compiler/rustc_query_system/src/query/job.rs - toolchain/rustc - Git at Google

 use crate::query::plumbing::CycleError;
 use crate::query::{QueryContext, QueryStackFrame};
 use rustc_hir::def::DefKind;

 use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::{
     struct_span_err, Diagnostic, DiagnosticBuilder, ErrorGuaranteed, Handler, Level,
 };
 use rustc_session::Session;
 use rustc_span::Span;

 use std::hash::Hash;
 use std::num::NonZeroU64;

 #[cfg(parallel_compiler)]
 use {
     parking_lot::{Condvar, Mutex},
     rustc_data_structures::fx::FxHashSet,
     rustc_data_structures::sync::Lock,
     rustc_data_structures::sync::Lrc,
     rustc_data_structures::{jobserver, OnDrop},
     rustc_rayon_core as rayon_core,
     rustc_span::DUMMY_SP,
     std::iter::{self, FromIterator},
     std::{mem, process},
 };

 /// Represents a span and a query key.
 #[derive(Clone, Debug)]
 pub struct QueryInfo {
     /// The span corresponding to the reason for which this query was required.
     pub span: Span,
     pub query: QueryStackFrame,
 }

 pub type QueryMap = FxHashMap<QueryJobId, QueryJobInfo>;

 /// A value uniquely identifying an active query job.
 #[derive(Copy, Clone, Eq, PartialEq, Hash)]
 pub struct QueryJobId(pub NonZeroU64);

 impl QueryJobId {
     fn query(self, map: &QueryMap) -> QueryStackFrame {
         map.get(&self).unwrap().query.clone()
     }

     #[cfg(parallel_compiler)]
     fn span(self, map: &QueryMap) -> Span {
         map.get(&self).unwrap().job.span
     }

     #[cfg(parallel_compiler)]
     fn parent(self, map: &QueryMap) -> Option<QueryJobId> {
         map.get(&self).unwrap().job.parent
     }

     #[cfg(parallel_compiler)]
     fn latch<'a>(self, map: &'a QueryMap) -> Option<&'a QueryLatch> {
         map.get(&self).unwrap().job.latch.as_ref()
     }
 }

 pub struct QueryJobInfo {
     pub query: QueryStackFrame,
     pub job: QueryJob,
 }

 /// Represents an active query job.
 #[derive(Clone)]
 pub struct QueryJob {
     pub id: QueryJobId,

     /// The span corresponding to the reason for which this query was required.
     pub span: Span,

     /// The parent query job which created this job and is implicitly waiting on it.
     pub parent: Option<QueryJobId>,

     /// The latch that is used to wait on this job.
     #[cfg(parallel_compiler)]
     latch: Option<QueryLatch>,
 }

 impl QueryJob {
     /// Creates a new query job.
     #[inline]
     pub fn new(id: QueryJobId, span: Span, parent: Option<QueryJobId>) -> Self {
         QueryJob {
             id,
             span,
             parent,
             #[cfg(parallel_compiler)]
             latch: None,
         }
     }

     #[cfg(parallel_compiler)]
     pub(super) fn latch(&mut self) -> QueryLatch {
         if self.latch.is_none() {
             self.latch = Some(QueryLatch::new());
         }
         self.latch.as_ref().unwrap().clone()
     }

     /// Signals to waiters that the query is complete.
     ///
     /// This does nothing for single threaded rustc,
     /// as there are no concurrent jobs which could be waiting on us
     #[inline]
     pub fn signal_complete(self) {
         #[cfg(parallel_compiler)]
         {
             if let Some(latch) = self.latch {
                 latch.set();
             }
         }
     }
 }

 #[cfg(not(parallel_compiler))]
 impl QueryJobId {
     #[cold]
     #[inline(never)]
     pub(super) fn find_cycle_in_stack(
         &self,
         query_map: QueryMap,
         current_job: &Option<QueryJobId>,
         span: Span,
     ) -> CycleError {
         // Find the waitee amongst `current_job` parents
         let mut cycle = Vec::new();
         let mut current_job = Option::clone(current_job);

         while let Some(job) = current_job {
             let info = query_map.get(&job).unwrap();
             cycle.push(QueryInfo { span: info.job.span, query: info.query.clone() });

             if job == *self {
                 cycle.reverse();

                 // This is the end of the cycle
                 // The span entry we included was for the usage
                 // of the cycle itself, and not part of the cycle
                 // Replace it with the span which caused the cycle to form
                 cycle[0].span = span;
                 // Find out why the cycle itself was used
                 let usage = info
                     .job
                     .parent
                     .as_ref()
                     .map(|parent| (info.job.span, parent.query(&query_map)));
                 return CycleError { usage, cycle };
             }

             current_job = info.job.parent;
         }

         panic!("did not find a cycle")
     }
 }

 #[cfg(parallel_compiler)]
 struct QueryWaiter {
     query: Option<QueryJobId>,
     condvar: Condvar,
     span: Span,
     cycle: Lock<Option<CycleError>>,
 }

