Auto merge of rust-lang#117658 - RalfJung:ptr-dangling, r=m-ou-se

rename ptr::invalid -> ptr::without_provenance

It has long bothered me that `ptr::invalid` returns a pointer that is actually valid for zero-sized memory accesses. In general, it doesn't even make sense to ask "is this pointer valid", you have to ask "is this pointer valid for a given memory access". We could say that a pointer is invalid if it is not valid for *any* memory access, but [the way this FCP is going](rust-lang/unsafe-code-guidelines#472), it looks like *all* pointers will be valid for zero-sized memory accesses.

Two possible alternative names emerged as people's favorites:
1. Something involving `dangling`, in analogy to `NonNull::dangling`. To avoid inconsistency with the `NonNull` method, the address-taking method could be called `dangling_at(addr: usize) -> *const T`.
2. `without_provenance`, to be symmetric with the inverse operation `ptr.addr_without_provenance()` (currently still called `ptr.addr()` but probably going to be renamed)

I have no idea which one of these is better. I read [this comment](rust-lang#117658 (comment)) as expressing a slight preference for something like the second option, so I went for that. I'm happy to go with `dangling_at` as well.

Cc `@rust-lang/opsem`

compiler/rustc_arena/src/lib.rs

@@ -95,7 +95,7 @@ impl<T> ArenaChunk<T> {
         unsafe {
             if mem::size_of::<T>() == 0 {
                 // A pointer as large as possible for zero-sized elements.
-                ptr::invalid_mut(!0)
+                ptr::without_provenance_mut(!0)
             } else {
                 self.start().add(self.storage.len())
             }

library/alloc/src/rc.rs

@@ -2804,7 +2804,9 @@ impl<T> Weak<T> {
     #[must_use]
     pub const fn new() -> Weak<T> {
         Weak {
-            ptr: unsafe { NonNull::new_unchecked(ptr::invalid_mut::<RcBox<T>>(usize::MAX)) },
+            ptr: unsafe {
+                NonNull::new_unchecked(ptr::without_provenance_mut::<RcBox<T>>(usize::MAX))
+            },
             alloc: Global,
         }
     }
@@ -2829,7 +2831,9 @@ impl<T, A: Allocator> Weak<T, A> {
     #[unstable(feature = "allocator_api", issue = "32838")]
     pub fn new_in(alloc: A) -> Weak<T, A> {
         Weak {
-            ptr: unsafe { NonNull::new_unchecked(ptr::invalid_mut::<RcBox<T>>(usize::MAX)) },
+            ptr: unsafe {
+                NonNull::new_unchecked(ptr::without_provenance_mut::<RcBox<T>>(usize::MAX))
+            },
             alloc,
         }
     }

library/alloc/src/sync.rs

@@ -2555,7 +2555,9 @@ impl<T> Weak<T> {
     #[must_use]
     pub const fn new() -> Weak<T> {
         Weak {
-            ptr: unsafe { NonNull::new_unchecked(ptr::invalid_mut::<ArcInner<T>>(usize::MAX)) },
+            ptr: unsafe {
+                NonNull::new_unchecked(ptr::without_provenance_mut::<ArcInner<T>>(usize::MAX))
+            },
             alloc: Global,
         }
     }
@@ -2583,7 +2585,9 @@ impl<T, A: Allocator> Weak<T, A> {
     #[unstable(feature = "allocator_api", issue = "32838")]
     pub fn new_in(alloc: A) -> Weak<T, A> {
         Weak {
-            ptr: unsafe { NonNull::new_unchecked(ptr::invalid_mut::<ArcInner<T>>(usize::MAX)) },
+            ptr: unsafe {
+                NonNull::new_unchecked(ptr::without_provenance_mut::<ArcInner<T>>(usize::MAX))
+            },
             alloc,
         }
     }

