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Add `ptr::Pointee` trait (for all types) and `ptr::metadata` function 

RFC: https://github.com/rust-lang/rfcs/pull/2580
This commit is contained in:
Simon Sapin 2020-10-19 15:38:11 +02:00
parent 9503ea19ed
commit 696b239f72
17 changed files with 349 additions and 10 deletions
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4
compiler/rustc_hir/src/lang_items.rs
View File

@ -201,6 +201,10 @@ language_item_table! {
// The associated item of `trait DiscriminantKind`.
Discriminant, sym::discriminant_type, discriminant_type, Target::AssocTy;
PointeeTrait, sym::pointee_trait, pointee_trait, Target::Trait;
Metadata, sym::metadata_type, metadata_type, Target::AssocTy;
DynMetadata, sym::dyn_metadata, dyn_metadata, Target::Struct;
Freeze, sym::freeze, freeze_trait, Target::Trait;
Drop, sym::drop, drop_trait, Target::Trait;

15
compiler/rustc_middle/src/traits/mod.rs
View File

@ -476,6 +476,9 @@ pub enum ImplSource<'tcx, N> {
/// ImplSource for a builtin `DeterminantKind` trait implementation.
DiscriminantKind(ImplSourceDiscriminantKindData),
/// ImplSource for a builtin `Pointee` trait implementation.
Pointee(ImplSourcePointeeData),
/// ImplSource automatically generated for a generator.
Generator(ImplSourceGeneratorData<'tcx, N>),
@ -494,7 +497,8 @@ impl<'tcx, N> ImplSource<'tcx, N> {
ImplSource::Generator(c) => c.nested,
ImplSource::Object(d) => d.nested,
ImplSource::FnPointer(d) => d.nested,
ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData) => Vec::new(),
ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData)
| ImplSource::Pointee(ImplSourcePointeeData) => Vec::new(),
ImplSource::TraitAlias(d) => d.nested,
}
}
@ -509,7 +513,8 @@ impl<'tcx, N> ImplSource<'tcx, N> {
ImplSource::Generator(c) => &c.nested[..],
ImplSource::Object(d) => &d.nested[..],
ImplSource::FnPointer(d) => &d.nested[..],
ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData) => &[],
ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData)
| ImplSource::Pointee(ImplSourcePointeeData) => &[],
ImplSource::TraitAlias(d) => &d.nested[..],
}
}
@ -554,6 +559,9 @@ impl<'tcx, N> ImplSource<'tcx, N> {
ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData) => {
ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData)
}
ImplSource::Pointee(ImplSourcePointeeData) => {
ImplSource::Pointee(ImplSourcePointeeData)
}
ImplSource::TraitAlias(d) => ImplSource::TraitAlias(ImplSourceTraitAliasData {
alias_def_id: d.alias_def_id,
substs: d.substs,
@ -632,6 +640,9 @@ pub struct ImplSourceFnPointerData<'tcx, N> {
#[derive(Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, HashStable)]
pub struct ImplSourceDiscriminantKindData;
#[derive(Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, HashStable)]
pub struct ImplSourcePointeeData;
#[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, TypeFoldable, Lift)]
pub struct ImplSourceTraitAliasData<'tcx, N> {
pub alias_def_id: DefId,

3
compiler/rustc_middle/src/traits/select.rs
View File

@ -125,6 +125,9 @@ pub enum SelectionCandidate<'tcx> {
/// Builtin implementation of `DiscriminantKind`.
DiscriminantKindCandidate,
/// Builtin implementation of `Pointee`.
PointeeCandidate,
TraitAliasCandidate(DefId),
/// Matching `dyn Trait` with a supertrait of `Trait`. The index is the

