56, 0x34, 0x12, 0x90, 0x78, \
@@ -816,7 +816,7 @@ impl u128 {
 #[lang = "usize"]
 impl usize {
     widening_impl! { usize, u32, 16 }
-    uint_impl! { usize, u16, 16, 65535, 4, "0xa003", "0x3a", "0x1234", "0x3412", "0x2c48",
+    uint_impl! { usize, u16, isize, 16, 65535, 4, "0xa003", "0x3a", "0x1234", "0x3412", "0x2c48",
     "[0x34, 0x12]", "[0x12, 0x34]",
     usize_isize_to_xe_bytes_doc!(), usize_isize_from_xe_bytes_doc!() }
 }
@@ -824,7 +824,7 @@ impl usize {
 #[lang = "usize"]
 impl usize {
     widening_impl! { usize, u64, 32 }
-    uint_impl! { usize, u32, 32, 4294967295, 8, "0x10000b3", "0xb301", "0x12345678",
+    uint_impl! { usize, u32, isize, 32, 4294967295, 8, "0x10000b3", "0xb301", "0x12345678",
     "0x78563412", "0x1e6a2c48", "[0x78, 0x56, 0x34, 0x12]", "[0x12, 0x34, 0x56, 0x78]",
     usize_isize_to_xe_bytes_doc!(), usize_isize_from_xe_bytes_doc!() }
 }
@@ -833,7 +833,7 @@ impl usize {
 #[lang = "usize"]
 impl usize {
     widening_impl! { usize, u128, 64 }
-    uint_impl! { usize, u64, 64, 18446744073709551615, 12, "0xaa00000000006e1", "0x6e10aa",
+    uint_impl! { usize, u64, isize, 64, 18446744073709551615, 12, "0xaa00000000006e1", "0x6e10aa",
     "0x1234567890123456", "0x5634129078563412", "0x6a2c48091e6a2c48",
     "[0x56, 0x34, 0x12, 0x90, 0x78, 0x56, 0x34, 0x12]",
      "[0x12, 0x34, 0x56, 0x78, 0x90, 0x12, 0x34, 0x56]",
diff --git a/library/core/src/num/uint_macros.rs b/library/core/src/num/uint_macros.rs
index 8ce8266263091..96375b82582ed 100644
--- a/library/core/src/num/uint_macros.rs
+++ b/library/core/src/num/uint_macros.rs
@@ -1,5 +1,5 @@
 macro_rules! uint_impl {
-    ($SelfT:ty, $ActualT:ident, $BITS:expr, $MaxV:expr,
+    ($SelfT:ty, $ActualT:ident, $SignedT:ident, $BITS:expr, $MaxV:expr,
         $rot:expr, $rot_op:expr, $rot_result:expr, $swap_op:expr, $swapped:expr,
         $reversed:expr, $le_bytes:expr, $be_bytes:expr,
         $to_xe_bytes_doc:expr, $from_xe_bytes_doc:expr) => {
@@ -442,6 +442,29 @@ macro_rules! uint_impl {
             unsafe { intrinsics::unchecked_add(self, rhs) }
         }
 
+        /// Checked addition with a signed integer. Computes `self + rhs`,
+        /// returning `None` if overflow occurred.
+        ///
+        /// # Examples
+        ///
+        /// Basic usage:
+        ///
+        /// ```
+        /// # #![feature(mixed_integer_ops)]
+        #[doc = concat!("assert_eq!(1", stringify!($SelfT), ".checked_add_signed(2), Some(3));")]
+        #[doc = concat!("assert_eq!(1", stringify!($SelfT), ".checked_add_signed(-2), None);")]
+        #[doc = concat!("assert_eq!((", stringify!($SelfT), "::MAX - 2).checked_add_signed(3), None);")]
+        /// ```
+        #[unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[rustc_const_unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[must_use = "this returns the result of the operation, \
+                      without modifying the original"]
+        #[inline]
+        pub const fn checked_add_signed(self, rhs: $SignedT) -> Option<Self> {
+            let (a, b) = self.overflowing_add_signed(rhs);
+            if unlikely!(b) {None} else {Some(a)}
+        }
+
         /// Checked integer subtraction. Computes `self - rhs`, returning
         /// `None` if overflow occurred.
         ///
@@ -995,6 +1018,35 @@ macro_rules! uint_impl {
             intrinsics::saturating_add(self, rhs)
         }
 