library/alloc/tests/fmt.rs

@@ -77,14 +77,14 @@ fn test_format_macro_interface() {
     t!(format!("{}", "foo"), "foo");
     t!(format!("{}", "foo".to_string()), "foo");
     if cfg!(target_pointer_width = "32") {
-        t!(format!("{:#p}", ptr::invalid::<isize>(0x1234)), "0x00001234");
-        t!(format!("{:#p}", ptr::invalid_mut::<isize>(0x1234)), "0x00001234");
+        t!(format!("{:#p}", ptr::without_provenance::<isize>(0x1234)), "0x00001234");
+        t!(format!("{:#p}", ptr::without_provenance_mut::<isize>(0x1234)), "0x00001234");
     } else {
-        t!(format!("{:#p}", ptr::invalid::<isize>(0x1234)), "0x0000000000001234");
-        t!(format!("{:#p}", ptr::invalid_mut::<isize>(0x1234)), "0x0000000000001234");
+        t!(format!("{:#p}", ptr::without_provenance::<isize>(0x1234)), "0x0000000000001234");
+        t!(format!("{:#p}", ptr::without_provenance_mut::<isize>(0x1234)), "0x0000000000001234");
     }
-    t!(format!("{:p}", ptr::invalid::<isize>(0x1234)), "0x1234");
-    t!(format!("{:p}", ptr::invalid_mut::<isize>(0x1234)), "0x1234");
+    t!(format!("{:p}", ptr::without_provenance::<isize>(0x1234)), "0x1234");
+    t!(format!("{:p}", ptr::without_provenance_mut::<isize>(0x1234)), "0x1234");
     t!(format!("{A:x}"), "aloha");
     t!(format!("{B:X}"), "adios");
     t!(format!("foo {} ☃☃☃☃☃☃", "bar"), "foo bar ☃☃☃☃☃☃");
@@ -208,7 +208,7 @@ fn test_format_macro_interface() {
     {
         let val = usize::MAX;
         let exp = format!("{val:#x}");
-        t!(format!("{:p}", std::ptr::invalid::<isize>(val)), exp);
+        t!(format!("{:p}", std::ptr::without_provenance::<isize>(val)), exp);
     }
 
     // Escaping

library/alloc/tests/vec.rs

@@ -2575,7 +2575,7 @@ fn test_box_zero_allocator() {
                 assert!(state.0.insert(addr));
                 state.1 += 1;
                 std::println!("allocating {addr}");
-                std::ptr::invalid_mut(addr)
+                std::ptr::without_provenance_mut(addr)
             } else {
                 unsafe { std::alloc::alloc(layout) }
             };

library/core/src/alloc/layout.rs

@@ -215,7 +215,7 @@ impl Layout {
     #[inline]
     pub const fn dangling(&self) -> NonNull<u8> {
         // SAFETY: align is guaranteed to be non-zero
-        unsafe { NonNull::new_unchecked(crate::ptr::invalid_mut::<u8>(self.align())) }
+        unsafe { NonNull::new_unchecked(crate::ptr::without_provenance_mut::<u8>(self.align())) }
     }
 
     /// Creates a layout describing the record that can hold a value

library/core/src/intrinsics.rs

@@ -1155,7 +1155,7 @@ extern "rust-intrinsic" {
     ///
     /// Transmuting pointers *to* integers in a `const` context is [undefined behavior][ub],
     /// unless the pointer was originally created *from* an integer.
-    /// (That includes this function specifically, integer-to-pointer casts, and helpers like [`invalid`][crate::ptr::invalid],
+    /// (That includes this function specifically, integer-to-pointer casts, and helpers like [`invalid`][crate::ptr::dangling],
     /// but also semantically-equivalent conversions such as punning through `repr(C)` union fields.)
     /// Any attempt to use the resulting value for integer operations will abort const-evaluation.
     /// (And even outside `const`, such transmutation is touching on many unspecified aspects of the

library/core/src/ptr/const_ptr.rs

@@ -181,7 +181,7 @@ impl<T: ?Sized> *const T {
     ///
     /// 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 [`invalid`][], which is undefined behavior to dereference. To
+    /// 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].
     ///

