Rollup merge of #137043 - Sky9x:unsafe-pinned-pt1-libs, r=tgross35,RalfJung,WaffleLapkin

Initial `UnsafePinned` implementation [Part 1: Libs]

Initial libs changes necessary to unblock further work on [RFC 3467](https://rust-lang.github.io/rfcs/3467-unsafe-pinned.html).
Tracking issue: #125735

This PR is split off from #136964, and includes just the libs changes:
- `UnsafePinned` struct
- private `UnsafeUnpin` structural auto trait
- Lang items for both
- Compiler changes necessary to block niches on `UnsafePinned`

This PR does not change codegen, miri, the existing `!Unpin` hack, or anything else. That work is to be split into later PRs.

---

cc ``@RalfJung`` ``@Noratrieb``

``@rustbot`` label F-unsafe_pinned T-libs-api

Author: jhpratt

compiler/rustc_abi/src/layout.rs

@@ -315,7 +315,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
         repr: &ReprOptions,
         variants: &IndexSlice<VariantIdx, IndexVec<FieldIdx, F>>,
         is_enum: bool,
-        is_unsafe_cell: bool,
+        is_special_no_niche: bool,
         scalar_valid_range: (Bound<u128>, Bound<u128>),
         discr_range_of_repr: impl Fn(i128, i128) -> (Integer, bool),
         discriminants: impl Iterator<Item = (VariantIdx, i128)>,
@@ -348,7 +348,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
                 repr,
                 variants,
                 is_enum,
-                is_unsafe_cell,
+                is_special_no_niche,
                 scalar_valid_range,
                 always_sized,
                 present_first,
@@ -505,7 +505,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
         repr: &ReprOptions,
         variants: &IndexSlice<VariantIdx, IndexVec<FieldIdx, F>>,
         is_enum: bool,
-        is_unsafe_cell: bool,
+        is_special_no_niche: bool,
         scalar_valid_range: (Bound<u128>, Bound<u128>),
         always_sized: bool,
         present_first: VariantIdx,
@@ -524,7 +524,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
         let mut st = self.univariant(&variants[v], repr, kind)?;
         st.variants = Variants::Single { index: v };
 
-        if is_unsafe_cell {
+        if is_special_no_niche {
             let hide_niches = |scalar: &mut _| match scalar {
                 Scalar::Initialized { value, valid_range } => {
                     *valid_range = WrappingRange::full(value.size(dl))

compiler/rustc_hir/src/lang_items.rs

@@ -182,6 +182,7 @@ language_item_table! {
     DynMetadata,             sym::dyn_metadata,        dyn_metadata,               Target::Struct,         GenericRequirement::None;
 
     Freeze,                  sym::freeze,              freeze_trait,               Target::Trait,          GenericRequirement::Exact(0);
+    UnsafeUnpin,             sym::unsafe_unpin,        unsafe_unpin_trait,         Target::Trait,          GenericRequirement::Exact(0);
 
     FnPtrTrait,              sym::fn_ptr_trait,        fn_ptr_trait,               Target::Trait,          GenericRequirement::Exact(0);
     FnPtrAddr,               sym::fn_ptr_addr,         fn_ptr_addr,                Target::Method(MethodKind::Trait { body: false }), GenericRequirement::None;
@@ -235,6 +236,8 @@ language_item_table! {
     IndexMut,                sym::index_mut,           index_mut_trait,            Target::Trait,          GenericRequirement::Exact(1);
 
     UnsafeCell,              sym::unsafe_cell,         unsafe_cell_type,           Target::Struct,         GenericRequirement::None;
+    UnsafePinned,            sym::unsafe_pinned,       unsafe_pinned_type,         Target::Struct,         GenericRequirement::None;
+
     VaList,                  sym::va_list,             va_list,                    Target::Struct,         GenericRequirement::None;
 
     Deref,                   sym::deref,               deref_trait,                Target::Trait,          GenericRequirement::Exact(0);

compiler/rustc_lint/src/types.rs

@@ -864,8 +864,8 @@ fn ty_is_known_nonnull<'tcx>(
                 return true;
             }
 
-            // `UnsafeCell` has its niche hidden.
-            if def.is_unsafe_cell() {
+            // `UnsafeCell` and `UnsafePinned` have their niche hidden.
+            if def.is_unsafe_cell() || def.is_unsafe_pinned() {
                 return false;
             }
 

compiler/rustc_middle/src/ty/adt.rs

@@ -53,6 +53,10 @@ bitflags::bitflags! {
         const IS_VARIANT_LIST_NON_EXHAUSTIVE = 1 << 8;
         /// Indicates whether the type is `UnsafeCell`.
         const IS_UNSAFE_CELL              = 1 << 9;
+        /// Indicates whether the type is `UnsafePinned`.
+        const IS_UNSAFE_PINNED              = 1 << 10;
+        /// Indicates whether the type is anonymous.
+        const IS_ANONYMOUS                = 1 << 11;
     }
 }
 rustc_data_structures::external_bitflags_debug! { AdtFlags }
@@ -302,6 +306,9 @@ impl AdtDefData {
         if tcx.is_lang_item(did, LangItem::UnsafeCell) {
             flags |= AdtFlags::IS_UNSAFE_CELL;
         }
+        if tcx.is_lang_item(did, LangItem::UnsafePinned) {
+            flags |= AdtFlags::IS_UNSAFE_PINNED;
+        }
 
         AdtDefData { did, variants, flags, repr }
     }
@@ -405,6 +412,12 @@ impl<'tcx> AdtDef<'tcx> {
         self.flags().contains(AdtFlags::IS_UNSAFE_CELL)
     }
 
