Feature gate: #![feature(const_pointer_is_aligned)]

This is a tracking issue for using ptr.is_aligned() and ptr.is_aligned_to(alignment) in const contexts.

Public API

impl<T: Sized> *(const|mut) T {
    pub const fn is_aligned(self) -> bool;
}

impl<T: ?Sized> *(const|mut) T {
    pub const fn is_aligned_to(self, align: usize) -> bool;
}

impl <T> NonNull<T> {
    pub const fn is_aligned(self) -> bool;
}

Steps / History

• Implementation: Constify is_aligned via align_offset #102795
• NonNull methods stabilized, constness moved to this gate Stabilize non_null_convenience #124498
• Allow const functions to behave differently during constant-evaluation and runtime: Relax const-eval restrictions rfcs#3352
• Final comment period (FCP)¹
• Stabilization PR

Unresolved Questions

• Do we want this at all or should alignment be runtime only / should users be using a stable const_eval_select instead?
• Should these be separate functions guaranteed_aligned{,to} instead?
• What behavior should these functions have during const eval? Some options include:
  1. Return true if the pointer is guaranteed to be aligned at runtime and false if we don't know whether the pointer is aligned at runtime. This is the current implementation.
  2. Same as above, but only document that the function may spuriously fail (during const eval), similar to align_offset. This allows changes to const alignment in the future.
  3. Assign a dummy address to all variables during const eval. Each allocation, including stack variables, would get its own "address space", which overlaps with other const eval address spaces and the runtime address space. This would make const eval alignment behavior similar to runtime, but a runtime pointer derived from a const eval pointer is still differently aligned. Pointer-to-integer casts are still not supported. The implementation for this would just remove this check: https://github.com/rust-lang/rust/blob/005f92d24acde37a02e0d084ce2ca8f40d2ee95a/compiler/rustc_const_eval/src/const_eval/machine.rs#L237
  4. Same as above, but we also allow pointer-to-integer casts, but not integer-to-pointer casts. Any program that assumes that different variables have different addresses bursts up in flame.

Related Links

• Tracking Issue for (runtime) is_aligned: Tracking Issue for pointer_is_aligned_to #96284
• Tracking Issue for const align_offset: Tracking Issue for const_align_offset #90962