library/core/src/ptr/mod.rs

@@ -4,13 +4,13 @@
 //!
 //! # Safety
 //!
-//! Many functions in this module take raw pointers as arguments and read from
-//! or write to them. For this to be safe, these pointers must be *valid*.
-//! Whether a pointer is valid depends on the operation it is used for
-//! (read or write), and the extent of the memory that is accessed (i.e.,
-//! how many bytes are read/written). Most functions use `*mut T` and `*const T`
-//! to access only a single value, in which case the documentation omits the size
-//! and implicitly assumes it to be `size_of::<T>()` bytes.
+//! Many functions in this module take raw pointers as arguments and read from or write to them. For
+//! this to be safe, these pointers must be *valid* for the given access. Whether a pointer is valid
+//! depends on the operation it is used for (read or write), and the extent of the memory that is
+//! accessed (i.e., how many bytes are read/written) -- it makes no sense to ask "is this pointer
+//! valid"; one has to ask "is this pointer valid for a given access". Most functions use `*mut T`
+//! and `*const T` to access only a single value, in which case the documentation omits the size and
+//! implicitly assumes it to be `size_of::<T>()` bytes.
 //!
 //! The precise rules for validity are not determined yet. The guarantees that are
 //! provided at this point are very minimal:
@@ -26,7 +26,7 @@
 //!   some memory happens to exist at that address and gets deallocated. This corresponds to writing
 //!   your own allocator: allocating zero-sized objects is not very hard. The canonical way to
 //!   obtain a pointer that is valid for zero-sized accesses is [`NonNull::dangling`].
-//FIXME: mention `ptr::invalid` above, once it is stable.
+//FIXME: mention `ptr::dangling` above, once it is stable.
 //! * All accesses performed by functions in this module are *non-atomic* in the sense
 //!   of [atomic operations] used to synchronize between threads. This means it is
 //!   undefined behavior to perform two concurrent accesses to the same location from different
@@ -44,6 +44,10 @@
 //! information, see the [book] as well as the section in the reference devoted
 //! to [undefined behavior][ub].
 //!
+//! We say that a pointer is "dangling" if it is not valid for any non-zero-sized accesses. This
+//! means out-of-bounds pointers, pointers to freed memory, null pointers, and pointers created with
+//! [`NonNull::dangling`] are all dangling.
+//!
 //! ## Alignment
 //!
 //! Valid raw pointers as defined above are not necessarily properly aligned (where
@@ -167,6 +171,7 @@
 //! * The **address-space** it is part of (e.g. "data" vs "code" in WASM).
 //! * The **address** it points to, which can be represented by a `usize`.
 //! * The **provenance** it has, defining the memory it has permission to access.
+//!   Provenance can be absent, in which case the pointer does not have permission to access any memory.
 //!
 //! Under Strict Provenance, a usize *cannot* accurately represent a pointer, and converting from
 //! a pointer to a usize is generally an operation which *only* extracts the address. It is
@@ -270,11 +275,12 @@
 //!
 //! But it *is* still sound to:
 //!
-//! * Create an invalid pointer from just an address (see [`ptr::invalid`][]). This can
-//!   be used for sentinel values like `null` *or* to represent a tagged pointer that will
-//!   never be dereferenceable. In general, it is always sound for an integer to pretend
-//!   to be a pointer "for fun" as long as you don't use operations on it which require
-//!   it to be valid (offset, read, write, etc).
+//! * Create a pointer without provenance from just an address (see [`ptr::dangling`][]). Such a
+//!   pointer cannot be used for memory accesses (except for zero-sized accesses). This can still be
+//!   useful for sentinel values like `null` *or* to represent a tagged pointer that will never be
+//!   dereferenceable. In general, it is always sound for an integer to pretend to be a pointer "for
+//!   fun" as long as you don't use operations on it which require it to be valid (non-zero-sized
+//!   offset, read, write, etc).
 //!
 //! * Forge an allocation of size zero at any sufficiently aligned non-null address.
 //!   i.e. the usual "ZSTs are fake, do what you want" rules apply *but* this only applies
@@ -283,7 +289,7 @@
 //!   that allocation and it will still get invalidated if the allocation gets deallocated.
 //!   In the future we may introduce an API to make such a forged allocation explicit.
 //!
-//! * [`wrapping_offset`][] a pointer outside its provenance. This includes invalid pointers
+//! * [`wrapping_offset`][] a pointer outside its provenance. This includes pointers
 //!   which have "no" provenance. Unfortunately there may be practical limits on this for a
 //!   particular platform, and it's an open question as to how to specify this (if at all).
 //!   Notably, [CHERI][] relies on a compression scheme that can't handle a
@@ -294,7 +300,7 @@
 //!   generous (think kilobytes, not bytes).
 //!
 //! * Compare arbitrary pointers by address. Addresses *are* just integers and so there is
-//!   always a coherent answer, even if the pointers are invalid or from different
+//!   always a coherent answer, even if the pointers are dangling or from different
 //!   address-spaces/provenances. Of course, comparing addresses from different address-spaces
 //!   is generally going to be *meaningless*, but so is comparing Kilograms to Meters, and Rust
 //!   doesn't prevent that either. Similarly, if you get "lucky" and notice that a pointer
@@ -367,7 +373,7 @@
 //! [`with_addr`]: pointer::with_addr
 //! [`map_addr`]: pointer::map_addr
 //! [`addr`]: pointer::addr
-//! [`ptr::invalid`]: core::ptr::invalid
+//! [`ptr::dangling`]: core::ptr::dangling
 //! [`expose_addr`]: pointer::expose_addr
 //! [`from_exposed_addr`]: from_exposed_addr
 //! [Miri]: https://github.com/rust-lang/miri
@@ -537,7 +543,7 @@ pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
 #[rustc_allow_const_fn_unstable(ptr_metadata)]
 #[rustc_diagnostic_item = "ptr_null"]
 pub const fn null<T: ?Sized + Thin>() -> *const T {
-    from_raw_parts(invalid(0), ())
+    from_raw_parts(without_provenance(0), ())
 }
 
