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Update wrapping_sh[lr] docs and examples #149837

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ff43366
Update `wrapping_sh[lr]` docs and examples
scottmcm Dec 10, 2025
f9b830c
Add more basic shift examples
scottmcm Dec 11, 2025
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28 changes: 24 additions & 4 deletions library/core/src/num/int_macros.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2288,6 +2288,13 @@ macro_rules! int_impl {
/// Panic-free bitwise shift-left; yields `self << mask(rhs)`, where `mask` removes
/// any high-order bits of `rhs` that would cause the shift to exceed the bitwidth of the type.
///
/// Beware that, unlike most other `wrapping_*` methods on integers, this
/// does *not* give the same result as doing the shift in infinite precision
/// then truncating as needed. The behaviour matches what shift instructions
/// do on many processors, and is what the `<<` operator does when overflow
/// checks are disabled, but numerically it's weird. Consider, instead,
/// using [`Self::unbounded_shl`] which has nicer behaviour.
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@workingjubilee workingjubilee Dec 13, 2025

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Wait, hm.

Is "nicer" quite right? "More numerical" behavior? "As an integer"? "More logical"? "Is more similar to the mul-by-pow-2 operation you were looking for"?

///
/// Note that this is *not* the same as a rotate-left; the RHS of a wrapping shift-left is restricted to
/// the range of the type, rather than the bits shifted out of the LHS being returned to the other end.
/// The primitive integer types all implement a [`rotate_left`](Self::rotate_left) function,
Expand All @@ -2296,8 +2303,11 @@ macro_rules! int_impl {
/// # Examples
///
/// ```
#[doc = concat!("assert_eq!((-1", stringify!($SelfT), ").wrapping_shl(7), -128);")]
#[doc = concat!("assert_eq!((-1", stringify!($SelfT), ").wrapping_shl(128), -1);")]
#[doc = concat!("assert_eq!((-1_", stringify!($SelfT), ").wrapping_shl(7), -128);")]
#[doc = concat!("assert_eq!(42_", stringify!($SelfT), ".wrapping_shl(", stringify!($BITS), "), 42);")]
#[doc = concat!("assert_eq!(42_", stringify!($SelfT), ".wrapping_shl(1).wrapping_shl(", stringify!($BITS_MINUS_ONE), "), 0);")]
#[doc = concat!("assert_eq!((-1_", stringify!($SelfT), ").wrapping_shl(128), -1);")]
#[doc = concat!("assert_eq!(5_", stringify!($SelfT), ".wrapping_shl(1025), 10);")]
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@JonathanBrouwer JonathanBrouwer Dec 10, 2025

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Might be nice to do some of these examples in binary (0b) or hex (0x), so one can see the bits move better

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@workingjubilee workingjubilee Dec 10, 2025 •
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Yeah, actually, demonstrating the shift from 0b0101 to 0b1010 sounds nice. Don't have to do it for all of them, ofc.

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@scottmcm scottmcm Dec 11, 2025

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Good call; I added a couple more basic examples to show the bits before the extreme examples.

