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use super::*;
use crate::infer::CombinedSnapshot;
use rustc_data_structures::{
fx::FxIndexMap,
graph::{scc::Sccs, vec_graph::VecGraph},
};
use rustc_index::Idx;
use rustc_middle::ty::error::TypeError;
use rustc_middle::ty::relate::RelateResult;
impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
/// Searches new universes created during `snapshot`, looking for
/// placeholders that may "leak" out from the universes they are contained
/// in. If any leaking placeholders are found, then an `Err` is returned
/// (typically leading to the snapshot being reversed). This algorithm
/// only looks at placeholders which cannot be named by `outer_universe`,
/// as this is the universe we're currently checking for a leak.
///
/// The leak check *used* to be the only way we had to handle higher-ranked
/// obligations. Now that we have integrated universes into the region
/// solvers, this is no longer the case, but we retain the leak check for
/// backwards compatibility purposes. In particular, it lets us make "early"
/// decisions about whether a region error will be reported that are used in
/// coherence and elsewhere -- see #56105 and #59490 for more details. The
/// eventual fate of the leak checker is not yet settled.
///
/// The leak checker works by searching for the following error patterns:
///
/// * P1: P2, where P1 != P2
/// * P1: R, where R is in some universe that cannot name P1
///
/// The idea here is that each of these patterns represents something that
/// the region solver would eventually report as an error, so we can detect
/// the error early. There is a fly in the ointment, though, in that this is
/// not entirely true. In particular, in the future, we may extend the
/// environment with implied bounds or other info about how placeholders
/// relate to regions in outer universes. In that case, `P1: R` for example
/// might become solvable.
///
/// # Summary of the implementation
///
/// The leak checks as follows. First, we construct a graph where `R2: R1`
/// implies `R2 -> R1`, and we compute the SCCs.
///
/// For each SCC S, we compute:
///
/// * what placeholder P it must be equal to, if any
/// * if there are multiple placeholders that must be equal, report an error because `P1: P2`
/// * the minimum universe of its constituents
///
/// Then we walk the SCCs in dependency order and compute
///
/// * what placeholder they must outlive transitively
/// * if they must also be equal to a placeholder, report an error because `P1: P2`
/// * minimum universe U of all SCCs they must outlive
/// * if they must also be equal to a placeholder P, and U cannot name P, report an error, as that
/// indicates `P: R` and `R` is in an incompatible universe
///
/// To improve performance and for the old trait solver caching to be sound, this takes
/// an optional snapshot in which case we only look at region constraints added in that
/// snapshot. If we were to not do that the `leak_check` during evaluation can rely on
/// region constraints added outside of that evaluation. As that is not reflected in the
/// cache key this would be unsound.
///
/// # Historical note
///
/// Older variants of the leak check used to report errors for these
/// patterns, but we no longer do:
///
/// * R: P1, even if R cannot name P1, because R = 'static is a valid sol'n
/// * R: P1, R: P2, as above
#[instrument(level = "debug", skip(self, tcx, only_consider_snapshot), ret)]
pub fn leak_check(
&mut self,
tcx: TyCtxt<'tcx>,
outer_universe: ty::UniverseIndex,
max_universe: ty::UniverseIndex,
only_consider_snapshot: Option<&CombinedSnapshot<'tcx>>,
) -> RelateResult<'tcx, ()> {
if outer_universe == max_universe {
return Ok(());
}
let mini_graph = &MiniGraph::new(tcx, &self, only_consider_snapshot);
let mut leak_check = LeakCheck::new(tcx, outer_universe, max_universe, mini_graph, self);
leak_check.assign_placeholder_values()?;
leak_check.propagate_scc_value()?;
Ok(())
}
}
struct LeakCheck<'me, 'tcx> {
tcx: TyCtxt<'tcx>,
outer_universe: ty::UniverseIndex,
mini_graph: &'me MiniGraph<'tcx>,
rcc: &'me RegionConstraintCollector<'me, 'tcx>,
// Initially, for each SCC S, stores a placeholder `P` such that `S = P`
// must hold.