 /// Creates a null mutable raw pointer.
@@ -563,32 +569,26 @@ pub const fn null<T: ?Sized + Thin>() -> *const T {
 #[rustc_allow_const_fn_unstable(ptr_metadata)]
 #[rustc_diagnostic_item = "ptr_null_mut"]
 pub const fn null_mut<T: ?Sized + Thin>() -> *mut T {
-    from_raw_parts_mut(invalid_mut(0), ())
+    from_raw_parts_mut(without_provenance_mut(0), ())
 }
 
-/// Creates an invalid pointer with the given address.
+/// Creates a pointer with the given address and no provenance.
+///
+/// Without provenance, this pointer is not associated with any actual allocation. Such a
+/// no-provenance pointer may be used for zero-sized memory accesses (if suitably aligned), but
+/// non-zero-sized memory accesses with a no-provenance pointer are UB. No-provenance pointers are
+/// little more than a usize address in disguise.
 ///
 /// This is different from `addr as *const T`, which creates a pointer that picks up a previously
 /// exposed provenance. See [`from_exposed_addr`] for more details on that operation.
 ///
-/// The module's top-level documentation discusses the precise meaning of an "invalid"
-/// pointer but essentially this expresses that the pointer is not associated
-/// with any actual allocation and is little more than a usize address in disguise.
-///
-/// This pointer will have no provenance associated with it and is therefore
-/// UB to read/write/offset. This mostly exists to facilitate things
-/// like `ptr::null` and `NonNull::dangling` which make invalid pointers.
-///
-/// (Standard "Zero-Sized-Types get to cheat and lie" caveats apply, although it
-/// may be desirable to give them their own API just to make that 100% clear.)
-///
 /// This API and its claimed semantics are part of the Strict Provenance experiment,
 /// see the [module documentation][crate::ptr] for details.
 #[inline(always)]
 #[must_use]
 #[rustc_const_stable(feature = "stable_things_using_strict_provenance", since = "1.61.0")]
 #[unstable(feature = "strict_provenance", issue = "95228")]
-pub const fn invalid<T>(addr: usize) -> *const T {
+pub const fn without_provenance<T>(addr: usize) -> *const T {
     // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
     // We use transmute rather than a cast so tools like Miri can tell that this
     // is *not* the same as from_exposed_addr.
@@ -597,29 +597,40 @@ pub const fn invalid<T>(addr: usize) -> *const T {
     unsafe { mem::transmute(addr) }
 }
 