 #[cfg(parallel_compiler)]
 impl QueryWaiter {
     fn notify(&self, registry: &rayon_core::Registry) {
         rayon_core::mark_unblocked(registry);
         self.condvar.notify_one();
     }
 }

 #[cfg(parallel_compiler)]
 struct QueryLatchInfo {
     complete: bool,
     waiters: Vec<Lrc<QueryWaiter>>,
 }

 #[cfg(parallel_compiler)]
 #[derive(Clone)]
 pub(super) struct QueryLatch {
     info: Lrc<Mutex<QueryLatchInfo>>,
 }

 #[cfg(parallel_compiler)]
 impl QueryLatch {
     fn new() -> Self {
         QueryLatch {
             info: Lrc::new(Mutex::new(QueryLatchInfo { complete: false, waiters: Vec::new() })),
         }
     }

     /// Awaits for the query job to complete.
     pub(super) fn wait_on(&self, query: Option<QueryJobId>, span: Span) -> Result<(), CycleError> {
         let waiter =
             Lrc::new(QueryWaiter { query, span, cycle: Lock::new(None), condvar: Condvar::new() });
         self.wait_on_inner(&waiter);
         // FIXME: Get rid of this lock. We have ownership of the QueryWaiter
         // although another thread may still have a Lrc reference so we cannot
         // use Lrc::get_mut
         let mut cycle = waiter.cycle.lock();
         match cycle.take() {
             None => Ok(()),
             Some(cycle) => Err(cycle),
         }
     }

     /// Awaits the caller on this latch by blocking the current thread.
     fn wait_on_inner(&self, waiter: &Lrc<QueryWaiter>) {
         let mut info = self.info.lock();
         if !info.complete {
             // We push the waiter on to the `waiters` list. It can be accessed inside
             // the `wait` call below, by 1) the `set` method or 2) by deadlock detection.
             // Both of these will remove it from the `waiters` list before resuming
             // this thread.
             info.waiters.push(waiter.clone());

             // If this detects a deadlock and the deadlock handler wants to resume this thread
             // we have to be in the `wait` call. This is ensured by the deadlock handler
             // getting the self.info lock.
             rayon_core::mark_blocked();
             jobserver::release_thread();
             waiter.condvar.wait(&mut info);
             // Release the lock before we potentially block in `acquire_thread`
             mem::drop(info);
             jobserver::acquire_thread();
         }
     }

     /// Sets the latch and resumes all waiters on it
     fn set(&self) {
         let mut info = self.info.lock();
         debug_assert!(!info.complete);
         info.complete = true;
         let registry = rayon_core::Registry::current();
         for waiter in info.waiters.drain(..) {
             waiter.notify(&registry);
         }
     }

     /// Removes a single waiter from the list of waiters.
     /// This is used to break query cycles.
     fn extract_waiter(&self, waiter: usize) -> Lrc<QueryWaiter> {
         let mut info = self.info.lock();
         debug_assert!(!info.complete);
         // Remove the waiter from the list of waiters
         info.waiters.remove(waiter)
     }
 }

 /// A resumable waiter of a query. The usize is the index into waiters in the query's latch
 #[cfg(parallel_compiler)]
 type Waiter = (QueryJobId, usize);

 /// Visits all the non-resumable and resumable waiters of a query.
 /// Only waiters in a query are visited.
 /// `visit` is called for every waiter and is passed a query waiting on `query_ref`
 /// and a span indicating the reason the query waited on `query_ref`.
 /// If `visit` returns Some, this function returns.
 /// For visits of non-resumable waiters it returns the return value of `visit`.
 /// For visits of resumable waiters it returns Some(Some(Waiter)) which has the
 /// required information to resume the waiter.
 /// If all `visit` calls returns None, this function also returns None.
 #[cfg(parallel_compiler)]
 fn visit_waiters<F>(query_map: &QueryMap, query: QueryJobId, mut visit: F) -> Option<Option<Waiter>>
 where
     F: FnMut(Span, QueryJobId) -> Option<Option<Waiter>>,
 {
     // Visit the parent query which is a non-resumable waiter since it's on the same stack
     if let Some(parent) = query.parent(query_map) {
         if let Some(cycle) = visit(query.span(query_map), parent) {
             return Some(cycle);
         }
     }

     // Visit the explicit waiters which use condvars and are resumable
     if let Some(latch) = query.latch(query_map) {
         for (i, waiter) in latch.info.lock().waiters.iter().enumerate() {
             if let Some(waiter_query) = waiter.query {
                 if visit(waiter.span, waiter_query).is_some() {
                     // Return a value which indicates that this waiter can be resumed
                     return Some(Some((query, i)));
                 }
             }
         }
     }

     None
 }

 /// Look for query cycles by doing a depth first search starting at `query`.
 /// `span` is the reason for the `query` to execute. This is initially DUMMY_SP.
 /// If a cycle is detected, this initial value is replaced with the span causing
 /// the cycle.
 #[cfg(parallel_compiler)]
 fn cycle_check(
     query_map: &QueryMap,
     query: QueryJobId,
     span: Span,
     stack: &mut Vec<(Span, QueryJobId)>,
     visited: &mut FxHashSet<QueryJobId>,
 ) -> Option<Option<Waiter>> {
     if !visited.insert(query) {
         return if let Some(p) = stack.iter().position(|q| q.1 == query) {
             // We detected a query cycle, fix up the initial span and return Some

