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Document consteval behavior of ub_checks, overflow_checks, is_val_statically_known. #154140

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rust-bors merged 2 commits into from
Mar 23, 2026
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Merged

Document consteval behavior of ub_checks, overflow_checks, is_val_statically_known. #154140

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8dcb257
Document consteval behavior of ub_checks & overflow_checks.
theemathas Mar 23, 2026
338deef
Remove outdated consteval docs for is_val_statically_known.
theemathas Mar 23, 2026
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20 changes: 12 additions & 8 deletions library/core/src/intrinsics/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2502,14 +2502,6 @@ pub(crate) macro const_eval_select {
/// particular value, ever. However, the compiler will generally make it
/// return `true` only if the value of the argument is actually known.
///
/// # Stability concerns
///
/// While it is safe to call, this intrinsic may behave differently in
/// a `const` context than otherwise. See the [`const_eval_select()`]
/// documentation for an explanation of the issues this can cause. Unlike
/// `const_eval_select`, this intrinsic isn't guaranteed to behave
/// deterministically even in a `const` context.
///
/// # Type Requirements
///
/// `T` must be either a `bool`, a `char`, a primitive numeric type (e.g. `f32`,
Expand Down Expand Up @@ -2590,6 +2582,12 @@ pub const unsafe fn typed_swap_nonoverlapping<T>(x: *mut T, y: *mut T) {
/// assertions are enabled whenever the *user crate* has UB checks enabled. However, if the
/// user has UB checks disabled, the checks will still get optimized out. This intrinsic is
/// primarily used by [`crate::ub_checks::assert_unsafe_precondition`].
///
/// # Consteval
///
/// In consteval, this function currently returns `true`. This is because the value of the `ub_checks`
/// configuration can differ across crates, but we need this function to always return the same
/// value in consteval in order to avoid unsoundness.
#[rustc_intrinsic_const_stable_indirect] // just for UB checks
#[inline(always)]
#[rustc_intrinsic]
Expand All @@ -2609,6 +2607,12 @@ pub const fn ub_checks() -> bool {
/// `#[inline]`), gating assertions on `overflow_checks()` rather than `cfg!(overflow_checks)` means that
/// assertions are enabled whenever the *user crate* has overflow checks enabled. However if the
/// user has overflow checks disabled, the checks will still get optimized out.
///
/// # Consteval
///
/// In consteval, this function currently returns `true`. This is because the value of the `overflow_checks`
/// configuration can differ across crates, but we need this function to always return the same
/// value in consteval in order to avoid unsoundness.
#[inline(always)]
#[rustc_intrinsic]
pub const fn overflow_checks() -> bool {
Expand Down
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