Rollup merge of rust-lang#90556 - scottmcm:carrying_comments, r=joshtriplett

Add more text and examples to `carrying_{add|mul}`

`feature(bigint_helper_methods)` tracking issue rust-lang#85532

cc ``@clarfonthey``

Author: matthiaskrgr

library/core/src/num/mod.rs

@@ -94,7 +94,13 @@ depending on the target pointer size.
 }
 
 macro_rules! widening_impl {
-    ($SelfT:ty, $WideT:ty, $BITS:literal) => {
+    ($SelfT:ty, $WideT:ty, $BITS:literal, unsigned) => {
+        widening_impl!($SelfT, $WideT, $BITS, "");
+    };
+    ($SelfT:ty, $WideT:ty, $BITS:literal, signed) => {
+        widening_impl!($SelfT, $WideT, $BITS, "# //");
+    };
+    ($SelfT:ty, $WideT:ty, $BITS:literal, $AdaptiveTestPrefix:literal) => {
         /// Calculates the complete product `self * rhs` without the possibility to overflow.
         ///
         /// This returns the low-order (wrapping) bits and the high-order (overflow) bits
@@ -148,6 +154,33 @@ macro_rules! widening_impl {
         /// assert_eq!(5u32.carrying_mul(2, 10), (20, 0));
         /// assert_eq!(1_000_000_000u32.carrying_mul(10, 0), (1410065408, 2));
         /// assert_eq!(1_000_000_000u32.carrying_mul(10, 10), (1410065418, 2));
+        #[doc = concat!($AdaptiveTestPrefix, "assert_eq!(",
+            stringify!($SelfT), "::MAX.carrying_mul(", stringify!($SelfT), "::MAX, ", stringify!($SelfT), "::MAX), ",
+            "(0, ", stringify!($SelfT), "::MAX));"
+        )]
+        /// ```
+        ///
+        /// If `carry` is zero, this is similar to [`overflowing_mul`](Self::overflowing_mul),
+        /// except that it gives the value of the overflow instead of just whether one happened:
+        ///
+        /// ```
+        /// #![feature(bigint_helper_methods)]
+        /// let r = u8::carrying_mul(7, 13, 0);
+        /// assert_eq!((r.0, r.1 != 0), u8::overflowing_mul(7, 13));
+        /// let r = u8::carrying_mul(13, 42, 0);
+        /// assert_eq!((r.0, r.1 != 0), u8::overflowing_mul(13, 42));
+        /// ```
+        ///
+        /// The value of the first field in the returned tuple matches what you'd get
+        /// by combining the [`wrapping_mul`](Self::wrapping_mul) and
+        /// [`wrapping_add`](Self::wrapping_add) methods:
+        ///
+        /// ```
+        /// #![feature(bigint_helper_methods)]
+        /// assert_eq!(
+        ///     789_u16.carrying_mul(456, 123).0,
+        ///     789_u16.wrapping_mul(456).wrapping_add(123),
+        /// );
         /// ```
         #[unstable(feature = "bigint_helper_methods", issue = "85532")]
         #[rustc_const_unstable(feature = "bigint_helper_methods", issue = "85532")]
@@ -168,29 +201,29 @@ macro_rules! widening_impl {
 
 #[lang = "i8"]
 impl i8 {
-    widening_impl! { i8, i16, 8 }
+    widening_impl! { i8, i16, 8, signed }
     int_impl! { i8, i8, u8, 8, 7, -128, 127, 2, "-0x7e", "0xa", "0x12", "0x12", "0x48",
     "[0x12]", "[0x12]", "", "" }
 }
 
 #[lang = "i16"]
 impl i16 {
-    widening_impl! { i16, i32, 16 }
+    widening_impl! { i16, i32, 16, signed }
     int_impl! { i16, i16, u16, 16, 15, -32768, 32767, 4, "-0x5ffd", "0x3a", "0x1234", "0x3412",
     "0x2c48", "[0x34, 0x12]", "[0x12, 0x34]", "", "" }
 }
 
 #[lang = "i32"]
 impl i32 {
-    widening_impl! { i32, i64, 32 }
+    widening_impl! { i32, i64, 32, signed }
     int_impl! { i32, i32, u32, 32, 31, -2147483648, 2147483647, 8, "0x10000b3", "0xb301",
     "0x12345678", "0x78563412", "0x1e6a2c48", "[0x78, 0x56, 0x34, 0x12]",
     "[0x12, 0x34, 0x56, 0x78]", "", "" }
 }
 
