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Sized Hierarchy #137944

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@davidtwco davidtwco commented Mar 3, 2025

This patch implements rust-lang/rfcs#3729. It introduces two new traits to the standard library, MetaSized and PointeeSized, and makes MetaSized and Sized into const traits (relying on unstable feature(const_trait_impl)). See the RFC for the rationale behind these traits and to discuss whether this change makes sense in the abstract.

These traits are unstable (as is their constness), so users cannot refer to them without opting-in to feature(sized_hierarchy). These traits are not behind cfgs as this would make implementation unfeasible, there would simply be too many cfgs required to add the necessary bounds everywhere. So, like Sized, these traits are automatically implemented by the compiler.

RFC 3729 describes migrations which are necessary to preserve backwards compatibility given the introduction of these traits, which are implemented and as follows:

  • On the current edition, Sized is rewritten as const Sized
    • If the sized_hierarchy feature is enabled, then an edition migration lint to rewrite the bound to const Sized will be emitted.
    • On the next edition, non-const Sized will resume being the default bound.
  • On the current edition, ?Sized is rewritten as const MetaSized
    • If the sized_hierarchy feature is enabled, then an edition migration lint to rewrite the bound to const MetaSized will be emitted.
    • On the next edition, writing ?Sized will be prohibited.
  • On the current edition, const MetaSized is added as a default supertrait for all traits w/out an explicit sizedness supertrait already.
    • If the sized_hierarchy feature is enabled, then an edition migration lint to add an explicit const MetaSized supertrait will be emitted.
    • On the next edition, there is no default const MetaSized supertrait.

Each of these migrations is not conditional on whether the item being migrated needs the migration to the stricter bound - this would be preferable but is not yet implemented (if it is possible to implement). All diagnostic output should remain the same (showing ?Sized even if the compiler sees const MetaSized) unless the sized_hierarchy feature is enabled.

Due to the use of unstable extern types in the standard library and rustc, some bounds in both projects have had to be relaxed already - this is unfortunate but unavoidable so that these extern types can continue to be used where they were before. Performing these relaxations in the standard library and rustc are desirable longer-term anyway, but some bounds are not as relaxed as they ideally would be due to the inability to relax Deref::Target (this will be investigated separately).

It is hoped that this is implemented such that it could be merged and these traits could exist "under the hood" without that being observable to the user (other than in any performance impact this has on the compiler, etc). Only once sized_hierarchy is stabilised would edition migration lints start to be emitted and diagnostic output show the "real" sizedness traits behind-the-scenes, rather than ?Sized. Some details might leak through due to the standard library relaxations, but this has not been observed in test output.

Notes:

  • Any commits starting with "upstream:" can be ignored, as these correspond to other upstream PRs that this is based on which have yet to be merged.
  • This best reviewed commit-by-commit. I've attempted to make the implementation easy to follow and keep similar changes and test output updates together.
    • Each commit has a short description describing its purpose.
    • This patch is large but it's primarily in the test suite (library: +573/-184, compiler: +1268/-310, tests: +3720/-452).
  • It is expected that this will have performance regressions initially and I'll aim to resolve those prior to merging if possible.
    • I'd appreciate feedback on how best to go about this from those familiar with the type system.
  • On my local machine, this passes all of the test suites, a stage two build and a tidy check.
  • PointeeSized is a different name from the RFC just to make it more obvious that it is different from std::ptr::Pointee but all the names are yet to be bikeshed anyway.

Fixes #79409.

r? @ghost (I'll discuss this with relevant teams to find a reviewer)

@rustbot rustbot added A-attributes Area: Attributes (`#[…]`, `#![…]`) A-run-make Area: port run-make Makefiles to rmake.rs A-rustdoc-json Area: Rustdoc JSON backend A-rustdoc-search Area: Rustdoc's search feature PG-exploit-mitigations Project group: Exploit mitigations S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. T-rustdoc Relevant to the rustdoc team, which will review and decide on the PR/issue. T-rustdoc-frontend Relevant to the rustdoc-frontend team, which will review and decide on the web UI/UX output. WG-trait-system-refactor The Rustc Trait System Refactor Initiative (-Znext-solver) labels Mar 3, 2025
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davidtwco commented Mar 3, 2025

I can reproduce this locally but I have no idea why it would be related to this patch. Clippy needed adjusting.

