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Fix post-monomorphization error note race in the parallel frontend #157282

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Fix post-monomorphization error note race in the parallel frontend #157282

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c1cd236
Attribute post-monomorphization error notes by emitting thread
xmakro Jun 5, 2026
2e128c7
Re-enable parallel frontend tests for post-monomorphization errors
xmakro Jun 5, 2026
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72 changes: 49 additions & 23 deletions compiler/rustc_errors/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -24,6 +24,7 @@ use std::io::Write;
use std::num::NonZero;
use std::ops::DerefMut;
use std::path::{Path, PathBuf};
use std::thread::ThreadId;
use std::{assert_matches, fmt, panic};

use Level::*;
Expand Down Expand Up @@ -297,11 +298,12 @@ impl<'a> std::ops::Deref for DiagCtxtHandle<'a> {
struct DiagCtxtInner {
flags: DiagCtxtFlags,

/// The error guarantees from all emitted errors. The length gives the error count.
err_guars: Vec<ErrorGuaranteed>,
/// The error guarantee from all emitted lint errors. The length gives the
/// lint error count.
lint_err_guars: Vec<ErrorGuaranteed>,
/// The error guarantees from all emitted errors, each paired with the
/// thread that emitted it. The length gives the error count.
err_guars: Vec<(ErrorGuaranteed, ThreadId)>,
/// The error guarantee from all emitted lint errors, each paired with the
/// thread that emitted it. The length gives the lint error count.
lint_err_guars: Vec<(ErrorGuaranteed, ThreadId)>,
/// The delayed bugs and their error guarantees.
delayed_bugs: Vec<(DelayedDiagInner, ErrorGuaranteed)>,

Expand Down Expand Up @@ -343,7 +345,7 @@ struct DiagCtxtInner {
/// `emit_stashed_diagnostics` by the time the `DiagCtxtInner` is dropped,
/// otherwise an assertion failure will occur.
stashed_diagnostics:
FxIndexMap<StashKey, FxIndexMap<Span, (DiagInner, Option<ErrorGuaranteed>)>>,
FxIndexMap<StashKey, FxIndexMap<Span, (DiagInner, Option<ErrorGuaranteed>, ThreadId)>>,

future_breakage_diagnostics: Vec<DiagInner>,

Expand Down Expand Up @@ -613,7 +615,7 @@ impl<'a> DiagCtxtHandle<'a> {
.stashed_diagnostics
.entry(key)
.or_default()
.insert(span.with_parent(None), (diag, guar));
.insert(span.with_parent(None), (diag, guar, std::thread::current().id()));

guar
}
Expand All @@ -623,7 +625,7 @@ impl<'a> DiagCtxtHandle<'a> {
/// error.
pub fn steal_non_err(self, span: Span, key: StashKey) -> Option<Diag<'a, ()>> {
// FIXME(#120456) - is `swap_remove` correct?
let (diag, guar) = self.inner.borrow_mut().stashed_diagnostics.get_mut(&key).and_then(
let (diag, guar, _) = self.inner.borrow_mut().stashed_diagnostics.get_mut(&key).and_then(
|stashed_diagnostics| stashed_diagnostics.swap_remove(&span.with_parent(None)),
)?;
assert!(!diag.is_error());
Expand All @@ -648,7 +650,7 @@ impl<'a> DiagCtxtHandle<'a> {
let err = self.inner.borrow_mut().stashed_diagnostics.get_mut(&key).and_then(
|stashed_diagnostics| stashed_diagnostics.swap_remove(&span.with_parent(None)),
);
err.map(|(err, guar)| {
err.map(|(err, guar, _)| {
// The use of `::<ErrorGuaranteed>` is safe because level is `Level::Error`.
assert_eq!(err.level, Error);
assert!(guar.is_some());
Expand All @@ -673,7 +675,7 @@ impl<'a> DiagCtxtHandle<'a> {
|stashed_diagnostics| stashed_diagnostics.swap_remove(&span.with_parent(None)),
);
match old_err {
Some((old_err, guar)) => {
Some((old_err, guar, _)) => {
assert_eq!(old_err.level, Error);
assert!(guar.is_some());
// Because `old_err` has already been counted, it can only be
Expand Down Expand Up @@ -710,7 +712,27 @@ impl<'a> DiagCtxtHandle<'a> {
+ inner
.stashed_diagnostics
.values()
.map(|a| a.values().filter(|(_, guar)| guar.is_some()).count())
.map(|a| a.values().filter(|(_, guar, _)| guar.is_some()).count())
.sum::<usize>()
}

