diff options
| -rw-r--r-- | library/core/src/ptr/const_ptr.rs | 41 | ||||
| -rw-r--r-- | library/core/src/ptr/mut_ptr.rs | 40 |
2 files changed, 61 insertions, 20 deletions
diff --git a/library/core/src/ptr/const_ptr.rs b/library/core/src/ptr/const_ptr.rs index c9af7f13e46..0b0449d6174 100644 --- a/library/core/src/ptr/const_ptr.rs +++ b/library/core/src/ptr/const_ptr.rs @@ -63,21 +63,22 @@ impl<T: ?Sized> *const T { self as _ } - /// Uses the pointer value in a new pointer of another type. + /// Uses the address value in a new pointer of another type. /// - /// In case `meta` is a (fat) pointer to an unsized type, this operation - /// will ignore the pointer part, whereas for (thin) pointers to sized - /// types, this has the same effect as a simple cast. + /// This operation will ignore the address part of its `meta` operand and discard existing + /// metadata of `self`. For pointers to a sized types (thin pointers), this has the same effect + /// as a simple cast. For pointers to an unsized type (fat pointers) this recombines the address + /// with new metadata such as slice lengths or `dyn`-vtable. /// - /// The resulting pointer will have provenance of `self`, i.e., for a fat - /// pointer, this operation is semantically the same as creating a new - /// fat pointer with the data pointer value of `self` but the metadata of - /// `meta`. + /// The resulting pointer will have provenance of `self`. This operation is semantically the + /// same as creating a new pointer with the data pointer value of `self` but the metadata of + /// `meta`, being fat or thin depending on the `meta` operand. /// /// # Examples /// - /// This function is primarily useful for allowing byte-wise pointer - /// arithmetic on potentially fat pointers: + /// This function is primarily useful for enabling pointer arithmetic on potentially fat + /// pointers. The pointer is cast to a sized pointee to utilize offset operations and then + /// recombined with its own original metadata. /// /// ``` /// #![feature(set_ptr_value)] @@ -91,6 +92,26 @@ impl<T: ?Sized> *const T { /// println!("{:?}", &*ptr); // will print "3" /// } /// ``` + /// + /// # *Incorrect* usage + /// + /// The provenance from pointers is *not* combined. The result must only be used to refer to the + /// address allowed by `self`. + /// + /// ```rust,no_run + /// #![feature(set_ptr_value)] + /// let x = 0u32; + /// let y = 1u32; + /// + /// let x = (&x) as *const u32; + /// let y = (&y) as *const u32; + /// + /// let offset = (x as usize - y as usize) / 4; + /// let bad = x.wrapping_add(offset).with_metadata_of(y); + /// + /// // This dereference is UB. The pointer only has provenance for `x` but points to `y`. + /// println!("{:?}", unsafe { &*bad }); + /// ``` #[unstable(feature = "set_ptr_value", issue = "75091")] #[rustc_const_stable(feature = "ptr_metadata_const", since = "CURRENT_RUSTC_VERSION")] #[must_use = "returns a new pointer rather than modifying its argument"] diff --git a/library/core/src/ptr/mut_ptr.rs b/library/core/src/ptr/mut_ptr.rs index ced6cb7d520..8b61da74d02 100644 --- a/library/core/src/ptr/mut_ptr.rs +++ b/library/core/src/ptr/mut_ptr.rs @@ -45,21 +45,22 @@ impl<T: ?Sized> *mut T { self as _ } - /// Uses the pointer value in a new pointer of another type. + /// Uses the address value in a new pointer of another type. /// - /// In case `meta` is a (fat) pointer to an unsized type, this operation - /// will ignore the pointer part, whereas for (thin) pointers to sized - /// types, this has the same effect as a simple cast. + /// This operation will ignore the address part of its `meta` operand and discard existing + /// metadata of `self`. For pointers to a sized types (thin pointers), this has the same effect + /// as a simple cast. For pointers to an unsized type (fat pointers) this recombines the address + /// with new metadata such as slice lengths or `dyn`-vtable. /// - /// The resulting pointer will have provenance of `self`, i.e., for a fat - /// pointer, this operation is semantically the same as creating a new - /// fat pointer with the data pointer value of `self` but the metadata of - /// `meta`. + /// The resulting pointer will have provenance of `self`. This operation is semantically the + /// same as creating a new pointer with the data pointer value of `self` but the metadata of + /// `meta`, being fat or thin depending on the `meta` operand. /// /// # Examples /// - /// This function is primarily useful for allowing byte-wise pointer - /// arithmetic on potentially fat pointers: + /// This function is primarily useful for enabling pointer arithmetic on potentially fat + /// pointers. The pointer is cast to a sized pointee to utilize offset operations and then + /// recombined with its own original metadata. /// /// ``` /// #![feature(set_ptr_value)] @@ -73,6 +74,25 @@ impl<T: ?Sized> *mut T { /// println!("{:?}", &*ptr); // will print "3" /// } /// ``` + /// + /// # *Incorrect* usage + /// + /// The provenance from pointers is *not* combined. The result must only be used to refer to the + /// address allowed by `self`. + /// + /// ```rust,no_run + /// #![feature(set_ptr_value)] + /// let mut x = 0u32; + /// let mut y = 1u32; + /// + /// let x = (&mut x) as *mut u32; + /// let y = (&mut y) as *mut u32; + /// + /// let offset = (x as usize - y as usize) / 4; + /// let bad = x.wrapping_add(offset).with_metadata_of(y); + /// + /// // This dereference is UB. The pointer only has provenance for `x` but points to `y`. + /// println!("{:?}", unsafe { &*bad }); #[unstable(feature = "set_ptr_value", issue = "75091")] #[rustc_const_stable(feature = "ptr_metadata_const", since = "CURRENT_RUSTC_VERSION")] #[must_use = "returns a new pointer rather than modifying its argument"] |