@rustbot rustbot added the T-bootstrap Relevant to the bootstrap subteam: Rust's build system (x.py and src/bootstrap) label Mar 3, 2025
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@traviscross traviscross added the T-lang Relevant to the language team, which will review and decide on the PR/issue. label Mar 4, 2025
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cc @rust-lang/lang

@fee1-dead fee1-dead self-assigned this Mar 4, 2025
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tmiasko commented Mar 4, 2025

Does this perhaps fix #127336 by rejecting it?

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Does this perhaps fix #127336 by rejecting it?

It doesn't currently.

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Undrafting now that CI passes

These should never be shown to users at the moment.
One clippy test is `no_core` and needs to have `MetaSized` and
`PointeeSized` added to it.
Existing lints that had special-casing for `Sized` predicates ought
to have these same special cases applied to `MetaSized` predicates.
Unexpected Clippy lint triggering is fixed in previous commits but
is necessary for `cfg(bootstrap)`.
It's unclear why this change in miri is necessary.
These error messages include lines of the standard library which have
changed and so need updated.
Unstability is propagated when staged API is not used and
`-Zforce-unstable-if-unmarked` is, but this only happened for const
functions not const traits, which ends up being an issue for some of the
minicore tests of codegen backends when introducing the sizedness traits.
As in many previous commits, adding the new traits and constness to
the minicore.
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bors commented Mar 12, 2025

☔ The latest upstream changes (presumably #138083) made this pull request unmergeable. Please resolve the merge conflicts.

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I've been meaning to run perf on this and see what the damage is:

@bors try @rust-timer queue

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@rustbot rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Mar 12, 2025
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@bors try @rust-timer queue

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bors commented Mar 12, 2025

⌛ Trying commit 78229bc with merge e8cd94d...

bors added a commit to rust-lang-ci/rust that referenced this pull request Mar 12, 2025
Sized Hierarchy

This patch implements rust-lang/rfcs#3729. It introduces two new traits to the standard library, `MetaSized` and `PointeeSized`, and makes `MetaSized` and `Sized` into const traits (relying on unstable `feature(const_trait_impl)`). See the RFC for the rationale behind these traits and to discuss whether this change makes sense in the abstract.

These traits are unstable (as is their constness), so users cannot refer to them without opting-in to `feature(sized_hierarchy)`. These traits are not behind `cfg`s as this would make implementation unfeasible, there would simply be too many `cfg`s required to add the necessary bounds everywhere. So, like `Sized`, these traits are automatically implemented by the compiler.

RFC 3729 describes migrations which are necessary to preserve backwards compatibility given the introduction of these traits, which are implemented and as follows:

- On the current edition, `Sized` is rewritten as `const Sized`
  - If the `sized_hierarchy` feature is enabled, then an edition migration lint to rewrite the bound to `const Sized` will be emitted.
  - On the next edition, non-const `Sized` will resume being the default bound.
- On the current edition, `?Sized` is rewritten as `const MetaSized`
  - If the `sized_hierarchy` feature is enabled, then an edition migration lint to rewrite the bound to `const MetaSized` will be emitted.
  - On the next edition, writing `?Sized` will be prohibited.
- On the current edition, `const MetaSized` is added as a default supertrait for all traits w/out an explicit sizedness supertrait already.
  - If the `sized_hierarchy` feature is enabled, then an edition migration lint to add an explicit `const MetaSized` supertrait will be emitted.
  - On the next edition, there is no default `const MetaSized` supertrait.

Each of these migrations is not conditional on whether the item being migrated *needs* the migration to the stricter bound - this would be preferable but is not yet implemented (if it is possible to implement). All diagnostic output should remain the same (showing `?Sized` even if the compiler sees `const MetaSized`) unless the `sized_hierarchy` feature is enabled.

Due to the use of unstable extern types in the standard library and rustc, some bounds in both projects have had to be relaxed already - this is unfortunate but unavoidable so that these extern types can continue to be used where they were before. Performing these relaxations in the standard library and rustc are desirable longer-term anyway, but some bounds are not as relaxed as they ideally would be due to the inability to relax `Deref::Target` (this will be investigated separately).