/// The number of errors that have been emitted on the *current thread*.
///
/// Like [`DiagCtxtHandle::err_count`], but only counts errors whose recorded
/// emitting thread is the calling thread.
pub fn err_count_on_current_thread(&self) -> usize {
let inner = self.inner.borrow();
let current = std::thread::current().id();
inner.err_guars.iter().filter(|(_, thread)| *thread == current).count()
+ inner.lint_err_guars.iter().filter(|(_, thread)| *thread == current).count()
+ inner
.stashed_diagnostics
.values()
.map(|a| {
a.values()
.filter(|(_, guar, thread)| guar.is_some() && *thread == current)
.count()
})
.sum::<usize>()
}
Comment on lines +723 to +737
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@zetanumbers zetanumbers Jun 4, 2026 •
edited by rustbot
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This method is misleading since instead of modifying the self argument it actually modifies a thread local. Instead we could store a ThreadId in the err_guars, lint_err_guars and stashed_diagnostics fields so that err_count_on_current_thread could filter and count errors almost like the old err_count, but also counting only matching ThreadIds.

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@zetanumbers zetanumbers Jun 4, 2026

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TBH we could use any type for this CountErrorsId as long as we set it in compiler/rustc_monomorphize/src/errors.rs and it is implicitly "passed down" to through the callstack, in a thread local for example.

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@xmakro xmakro Jun 4, 2026 •
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Thanks, I added the ThreadId explicitly to the relevant vecs holding errors, do you prefer type ErrCountId = ThreadId?

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Please no type alias. This is fine.


Expand Down Expand Up @@ -879,7 +901,8 @@ impl<'a> DiagCtxtHandle<'a> {
// This `unchecked_error_guaranteed` is valid. It is where the
// `ErrorGuaranteed` for unused_extern errors originates.
#[allow(deprecated)]
inner.lint_err_guars.push(ErrorGuaranteed::unchecked_error_guaranteed());
let guar = ErrorGuaranteed::unchecked_error_guaranteed();
inner.lint_err_guars.push((guar, std::thread::current().id()));
inner.panic_if_treat_err_as_bug();
}

Expand Down Expand Up @@ -1178,7 +1201,7 @@ impl DiagCtxtInner {
let mut guar = None;
let has_errors = !self.err_guars.is_empty();
for (_, stashed_diagnostics) in std::mem::take(&mut self.stashed_diagnostics).into_iter() {
for (_, (diag, _guar)) in stashed_diagnostics {
for (_, (diag, _guar, _thread)) in stashed_diagnostics {
if !diag.is_error() {
// Unless they're forced, don't flush stashed warnings when
// there are errors, to avoid causing warning overload. The
Expand Down Expand Up @@ -1347,13 +1370,14 @@ impl DiagCtxtInner {
// `ErrorGuaranteed` for errors and lint errors originates.
#[allow(deprecated)]
let guar = ErrorGuaranteed::unchecked_error_guaranteed();
let thread = std::thread::current().id();
if is_lint {
self.lint_err_guars.push(guar);
self.lint_err_guars.push((guar, thread));
} else {
if let Some(taint) = taint {
taint.set(Some(guar));
}
self.err_guars.push(guar);
self.err_guars.push((guar, thread));
}
self.panic_if_treat_err_as_bug();
Some(guar)
Expand All @@ -1377,12 +1401,12 @@ impl DiagCtxtInner {
}