3
compiler/rustc_middle/src/traits/structural_impls.rs
View File

@ -19,6 +19,8 @@ impl<'tcx, N: fmt::Debug> fmt::Debug for traits::ImplSource<'tcx, N> {
super::ImplSource::DiscriminantKind(ref d) => write!(f, "{:?}", d),
super::ImplSource::Pointee(ref d) => write!(f, "{:?}", d),
super::ImplSource::Object(ref d) => write!(f, "{:?}", d),
super::ImplSource::Param(ref n, ct) => {
@ -110,4 +112,5 @@ impl<'tcx, N: fmt::Debug> fmt::Debug for traits::ImplSourceTraitAliasData<'tcx,
TrivialTypeFoldableAndLiftImpls! {
super::IfExpressionCause,
super::ImplSourceDiscriminantKindData,
super::ImplSourcePointeeData,
}

45
compiler/rustc_middle/src/ty/sty.rs
View File

@ -2133,6 +2133,51 @@ impl<'tcx> TyS<'tcx> {
}
}
/// Returns the type of metadata for (potentially fat) pointers to this type.
pub fn ptr_metadata_ty(&'tcx self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
// FIXME: should this normalize?
let tail = tcx.struct_tail_without_normalization(self);
match tail.kind() {
// Sized types
ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
| ty::Uint(_)
| ty::Int(_)
| ty::Bool
| ty::Float(_)
| ty::FnDef(..)
| ty::FnPtr(_)
| ty::RawPtr(..)
| ty::Char
| ty::Ref(..)
| ty::Generator(..)
| ty::GeneratorWitness(..)
| ty::Array(..)
| ty::Closure(..)
| ty::Never
| ty::Error(_)
| ty::Foreign(..)
// If returned by `struct_tail_without_normalization` this is a unit struct
// without any fields, or not a struct, and therefore is Sized.
| ty::Adt(..)
// If returned by `struct_tail_without_normalization` this is the empty tuple,
// a.k.a. unit type, which is Sized
| ty::Tuple(..) => tcx.types.unit,
ty::Str | ty::Slice(_) => tcx.types.usize,
ty::Dynamic(..) => tcx.type_of(tcx.lang_items().dyn_metadata().unwrap()),
ty::Projection(_)
| ty::Param(_)
| ty::Opaque(..)
| ty::Infer(ty::TyVar(_))
| ty::Bound(..)
| ty::Placeholder(..)
| ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
bug!("`ptr_metadata_ty` applied to unexpected type: {:?}", tail)
}
}
}
/// When we create a closure, we record its kind (i.e., what trait
/// it implements) into its `ClosureSubsts` using a type
/// parameter. This is kind of a phantom type, except that the

3
compiler/rustc_span/src/symbol.rs
View File

@ -476,6 +476,7 @@ symbols! {
dropck_eyepatch,
dropck_parametricity,
dylib,
dyn_metadata,
dyn_trait,
edition_macro_pats,
eh_catch_typeinfo,
@ -710,6 +711,7 @@ symbols! {
memory,
message,
meta,
metadata_type,
min_align_of,
min_align_of_val,
min_const_fn,
@ -832,6 +834,7 @@ symbols! {
plugin,
plugin_registrar,
plugins,
pointee_trait,
pointer,
pointer_trait,
pointer_trait_fmt,