It is hoped that this is implemented such that it could be merged and these traits could exist "under the hood" without that being observable to the user (other than in any performance impact this has on the compiler, etc). Only once `sized_hierarchy` is stabilised would edition migration lints start to be emitted and diagnostic output show the "real" sizedness traits behind-the-scenes, rather than `?Sized`. Some details might leak through due to the standard library relaxations, but this has not been observed in test output.

**Notes:**

- Any commits starting with "upstream:" can be ignored, as these correspond to other upstream PRs that this is based on which have yet to be merged.
- This best reviewed commit-by-commit. I've attempted to make the implementation easy to follow and keep similar changes and test output updates together.
  - Each commit has a short description describing its purpose.
  - This patch is large but it's primarily in the test suite (library: +573/-184, compiler: +1268/-310, tests: +3720/-452).
- It is expected that this will have performance regressions initially and I'll aim to resolve those prior to merging if possible.
  - I'd appreciate feedback on how best to go about this from those familiar with the type system.
- On my local machine, this passes all of the test suites, a stage two build and a tidy check.
- `PointeeSized` is a different name from the RFC just to make it more obvious that it is different from `std::ptr::Pointee` but all the names are yet to be bikeshed anyway.

Fixes rust-lang#79409.

r? `@ghost` (I'll discuss this with relevant teams to find a reviewer)
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bors commented Mar 12, 2025

☀️ Try build successful - checks-actions
Build commit: e8cd94d (e8cd94d5edf3dfb6250fc2a675d4bde945c511e7)

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Finished benchmarking commit (e8cd94d): comparison URL.

Overall result: ❌ regressions - please read the text below

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

mean range count
Regressions ❌
(primary)
2.6% [0.2%, 17.3%] 229
Regressions ❌
(secondary)
3.8% [0.2%, 20.3%] 150
Improvements ✅
(primary)
- - 0
Improvements ✅
(secondary)
- - 0
All ❌✅ (primary) 2.6% [0.2%, 17.3%] 229

Max RSS (memory usage)

Results (primary 2.2%, secondary 3.2%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

mean range count
Regressions ❌
(primary)
2.6% [1.1%, 4.6%] 11
Regressions ❌
(secondary)
3.2% [1.1%, 5.1%] 12
Improvements ✅
(primary)
-1.9% [-1.9%, -1.9%] 1
Improvements ✅
(secondary)
- - 0
All ❌✅ (primary) 2.2% [-1.9%, 4.6%] 12

Cycles

Results (primary 4.8%, secondary 7.2%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

mean range count
Regressions ❌
(primary)
4.8% [1.0%, 16.0%] 109
Regressions ❌
(secondary)
7.3% [1.4%, 23.6%] 66
Improvements ✅
(primary)
- - 0
Improvements ✅
(secondary)
-2.6% [-2.6%, -2.6%] 1
All ❌✅ (primary) 4.8% [1.0%, 16.0%] 109

Binary size

Results (primary 0.3%, secondary 0.3%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

mean range count
Regressions ❌
(primary)
0.3% [0.0%, 1.3%] 76
Regressions ❌
(secondary)
0.3% [0.0%, 0.6%] 31
Improvements ✅
(primary)
- - 0
Improvements ✅
(secondary)
- - 0
All ❌✅ (primary) 0.3% [0.0%, 1.3%] 76

Bootstrap: 779.458s -> 790.01s (1.35%)
Artifact size: 365.27 MiB -> 365.35 MiB (0.02%)

@rustbot rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Mar 13, 2025
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Okay, so there's some work to do on perf, as expected.