fn has_errors_excluding_lint_errors(&self) -> Option<ErrorGuaranteed> {
self.err_guars.get(0).copied().or_else(|| {
if let Some((_diag, guar)) = self
self.err_guars.get(0).map(|(guar, _)| *guar).or_else(|| {
if let Some((_diag, guar, _)) = self
.stashed_diagnostics
.values()
.flat_map(|stashed_diagnostics| stashed_diagnostics.values())
.find(|(diag, guar)| guar.is_some() && diag.is_lint.is_none())
.find(|(diag, guar, _)| guar.is_some() && diag.is_lint.is_none())
{
*guar
} else {
Expand All @@ -1392,13 +1416,15 @@ impl DiagCtxtInner {
}

fn has_errors(&self) -> Option<ErrorGuaranteed> {
self.err_guars.get(0).copied().or_else(|| self.lint_err_guars.get(0).copied()).or_else(
|| {
self.err_guars
.get(0)
.map(|(guar, _)| *guar)
.or_else(|| self.lint_err_guars.get(0).map(|(guar, _)| *guar))
.or_else(|| {
self.stashed_diagnostics.values().find_map(|stashed_diagnostics| {
stashed_diagnostics.values().find_map(|(_, guar)| *guar)
stashed_diagnostics.values().find_map(|(_, guar, _)| *guar)
})
},
)
})
}

fn has_errors_or_delayed_bugs(&self) -> Option<ErrorGuaranteed> {
Expand Down
4 changes: 2 additions & 2 deletions compiler/rustc_monomorphize/src/collector.rs
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Expand Up @@ -413,7 +413,7 @@ fn collect_items_rec<'tcx>(
// current step of mono items collection.
//
// FIXME: don't rely on global state, instead bubble up errors. Note: this is very hard to do.
let error_count = tcx.dcx().err_count();
let error_count = tcx.dcx().err_count_on_current_thread();

// In `mentioned_items` we collect items that were mentioned in this MIR but possibly do not
// need to be monomorphized. This is done to ensure that optimizing away function calls does not
Expand Down Expand Up @@ -538,7 +538,7 @@ fn collect_items_rec<'tcx>(

// Check for PMEs and emit a diagnostic if one happened. To try to show relevant edges of the
// mono item graph.
if tcx.dcx().err_count() > error_count
if tcx.dcx().err_count_on_current_thread() > error_count
&& starting_item.node.is_generic_fn()
&& starting_item.node.is_user_defined()
{
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/abi/simd-abi-checks-avx.rs
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@@ -1,7 +1,7 @@
//@ only-x86_64
//@ build-fail
//@ compile-flags: -C target-feature=-avx
//@ ignore-parallel-frontend post-monomorphization errors
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#![feature(portable_simd)]
#![feature(simd_ffi)]
#![allow(improper_ctypes_definitions)]
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/const-eval/index-out-of-bounds-never-type.rs
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@@ -1,5 +1,5 @@
//@ build-fail
//@ ignore-parallel-frontend post-monomorphization errors

// Regression test for #66975
#![warn(unconditional_panic)]
#![feature(never_type)]
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/const-eval/issue-50814-2.rs
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Expand Up @@ -2,7 +2,7 @@
//@ revisions: normal mir-opt
//@ [mir-opt]compile-flags: -Zmir-opt-level=4
//@ dont-require-annotations: NOTE
//@ ignore-parallel-frontend post-monomorphization errors

trait C {
const BOO: usize;
}
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/const-eval/issue-50814.rs
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@@ -1,6 +1,6 @@
//@ build-fail
//@ dont-require-annotations: NOTE
//@ ignore-parallel-frontend post-monomorphization errors

trait Unsigned {
const MAX: u8;
}
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/mono-reachable-invalid-const.rs
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@@ -1,5 +1,5 @@
//@ build-fail
//@ ignore-parallel-frontend post-monomorphization errors

struct Bar<const BITS: usize>;

impl<const BITS: usize> Bar<BITS> {
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/required-consts/collect-in-called-fn.rs
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Expand Up @@ -4,7 +4,7 @@
//@[opt] compile-flags: -O
//! Make sure we detect erroneous constants post-monomorphization even when they are unused. This is
//! crucial, people rely on it for soundness. (https://github.com/rust-lang/rust/issues/112090)
//@ ignore-parallel-frontend post-monomorphization errors