//
// Later, during the [`LeakCheck::propagate_scc_value`] function, this array
// is repurposed to store some placeholder `P` such that the weaker
// condition `S: P` must hold. (This is true if `S: S1` transitively and `S1
// = P`.)
scc_placeholders: IndexVec<LeakCheckScc, Option<ty::PlaceholderRegion>>,
// For each SCC S, track the minimum universe that flows into it. Note that
// this is both the minimum of the universes for every region that is a
// member of the SCC, but also if you have `R1: R2`, then the universe of
// `R2` must be less than the universe of `R1` (i.e., `R1` flows `R2`). To
// see that, imagine that you have `P1: R` -- in that case, `R` must be
// either the placeholder `P1` or the empty region in that same universe.
//
// To detect errors, we look for an SCC S where the values in
// `scc_values[S]` (if any) cannot be stored into `scc_universes[S]`.
scc_universes: IndexVec<LeakCheckScc, SccUniverse<'tcx>>,
}
impl<'me, 'tcx> LeakCheck<'me, 'tcx> {
fn new(
tcx: TyCtxt<'tcx>,
outer_universe: ty::UniverseIndex,
max_universe: ty::UniverseIndex,
mini_graph: &'me MiniGraph<'tcx>,
rcc: &'me RegionConstraintCollector<'me, 'tcx>,
) -> Self {
let dummy_scc_universe = SccUniverse { universe: max_universe, region: None };
Self {
tcx,
outer_universe,
mini_graph,
rcc,
scc_placeholders: IndexVec::from_elem_n(None, mini_graph.sccs.num_sccs()),
scc_universes: IndexVec::from_elem_n(dummy_scc_universe, mini_graph.sccs.num_sccs()),
}
}
/// Compute what placeholders (if any) each SCC must be equal to.
/// Also compute the minimum universe of all the regions in each SCC.
fn assign_placeholder_values(&mut self) -> RelateResult<'tcx, ()> {
// First walk: find each placeholder that is from a newly created universe.
for (region, leak_check_node) in &self.mini_graph.nodes {
let scc = self.mini_graph.sccs.scc(*leak_check_node);
// Set the universe of each SCC to be the minimum of its constituent universes
let universe = self.rcc.universe(*region);
debug!(
"assign_placeholder_values: scc={:?} universe={:?} region={:?}",
scc, universe, region
);
self.scc_universes[scc].take_min(universe, *region);
// Detect those SCCs that directly contain a placeholder
if let ty::RePlaceholder(placeholder) = **region {
if self.outer_universe.cannot_name(placeholder.universe) {
self.assign_scc_value(scc, placeholder)?;
}
}
}
Ok(())
}
// assign_scc_value(S, P): Update `scc_values` to account for the fact that `P: S` must hold.
// This may create an error.
fn assign_scc_value(
&mut self,
scc: LeakCheckScc,
placeholder: ty::PlaceholderRegion,
) -> RelateResult<'tcx, ()> {
match self.scc_placeholders[scc] {
Some(p) => {
assert_ne!(p, placeholder);
return Err(self.placeholder_error(p, placeholder));
}
None => {
self.scc_placeholders[scc] = Some(placeholder);
}
};
Ok(())
}
/// For each SCC S, iterate over each successor S1 where `S: S1`:
///
/// * Compute
/// Iterate over each SCC `S` and ensure that, for each `S1` where `S1: S`,
/// `universe(S) <= universe(S1)`. This executes after
/// `assign_placeholder_values`, so `universe(S)` is already the minimum
/// universe of any of its direct constituents.