             // Remove previous stack entries
             stack.drain(0..p);
             // Replace the span for the first query with the cycle cause
             stack[0].0 = span;
             Some(None)
         } else {
             None
         };
     }

     // Query marked as visited is added it to the stack
     stack.push((span, query));

     // Visit all the waiters
     let r = visit_waiters(query_map, query, |span, successor| {
         cycle_check(query_map, successor, span, stack, visited)
     });

     // Remove the entry in our stack if we didn't find a cycle
     if r.is_none() {
         stack.pop();
     }

     r
 }

 /// Finds out if there's a path to the compiler root (aka. code which isn't in a query)
 /// from `query` without going through any of the queries in `visited`.
 /// This is achieved with a depth first search.
 #[cfg(parallel_compiler)]
 fn connected_to_root(
     query_map: &QueryMap,
     query: QueryJobId,
     visited: &mut FxHashSet<QueryJobId>,
 ) -> bool {
     // We already visited this or we're deliberately ignoring it
     if !visited.insert(query) {
         return false;
     }

     // This query is connected to the root (it has no query parent), return true
     if query.parent(query_map).is_none() {
         return true;
     }

     visit_waiters(query_map, query, |_, successor| {
         connected_to_root(query_map, successor, visited).then_some(None)
     })
     .is_some()
 }

 // Deterministically pick an query from a list
 #[cfg(parallel_compiler)]
 fn pick_query<'a, T, F>(query_map: &QueryMap, queries: &'a [T], f: F) -> &'a T
 where
     F: Fn(&T) -> (Span, QueryJobId),
 {
     // Deterministically pick an entry point
     // FIXME: Sort this instead
     queries
         .iter()
         .min_by_key(|v| {
             let (span, query) = f(v);
             let hash = query.query(query_map).hash;
             // Prefer entry points which have valid spans for nicer error messages
             // We add an integer to the tuple ensuring that entry points
             // with valid spans are picked first
             let span_cmp = if span == DUMMY_SP { 1 } else { 0 };
             (span_cmp, hash)
         })
         .unwrap()
 }

 /// Looks for query cycles starting from the last query in `jobs`.
 /// If a cycle is found, all queries in the cycle is removed from `jobs` and
 /// the function return true.
 /// If a cycle was not found, the starting query is removed from `jobs` and
 /// the function returns false.
 #[cfg(parallel_compiler)]
 fn remove_cycle(
     query_map: &QueryMap,
     jobs: &mut Vec<QueryJobId>,
     wakelist: &mut Vec<Lrc<QueryWaiter>>,
 ) -> bool {
     let mut visited = FxHashSet::default();
     let mut stack = Vec::new();
     // Look for a cycle starting with the last query in `jobs`
     if let Some(waiter) =
         cycle_check(query_map, jobs.pop().unwrap(), DUMMY_SP, &mut stack, &mut visited)
     {
         // The stack is a vector of pairs of spans and queries; reverse it so that
         // the earlier entries require later entries
         let (mut spans, queries): (Vec<_>, Vec<_>) = stack.into_iter().rev().unzip();

         // Shift the spans so that queries are matched with the span for their waitee
         spans.rotate_right(1);

         // Zip them back together
         let mut stack: Vec<_> = iter::zip(spans, queries).collect();

         // Remove the queries in our cycle from the list of jobs to look at
         for r in &stack {
             if let Some(pos) = jobs.iter().position(|j| j == &r.1) {
                 jobs.remove(pos);
             }
         }

         // Find the queries in the cycle which are
         // connected to queries outside the cycle
         let entry_points = stack
             .iter()
             .filter_map(|&(span, query)| {
                 if query.parent(query_map).is_none() {
                     // This query is connected to the root (it has no query parent)
                     Some((span, query, None))
                 } else {
                     let mut waiters = Vec::new();
                     // Find all the direct waiters who lead to the root
                     visit_waiters(query_map, query, |span, waiter| {
                         // Mark all the other queries in the cycle as already visited
                         let mut visited = FxHashSet::from_iter(stack.iter().map(|q| q.1));

                         if connected_to_root(query_map, waiter, &mut visited) {
                             waiters.push((span, waiter));
                         }

                         None
                     });
                     if waiters.is_empty() {
                         None
                     } else {
                         // Deterministically pick one of the waiters to show to the user
                         let waiter = *pick_query(query_map, &waiters, |s| *s);
                         Some((span, query, Some(waiter)))
                     }
                 }
             })
             .collect::<Vec<(Span, QueryJobId, Option<(Span, QueryJobId)>)>>();

         // Deterministically pick an entry point
         let (_, entry_point, usage) = pick_query(query_map, &entry_points, |e| (e.0, e.1));

         // Shift the stack so that our entry point is first
         let entry_point_pos = stack.iter().position(|(_, query)| query == entry_point);
         if let Some(pos) = entry_point_pos {
             stack.rotate_left(pos);
         }

         let usage = usage.as_ref().map(|(span, query)| (*span, query.query(query_map)));