struct Fail<T>(T);
impl<T> Fail<T> {
const C: () = panic!(); //~ERROR evaluation panicked: explicit panic
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/required-consts/collect-in-dead-closure.rs
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Expand Up @@ -3,7 +3,7 @@
//@[noopt] compile-flags: -Copt-level=0
//@[opt] compile-flags: -O
//! This fails without optimizations, so it should also fail with optimizations.
//@ ignore-parallel-frontend post-monomorphization errors

struct Fail<T>(T);
impl<T> Fail<T> {
const C: () = panic!(); //~ERROR evaluation panicked: explicit panic
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/required-consts/collect-in-dead-drop.rs
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Expand Up @@ -3,7 +3,7 @@
//@[noopt] compile-flags: -Copt-level=0
//@[opt] compile-flags: -O
//! This fails without optimizations, so it should also fail with optimizations.
//@ ignore-parallel-frontend post-monomorphization errors

struct Fail<T>(T);
impl<T> Fail<T> {
const C: () = panic!(); //~ERROR evaluation panicked: explicit panic
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/required-consts/collect-in-dead-fn-behind-assoc-type.rs
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Expand Up @@ -4,7 +4,7 @@
//@[noopt] compile-flags: -Copt-level=0
//@[opt] compile-flags: -O
//! This fails without optimizations, so it should also fail with optimizations.
//@ ignore-parallel-frontend post-monomorphization errors

struct Fail<T>(T);
impl<T> Fail<T> {
const C: () = panic!(); //~ERROR evaluation panicked: explicit panic
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/required-consts/collect-in-dead-fn-behind-generic.rs
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Expand Up @@ -3,7 +3,7 @@
//@[noopt] compile-flags: -Copt-level=0
//@[opt] compile-flags: -O
//! This fails without optimizations, so it should also fail with optimizations.
//@ ignore-parallel-frontend post-monomorphization errors

struct Fail<T>(T);
impl<T> Fail<T> {
const C: () = panic!(); //~ERROR evaluation panicked: explicit panic
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/required-consts/collect-in-dead-fn-behind-opaque-type.rs
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Expand Up @@ -4,7 +4,7 @@
//@[opt] compile-flags: -O
//! This fails without optimizations, so it should also fail with optimizations.
#![feature(type_alias_impl_trait)]
//@ ignore-parallel-frontend post-monomorphization errors

mod m {
struct Fail<T>(T);
impl<T> Fail<T> {
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/required-consts/collect-in-dead-fn.rs
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Expand Up @@ -3,7 +3,7 @@
//@[noopt] compile-flags: -Copt-level=0
//@[opt] compile-flags: -O
//! This fails without optimizations, so it should also fail with optimizations.
//@ ignore-parallel-frontend post-monomorphization errors

struct Fail<T>(T);
impl<T> Fail<T> {
const C: () = panic!(); //~ERROR evaluation panicked: explicit panic
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/required-consts/collect-in-dead-fnptr-in-const.rs
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Expand Up @@ -3,7 +3,7 @@
//@[noopt] compile-flags: -Copt-level=0
//@[opt] compile-flags: -O
//! This fails without optimizations, so it should also fail with optimizations.
//@ ignore-parallel-frontend post-monomorphization errors

struct Late<T>(T);
impl<T> Late<T> {
const FAIL: () = panic!(); //~ERROR evaluation panicked: explicit panic
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/required-consts/collect-in-dead-fnptr.rs
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Expand Up @@ -3,7 +3,7 @@
//@[noopt] compile-flags: -Copt-level=0
//@[opt] compile-flags: -O
//! This fails without optimizations, so it should also fail with optimizations.
//@ ignore-parallel-frontend post-monomorphization errors