fn propagate_scc_value(&mut self) -> RelateResult<'tcx, ()> {
// Loop invariants:
//
// On start of the loop iteration for `scc1`:
//
// * `scc_universes[scc1]` contains the minimum universe of the
// constituents of `scc1`
// * `scc_placeholder[scc1]` stores the placeholder that `scc1` must
// be equal to (if any)
//
// For each successor `scc2` where `scc1: scc2`:
//
// * `scc_placeholder[scc2]` stores some placeholder `P` where
// `scc2: P` (if any)
// * `scc_universes[scc2]` contains the minimum universe of the
// constituents of `scc2` and any of its successors
for scc1 in self.mini_graph.sccs.all_sccs() {
debug!(
"propagate_scc_value: scc={:?} with universe {:?}",
scc1, self.scc_universes[scc1]
);
// Walk over each `scc2` such that `scc1: scc2` and compute:
//
// * `scc1_universe`: the minimum universe of `scc2` and the constituents of `scc1`
// * `succ_bound`: placeholder `P` that the successors must outlive, if any (if there are multiple,
// we pick one arbitrarily)
let mut scc1_universe = self.scc_universes[scc1];
let mut succ_bound = None;
for &scc2 in self.mini_graph.sccs.successors(scc1) {
let SccUniverse { universe: scc2_universe, region: scc2_region } =
self.scc_universes[scc2];
scc1_universe.take_min(scc2_universe, scc2_region.unwrap());
if let Some(b) = self.scc_placeholders[scc2] {
succ_bound = Some(b);
}
}
// Update minimum universe of scc1.
self.scc_universes[scc1] = scc1_universe;
// At this point, `scc_placeholders[scc1]` stores the placeholder that
// `scc1` must be equal to, if any.
if let Some(scc1_placeholder) = self.scc_placeholders[scc1] {
debug!(
"propagate_scc_value: scc1={:?} placeholder={:?} scc1_universe={:?}",
scc1, scc1_placeholder, scc1_universe
);
// Check if `P1: R` for some `R` in a universe that cannot name
// P1. That's an error.
if scc1_universe.universe.cannot_name(scc1_placeholder.universe) {
return Err(self.error(scc1_placeholder, scc1_universe.region.unwrap()));
}
// Check if we have some placeholder where `S: P2`
// (transitively). In that case, since `S = P1`, that implies
// `P1: P2`, which is an error condition.
if let Some(scc2_placeholder) = succ_bound {
assert_ne!(scc1_placeholder, scc2_placeholder);
return Err(self.placeholder_error(scc1_placeholder, scc2_placeholder));
}
} else {
// Otherwise, we can reach a placeholder if some successor can.
self.scc_placeholders[scc1] = succ_bound;
}
// At this point, `scc_placeholder[scc1]` stores some placeholder that `scc1` must outlive (if any).
}
Ok(())
}
fn placeholder_error(
&self,
placeholder1: ty::PlaceholderRegion,
placeholder2: ty::PlaceholderRegion,
) -> TypeError<'tcx> {
self.error(placeholder1, ty::Region::new_placeholder(self.tcx, placeholder2))
}
fn error(
&self,
placeholder: ty::PlaceholderRegion,
other_region: ty::Region<'tcx>,
) -> TypeError<'tcx> {
debug!("error: placeholder={:?}, other_region={:?}", placeholder, other_region);
TypeError::RegionsInsufficientlyPolymorphic(placeholder.bound.kind, other_region)
}
}
// States we need to distinguish:
//
// * must be equal to a placeholder (i.e., a placeholder is in the SCC)
// * it could conflict with some other regions in the SCC in different universes
// * or a different placeholder
// * `P1: S` and `S` must be equal to a placeholder
// * `P1: S` and `S` is in an incompatible universe
//
// So if we
//
// (a) compute which placeholder (if any) each SCC must be equal to
// (b) compute its minimum universe
// (c) compute *some* placeholder where `S: P1` (any one will do)
//
// then we get an error if:
//
// - it must be equal to a placeholder `P1` and minimum universe cannot name `P1`
// - `S: P1` and minimum universe cannot name `P1`
// - `S: P1` and we must be equal to `P2`
//
// So we want to track:
//
// * Equal placeholder (if any)
// * Some bounding placeholder (if any)
// * Minimum universe
//
// * We compute equal placeholder + minimum universe of constituents in first pass
// * Then we walk in order and compute from our dependencies `S1` where `S: S1` (`S -> S1`)
// * bounding placeholder (if any)
// * minimum universe
// * And if we must be equal to a placeholder then we check it against
// * minimum universe
// * no bounding placeholder
/// Tracks the "minimum universe" for each SCC, along with some region that
/// caused it to change.
