stabilize Strict Provenance and Exposed Provenance

This comes with a big docs rewrite. It's mostly just rewording, except: - I removed the discussion of how pointers also have an associated "address space" -- that is not actually tracked in the pointer value, it is tracked in the type, so IMO it just distracts from the core point of provenance. - I also adjusted the docs for with_exposed_provenance to make it clear that we cannot guarantee much about this function, it's all best-effort.
rust-lang · Sep 14, 2024 · 18c72d0 · 18c72d0
1 parent f9567d0
commit 18c72d0
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Showing 4 changed files with 237 additions and 309 deletions.
diff --git a/library/core/src/ptr/const_ptr.rs b/library/core/src/ptr/const_ptr.rs
@@ -116,10 +116,11 @@ impl<T: ?Sized> *const T {
 
     /// Gets the "address" portion of the pointer.
     ///
-    /// This is similar to `self as usize`, which semantically discards *provenance* and
-    /// *address-space* information. However, unlike `self as usize`, casting the returned address
-    /// back to a pointer yields a [pointer without provenance][without_provenance], which is undefined behavior to dereference. To
-    /// properly restore the lost information and obtain a dereferenceable pointer, use
+    /// This is similar to `self as usize`, except that the [provenance][crate::ptr#provenance] of
+    /// the pointer is discarded and not [exposed][crate::ptr#exposed-provenance]. This means that
+    /// casting the returned address back to a pointer yields a [pointer without
+    /// provenance][without_provenance], which is undefined behavior to dereference. To properly
+    /// restore the lost information and obtain a dereferenceable pointer, use
     /// [`with_addr`][pointer::with_addr] or [`map_addr`][pointer::map_addr].
     ///
     /// If using those APIs is not possible because there is no way to preserve a pointer with the
@@ -134,67 +135,61 @@ impl<T: ?Sized> *const T {
     /// perform a change of representation to produce a value containing only the address
     /// portion of the pointer. What that means is up to the platform to define.
     ///
-    /// This API and its claimed semantics are part of the Strict Provenance experiment, and as such
-    /// might change in the future (including possibly weakening this so it becomes wholly
-    /// equivalent to `self as usize`). See the [module documentation][crate::ptr] for details.
+    /// This is a [Strict Provenance][crate::ptr#strict-provenance] API.
     #[must_use]
     #[inline(always)]
-    #[unstable(feature = "strict_provenance", issue = "95228")]
+    #[stable(feature = "strict_provenance", since = "CURRENT_RUSTC_VERSION")]
     pub fn addr(self) -> usize {
-        // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
+        // A pointer-to-integer transmute has exactly the right semantics: it returns
+        // the address without exposing the provenance.
         // SAFETY: Pointer-to-integer transmutes are valid (if you are okay with losing the
         // provenance).
         unsafe { mem::transmute(self.cast::<()>()) }
     }
 
-    /// Exposes the "provenance" part of the pointer for future use in
-    /// [`with_exposed_provenance`][] and returns the "address" portion.
+    /// Exposes the ["provenance"][crate::ptr#provenance] part of the pointer for future use in
+    /// [`with_exposed_provenance`] and returns the "address" portion.
     ///
-    /// This is equivalent to `self as usize`, which semantically discards *provenance* and
-    /// *address-space* information. Furthermore, this (like the `as` cast) has the implicit
-    /// side-effect of marking the provenance as 'exposed', so on platforms that support it you can
-    /// later call [`with_exposed_provenance`][] to reconstitute the original pointer including its
-    /// provenance. (Reconstructing address space information, if required, is your responsibility.)
+    /// This is equivalent to `self as usize`, which semantically discards provenance information.
+    /// Furthermore, this (like the `as` cast) has the implicit side-effect of marking the
+    /// provenance as 'exposed', so on platforms that support it you can later call
+    /// [`with_exposed_provenance`] to reconstitute the original pointer including its provenance.
     ///
-    /// Using this method means that code is *not* following [Strict
-    /// Provenance][super#strict-provenance] rules. Supporting
-    /// [`with_exposed_provenance`][] complicates specification and reasoning and may not be supported by
-    /// tools that help you to stay conformant with the Rust memory model, so it is recommended to
-    /// use [`addr`][pointer::addr] wherever possible.
+    /// Due to its inherent ambiguity, [`with_exposed_provenance`] may not be supported by tools
+    /// that help you to stay conformant with the Rust memory model. It is recommended to use
+    /// [Strict Provenance][crate::ptr#strict-provenance] APIs such as [`with_addr`][pointer::with_addr]
+    /// wherever possible, in which case [`addr`][pointer::addr] should be used instead of `expose_provenance`.
     ///
     /// On most platforms this will produce a value with the same bytes as the original pointer,
     /// because all the bytes are dedicated to describing the address. Platforms which need to store
     /// additional information in the pointer may not support this operation, since the 'expose'
-    /// side-effect which is required for [`with_exposed_provenance`][] to work is typically not
+    /// side-effect which is required for [`with_exposed_provenance`] to work is typically not
     /// available.
     ///
-    /// It is unclear whether this method can be given a satisfying unambiguous specification. This
-    /// API and its claimed semantics are part of [Exposed Provenance][super#exposed-provenance].
+    /// This is an [Exposed Provenance][crate::ptr#exposed-provenance] API.
     ///
     /// [`with_exposed_provenance`]: with_exposed_provenance
     #[must_use]
     #[inline(always)]
-    #[unstable(feature = "exposed_provenance", issue = "95228")]
+    #[stable(feature = "exposed_provenance", since = "CURRENT_RUSTC_VERSION")]
     pub fn expose_provenance(self) -> usize {
-        // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
         self.cast::<()>() as usize
     }
 