68
compiler/rustc_trait_selection/src/traits/project.rs
View File

@ -12,7 +12,7 @@ use super::SelectionContext;
use super::SelectionError;
use super::{
ImplSourceClosureData, ImplSourceDiscriminantKindData, ImplSourceFnPointerData,
ImplSourceGeneratorData, ImplSourceUserDefinedData,
ImplSourceGeneratorData, ImplSourcePointeeData, ImplSourceUserDefinedData,
};
use super::{Normalized, NormalizedTy, ProjectionCacheEntry, ProjectionCacheKey};
@ -1069,6 +1069,51 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
| ty::Error(_) => false,
}
}
super::ImplSource::Pointee(..) => {
// While `Pointee` is automatically implemented for every type,
// the concrete metadata type may not be known yet.
//
// Any type with multiple potential metadata types is therefore not eligible.
let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
// FIXME: should this normalize?
let tail = selcx.tcx().struct_tail_without_normalization(self_ty);
match tail.kind() {
ty::Bool
| ty::Char
| ty::Int(_)
| ty::Uint(_)
| ty::Float(_)
| ty::Foreign(_)
| ty::Str
| ty::Array(..)
| ty::Slice(_)
| ty::RawPtr(..)
| ty::Ref(..)
| ty::FnDef(..)
| ty::FnPtr(..)
| ty::Dynamic(..)
| ty::Closure(..)
| ty::Generator(..)
| ty::GeneratorWitness(..)
| ty::Never
// If returned by `struct_tail_without_normalization` this is a unit struct
// without any fields, or not a struct, and therefore is Sized.
| ty::Adt(..)
// If returned by `struct_tail_without_normalization` this is the empty tuple.
| ty::Tuple(..)
// Integers and floats are always Sized, and so have unit type metadata.
| ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(..)) => true,
ty::Projection(..)
| ty::Opaque(..)
| ty::Param(..)
| ty::Bound(..)
| ty::Placeholder(..)
| ty::Infer(..)
| ty::Error(_) => false,
}
}
super::ImplSource::Param(..) => {
// This case tell us nothing about the value of an
// associated type. Consider:
@ -1169,6 +1214,7 @@ fn confirm_select_candidate<'cx, 'tcx>(
super::ImplSource::DiscriminantKind(data) => {
confirm_discriminant_kind_candidate(selcx, obligation, data)
}
super::ImplSource::Pointee(data) => confirm_pointee_candidate(selcx, obligation, data),
super::ImplSource::Object(_)
| super::ImplSource::AutoImpl(..)
| super::ImplSource::Param(..)
@ -1256,6 +1302,26 @@ fn confirm_discriminant_kind_candidate<'cx, 'tcx>(
confirm_param_env_candidate(selcx, obligation, ty::Binder::dummy(predicate), false)
}
fn confirm_pointee_candidate<'cx, 'tcx>(
selcx: &mut SelectionContext<'cx, 'tcx>,
obligation: &ProjectionTyObligation<'tcx>,
_: ImplSourcePointeeData,
) -> Progress<'tcx> {
let tcx = selcx.tcx();
let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
let substs = tcx.mk_substs([self_ty.into()].iter());
let metadata_def_id = tcx.require_lang_item(LangItem::Metadata, None);
let predicate = ty::ProjectionPredicate {
projection_ty: ty::ProjectionTy { substs, item_def_id: metadata_def_id },
ty: self_ty.ptr_metadata_ty(tcx),
};
confirm_param_env_candidate(selcx, obligation, ty::Binder::bind(predicate), false)
}
fn confirm_fn_pointer_candidate<'cx, 'tcx>(
selcx: &mut SelectionContext<'cx, 'tcx>,
obligation: &ProjectionTyObligation<'tcx>,

3
compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
View File

@ -267,6 +267,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
} else if lang_items.discriminant_kind_trait() == Some(def_id) {
// `DiscriminantKind` is automatically implemented for every type.
candidates.vec.push(DiscriminantKindCandidate);
} else if lang_items.pointee_trait() == Some(def_id) {
// `Pointee` is automatically implemented for every type.
candidates.vec.push(PointeeCandidate);
} else if lang_items.sized_trait() == Some(def_id) {
// Sized is never implementable by end-users, it is
// always automatically computed.