#[derive(Copy, Clone, Debug)]
struct SccUniverse<'tcx> {
/// For some SCC S, the minimum universe of:
///
/// * each region R in S
/// * each SCC S1 such that S: S1
universe: ty::UniverseIndex,
/// Some region that caused `universe` to be what it is.
region: Option<ty::Region<'tcx>>,
}
impl<'tcx> SccUniverse<'tcx> {
/// If `universe` is less than our current universe, then update
/// `self.universe` and `self.region`.
fn take_min(&mut self, universe: ty::UniverseIndex, region: ty::Region<'tcx>) {
if universe < self.universe || self.region.is_none() {
self.universe = universe;
self.region = Some(region);
}
}
}
rustc_index::newtype_index! {
#[debug_format = "LeakCheckNode({})"]
struct LeakCheckNode {}
}
rustc_index::newtype_index! {
#[debug_format = "LeakCheckScc({})"]
struct LeakCheckScc {}
}
/// Represents the graph of constraints. For each `R1: R2` constraint we create
/// an edge `R1 -> R2` in the graph.
struct MiniGraph<'tcx> {
/// Map from a region to the index of the node in the graph.
nodes: FxIndexMap<ty::Region<'tcx>, LeakCheckNode>,
/// Map from node index to SCC, and stores the successors of each SCC. All
/// the regions in the same SCC are equal to one another, and if `S1 -> S2`,
/// then `S1: S2`.
sccs: Sccs<LeakCheckNode, LeakCheckScc>,
}
impl<'tcx> MiniGraph<'tcx> {
fn new(
tcx: TyCtxt<'tcx>,
region_constraints: &RegionConstraintCollector<'_, 'tcx>,
only_consider_snapshot: Option<&CombinedSnapshot<'tcx>>,
) -> Self {
let mut nodes = FxIndexMap::default();
let mut edges = Vec::new();
// Note that if `R2: R1`, we get a callback `r1, r2`, so `target` is first parameter.
Self::iterate_region_constraints(
tcx,
region_constraints,
only_consider_snapshot,
|target, source| {
let source_node = Self::add_node(&mut nodes, source);
let target_node = Self::add_node(&mut nodes, target);
edges.push((source_node, target_node));
},
);
let graph = VecGraph::new(nodes.len(), edges);
let sccs = Sccs::new(&graph);
Self { nodes, sccs }
}
/// Invokes `each_edge(R1, R2)` for each edge where `R2: R1`
fn iterate_region_constraints(
tcx: TyCtxt<'tcx>,
region_constraints: &RegionConstraintCollector<'_, 'tcx>,
only_consider_snapshot: Option<&CombinedSnapshot<'tcx>>,
mut each_edge: impl FnMut(ty::Region<'tcx>, ty::Region<'tcx>),
) {
let mut each_constraint = |constraint| match constraint {
&Constraint::VarSubVar(a, b) => {
each_edge(ty::Region::new_var(tcx, a), ty::Region::new_var(tcx, b));
}
&Constraint::RegSubVar(a, b) => {
each_edge(a, ty::Region::new_var(tcx, b));
}
&Constraint::VarSubReg(a, b) => {
each_edge(ty::Region::new_var(tcx, a), b);
}
&Constraint::RegSubReg(a, b) => {
each_edge(a, b);
}
};
if let Some(snapshot) = only_consider_snapshot {
for undo_entry in
region_constraints.undo_log.region_constraints_in_snapshot(&snapshot.undo_snapshot)
{
match undo_entry {
AddConstraint(constraint) => {
each_constraint(constraint);
}
&AddVerify(i) => span_bug!(
region_constraints.data().verifys[i].origin.span(),
"we never add verifications while doing higher-ranked things",
),
&AddCombination(..) | &AddVar(..) => {}
}
}
} else {
region_constraints
.data()
.constraints
.keys()
.for_each(|constraint| each_constraint(constraint));
}
}
fn add_node(
nodes: &mut FxIndexMap<ty::Region<'tcx>, LeakCheckNode>,
r: ty::Region<'tcx>,
) -> LeakCheckNode {
let l = nodes.len();
*nodes.entry(r).or_insert(LeakCheckNode::new(l))
}
}