-/// Creates an invalid mutable pointer with the given address.
+/// Creates a new pointer that is dangling, but well-aligned.
 ///
-/// This is different from `addr as *mut T`, which creates a pointer that picks up a previously
-/// exposed provenance. See [`from_exposed_addr_mut`] for more details on that operation.
+/// This is useful for initializing types which lazily allocate, like
+/// `Vec::new` does.
 ///
-/// The module's top-level documentation discusses the precise meaning of an "invalid"
-/// pointer but essentially this expresses that the pointer is not associated
-/// with any actual allocation and is little more than a usize address in disguise.
+/// Note that the pointer value may potentially represent a valid pointer to
+/// a `T`, which means this must not be used as a "not yet initialized"
+/// sentinel value. Types that lazily allocate must track initialization by
+/// some other means.
+#[inline(always)]
+#[must_use]
+#[rustc_const_stable(feature = "stable_things_using_strict_provenance", since = "1.61.0")]
+#[unstable(feature = "strict_provenance", issue = "95228")]
+pub const fn dangling<T>() -> *const T {
+    without_provenance(mem::align_of::<T>())
+}
+
+/// Creates a pointer with the given address and no provenance.
 ///
-/// This pointer will have no provenance associated with it and is therefore
-/// UB to read/write/offset. This mostly exists to facilitate things
-/// like `ptr::null` and `NonNull::dangling` which make invalid pointers.
+/// Without provenance, this pointer is not associated with any actual allocation. Such a
+/// no-provenance pointer may be used for zero-sized memory accesses (if suitably aligned), but
+/// non-zero-sized memory accesses with a no-provenance pointer are UB. No-provenance pointers are
+/// little more than a usize address in disguise.
 ///
-/// (Standard "Zero-Sized-Types get to cheat and lie" caveats apply, although it
-/// may be desirable to give them their own API just to make that 100% clear.)
+/// This is different from `addr as *mut T`, which creates a pointer that picks up a previously
+/// exposed provenance. See [`from_exposed_addr_mut`] for more details on that operation.
 ///
 /// This API and its claimed semantics are part of the Strict Provenance experiment,
 /// see the [module documentation][crate::ptr] for details.
 #[inline(always)]
 #[must_use]
 #[rustc_const_stable(feature = "stable_things_using_strict_provenance", since = "1.61.0")]
 #[unstable(feature = "strict_provenance", issue = "95228")]
-pub const fn invalid_mut<T>(addr: usize) -> *mut T {
+pub const fn without_provenance_mut<T>(addr: usize) -> *mut T {
     // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
     // We use transmute rather than a cast so tools like Miri can tell that this
     // is *not* the same as from_exposed_addr.
@@ -628,6 +639,23 @@ pub const fn invalid_mut<T>(addr: usize) -> *mut T {
     unsafe { mem::transmute(addr) }
 }
 
+/// Creates a new pointer that is dangling, but well-aligned.
+///
+/// This is useful for initializing types which lazily allocate, like
+/// `Vec::new` does.
+///
+/// Note that the pointer value may potentially represent a valid pointer to
+/// a `T`, which means this must not be used as a "not yet initialized"
+/// sentinel value. Types that lazily allocate must track initialization by
+/// some other means.
+#[inline(always)]
+#[must_use]
+#[rustc_const_stable(feature = "stable_things_using_strict_provenance", since = "1.61.0")]
+#[unstable(feature = "strict_provenance", issue = "95228")]
+pub const fn dangling_mut<T>() -> *mut T {
+    without_provenance_mut(mem::align_of::<T>())
+}
+
 /// Convert an address back to a pointer, picking up a previously 'exposed' provenance.
 ///
 /// This is a more rigorously specified alternative to `addr as *const T`. The provenance of the

library/core/src/ptr/mut_ptr.rs

@@ -188,9 +188,10 @@ impl<T: ?Sized> *mut T {
     ///
     /// 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 [`invalid`][], 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].
+    /// back to a pointer yields yields a [pointer without provenance][without_provenance_mut], 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
     /// required provenance, then Strict Provenance might not be for you. Use pointer-integer casts

library/core/src/ptr/non_null.rs

@@ -4,8 +4,7 @@ use crate::hash;
 use crate::intrinsics;
 use crate::intrinsics::assert_unsafe_precondition;
 use crate::marker::Unsize;
-use crate::mem::SizedTypeProperties;
-use crate::mem::{self, MaybeUninit};
+use crate::mem::{MaybeUninit, SizedTypeProperties};
 use crate::num::{NonZero, NonZeroUsize};
 use crate::ops::{CoerceUnsized, DispatchFromDyn};
 use crate::ptr;
@@ -114,7 +113,7 @@ impl<T: Sized> NonNull<T> {
         // to a *mut T. Therefore, `ptr` is not null and the conditions for
         // calling new_unchecked() are respected.
         unsafe {
-            let ptr = crate::ptr::invalid_mut::<T>(mem::align_of::<T>());
+            let ptr = crate::ptr::dangling_mut::<T>();
             NonNull::new_unchecked(ptr)
         }
     }