/// ```
#[stable(feature = "num_wrapping", since = "1.2.0")]
#[rustc_const_stable(feature = "const_int_methods", since = "1.32.0")]
Expand All @@ -2315,6 +2325,13 @@ macro_rules! int_impl {
/// Panic-free bitwise shift-right; yields `self >> mask(rhs)`, where `mask`
/// removes any high-order bits of `rhs` that would cause the shift to exceed the bitwidth of the type.
///
/// Beware that, unlike most other `wrapping_*` methods on integers, this
/// does *not* give the same result as doing the shift in infinite precision
/// then truncating as needed. The behaviour matches what shift instructions
/// do on many processors, and is what the `>>` operator does when overflow
/// checks are disabled, but numerically it's weird. Consider, instead,
/// using [`Self::unbounded_shr`] which has nicer behaviour.
///
/// Note that this is *not* the same as a rotate-right; the RHS of a wrapping shift-right is restricted
/// to the range of the type, rather than the bits shifted out of the LHS being returned to the other
/// end. The primitive integer types all implement a [`rotate_right`](Self::rotate_right) function,
Expand All @@ -2323,8 +2340,11 @@ macro_rules! int_impl {
/// # Examples
///
/// ```
#[doc = concat!("assert_eq!((-128", stringify!($SelfT), ").wrapping_shr(7), -1);")]
/// assert_eq!((-128i16).wrapping_shr(64), -128);
#[doc = concat!("assert_eq!((-128_", stringify!($SelfT), ").wrapping_shr(7), -1);")]
#[doc = concat!("assert_eq!(42_", stringify!($SelfT), ".wrapping_shr(", stringify!($BITS), "), 42);")]
#[doc = concat!("assert_eq!(42_", stringify!($SelfT), ".wrapping_shr(1).wrapping_shr(", stringify!($BITS_MINUS_ONE), "), 0);")]
/// assert_eq!((-128_i16).wrapping_shr(64), -128);
#[doc = concat!("assert_eq!(10_", stringify!($SelfT), ".wrapping_shr(1025), 5);")]
/// ```
#[stable(feature = "num_wrapping", since = "1.2.0")]
#[rustc_const_stable(feature = "const_int_methods", since = "1.32.0")]
Expand Down
36 changes: 32 additions & 4 deletions library/core/src/num/uint_macros.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2593,6 +2593,13 @@ macro_rules! uint_impl {
/// where `mask` removes any high-order bits of `rhs` that
/// would cause the shift to exceed the bitwidth of the type.
///
/// Beware that, unlike most other `wrapping_*` methods on integers, this
/// does *not* give the same result as doing the shift in infinite precision
/// then truncating as needed. The behaviour matches what shift instructions
/// do on many processors, and is what the `<<` operator does when overflow
/// checks are disabled, but numerically it's weird. Consider, instead,
/// using [`Self::unbounded_shl`] which has nicer behaviour.
///
/// Note that this is *not* the same as a rotate-left; the
/// RHS of a wrapping shift-left is restricted to the range
/// of the type, rather than the bits shifted out of the LHS
Expand All @@ -2603,8 +2610,15 @@ macro_rules! uint_impl {
/// # Examples
///
/// ```
#[doc = concat!("assert_eq!(1", stringify!($SelfT), ".wrapping_shl(7), 128);")]
#[doc = concat!("assert_eq!(1", stringify!($SelfT), ".wrapping_shl(128), 1);")]
#[doc = concat!("assert_eq!(1_", stringify!($SelfT), ".wrapping_shl(7), 128);")]
#[doc = concat!("assert_eq!(0b101_", stringify!($SelfT), ".wrapping_shl(0), 0b101);")]
#[doc = concat!("assert_eq!(0b101_", stringify!($SelfT), ".wrapping_shl(1), 0b1010);")]
#[doc = concat!("assert_eq!(0b101_", stringify!($SelfT), ".wrapping_shl(2), 0b10100);")]
#[doc = concat!("assert_eq!(", stringify!($SelfT), "::MAX.wrapping_shl(2), ", stringify!($SelfT), "::MAX - 3);")]
#[doc = concat!("assert_eq!(42_", stringify!($SelfT), ".wrapping_shl(", stringify!($BITS), "), 42);")]
#[doc = concat!("assert_eq!(42_", stringify!($SelfT), ".wrapping_shl(1).wrapping_shl(", stringify!($BITS_MINUS_ONE), "), 0);")]
#[doc = concat!("assert_eq!(1_", stringify!($SelfT), ".wrapping_shl(128), 1);")]
#[doc = concat!("assert_eq!(5_", stringify!($SelfT), ".wrapping_shl(1025), 10);")]
/// ```
#[stable(feature = "num_wrapping", since = "1.2.0")]
#[rustc_const_stable(feature = "const_wrapping_math", since = "1.32.0")]
Expand All @@ -2623,6 +2637,13 @@ macro_rules! uint_impl {
/// where `mask` removes any high-order bits of `rhs` that
/// would cause the shift to exceed the bitwidth of the type.
///
/// Beware that, unlike most other `wrapping_*` methods on integers, this
/// does *not* give the same result as doing the shift in infinite precision
/// then truncating as needed. The behaviour matches what shift instructions
/// do on many processors, and is what the `>>` operator does when overflow
/// checks are disabled, but numerically it's weird. Consider, instead,
/// using [`Self::unbounded_shr`] which has nicer behaviour.
///
/// Note that this is *not* the same as a rotate-right; the
/// RHS of a wrapping shift-right is restricted to the range
/// of the type, rather than the bits shifted out of the LHS
Expand All @@ -2633,8 +2654,15 @@ macro_rules! uint_impl {
/// # Examples
///
/// ```
#[doc = concat!("assert_eq!(128", stringify!($SelfT), ".wrapping_shr(7), 1);")]
#[doc = concat!("assert_eq!(128", stringify!($SelfT), ".wrapping_shr(128), 128);")]
#[doc = concat!("assert_eq!(128_", stringify!($SelfT), ".wrapping_shr(7), 1);")]
#[doc = concat!("assert_eq!(0b1010_", stringify!($SelfT), ".wrapping_shr(0), 0b1010);")]
#[doc = concat!("assert_eq!(0b1010_", stringify!($SelfT), ".wrapping_shr(1), 0b101);")]
#[doc = concat!("assert_eq!(0b1010_", stringify!($SelfT), ".wrapping_shr(2), 0b10);")]
#[doc = concat!("assert_eq!(", stringify!($SelfT), "::MAX.wrapping_shr(1), ", stringify!($SignedT), "::MAX.cast_unsigned());")]
#[doc = concat!("assert_eq!(42_", stringify!($SelfT), ".wrapping_shr(", stringify!($BITS), "), 42);")]
#[doc = concat!("assert_eq!(42_", stringify!($SelfT), ".wrapping_shr(1).wrapping_shr(", stringify!($BITS_MINUS_ONE), "), 0);")]
#[doc = concat!("assert_eq!(128_", stringify!($SelfT), ".wrapping_shr(128), 128);")]
#[doc = concat!("assert_eq!(10_", stringify!($SelfT), ".wrapping_shr(1025), 5);")]
/// ```
#[stable(feature = "num_wrapping", since = "1.2.0")]
#[rustc_const_stable(feature = "const_wrapping_math", since = "1.32.0")]
Expand Down
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