Partial-stabilize the basics from bigint_helper_methods

Author: scottmcm

library/core/src/num/uint_macros.rs

@@ -2421,7 +2421,7 @@ macro_rules! uint_impl {
         }
 
         /// Calculates `self` + `rhs` + `carry` and returns a tuple containing
-        /// the sum and the output carry.
+        /// the sum and the output carry (in that order).
         ///
         /// Performs "ternary addition" of two integer operands and a carry-in
         /// bit, and returns an output integer and a carry-out bit. This allows
@@ -2439,8 +2439,6 @@ macro_rules! uint_impl {
         /// # Examples
         ///
         /// ```
-        /// #![feature(bigint_helper_methods)]
-        ///
         #[doc = concat!("//    3  MAX    (a = 3 × 2^", stringify!($BITS), " + 2^", stringify!($BITS), " - 1)")]
         #[doc = concat!("// +  5    7    (b = 5 × 2^", stringify!($BITS), " + 7)")]
         /// // ---------
@@ -2457,7 +2455,7 @@ macro_rules! uint_impl {
         ///
         /// assert_eq!((sum1, sum0), (9, 6));
         /// ```
-        #[unstable(feature = "bigint_helper_methods", issue = "85532")]
+        #[stable(feature = "min_bigint_helpers", since = "CURRENT_RUSTC_VERSION")]
         #[rustc_const_unstable(feature = "bigint_helper_methods", issue = "85532")]
         #[must_use = "this returns the result of the operation, \
                       without modifying the original"]
@@ -2644,25 +2642,41 @@ macro_rules! uint_impl {
             (a as Self, b)
         }
 
-        /// Calculates the complete product `self * rhs` without the possibility to overflow.
+        /// Calculates the complete double-width product `self * rhs`.
         ///
         /// This returns the low-order (wrapping) bits and the high-order (overflow) bits
-        /// of the result as two separate values, in that order.
+        /// of the result as two separate values, in that order. As such,
+        /// `a.widening_mul(b).0` produces the same result as `a.wrapping_mul(b)`.
+        ///
+        /// If you also need to add a value and/or carry to the wide result, then you want
+        /// [`Self::carrying_mul_add`] instead.
         ///
-        /// If you also need to add a carry to the wide result, then you want
-        /// [`Self::carrying_mul`] instead.
+        /// If you just want to know *whether* the multiplication overflowed, then you
+        /// want [`Self::overflowing_mul`] instead.
         ///
         /// # Examples
         ///
+        /// ```
+        #[doc = concat!("assert_eq!(5_", stringify!($SelfT), ".widening_mul(7), (35, 0));")]
+        #[doc = concat!("assert_eq!(", stringify!($SelfT), "::MAX.widening_mul(", stringify!($SelfT), "::MAX), (1, ", stringify!($SelfT), "::MAX - 1));")]
+        /// ```
+        ///
+        /// Compared to other `*_mul` methods:
+        /// ```
+        #[doc = concat!("assert_eq!(", stringify!($SelfT), "::widening_mul(1 << ", stringify!($BITS_MINUS_ONE), ", 6), (0, 3));")]
+        #[doc = concat!("assert_eq!(", stringify!($SelfT), "::overflowing_mul(1 << ", stringify!($BITS_MINUS_ONE), ", 6), (0, true));")]
+        #[doc = concat!("assert_eq!(", stringify!($SelfT), "::wrapping_mul(1 << ", stringify!($BITS_MINUS_ONE), ", 6), 0);")]
+        #[doc = concat!("assert_eq!(", stringify!($SelfT), "::checked_mul(1 << ", stringify!($BITS_MINUS_ONE), ", 6), None);")]
+        /// ```
+        ///
         /// Please note that this example is shared between integer types.
         /// Which explains why `u32` is used here.
         ///
         /// ```
-        /// #![feature(bigint_helper_methods)]
         /// assert_eq!(5u32.widening_mul(2), (10, 0));
         /// assert_eq!(1_000_000_000u32.widening_mul(10), (1410065408, 2));
         /// ```
-        #[unstable(feature = "bigint_helper_methods", issue = "85532")]
+        #[stable(feature = "min_bigint_helpers", since = "CURRENT_RUSTC_VERSION")]
         #[rustc_const_unstable(feature = "bigint_helper_methods", issue = "85532")]
         #[must_use = "this returns the result of the operation, \
                       without modifying the original"]
@@ -2756,26 +2770,27 @@ macro_rules! uint_impl {
             Self::carrying_mul_add(self, rhs, carry, 0)
         }
 