struct Fail<T>(T);
impl<T> Fail<T> {
const C: () = panic!(); //~ERROR evaluation panicked: explicit panic
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/required-consts/collect-in-dead-forget.rs
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Expand Up @@ -3,7 +3,7 @@
//@[noopt] compile-flags: -Copt-level=0
//@[opt] compile-flags: -O
//! This passes without optimizations, so it can (and should) also pass with optimizations.
//@ ignore-parallel-frontend post-monomorphization errors

struct Fail<T>(T);
impl<T> Fail<T> {
const C: () = panic!();
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/required-consts/collect-in-dead-move.rs
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Expand Up @@ -3,7 +3,7 @@
//@[noopt] compile-flags: -Copt-level=0
//@[opt] compile-flags: -O
//! This fails without optimizations, so it should also fail with optimizations.
//@ ignore-parallel-frontend post-monomorphization errors

struct Fail<T>(T);
impl<T> Fail<T> {
const C: () = panic!(); //~ERROR evaluation panicked: explicit panic
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/required-consts/collect-in-dead-vtable.rs
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Expand Up @@ -3,7 +3,7 @@
//@[noopt] compile-flags: -Copt-level=0
//@[opt] compile-flags: -O
//! This fails without optimizations, so it should also fail with optimizations.
//@ ignore-parallel-frontend post-monomorphization errors

struct Fail<T>(T);
impl<T> Fail<T> {
const C: () = panic!(); //~ERROR evaluation panicked: explicit panic
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/required-consts/collect-in-promoted-const.rs
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Expand Up @@ -3,7 +3,7 @@
//@[noopt] compile-flags: -Copt-level=0
//@[opt] compile-flags: -O
//! Make sure we error on erroneous consts even if they get promoted.
//@ ignore-parallel-frontend post-monomorphization errors

struct Fail<T>(T);
impl<T> Fail<T> {
const C: () = panic!(); //~ERROR evaluation panicked: explicit panic
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/required-consts/interpret-in-const-called-fn.rs
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Expand Up @@ -2,7 +2,7 @@
//@[noopt] compile-flags: -Copt-level=0
//@[opt] compile-flags: -O
//! Make sure we error on erroneous consts even if they are unused.
//@ ignore-parallel-frontend post-monomorphization errors

struct Fail<T>(T);
impl<T> Fail<T> {
const C: () = panic!(); //~ERROR explicit panic
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/required-consts/interpret-in-promoted.rs
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Expand Up @@ -2,7 +2,7 @@
//@[noopt] compile-flags: -Copt-level=0
//@[opt] compile-flags: -O
//@ dont-require-annotations: NOTE
//@ ignore-parallel-frontend post-monomorphization errors

//! Make sure we evaluate const fn calls even if they get promoted and their result ignored.

const unsafe fn ub() {
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/required-consts/interpret-in-static.rs
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Expand Up @@ -2,7 +2,7 @@
//@[noopt] compile-flags: -Copt-level=0
//@[opt] compile-flags: -O
//! Make sure we error on erroneous consts even if they are unused.
//@ ignore-parallel-frontend post-monomorphization errors

struct Fail<T>(T);
impl<T> Fail<T> {
const C: () = panic!(); //~ERROR explicit panic
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/inline-const/required-const.rs
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@@ -1,6 +1,6 @@
//@ build-fail
//@ compile-flags: -Zmir-opt-level=3
//@ ignore-parallel-frontend post-monomorphization errors

fn foo<T>() {
if false {
const { panic!() } //~ ERROR E0080
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/lint/large_assignments/copy_into_box_rc_arc.rs
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Expand Up @@ -3,7 +3,7 @@
#![move_size_limit = "1000"]
//@ build-fail
//@ only-64bit
//@ ignore-parallel-frontend post-monomorphization errors

//@ edition:2018
//@ compile-flags: -Zmir-opt-level=1

Expand Down
2 changes: 1 addition & 1 deletion tests/ui/lint/large_assignments/copy_into_fn.rs
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@@ -1,5 +1,5 @@
//@ build-fail
//@ ignore-parallel-frontend post-monomorphization errors

#![feature(large_assignments)]
#![move_size_limit = "1000"]
#![deny(large_assignments)]
Expand Down
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