5
compiler/rustc_trait_selection/src/traits/select/confirmation.rs
View File

@ -30,7 +30,8 @@ use crate::traits::{BuiltinDerivedObligation, ImplDerivedObligation};
use crate::traits::{
ImplSourceAutoImplData, ImplSourceBuiltinData, ImplSourceClosureData,
ImplSourceDiscriminantKindData, ImplSourceFnPointerData, ImplSourceGeneratorData,
ImplSourceObjectData, ImplSourceTraitAliasData, ImplSourceUserDefinedData,
ImplSourceObjectData, ImplSourcePointeeData, ImplSourceTraitAliasData,
ImplSourceUserDefinedData,
};
use crate::traits::{ObjectCastObligation, PredicateObligation, TraitObligation};
use crate::traits::{Obligation, ObligationCause};
@ -99,6 +100,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
Ok(ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData))
}
PointeeCandidate => Ok(ImplSource::Pointee(ImplSourcePointeeData)),
TraitAliasCandidate(alias_def_id) => {
let data = self.confirm_trait_alias_candidate(obligation, alias_def_id);
Ok(ImplSource::TraitAlias(data))

18
compiler/rustc_trait_selection/src/traits/select/mod.rs
View File

@ -1318,8 +1318,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
let is_global =
|cand: &ty::PolyTraitRef<'_>| cand.is_global() && !cand.has_late_bound_regions();
// (*) Prefer `BuiltinCandidate { has_nested: false }` and `DiscriminantKindCandidate`
// to anything else.
// (*) Prefer `BuiltinCandidate { has_nested: false }`, `PointeeCandidate`,
// and `DiscriminantKindCandidate` to anything else.
//
// This is a fix for #53123 and prevents winnowing from accidentally extending the
// lifetime of a variable.
@ -1332,8 +1332,18 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
}
// (*)
(BuiltinCandidate { has_nested: false } | DiscriminantKindCandidate, _) => true,
(_, BuiltinCandidate { has_nested: false } | DiscriminantKindCandidate) => false,
(
BuiltinCandidate { has_nested: false }
| DiscriminantKindCandidate
| PointeeCandidate,
_,
) => true,
(
_,
BuiltinCandidate { has_nested: false }
| DiscriminantKindCandidate
| PointeeCandidate,
) => false,
(ParamCandidate(other), ParamCandidate(victim)) => {
if other.value == victim.value && victim.constness == Constness::NotConst {

3
compiler/rustc_ty_utils/src/instance.rs
View File

@ -274,7 +274,8 @@ fn resolve_associated_item<'tcx>(
traits::ImplSource::AutoImpl(..)
| traits::ImplSource::Param(..)
| traits::ImplSource::TraitAlias(..)
| traits::ImplSource::DiscriminantKind(..) => None,
| traits::ImplSource::DiscriminantKind(..)
| traits::ImplSource::Pointee(..) => None,
})
}

15
compiler/rustc_typeck/src/coherence/mod.rs
View File

@ -48,7 +48,20 @@ fn enforce_trait_manually_implementable(
let did = Some(trait_def_id);
let li = tcx.lang_items();
// Disallow *all* explicit impls of `DiscriminantKind`, `Sized` and `Unsize` for now.
// Disallow *all* explicit impls of `Pointee`, `DiscriminantKind`, `Sized` and `Unsize` for now.
if did == li.pointee_trait() {
let span = impl_header_span(tcx, impl_def_id);
struct_span_err!(
tcx.sess,
span,
E0322,
"explicit impls for the `Pointee` trait are not permitted"
)
.span_label(span, "impl of 'Pointee' not allowed")
.emit();
return;
}
if did == li.discriminant_kind_trait() {
let span = impl_header_span(tcx, impl_def_id);
struct_span_err!(

1
library/core/src/lib.rs
View File

@ -133,6 +133,7 @@
#![feature(stmt_expr_attributes)]
#![feature(str_split_as_str)]
#![feature(str_split_inclusive_as_str)]
#![feature(trait_alias)]
#![feature(transparent_unions)]
#![feature(try_blocks)]
#![feature(unboxed_closures)]