         // Create the cycle error
         let error = CycleError {
             usage,
             cycle: stack
                 .iter()
                 .map(|&(s, ref q)| QueryInfo { span: s, query: q.query(query_map) })
                 .collect(),
         };

         // We unwrap `waiter` here since there must always be one
         // edge which is resumable / waited using a query latch
         let (waitee_query, waiter_idx) = waiter.unwrap();

         // Extract the waiter we want to resume
         let waiter = waitee_query.latch(query_map).unwrap().extract_waiter(waiter_idx);

         // Set the cycle error so it will be picked up when resumed
         *waiter.cycle.lock() = Some(error);

         // Put the waiter on the list of things to resume
         wakelist.push(waiter);

         true
     } else {
         false
     }
 }

 /// Detects query cycles by using depth first search over all active query jobs.
 /// If a query cycle is found it will break the cycle by finding an edge which
 /// uses a query latch and then resuming that waiter.
 /// There may be multiple cycles involved in a deadlock, so this searches
 /// all active queries for cycles before finally resuming all the waiters at once.
 #[cfg(parallel_compiler)]
 pub fn deadlock(query_map: QueryMap, registry: &rayon_core::Registry) {
     let on_panic = OnDrop(|| {
         eprintln!("deadlock handler panicked, aborting process");
         process::abort();
     });

     let mut wakelist = Vec::new();
     let mut jobs: Vec<QueryJobId> = query_map.keys().cloned().collect();

     let mut found_cycle = false;

     while jobs.len() > 0 {
         if remove_cycle(&query_map, &mut jobs, &mut wakelist) {
             found_cycle = true;
         }
     }

     // Check that a cycle was found. It is possible for a deadlock to occur without
     // a query cycle if a query which can be waited on uses Rayon to do multithreading
     // internally. Such a query (X) may be executing on 2 threads (A and B) and A may
     // wait using Rayon on B. Rayon may then switch to executing another query (Y)
     // which in turn will wait on X causing a deadlock. We have a false dependency from
     // X to Y due to Rayon waiting and a true dependency from Y to X. The algorithm here
     // only considers the true dependency and won't detect a cycle.
     assert!(found_cycle);

     // FIXME: Ensure this won't cause a deadlock before we return
     for waiter in wakelist.into_iter() {
         waiter.notify(registry);
     }

     on_panic.disable();
 }

 #[inline(never)]
 #[cold]
 pub(crate) fn report_cycle<'a>(
     sess: &'a Session,
     CycleError { usage, cycle: stack }: CycleError,
 ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
     assert!(!stack.is_empty());

     let span = stack[0].query.default_span(stack[1 % stack.len()].span);
     let mut err =
         struct_span_err!(sess, span, E0391, "cycle detected when {}", stack[0].query.description);

     for i in 1..stack.len() {
         let query = &stack[i].query;
         let span = query.default_span(stack[(i + 1) % stack.len()].span);
         err.span_note(span, &format!("...which requires {}...", query.description));
     }

     if stack.len() == 1 {
         err.note(&format!("...which immediately requires {} again", stack[0].query.description));
     } else {
         err.note(&format!(
             "...which again requires {}, completing the cycle",
             stack[0].query.description
         ));
     }

     if stack.iter().all(|entry| {
         entry
             .query
             .def_kind
             .map_or(false, |def_kind| matches!(def_kind, DefKind::TyAlias | DefKind::TraitAlias))
     }) {
         if stack.iter().all(|entry| {
             entry.query.def_kind.map_or(false, |def_kind| matches!(def_kind, DefKind::TyAlias))
         }) {
             err.note("type aliases cannot be recursive");
             err.help("consider using a struct, enum, or union instead to break the cycle");
             err.help("see <https://doc.rust-lang.org/reference/types.html#recursive-types> for more information");
         } else {
             err.note("trait aliases cannot be recursive");
         }
     }

     if let Some((span, query)) = usage {
         err.span_note(query.default_span(span), &format!("cycle used when {}", query.description));
     }

     err
 }

 pub fn print_query_stack<CTX: QueryContext>(
     tcx: CTX,
     mut current_query: Option<QueryJobId>,
     handler: &Handler,
     num_frames: Option<usize>,
 ) -> usize {
     // Be careful relying on global state here: this code is called from
     // a panic hook, which means that the global `Handler` may be in a weird
     // state if it was responsible for triggering the panic.
     let mut i = 0;
     let query_map = tcx.try_collect_active_jobs();

     while let Some(query) = current_query {
         if Some(i) == num_frames {
             break;
         }
         let Some(query_info) = query_map.as_ref().and_then(|map| map.get(&query)) else {
             break;
         };
         let mut diag = Diagnostic::new(
             Level::FailureNote,
             &format!("#{} [{}] {}", i, query_info.query.name, query_info.query.description),
         );
         diag.span = query_info.job.span.into();
         handler.force_print_diagnostic(diag);

         current_query = query_info.job.parent;
         i += 1;
     }

     i
 }
	use crate::query::plumbing::CycleError;
	use crate::query::{QueryContext, QueryStackFrame};
	use rustc_hir::def::DefKind;

	use rustc_data_structures::fx::FxHashMap;
	use rustc_errors::{
	struct_span_err, Diagnostic, DiagnosticBuilder, ErrorGuaranteed, Handler, Level,
	};
	use rustc_session::Session;
	use rustc_span::Span;

	use std::hash::Hash;
	use std::num::NonZeroU64;