-        /// Calculates the "full multiplication" `self * rhs + carry1 + carry2`
-        /// without the possibility to overflow.
+        /// Calculates the "full multiplication" `self * rhs + carry1 + carry2`.
         ///
         /// This returns the low-order (wrapping) bits and the high-order (overflow) bits
         /// of the result as two separate values, in that order.
         ///
+        /// This cannot overflow, as the double-width result has exactly enough
+        /// space for the largest possible result. This is equivalent to how, in
+        /// decimal, 9 × 9 + 9 + 9 = 81 + 18 = 99 = 9×10⁰ + 9×10¹ = 10² - 1.
+        ///
         /// Performs "long multiplication" which takes in an extra amount to add, and may return an
         /// additional amount of overflow. This allows for chaining together multiple
         /// multiplications to create "big integers" which represent larger values.
         ///
-        /// If you don't need either `carry`, then you can use [`Self::widening_mul`] instead,
-        /// and if you only need one `carry`, then you can use [`Self::carrying_mul`] instead.
+        /// If you don't need either `carry`, then you can use [`Self::widening_mul`] instead.
         ///
         /// # Examples
         ///
         /// Please note that this example is shared between integer types,
         /// which explains why `u32` is used here.
         ///
         /// ```
-        /// #![feature(bigint_helper_methods)]
         /// assert_eq!(5u32.carrying_mul_add(2, 0, 0), (10, 0));
         /// assert_eq!(5u32.carrying_mul_add(2, 10, 10), (30, 0));
         /// assert_eq!(1_000_000_000u32.carrying_mul_add(10, 0, 0), (1410065408, 2));
@@ -2792,8 +2807,6 @@ macro_rules! uint_impl {
         /// using `u8` for simplicity of the demonstration.
         ///
         /// ```
-        /// #![feature(bigint_helper_methods)]
-        ///
         /// fn quadratic_mul<const N: usize>(a: [u8; N], b: [u8; N]) -> [u8; N] {
         ///     let mut out = [0; N];
         ///     for j in 0..N {
@@ -2808,13 +2821,13 @@ macro_rules! uint_impl {
         /// // -1 * -1 == 1
         /// assert_eq!(quadratic_mul([0xFF; 3], [0xFF; 3]), [1, 0, 0]);
         ///
-        /// assert_eq!(u32::wrapping_mul(0x9e3779b9, 0x7f4a7c15), 0xCFFC982D);
+        /// assert_eq!(u32::wrapping_mul(0x9e3779b9, 0x7f4a7c15), 0xcffc982d);
         /// assert_eq!(
         ///     quadratic_mul(u32::to_le_bytes(0x9e3779b9), u32::to_le_bytes(0x7f4a7c15)),
-        ///     u32::to_le_bytes(0xCFFC982D)
+        ///     u32::to_le_bytes(0xcffc982d)
         /// );
         /// ```
-        #[unstable(feature = "bigint_helper_methods", issue = "85532")]
+        #[stable(feature = "min_bigint_helpers", since = "CURRENT_RUSTC_VERSION")]
         #[rustc_const_unstable(feature = "bigint_helper_methods", issue = "85532")]
         #[must_use = "this returns the result of the operation, \
                       without modifying the original"]