 #[lang = "i64"]
 impl i64 {
-    widening_impl! { i64, i128, 64 }
+    widening_impl! { i64, i128, 64, signed }
     int_impl! { i64, i64, u64, 64, 63, -9223372036854775808, 9223372036854775807, 12,
     "0xaa00000000006e1", "0x6e10aa", "0x1234567890123456", "0x5634129078563412",
     "0x6a2c48091e6a2c48", "[0x56, 0x34, 0x12, 0x90, 0x78, 0x56, 0x34, 0x12]",
@@ -212,7 +245,7 @@ impl i128 {
 #[cfg(target_pointer_width = "16")]
 #[lang = "isize"]
 impl isize {
-    widening_impl! { isize, i32, 16 }
+    widening_impl! { isize, i32, 16, signed }
     int_impl! { isize, i16, usize, 16, 15, -32768, 32767, 4, "-0x5ffd", "0x3a", "0x1234",
     "0x3412", "0x2c48", "[0x34, 0x12]", "[0x12, 0x34]",
     usize_isize_to_xe_bytes_doc!(), usize_isize_from_xe_bytes_doc!() }
@@ -221,7 +254,7 @@ impl isize {
 #[cfg(target_pointer_width = "32")]
 #[lang = "isize"]
 impl isize {
-    widening_impl! { isize, i64, 32 }
+    widening_impl! { isize, i64, 32, signed }
     int_impl! { isize, i32, usize, 32, 31, -2147483648, 2147483647, 8, "0x10000b3", "0xb301",
     "0x12345678", "0x78563412", "0x1e6a2c48", "[0x78, 0x56, 0x34, 0x12]",
     "[0x12, 0x34, 0x56, 0x78]",
@@ -231,7 +264,7 @@ impl isize {
 #[cfg(target_pointer_width = "64")]
 #[lang = "isize"]
 impl isize {
-    widening_impl! { isize, i128, 64 }
+    widening_impl! { isize, i128, 64, signed }
     int_impl! { isize, i64, usize, 64, 63, -9223372036854775808, 9223372036854775807,
     12, "0xaa00000000006e1", "0x6e10aa",  "0x1234567890123456", "0x5634129078563412",
      "0x6a2c48091e6a2c48", "[0x56, 0x34, 0x12, 0x90, 0x78, 0x56, 0x34, 0x12]",
@@ -244,7 +277,7 @@ const ASCII_CASE_MASK: u8 = 0b0010_0000;
 
 #[lang = "u8"]
 impl u8 {
-    widening_impl! { u8, u16, 8 }
+    widening_impl! { u8, u16, 8, unsigned }
     uint_impl! { u8, u8, i8, 8, 255, 2, "0x82", "0xa", "0x12", "0x12", "0x48", "[0x12]",
     "[0x12]", "", "" }
 
@@ -793,21 +826,21 @@ impl u8 {
 
 #[lang = "u16"]
 impl u16 {
-    widening_impl! { u16, u32, 16 }
+    widening_impl! { u16, u32, 16, unsigned }
     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 }
+    widening_impl! { u32, u64, 32, unsigned }
     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 }
+    widening_impl! { u64, u128, 64, unsigned }
     uint_impl! { u64, u64, i64, 64, 18446744073709551615, 12, "0xaa00000000006e1", "0x6e10aa",
     "0x1234567890123456", "0x5634129078563412", "0x6a2c48091e6a2c48",
     "[0x56, 0x34, 0x12, 0x90, 0x78, 0x56, 0x34, 0x12]",
@@ -830,15 +863,15 @@ impl u128 {
 #[cfg(target_pointer_width = "16")]
 #[lang = "usize"]
 impl usize {
-    widening_impl! { usize, u32, 16 }
+    widening_impl! { usize, u32, 16, unsigned }
     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!() }
 }
 #[cfg(target_pointer_width = "32")]
 #[lang = "usize"]
 impl usize {
-    widening_impl! { usize, u64, 32 }
+    widening_impl! { usize, u64, 32, unsigned }
     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!() }
@@ -847,7 +880,7 @@ impl usize {
 #[cfg(target_pointer_width = "64")]
 #[lang = "usize"]
 impl usize {
-    widening_impl! { usize, u128, 64 }
+    widening_impl! { usize, u128, 64, unsigned }
     uint_impl! { usize, u64, isize, 64, 18446744073709551615, 12, "0xaa00000000006e1", "0x6e10aa",
     "0x1234567890123456", "0x5634129078563412", "0x6a2c48091e6a2c48",
     "[0x56, 0x34, 0x12, 0x90, 0x78, 0x56, 0x34, 0x12]",

library/core/src/num/uint_macros.rs

@@ -1504,6 +1504,8 @@ macro_rules! uint_impl {
         /// additional bit of overflow. This allows for chaining together multiple additions
         /// to create "big integers" which represent larger values.
         ///
+        #[doc = concat!("This can be thought of as a ", stringify!($BITS), "-bit \"full adder\", in the electronics sense.")]
+        ///
         /// # Examples
         ///
         /// Basic usage
@@ -1513,7 +1515,20 @@ macro_rules! uint_impl {
         #[doc = concat!("assert_eq!(5", stringify!($SelfT), ".carrying_add(2, false), (7, false));")]
         #[doc = concat!("assert_eq!(5", stringify!($SelfT), ".carrying_add(2, true), (8, false));")]
         #[doc = concat!("assert_eq!(", stringify!($SelfT), "::MAX.carrying_add(1, false), (0, true));")]
+        #[doc = concat!("assert_eq!(", stringify!($SelfT), "::MAX.carrying_add(0, true), (0, true));")]
         #[doc = concat!("assert_eq!(", stringify!($SelfT), "::MAX.carrying_add(1, true), (1, true));")]
+        #[doc = concat!("assert_eq!(",
+            stringify!($SelfT), "::MAX.carrying_add(", stringify!($SelfT), "::MAX, true), ",
+            "(", stringify!($SelfT), "::MAX, true));"
+        )]
+        /// ```
+        ///
+        /// If `carry` is false, this method is equivalent to [`overflowing_add`](Self::overflowing_add):
+        ///
+        /// ```
+        /// #![feature(bigint_helper_methods)]
+        #[doc = concat!("assert_eq!(5_", stringify!($SelfT), ".carrying_add(2, false), 5_", stringify!($SelfT), ".overflowing_add(2));")]
+        #[doc = concat!("assert_eq!(", stringify!($SelfT), "::MAX.carrying_add(1, false), ", stringify!($SelfT), "::MAX.overflowing_add(1));")]
         /// ```
         #[unstable(feature = "bigint_helper_methods", issue = "85532")]
         #[rustc_const_unstable(feature = "const_bigint_helper_methods", issue = "85532")]