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Second pass. Will spend more time on it today after I get to my PC and not from my laptop :)


/// Types that may or may not have a size.
#[unstable(feature = "sized_hierarchy", issue = "none")]
#[cfg_attr(not(bootstrap), lang = "pointeesized")]
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lang = "pointee_sized"

Comment on lines +155 to +157
/// [`structural_traits::instantiate_constituent_tys_for_sized_trait`],
/// [`structural_traits::instantiate_constituent_tys_for_metasized_trait`] and
/// [`structural_traits::instantiate_constituent_tys_for_pointeesized_trait`].
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Maybe better to just refer to instantiate_constituent_tys_for_sizedness_trait only

Comment on lines +232 to +242
match sizedness {
SizedTraitKind::Sized => {
structural_traits::instantiate_constituent_tys_for_sized_trait
}
SizedTraitKind::MetaSized => {
structural_traits::instantiate_constituent_tys_for_metasized_trait
}
SizedTraitKind::PointeeSized => {
structural_traits::instantiate_constituent_tys_for_pointeesized_trait
}
},
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I don't think we this extra layer of indirection as there is already instantiate_constituent_tys_for_sizedness_trait which takes any SizedTraitKind

Comment on lines +197 to +205
// impl {Meta,}Sized for Adt<Args...> where {meta,}sized_constraint(Adt)<Args...>: {Meta,}Sized
// `{meta,}sized_constraint(Adt)` is the deepest struct trail that can be determined
// by the definition of `Adt`, independent of the generic args.
// impl Sized for Adt<Args...> if sized_constraint(Adt) == None
// As a performance optimization, `sized_constraint(Adt)` can return `None`
// if the ADTs definition implies that it is sized by for all possible args.
// impl {Meta,}Sized for Adt<Args...> if {meta,}sized_constraint(Adt) == None
// As a performance optimization, `{meta,}sized_constraint(Adt)` can return `None`
// if the ADTs definition implies that it is {meta,}sized by for all possible args.
// In this case, the builtin impl will have no nested subgoals. This is a
// "best effort" optimization and `sized_constraint` may return `Some`, even
// if the ADT is sized for all possible args.
// "best effort" optimization and `{meta,}sized_constraint` may return `Some`, even
// if the ADT is {meta,}sized for all possible args.
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these should all contain pointee also

Comment on lines 101 to 107
if ptr.trait_ref.path.res == Res::Def(DefKind::Trait, pointeesized_did) {
match ptr.modifiers.polarity {
hir::BoundPolarity::Maybe(_) => pointeesized.maybe = true,
hir::BoundPolarity::Negative(_) => pointeesized.negative = true,
hir::BoundPolarity::Positive => pointeesized.positive = true,
}
}
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This part can be deduplicated through doing something like

let collected = if ... { &mut sized } else if ... { &mut meta_sized } else if { &mut pointee_sized } else { continue };  

Comment on lines +141 to +152
let mut idx = None;
for (cur_idx, (clause, _)) in bounds.into_iter().enumerate() {
if let Some(clause) = clause.as_trait_clause()
&& clause.skip_binder().def_id() == did
{
idx = Some(cur_idx);
}
}

if let Some(idx) = idx {
bounds.remove(idx);
}
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Any chance for duplication? In any case we should use

bounds.retain(|(clause, _)| clause.as_trait_clause().map_or(false, |clause| clause.skip_binder().def_id() != did))

instead.

Comment on lines +6 to +21
trait Sized_: Sized { }

trait NegSized: ?Sized { }
//~^ ERROR `?Trait` is not permitted in supertraits

trait MetaSized_: MetaSized { }

trait NegMetaSized: ?MetaSized { }
//~^ ERROR `?Trait` is not permitted in supertraits


trait PointeeSized_: PointeeSized { }

trait NegPointeeSized: ?PointeeSized { }
//~^ ERROR `?Trait` is not permitted in supertraits

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Should add a trait Bare {} here and test

Comment on lines 1 to 24
//@ check-pass
#![feature(sized_hierarchy)]

use std::marker::PhantomData;

pub trait Bar<'a> {
type Foo;
}

pub struct Foo<'a, T: Bar<'a>> {
phantom: PhantomData<&'a T>,
}

impl<'a, 'b, T> PartialEq<Foo<'b, T>> for Foo<'a, T>
where
T: for<'c> Bar<'c>,
<T as Bar<'a>>::Foo: PartialEq<<T as Bar<'b>>::Foo>,
{
fn eq(&self, _: &Foo<'b, T>) -> bool {
loop {}
}
}

fn main() { }
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Hmmm. Where does this test come from?