	#[cfg(parallel_compiler)]
	use {
	parking_lot::{Condvar, Mutex},
	rustc_data_structures::fx::FxHashSet,
	rustc_data_structures::sync::Lock,
	rustc_data_structures::sync::Lrc,
	rustc_data_structures::{jobserver, OnDrop},
	rustc_rayon_core as rayon_core,
	rustc_span::DUMMY_SP,
	std::iter::{self, FromIterator},
	std::{mem, process},
	};

	/// Represents a span and a query key.
	#[derive(Clone, Debug)]
	pub struct QueryInfo {
	/// The span corresponding to the reason for which this query was required.
	pub span: Span,
	pub query: QueryStackFrame,
	}

	pub type QueryMap = FxHashMap<QueryJobId, QueryJobInfo>;

	/// A value uniquely identifying an active query job.
	#[derive(Copy, Clone, Eq, PartialEq, Hash)]
	pub struct QueryJobId(pub NonZeroU64);

	impl QueryJobId {
	fn query(self, map: &QueryMap) -> QueryStackFrame {
	map.get(&self).unwrap().query.clone()
	}

	#[cfg(parallel_compiler)]
	fn span(self, map: &QueryMap) -> Span {
	map.get(&self).unwrap().job.span
	}

	#[cfg(parallel_compiler)]
	fn parent(self, map: &QueryMap) -> Option<QueryJobId> {
	map.get(&self).unwrap().job.parent
	}

	#[cfg(parallel_compiler)]
	fn latch<'a>(self, map: &'a QueryMap) -> Option<&'a QueryLatch> {
	map.get(&self).unwrap().job.latch.as_ref()
	}
	}

	pub struct QueryJobInfo {
	pub query: QueryStackFrame,
	pub job: QueryJob,
	}

	/// Represents an active query job.
	#[derive(Clone)]
	pub struct QueryJob {
	pub id: QueryJobId,

	/// The span corresponding to the reason for which this query was required.
	pub span: Span,

	/// The parent query job which created this job and is implicitly waiting on it.
	pub parent: Option<QueryJobId>,

	/// The latch that is used to wait on this job.
	#[cfg(parallel_compiler)]
	latch: Option<QueryLatch>,
	}

	impl QueryJob {
	/// Creates a new query job.
	#[inline]
	pub fn new(id: QueryJobId, span: Span, parent: Option<QueryJobId>) -> Self {
	QueryJob {
	id,
	span,
	parent,
	#[cfg(parallel_compiler)]
	latch: None,
	}
	}

	#[cfg(parallel_compiler)]
	pub(super) fn latch(&mut self) -> QueryLatch {
	if self.latch.is_none() {
	self.latch = Some(QueryLatch::new());
	}
	self.latch.as_ref().unwrap().clone()
	}

	/// Signals to waiters that the query is complete.
	///
	/// This does nothing for single threaded rustc,
	/// as there are no concurrent jobs which could be waiting on us
	#[inline]
	pub fn signal_complete(self) {
	#[cfg(parallel_compiler)]
	{
	if let Some(latch) = self.latch {
	latch.set();
	}
	}
	}
	}

	#[cfg(not(parallel_compiler))]
	impl QueryJobId {
	#[cold]
	#[inline(never)]
	pub(super) fn find_cycle_in_stack(
	&self,
	query_map: QueryMap,
	current_job: &Option<QueryJobId>,
	span: Span,
	) -> CycleError {
	// Find the waitee amongst `current_job` parents
	let mut cycle = Vec::new();
	let mut current_job = Option::clone(current_job);

	while let Some(job) = current_job {
	let info = query_map.get(&job).unwrap();
	cycle.push(QueryInfo { span: info.job.span, query: info.query.clone() });

	if job == *self {
	cycle.reverse();

	// This is the end of the cycle
	// The span entry we included was for the usage
	// of the cycle itself, and not part of the cycle
	// Replace it with the span which caused the cycle to form
	cycle[0].span = span;
	// Find out why the cycle itself was used
	let usage = info
	.job
	.parent
	.as_ref()
	.map(\|parent\| (info.job.span, parent.query(&query_map)));
	return CycleError { usage, cycle };
	}

	current_job = info.job.parent;
	}

	panic!("did not find a cycle")
	}
	}

	#[cfg(parallel_compiler)]
	struct QueryWaiter {
	query: Option<QueryJobId>,
	condvar: Condvar,
	span: Span,
	cycle: Lock<Option<CycleError>>,
	}

	#[cfg(parallel_compiler)]
	impl QueryWaiter {
	fn notify(&self, registry: &rayon_core::Registry) {
	rayon_core::mark_unblocked(registry);
	self.condvar.notify_one();
	}
	}