+    /// Returns `true` if this is `UnsafePinned<T>`.
+    #[inline]
+    pub fn is_unsafe_pinned(self) -> bool {
+        self.flags().contains(AdtFlags::IS_UNSAFE_PINNED)
+    }
+
     /// Returns `true` if this is `ManuallyDrop<T>`.
     #[inline]
     pub fn is_manually_drop(self) -> bool {

compiler/rustc_span/src/symbol.rs

@@ -2215,6 +2215,8 @@ symbols! {
         unsafe_fields,
         unsafe_no_drop_flag,
         unsafe_pin_internals,
+        unsafe_pinned,
+        unsafe_unpin,
         unsize,
         unsized_const_param_ty,
         unsized_const_params,

compiler/rustc_ty_utils/src/layout.rs

@@ -514,6 +514,9 @@ fn layout_of_uncached<'tcx>(
                 return map_layout(cx.calc.layout_of_union(&def.repr(), &variants));
             }
 
+            // UnsafeCell and UnsafePinned both disable niche optimizations
+            let is_special_no_niche = def.is_unsafe_cell() || def.is_unsafe_pinned();
+
             let get_discriminant_type =
                 |min, max| abi::Integer::repr_discr(tcx, ty, &def.repr(), min, max);
 
@@ -542,7 +545,7 @@ fn layout_of_uncached<'tcx>(
                     &def.repr(),
                     &variants,
                     def.is_enum(),
-                    def.is_unsafe_cell(),
+                    is_special_no_niche,
                     tcx.layout_scalar_valid_range(def.did()),
                     get_discriminant_type,
                     discriminants_iter(),
@@ -568,7 +571,7 @@ fn layout_of_uncached<'tcx>(
                     &def.repr(),
                     &variants,
                     def.is_enum(),
-                    def.is_unsafe_cell(),
+                    is_special_no_niche,
                     tcx.layout_scalar_valid_range(def.did()),
                     get_discriminant_type,
                     discriminants_iter(),

library/core/src/lib.rs

@@ -127,6 +127,7 @@
 #![feature(ub_checks)]
 #![feature(unchecked_neg)]
 #![feature(unchecked_shifts)]
+#![feature(unsafe_pinned)]
 #![feature(utf16_extra)]
 #![feature(variant_count)]
 // tidy-alphabetical-end

library/core/src/marker.rs

@@ -17,6 +17,7 @@ use crate::cell::UnsafeCell;
 use crate::cmp;
 use crate::fmt::Debug;
 use crate::hash::{Hash, Hasher};
+use crate::pin::UnsafePinned;
 
 /// Implements a given marker trait for multiple types at the same time.
 ///
@@ -878,6 +879,23 @@ marker_impls! {
         {T: ?Sized} &mut T,
 }
 
+/// Used to determine whether a type contains any `UnsafePinned` (or `PhantomPinned`) internally,
+/// but not through an indirection. This affects, for example, whether we emit `noalias` metadata
+/// for `&mut T` or not.
+///
+/// This is part of [RFC 3467](https://rust-lang.github.io/rfcs/3467-unsafe-pinned.html), and is
+/// tracked by [#125735](https://github.com/rust-lang/rust/issues/125735).
+#[cfg_attr(not(bootstrap), lang = "unsafe_unpin")]
+#[cfg_attr(bootstrap, allow(dead_code))]
+pub(crate) unsafe auto trait UnsafeUnpin {}
+
+impl<T: ?Sized> !UnsafeUnpin for UnsafePinned<T> {}
+unsafe impl<T: ?Sized> UnsafeUnpin for PhantomData<T> {}
+unsafe impl<T: ?Sized> UnsafeUnpin for *const T {}
+unsafe impl<T: ?Sized> UnsafeUnpin for *mut T {}
+unsafe impl<T: ?Sized> UnsafeUnpin for &T {}
+unsafe impl<T: ?Sized> UnsafeUnpin for &mut T {}
+
 /// Types that do not require any pinning guarantees.
 ///
 /// For information on what "pinning" is, see the [`pin` module] documentation.
@@ -953,13 +971,24 @@ pub auto trait Unpin {}
 /// A marker type which does not implement `Unpin`.
 ///
 /// If a type contains a `PhantomPinned`, it will not implement `Unpin` by default.
+//
+// FIXME(unsafe_pinned): This is *not* a stable guarantee we want to make, at least not yet.
+// Note that for backwards compatibility with the new [`UnsafePinned`] wrapper type, placing this
+// marker in your struct acts as if you wrapped the entire struct in an `UnsafePinned`. This type
+// will likely eventually be deprecated, and all new code should be using `UnsafePinned` instead.
 #[stable(feature = "pin", since = "1.33.0")]
 #[derive(Debug, Default, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
 pub struct PhantomPinned;
 
 #[stable(feature = "pin", since = "1.33.0")]
 impl !Unpin for PhantomPinned {}
 
+// This is a small hack to allow existing code which uses PhantomPinned to opt-out of noalias to
+// continue working. Ideally PhantomPinned could just wrap an `UnsafePinned<()>` to get the same
+// effect, but we can't add a new field to an already stable unit struct -- that would be a breaking
+// change.
+impl !UnsafeUnpin for PhantomPinned {}
+
 marker_impls! {
     #[stable(feature = "pin", since = "1.33.0")]
     Unpin for

library/core/src/pin.rs

@@ -931,6 +931,11 @@ use crate::{
 };
 use crate::{cmp, fmt};
 
+mod unsafe_pinned;
+
+#[unstable(feature = "unsafe_pinned", issue = "125735")]
+pub use self::unsafe_pinned::UnsafePinned;
+
 /// A pointer which pins its pointee in place.
 ///
 /// [`Pin`] is a wrapper around some kind of pointer `Ptr` which makes that pointer "pin" its