77
library/core/src/ptr/metadata.rs Normal file
View File

@ -0,0 +1,77 @@
#![unstable(feature = "ptr_metadata", issue = /* FIXME */ "none")]
use crate::fmt;
use crate::hash::Hash;
use crate::ptr::NonNull;
/// FIXME docs
#[lang = "pointee_trait"]
pub trait Pointee {
/// The type for metadata in pointers and references to `Self`.
#[lang = "metadata_type"]
// NOTE: Keep trait bounds in `static_assert_expected_bounds_for_metadata`
// in `library/core/src/ptr/metadata.rs`
// in sync with those here:
type Metadata: Copy + Send + Sync + Ord + Hash + Unpin;
}
/// Pointers to types implementing this trait alias are “thin”
///
/// ```rust
/// #![feature(ptr_metadata)]
///
/// fn this_never_panics<T: std::ptr::Thin>() {
/// assert_eq!(std::mem::size_of::<&T>(), std::mem::size_of::<usize>())
/// }
/// ```
#[unstable(feature = "ptr_metadata", issue = /* FIXME */ "none")]
// NOTE: don’t stabilize this before trait aliases are stable in the language?
pub trait Thin = Pointee<Metadata = ()>;
/// Extract the metadata component of a pointer.
#[inline]
pub fn metadata<T: ?Sized>(ptr: *const T) -> <T as Pointee>::Metadata {
// SAFETY: Accessing the value from the `PtrRepr` union is safe since *const T
// and PtrComponents<T> have the same memory layouts. Only std can make this
// guarantee.
unsafe { PtrRepr { const_ptr: ptr }.components.metadata }
}
#[repr(C)]
union PtrRepr<T: ?Sized> {
const_ptr: *const T,
components: PtrComponents<T>,
}
#[repr(C)]
struct PtrComponents<T: ?Sized> {
data_address: usize,
metadata: <T as Pointee>::Metadata,
}
// Manual impl needed to avoid `T: Copy` bound.
impl<T: ?Sized> Copy for PtrComponents<T> {}
// Manual impl needed to avoid `T: Clone` bound.
impl<T: ?Sized> Clone for PtrComponents<T> {
fn clone(&self) -> Self {
*self
}
}
/// The metadata for a `dyn SomeTrait` trait object type.
#[lang = "dyn_metadata"]
#[derive(Copy, Clone, Ord, PartialOrd, Eq, PartialEq, Hash)]
pub struct DynMetadata {
#[allow(unused)]
vtable_ptr: NonNull<()>,
}
unsafe impl Send for DynMetadata {}
unsafe impl Sync for DynMetadata {}
impl fmt::Debug for DynMetadata {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.write_str("DynMetadata { … }")
}
}

6
library/core/src/ptr/mod.rs
View File

@ -82,6 +82,12 @@ pub use crate::intrinsics::copy;
#[doc(inline)]
pub use crate::intrinsics::write_bytes;
#[cfg(not(bootstrap))]
mod metadata;
#[cfg(not(bootstrap))]
#[unstable(feature = "ptr_metadata", issue = /* FIXME */ "none")]
pub use metadata::{metadata, DynMetadata, Pointee, Thin};
mod non_null;
#[stable(feature = "nonnull", since = "1.25.0")]
pub use non_null::NonNull;

3
library/core/tests/lib.rs
View File

@ -27,6 +27,7 @@
#![feature(duration_saturating_ops)]
#![feature(duration_zero)]
#![feature(exact_size_is_empty)]
#![feature(extern_types)]
#![feature(fixed_size_array)]
#![feature(flt2dec)]
#![feature(fmt_internals)]
@ -67,8 +68,10 @@
#![feature(option_result_unwrap_unchecked)]
#![feature(option_unwrap_none)]
#![feature(peekable_peek_mut)]
#![feature(ptr_metadata)]
#![feature(once_cell)]
#![feature(unsafe_block_in_unsafe_fn)]
#![feature(unsized_tuple_coercion)]
#![feature(int_bits_const)]
#![feature(nonzero_leading_trailing_zeros)]
#![feature(const_option)]