	#[cfg(parallel_compiler)]
	struct QueryLatchInfo {
	complete: bool,
	waiters: Vec<Lrc<QueryWaiter>>,
	}

	#[cfg(parallel_compiler)]
	#[derive(Clone)]
	pub(super) struct QueryLatch {
	info: Lrc<Mutex<QueryLatchInfo>>,
	}

	#[cfg(parallel_compiler)]
	impl QueryLatch {
	fn new() -> Self {
	QueryLatch {
	info: Lrc::new(Mutex::new(QueryLatchInfo { complete: false, waiters: Vec::new() })),
	}
	}

	/// Awaits for the query job to complete.
	pub(super) fn wait_on(&self, query: Option<QueryJobId>, span: Span) -> Result<(), CycleError> {
	let waiter =
	Lrc::new(QueryWaiter { query, span, cycle: Lock::new(None), condvar: Condvar::new() });
	self.wait_on_inner(&waiter);
	// FIXME: Get rid of this lock. We have ownership of the QueryWaiter
	// although another thread may still have a Lrc reference so we cannot
	// use Lrc::get_mut
	let mut cycle = waiter.cycle.lock();
	match cycle.take() {
	None => Ok(()),
	Some(cycle) => Err(cycle),
	}
	}

	/// Awaits the caller on this latch by blocking the current thread.
	fn wait_on_inner(&self, waiter: &Lrc<QueryWaiter>) {
	let mut info = self.info.lock();
	if !info.complete {
	// We push the waiter on to the `waiters` list. It can be accessed inside
	// the `wait` call below, by 1) the `set` method or 2) by deadlock detection.
	// Both of these will remove it from the `waiters` list before resuming
	// this thread.
	info.waiters.push(waiter.clone());

	// If this detects a deadlock and the deadlock handler wants to resume this thread
	// we have to be in the `wait` call. This is ensured by the deadlock handler
	// getting the self.info lock.
	rayon_core::mark_blocked();
	jobserver::release_thread();
	waiter.condvar.wait(&mut info);
	// Release the lock before we potentially block in `acquire_thread`
	mem::drop(info);
	jobserver::acquire_thread();
	}
	}

	/// Sets the latch and resumes all waiters on it
	fn set(&self) {
	let mut info = self.info.lock();
	debug_assert!(!info.complete);
	info.complete = true;
	let registry = rayon_core::Registry::current();
	for waiter in info.waiters.drain(..) {
	waiter.notify(&registry);
	}
	}

	/// Removes a single waiter from the list of waiters.
	/// This is used to break query cycles.
	fn extract_waiter(&self, waiter: usize) -> Lrc<QueryWaiter> {
	let mut info = self.info.lock();
	debug_assert!(!info.complete);
	// Remove the waiter from the list of waiters
	info.waiters.remove(waiter)
	}
	}

	/// A resumable waiter of a query. The usize is the index into waiters in the query's latch
	#[cfg(parallel_compiler)]
	type Waiter = (QueryJobId, usize);

	/// Visits all the non-resumable and resumable waiters of a query.
	/// Only waiters in a query are visited.
	/// `visit` is called for every waiter and is passed a query waiting on `query_ref`
	/// and a span indicating the reason the query waited on `query_ref`.
	/// If `visit` returns Some, this function returns.
	/// For visits of non-resumable waiters it returns the return value of `visit`.
	/// For visits of resumable waiters it returns Some(Some(Waiter)) which has the
	/// required information to resume the waiter.
	/// If all `visit` calls returns None, this function also returns None.
	#[cfg(parallel_compiler)]
	fn visit_waiters<F>(query_map: &QueryMap, query: QueryJobId, mut visit: F) -> Option<Option<Waiter>>
	where
	F: FnMut(Span, QueryJobId) -> Option<Option<Waiter>>,
	{
	// Visit the parent query which is a non-resumable waiter since it's on the same stack
	if let Some(parent) = query.parent(query_map) {
	if let Some(cycle) = visit(query.span(query_map), parent) {
	return Some(cycle);
	}
	}

	// Visit the explicit waiters which use condvars and are resumable
	if let Some(latch) = query.latch(query_map) {
	for (i, waiter) in latch.info.lock().waiters.iter().enumerate() {
	if let Some(waiter_query) = waiter.query {
	if visit(waiter.span, waiter_query).is_some() {
	// Return a value which indicates that this waiter can be resumed
	return Some(Some((query, i)));
	}
	}
	}
	}

	None
	}

	/// Look for query cycles by doing a depth first search starting at `query`.
	/// `span` is the reason for the `query` to execute. This is initially DUMMY_SP.
	/// If a cycle is detected, this initial value is replaced with the span causing
	/// the cycle.
	#[cfg(parallel_compiler)]
	fn cycle_check(
	query_map: &QueryMap,
	query: QueryJobId,
	span: Span,
	stack: &mut Vec<(Span, QueryJobId)>,
	visited: &mut FxHashSet<QueryJobId>,
	) -> Option<Option<Waiter>> {
	if !visited.insert(query) {
	return if let Some(p) = stack.iter().position(\|q\| q.1 == query) {
	// We detected a query cycle, fix up the initial span and return Some

