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clarify pointer offset function safety concerns #52668

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Jul 25, 2018
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clarify pointer offset function safety concerns #52668

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20 changes: 16 additions & 4 deletions src/libcore/ptr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -591,7 +591,7 @@ impl<T: ?Sized> *const T {
/// Behavior:
///
/// * Both the starting and resulting pointer must be either in bounds or one
/// byte past the end of an allocated object.
/// byte past the end of *the same* allocated object.
///
/// * The computed offset, **in bytes**, cannot overflow an `isize`.
///
Expand Down Expand Up @@ -643,9 +643,15 @@ impl<T: ?Sized> *const T {
///
/// The resulting pointer does not need to be in bounds, but it is
/// potentially hazardous to dereference (which requires `unsafe`).
/// In particular, the resulting pointer may *not* be used to access a
/// different allocated object than the one `self` points to. In other
/// words, `x.wrapping_offset(y.wrapping_offset_from(x))` is
/// *not* the same as `y`, and dereferencing it is undefined behavior
/// unless `x` and `y` point into the same allocated object.
///
/// Always use `.offset(count)` instead when possible, because `offset`
/// allows the compiler to optimize better.
/// allows the compiler to optimize better. If you need to cross object
/// boundaries, cast the pointer to an integer and do the arithmetic there.
///
/// # Examples
///
Expand Down Expand Up @@ -1340,7 +1346,7 @@ impl<T: ?Sized> *mut T {
/// Behavior:
///
/// * Both the starting and resulting pointer must be either in bounds or one
/// byte past the end of an allocated object.
/// byte past the end of *the same* allocated object.
///
/// * The computed offset, **in bytes**, cannot overflow an `isize`.
///
Expand Down Expand Up @@ -1391,9 +1397,15 @@ impl<T: ?Sized> *mut T {
///
/// The resulting pointer does not need to be in bounds, but it is
/// potentially hazardous to dereference (which requires `unsafe`).
/// In particular, the resulting pointer may *not* be used to access a
/// different allocated object than the one `self` points to. In other
/// words, `x.wrapping_offset(y.wrapping_offset_from(x))` is
/// *not* the same as `y`, and dereferencing it is undefined behavior
/// unless `x` and `y` point into the same allocated object.
///
/// Always use `.offset(count)` instead when possible, because `offset`
/// allows the compiler to optimize better.
/// allows the compiler to optimize better. If you need to cross object
/// boundaries, cast the pointer to an integer and do the arithmetic there.
///
/// # Examples
///
Expand Down