@rustbot rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Mar 14, 2025
Comment on lines +608 to +611
let pointeesized_predicate = {
let pointeesized_did = tcx.require_lang_item(LangItem::PointeeSized, None);
ty::TraitRef::new(tcx, pointeesized_did, [unsized_self_ty]).upcast(tcx)
};
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Does this need to be proven? It's not referenced in the commit message, and also U: MetaSized already implies it.

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How would the pretty printer work if we're working with a dependency crate that uses the sized_hierarchy feature but the caller does not enable it? Add a test please

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I have some general issues with the commits starting from "tests: PointeeSized bounds with extern types" to "tests: bless remaining tests".

In those tests' cases, were enabling the sized_hierarchy required for some tests to work? For example, before the feature was enabled, do functions like fn foo<T: ?Sized>() still accept opaque types? If they don't and need the feature to be enabled to work again, then that's a potential breakage.

Comment on lines -9 to 15
impl<V: ?Sized> Callable for () {
impl<V: PointeeSized> Callable for () {
//~^ ERROR the type parameter `V` is not constrained by the impl trait, self type, or predicates
fn call() {}
}
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This error still fires for impl<V: ?Sized> right? They probably don't need to use feature(sized_hierarchy)?

Comment on lines 2 to 13

#![feature(sized_hierarchy)]
#![feature(non_lifetime_binders)]
//~^ WARN the feature `non_lifetime_binders` is incomplete

trait Trait<T: ?Sized> {}
use std::marker::PointeeSized;

impl<T: ?Sized> Trait<T> for i32 {}
trait Trait<T: PointeeSized> {}

impl<T: PointeeSized> Trait<T> for i32 {}

fn produce() -> impl for<T> Trait<T> {
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After the sized hierarchy is introduced, does this need sized_hierarchy enabled to work? If yes, that is a potential concern. If not, these tests should remain without using the feature, while you can add the tests to under the experimental feature.

@@ -688,6 +688,10 @@ passes_rustc_lint_opt_ty =
`#[rustc_lint_opt_ty]` should be applied to a struct
.label = not a struct

passes_rustc_non_const_sized =
attribute should be applied to a struct or enum
.label = not an struct or enum
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not a struct or enum

Comment on lines +239 to +248
if !self_ty.has_non_const_sizedness() && matches!(sizedness, SizedTraitKind::Sized) {
let metasized_def_id = cx.require_lang_item(TraitSolverLangItem::MetaSized);
let metasized_trait_ref = ty::TraitRef::new(cx, metasized_def_id, [self_ty]);
Ok(vec![metasized_trait_ref])
} else if !self_ty.has_non_const_sizedness() {
// `MetaSized` has no conditionally const supertrait
Ok(vec![])
} else {
Err(NoSolution)
}
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rewrite this into first if self_ty.has_non_const_sizedness() { return Err(NoSolution) }, then explicitly match the sizedness, handle the PointeeSized variant too.

Comment on lines +267 to +281
if !self_ty.has_non_const_sizedness() && matches!(sizedness, SizedTraitKind::Sized) {
let metasized_def_id = tcx.require_lang_item(LangItem::MetaSized, None);
let metasized_trait_ref = ty::TraitRef::new(tcx, metasized_def_id, [self_ty]);
let metasized_obligation = obligation.with(
tcx,
ty::Binder::dummy(metasized_trait_ref)
.to_host_effect_clause(tcx, obligation.predicate.constness),
);
Ok(thin_vec![metasized_obligation])
} else if !self_ty.has_non_const_sizedness() {
// `MetaSized` has no conditionally const supertrait
Ok(thin_vec![])
} else {
Err(EvaluationFailure::NoSolution)
}
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same as above

Comment on lines +1 to +14
//@ check-pass
//@ compile-flags: --crate-type=lib
#![feature(const_trait_impl)]

// This test can fail because of the implicit const bounds/supertraits.

pub struct Bar;

#[const_trait]
pub trait Foo {}

impl const Foo for Bar {}

pub const fn bar<T: ~const Foo>() {}
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where does this test come from?

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ICE with unsizing an extern type
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