	// Remove previous stack entries
	stack.drain(0..p);
	// Replace the span for the first query with the cycle cause
	stack[0].0 = span;
	Some(None)
	} else {
	None
	};
	}

	// Query marked as visited is added it to the stack
	stack.push((span, query));

	// Visit all the waiters
	let r = visit_waiters(query_map, query, \|span, successor\| {
	cycle_check(query_map, successor, span, stack, visited)
	});

	// Remove the entry in our stack if we didn't find a cycle
	if r.is_none() {
	stack.pop();
	}

	r
	}

	/// Finds out if there's a path to the compiler root (aka. code which isn't in a query)
	/// from `query` without going through any of the queries in `visited`.
	/// This is achieved with a depth first search.
	#[cfg(parallel_compiler)]
	fn connected_to_root(
	query_map: &QueryMap,
	query: QueryJobId,
	visited: &mut FxHashSet<QueryJobId>,
	) -> bool {
	// We already visited this or we're deliberately ignoring it
	if !visited.insert(query) {
	return false;
	}

	// This query is connected to the root (it has no query parent), return true
	if query.parent(query_map).is_none() {
	return true;
	}

	visit_waiters(query_map, query, \|_, successor\| {
	connected_to_root(query_map, successor, visited).then_some(None)
	})
	.is_some()
	}

	// Deterministically pick an query from a list
	#[cfg(parallel_compiler)]
	fn pick_query<'a, T, F>(query_map: &QueryMap, queries: &'a [T], f: F) -> &'a T
	where
	F: Fn(&T) -> (Span, QueryJobId),
	{
	// Deterministically pick an entry point
	// FIXME: Sort this instead
	queries
	.iter()
	.min_by_key(\|v\| {
	let (span, query) = f(v);
	let hash = query.query(query_map).hash;
	// Prefer entry points which have valid spans for nicer error messages
	// We add an integer to the tuple ensuring that entry points
	// with valid spans are picked first
	let span_cmp = if span == DUMMY_SP { 1 } else { 0 };
	(span_cmp, hash)
	})
	.unwrap()
	}

	/// Looks for query cycles starting from the last query in `jobs`.
	/// If a cycle is found, all queries in the cycle is removed from `jobs` and
	/// the function return true.
	/// If a cycle was not found, the starting query is removed from `jobs` and
	/// the function returns false.
	#[cfg(parallel_compiler)]
	fn remove_cycle(
	query_map: &QueryMap,
	jobs: &mut Vec<QueryJobId>,
	wakelist: &mut Vec<Lrc<QueryWaiter>>,
	) -> bool {
	let mut visited = FxHashSet::default();
	let mut stack = Vec::new();
	// Look for a cycle starting with the last query in `jobs`
	if let Some(waiter) =
	cycle_check(query_map, jobs.pop().unwrap(), DUMMY_SP, &mut stack, &mut visited)
	{
	// The stack is a vector of pairs of spans and queries; reverse it so that
	// the earlier entries require later entries
	let (mut spans, queries): (Vec<_>, Vec<_>) = stack.into_iter().rev().unzip();

	// Shift the spans so that queries are matched with the span for their waitee
	spans.rotate_right(1);

	// Zip them back together
	let mut stack: Vec<_> = iter::zip(spans, queries).collect();

	// Remove the queries in our cycle from the list of jobs to look at
	for r in &stack {
	if let Some(pos) = jobs.iter().position(\|j\| j == &r.1) {
	jobs.remove(pos);
	}
	}

	// Find the queries in the cycle which are
	// connected to queries outside the cycle
	let entry_points = stack
	.iter()
	.filter_map(\|&(span, query)\| {
	if query.parent(query_map).is_none() {
	// This query is connected to the root (it has no query parent)
	Some((span, query, None))
	} else {
	let mut waiters = Vec::new();
	// Find all the direct waiters who lead to the root
	visit_waiters(query_map, query, \|span, waiter\| {
	// Mark all the other queries in the cycle as already visited
	let mut visited = FxHashSet::from_iter(stack.iter().map(\|q\| q.1));

	if connected_to_root(query_map, waiter, &mut visited) {
	waiters.push((span, waiter));
	}

	None
	});
	if waiters.is_empty() {
	None
	} else {
	// Deterministically pick one of the waiters to show to the user
	let waiter = pick_query(query_map, &waiters, \|s\| s);
	Some((span, query, Some(waiter)))
	}
	}
	})
	.collect::<Vec<(Span, QueryJobId, Option<(Span, QueryJobId)>)>>();

	// Deterministically pick an entry point
	let (_, entry_point, usage) = pick_query(query_map, &entry_points, \|e\| (e.0, e.1));

	// Shift the stack so that our entry point is first
	let entry_point_pos = stack.iter().position(\|(_, query)\| query == entry_point);
	if let Some(pos) = entry_point_pos {
	stack.rotate_left(pos);
	}

	let usage = usage.as_ref().map(\|(span, query)\| (*span, query.query(query_map)));