-    /// Creates a new pointer with the given address.
+    /// Creates a new pointer with the given address and the [provenance][crate::ptr#provenance] of
+    /// `self`.
     ///
-    /// This performs the same operation as an `addr as ptr` cast, but copies
-    /// the *address-space* and *provenance* of `self` to the new pointer.
-    /// This allows us to dynamically preserve and propagate this important
-    /// information in a way that is otherwise impossible with a unary cast.
+    /// This is similar to a `addr as *const T` cast, but copies
+    /// the *provenance* of `self` to the new pointer.
+    /// This avoids the inherent ambiguity of the unary cast.
     ///
     /// This is equivalent to using [`wrapping_offset`][pointer::wrapping_offset] to offset
     /// `self` to the given address, and therefore has all the same capabilities and restrictions.
     ///
-    /// This API and its claimed semantics are part of the Strict Provenance experiment,
-    /// see the [module documentation][crate::ptr] for details.
+    /// This is a [Strict Provenance][crate::ptr#strict-provenance] API.
     #[must_use]
     #[inline]
-    #[unstable(feature = "strict_provenance", issue = "95228")]
+    #[stable(feature = "strict_provenance", since = "CURRENT_RUSTC_VERSION")]
     pub fn with_addr(self, addr: usize) -> Self {
         // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
         //
@@ -209,15 +204,15 @@ impl<T: ?Sized> *const T {
         self.wrapping_byte_offset(offset)
     }
 
-    /// Creates a new pointer by mapping `self`'s address to a new one.
+    /// Creates a new pointer by mapping `self`'s address to a new one, preserving the
+    /// [provenance][crate::ptr#provenance] of `self`.
     ///
     /// This is a convenience for [`with_addr`][pointer::with_addr], see that method for details.
     ///
-    /// This API and its claimed semantics are part of the Strict Provenance experiment,
-    /// see the [module documentation][crate::ptr] for details.
+    /// This is a [Strict Provenance][crate::ptr#strict-provenance] API.
     #[must_use]
     #[inline]
-    #[unstable(feature = "strict_provenance", issue = "95228")]
+    #[stable(feature = "strict_provenance", since = "CURRENT_RUSTC_VERSION")]
     pub fn map_addr(self, f: impl FnOnce(usize) -> usize) -> Self {
         self.with_addr(f(self.addr()))
     }
@@ -357,7 +352,7 @@ impl<T: ?Sized> *const T {
     ///
     /// * The computed offset, `count * size_of::<T>()` bytes, must not overflow `isize`.
     ///
-    /// * If the computed offset is non-zero, then `self` must be derived from a pointer to some
+    /// * If the computed offset is non-zero, then `self` must be [derived from][crate::ptr#provenance] a pointer to some
     ///   [allocated object], and the entire memory range between `self` and the result must be in
     ///   bounds of that allocated object. In particular, this range must not "wrap around" the edge
     ///   of the address space.
@@ -561,7 +556,7 @@ impl<T: ?Sized> *const T {
     /// * `self` and `origin` must either
     ///
     ///   * point to the same address, or
-    ///   * both be *derived from* a pointer to the same [allocated object], and the memory range between
+    ///   * both be [derived from][crate::ptr#provenance] a pointer to the same [allocated object], and the memory range between
     ///     the two pointers must be in bounds of that object. (See below for an example.)
     ///
     /// * The distance between the pointers, in bytes, must be an exact multiple
@@ -818,7 +813,7 @@ impl<T: ?Sized> *const T {
     ///
     /// * The computed offset, `count * size_of::<T>()` bytes, must not overflow `isize`.
     ///
-    /// * If the computed offset is non-zero, then `self` must be derived from a pointer to some
+    /// * If the computed offset is non-zero, then `self` must be [derived from][crate::ptr#provenance] a pointer to some
     ///   [allocated object], and the entire memory range between `self` and the result must be in
     ///   bounds of that allocated object. In particular, this range must not "wrap around" the edge
     ///   of the address space.
@@ -892,7 +887,7 @@ impl<T: ?Sized> *const T {
     ///
     /// * The computed offset, `count * size_of::<T>()` bytes, must not overflow `isize`.
     ///
-    /// * If the computed offset is non-zero, then `self` must be derived from a pointer to some
+    /// * If the computed offset is non-zero, then `self` must be [derived from][crate::ptr#provenance] a pointer to some
     ///   [allocated object], and the entire memory range between `self` and the result must be in
     ///   bounds of that allocated object. In particular, this range must not "wrap around" the edge
     ///   of the address space.