+        /// Saturating addition with a signed integer. Computes `self + rhs`,
+        /// saturating at the numeric bounds instead of overflowing.
+        ///
+        /// # Examples
+        ///
+        /// Basic usage:
+        ///
+        /// ```
+        /// # #![feature(mixed_integer_ops)]
+        #[doc = concat!("assert_eq!(1", stringify!($SelfT), ".saturating_add_signed(2), 3);")]
+        #[doc = concat!("assert_eq!(1", stringify!($SelfT), ".saturating_add_signed(-2), 0);")]
+        #[doc = concat!("assert_eq!((", stringify!($SelfT), "::MAX - 2).saturating_add_signed(4), ", stringify!($SelfT), "::MAX);")]
+        /// ```
+        #[unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[rustc_const_unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[must_use = "this returns the result of the operation, \
+                      without modifying the original"]
+        #[inline]
+        pub const fn saturating_add_signed(self, rhs: $SignedT) -> Self {
+            let (res, overflow) = self.overflowing_add(rhs as Self);
+            if overflow == (rhs < 0) {
+                res
+            } else if overflow {
+                Self::MAX
+            } else {
+                0
+            }
+        }
+
         /// Saturating integer subtraction. Computes `self - rhs`, saturating
         /// at the numeric bounds instead of overflowing.
         ///
@@ -1111,6 +1163,28 @@ macro_rules! uint_impl {
             intrinsics::wrapping_add(self, rhs)
         }
 
+        /// Wrapping (modular) addition with a signed integer. Computes
+        /// `self + rhs`, wrapping around at the boundary of the type.
+        ///
+        /// # Examples
+        ///
+        /// Basic usage:
+        ///
+        /// ```
+        /// # #![feature(mixed_integer_ops)]
+        #[doc = concat!("assert_eq!(1", stringify!($SelfT), ".wrapping_add_signed(2), 3);")]
+        #[doc = concat!("assert_eq!(1", stringify!($SelfT), ".wrapping_add_signed(-2), ", stringify!($SelfT), "::MAX);")]
+        #[doc = concat!("assert_eq!((", stringify!($SelfT), "::MAX - 2).wrapping_add_signed(4), 1);")]
+        /// ```
+        #[unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[rustc_const_unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[must_use = "this returns the result of the operation, \
+                      without modifying the original"]
+        #[inline]
+        pub const fn wrapping_add_signed(self, rhs: $SignedT) -> Self {
+            self.wrapping_add(rhs as Self)
+        }
+
         /// Wrapping (modular) subtraction. Computes `self - rhs`,
         /// wrapping around at the boundary of the type.
         ///
@@ -1435,6 +1509,32 @@ macro_rules! uint_impl {
             (c, b | d)
         }
 
+        /// Calculates `self` + `rhs` with a signed `rhs`
+        ///
+        /// Returns a tuple of the addition along with a boolean indicating
+        /// whether an arithmetic overflow would occur. If an overflow would
+        /// have occurred then the wrapped value is returned.
+        ///
+        /// # Examples
+        ///
+        /// Basic usage:
+        ///
+        /// ```
+        /// # #![feature(mixed_integer_ops)]
+        #[doc = concat!("assert_eq!(1", stringify!($SelfT), ".overflowing_add_signed(2), (3, false));")]
+        #[doc = concat!("assert_eq!(1", stringify!($SelfT), ".overflowing_add_signed(-2), (", stringify!($SelfT), "::MAX, true));")]
+        #[doc = concat!("assert_eq!((", stringify!($SelfT), "::MAX - 2).overflowing_add_signed(4), (1, true));")]
+        /// ```
+        #[unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[rustc_const_unstable(feature = "mixed_integer_ops", issue = "87840")]
+        #[must_use = "this returns the result of the operation, \
+                      without modifying the original"]
+        #[inline]
+        pub const fn overflowing_add_signed(self, rhs: $SignedT) -> (Self, bool) {
+            let (res, overflowed) = self.overflowing_add(rhs as Self);
+            (res, overflowed ^ (rhs < 0))
+        }
+
         /// Calculates `self` - `rhs`
         ///
         /// Returns a tuple of the subtraction along with a boolean indicating
diff --git a/library/std/src/io/cursor.rs b/library/std/src/io/cursor.rs
index 25cc5e67ad14e..980b2531192e8 100644
--- a/library/std/src/io/cursor.rs
+++ b/library/std/src/io/cursor.rs
@@ -292,12 +292,7 @@ where
             SeekFrom::End(n) => (self.inner.as_ref().len() as u64, n),
             SeekFrom::Current(n) => (self.pos, n),
         };
-        let new_pos = if offset >= 0 {
-            base_pos.checked_add(offset as u64)
-        } else {
-            base_pos.checked_sub((offset.wrapping_neg()) as u64)
-        };
-        match new_pos {
+        match base_pos.checked_add_signed(offset) {
             Some(n) => {
                 self.pos = n;
                 Ok(self.pos)
diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index b33a3c5d22fe1..1781a3cb7c402 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -297,6 +297,7 @@
 #![feature(maybe_uninit_slice)]
 #![feature(maybe_uninit_uninit_array)]
 #![feature(min_specialization)]
+#![feature(mixed_integer_ops)]
 #![cfg_attr(not(bootstrap), feature(must_not_suspend))]
 #![feature(needs_panic_runtime)]
 #![feature(negative_impls)]