	// Create the cycle error
	let error = CycleError {
	usage,
	cycle: stack
	.iter()
	.map(\|&(s, ref q)\| QueryInfo { span: s, query: q.query(query_map) })
	.collect(),
	};

	// We unwrap `waiter` here since there must always be one
	// edge which is resumable / waited using a query latch
	let (waitee_query, waiter_idx) = waiter.unwrap();

	// Extract the waiter we want to resume
	let waiter = waitee_query.latch(query_map).unwrap().extract_waiter(waiter_idx);

	// Set the cycle error so it will be picked up when resumed
	*waiter.cycle.lock() = Some(error);

	// Put the waiter on the list of things to resume
	wakelist.push(waiter);

	true
	} else {
	false
	}
	}

	/// Detects query cycles by using depth first search over all active query jobs.
	/// If a query cycle is found it will break the cycle by finding an edge which
	/// uses a query latch and then resuming that waiter.
	/// There may be multiple cycles involved in a deadlock, so this searches
	/// all active queries for cycles before finally resuming all the waiters at once.
	#[cfg(parallel_compiler)]
	pub fn deadlock(query_map: QueryMap, registry: &rayon_core::Registry) {
	let on_panic = OnDrop(\|\| {
	eprintln!("deadlock handler panicked, aborting process");
	process::abort();
	});

	let mut wakelist = Vec::new();
	let mut jobs: Vec<QueryJobId> = query_map.keys().cloned().collect();

	let mut found_cycle = false;

	while jobs.len() > 0 {
	if remove_cycle(&query_map, &mut jobs, &mut wakelist) {
	found_cycle = true;
	}
	}

	// Check that a cycle was found. It is possible for a deadlock to occur without
	// a query cycle if a query which can be waited on uses Rayon to do multithreading
	// internally. Such a query (X) may be executing on 2 threads (A and B) and A may
	// wait using Rayon on B. Rayon may then switch to executing another query (Y)
	// which in turn will wait on X causing a deadlock. We have a false dependency from
	// X to Y due to Rayon waiting and a true dependency from Y to X. The algorithm here
	// only considers the true dependency and won't detect a cycle.
	assert!(found_cycle);

	// FIXME: Ensure this won't cause a deadlock before we return
	for waiter in wakelist.into_iter() {
	waiter.notify(registry);
	}

	on_panic.disable();
	}

	#[inline(never)]
	#[cold]
	pub(crate) fn report_cycle<'a>(
	sess: &'a Session,
	CycleError { usage, cycle: stack }: CycleError,
	) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
	assert!(!stack.is_empty());

	let span = stack[0].query.default_span(stack[1 % stack.len()].span);
	let mut err =
	struct_span_err!(sess, span, E0391, "cycle detected when {}", stack[0].query.description);

	for i in 1..stack.len() {
	let query = &stack[i].query;
	let span = query.default_span(stack[(i + 1) % stack.len()].span);
	err.span_note(span, &format!("...which requires {}...", query.description));
	}

	if stack.len() == 1 {
	err.note(&format!("...which immediately requires {} again", stack[0].query.description));
	} else {
	err.note(&format!(
	"...which again requires {}, completing the cycle",
	stack[0].query.description
	));
	}

	if stack.iter().all(\|entry\| {
	entry
	.query
	.def_kind
	.map_or(false, \|def_kind\| matches!(def_kind, DefKind::TyAlias \| DefKind::TraitAlias))
	}) {
	if stack.iter().all(\|entry\| {
	entry.query.def_kind.map_or(false, \|def_kind\| matches!(def_kind, DefKind::TyAlias))
	}) {
	err.note("type aliases cannot be recursive");
	err.help("consider using a struct, enum, or union instead to break the cycle");
	err.help("see <https://doc.rust-lang.org/reference/types.html#recursive-types> for more information");
	} else {
	err.note("trait aliases cannot be recursive");
	}
	}

	if let Some((span, query)) = usage {
	err.span_note(query.default_span(span), &format!("cycle used when {}", query.description));
	}

	err
	}

	pub fn print_query_stack<CTX: QueryContext>(
	tcx: CTX,
	mut current_query: Option<QueryJobId>,
	handler: &Handler,
	num_frames: Option<usize>,
	) -> usize {
	// Be careful relying on global state here: this code is called from
	// a panic hook, which means that the global `Handler` may be in a weird
	// state if it was responsible for triggering the panic.
	let mut i = 0;
	let query_map = tcx.try_collect_active_jobs();

	while let Some(query) = current_query {
	if Some(i) == num_frames {
	break;
	}
	let Some(query_info) = query_map.as_ref().and_then(\|map\| map.get(&query)) else {
	break;
	};
	let mut diag = Diagnostic::new(
	Level::FailureNote,
	&format!("#{} [{}] {}", i, query_info.query.name, query_info.query.description),
	);
	diag.span = query_info.job.span.into();
	handler.force_print_diagnostic(diag);

	current_query = query_info.job.parent;
	i += 1;
	}

	i
	}