87
library/core/tests/ptr.rs
View File

@ -1,5 +1,6 @@
use core::cell::RefCell;
use core::ptr::*;
use std::fmt::Display;
#[test]
fn test_const_from_raw_parts() {
@ -413,3 +414,89 @@ fn offset_from() {
assert_eq!(ptr2.offset(-2), ptr1);
}
}
#[test]
#[cfg(not(bootstrap))]
fn ptr_metadata() {
struct Unit;
struct Pair<A, B: ?Sized>(A, B);
extern "C" {
type Extern;
}
let () = metadata(&());
let () = metadata(&Unit);
let () = metadata(&4_u32);
let () = metadata(&String::new());
let () = metadata(&Some(4_u32));
let () = metadata(&ptr_metadata);
let () = metadata(&|| {});
let () = metadata(&[4, 7]);
let () = metadata(&(4, String::new()));
let () = metadata(&Pair(4, String::new()));
let () = metadata(0 as *const Extern);
let () = metadata(0 as *const <&u32 as std::ops::Deref>::Target);
assert_eq!(metadata("foo"), 3_usize);
assert_eq!(metadata(&[4, 7][..]), 2_usize);
let dst_tuple: &(bool, [u8]) = &(true, [0x66, 0x6F, 0x6F]);
let dst_struct: &Pair<bool, [u8]> = &Pair(true, [0x66, 0x6F, 0x6F]);
assert_eq!(metadata(dst_tuple), 3_usize);
assert_eq!(metadata(dst_struct), 3_usize);
unsafe {
let dst_tuple: &(bool, str) = std::mem::transmute(dst_tuple);
let dst_struct: &Pair<bool, str> = std::mem::transmute(dst_struct);
assert_eq!(&dst_tuple.1, "foo");
assert_eq!(&dst_struct.1, "foo");
assert_eq!(metadata(dst_tuple), 3_usize);
assert_eq!(metadata(dst_struct), 3_usize);
}
let vtable_1: DynMetadata = metadata(&4_u32 as &dyn Display);
let vtable_2: DynMetadata = metadata(&(true, 7_u32) as &(bool, dyn Display));
let vtable_3: DynMetadata = metadata(&Pair(true, 7_u32) as &Pair<bool, dyn Display>);
let vtable_4: DynMetadata = metadata(&4_u16 as &dyn Display);
unsafe {
let address_1: usize = std::mem::transmute(vtable_1);
let address_2: usize = std::mem::transmute(vtable_2);
let address_3: usize = std::mem::transmute(vtable_3);
let address_4: usize = std::mem::transmute(vtable_4);
// Same erased type and same trait: same vtable pointer
assert_eq!(address_1, address_2);
assert_eq!(address_1, address_3);
// Different erased type: different vtable pointer
assert_ne!(address_1, address_4);
}
}
#[test]
#[cfg(not(bootstrap))]
fn ptr_metadata_bounds() {
fn metadata_eq_method_address<T: ?Sized>() -> usize {
// The `Metadata` associated type has an `Ord` bound, so this is valid:
<<T as Pointee>::Metadata as PartialEq>::eq as usize
}
// "Synthetic" trait impls generated by the compiler like those of `Pointee`
// are not checked for bounds of associated type.
// So with a buggy libcore we could have both:
// * `<dyn Display as Pointee>::Metadata == DynMetadata`
// * `DynMetadata: !PartialEq`
// … and cause an ICE here:
metadata_eq_method_address::<dyn Display>();
// For this reason, let’s check here that bounds are satisfied:
static_assert_expected_bounds_for_metadata::<()>();
static_assert_expected_bounds_for_metadata::<usize>();
static_assert_expected_bounds_for_metadata::<DynMetadata>();
fn static_assert_associated_type<T: ?Sized>() {
static_assert_expected_bounds_for_metadata::<<T as Pointee>::Metadata>()
}
fn static_assert_expected_bounds_for_metadata<Meta>()
where
// Keep this in sync with the associated type in `library/core/src/ptr/metadata.rs`
Meta: Copy + Send + Sync + Ord + std::hash::Hash + Unpin,
{
}
}