# rustc_borrowck/constraints/mod.rs

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```
use std::fmt;
use std::ops::Index;
use rustc_index::{IndexSlice, IndexVec};
use rustc_middle::mir::ConstraintCategory;
use rustc_middle::ty::{RegionVid, TyCtxt, VarianceDiagInfo};
use rustc_span::Span;
use tracing::{debug, instrument};
use crate::region_infer::{ConstraintSccs, RegionDefinition, RegionTracker};
use crate::type_check::Locations;
use crate::universal_regions::UniversalRegions;
pub(crate) mod graph;
/// A set of NLL region constraints. These include "outlives"
/// constraints of the form `R1: R2`. Each constraint is identified by
/// a unique `OutlivesConstraintIndex` and you can index into the set
/// (`constraint_set[i]`) to access the constraint details.
#[derive(Clone, Debug, Default)]
pub(crate) struct OutlivesConstraintSet<'tcx> {
outlives: IndexVec<OutlivesConstraintIndex, OutlivesConstraint<'tcx>>,
}
impl<'tcx> OutlivesConstraintSet<'tcx> {
pub(crate) fn push(&mut self, constraint: OutlivesConstraint<'tcx>) {
debug!("OutlivesConstraintSet::push({:?})", constraint);
if constraint.sup == constraint.sub {
// 'a: 'a is pretty uninteresting
return;
}
self.outlives.push(constraint);
}
/// Constructs a "normal" graph from the constraint set; the graph makes it
/// easy to find the constraints affecting a particular region.
///
/// N.B., this graph contains a "frozen" view of the current
/// constraints. Any new constraints added to the `OutlivesConstraintSet`
/// after the graph is built will not be present in the graph.
pub(crate) fn graph(&self, num_region_vars: usize) -> graph::NormalConstraintGraph {
graph::ConstraintGraph::new(graph::Normal, self, num_region_vars)
}
/// Like `graph`, but constraints a reverse graph where `R1: R2`
/// represents an edge `R2 -> R1`.
pub(crate) fn reverse_graph(&self, num_region_vars: usize) -> graph::ReverseConstraintGraph {
graph::ConstraintGraph::new(graph::Reverse, self, num_region_vars)
}
pub(crate) fn outlives(
&self,
) -> &IndexSlice<OutlivesConstraintIndex, OutlivesConstraint<'tcx>> {
&self.outlives
}
/// Computes cycles (SCCs) in the graph of regions. In particular,
/// find all regions R1, R2 such that R1: R2 and R2: R1 and group
/// them into an SCC, and find the relationships between SCCs.
pub(crate) fn compute_sccs(
&self,
static_region: RegionVid,
definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
) -> ConstraintSccs {
let constraint_graph = self.graph(definitions.len());
let region_graph = &constraint_graph.region_graph(self, static_region);
ConstraintSccs::new_with_annotation(®ion_graph, |r| {
RegionTracker::new(r, &definitions[r])
})
}
/// This method handles Universe errors by rewriting the constraint
/// graph. For each strongly connected component in the constraint
/// graph such that there is a series of constraints
/// A: B: C: ... : X where
/// A's universe is smaller than X's and A is a placeholder,
/// add a constraint that A: 'static. This is a safe upper bound
/// in the face of borrow checker/trait solver limitations that will
/// eventually go away.
///
/// For a more precise definition, see the documentation for
/// [`RegionTracker::has_incompatible_universes()`].
///
/// This edge case used to be handled during constraint propagation
/// by iterating over the strongly connected components in the constraint
/// graph while maintaining a set of bookkeeping mappings similar
/// to what is stored in `RegionTracker` and manually adding 'sttaic as
/// needed.
///
/// It was rewritten as part of the Polonius project with the goal of moving
/// higher-kindedness concerns out of the path of the borrow checker,
/// for two reasons:
///
/// 1. Implementing Polonius is difficult enough without also
/// handling them.
/// 2. The long-term goal is to handle higher-kinded concerns
/// in the trait solver, where they belong. This avoids
/// logic duplication and allows future trait solvers
/// to compute better bounds than for example our
/// "must outlive 'static" here.
///
/// This code is a stop-gap measure in preparation for the future trait solver.
///
/// Every constraint added by this method is an
/// internal `IllegalUniverse` constraint.
#[instrument(skip(self, universal_regions, definitions))]
pub(crate) fn add_outlives_static(
&mut self,
universal_regions: &UniversalRegions<'tcx>,
definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
) -> ConstraintSccs {
let fr_static = universal_regions.fr_static;
let sccs = self.compute_sccs(fr_static, definitions);
// Changed to `true` if we added any constraints to `self` and need to
// recompute SCCs.
let mut added_constraints = false;
for scc in sccs.all_sccs() {
// No point in adding 'static: 'static!
// This micro-optimisation makes somewhat sense
// because static outlives *everything*.
if scc == sccs.scc(fr_static) {
continue;
}
let annotation = sccs.annotation(scc);
// If this SCC participates in a universe violation,
// e.g. if it reaches a region with a universe smaller than
// the largest region reached, add a requirement that it must
// outlive `'static`.
if annotation.has_incompatible_universes() {
// Optimisation opportunity: this will add more constraints than
// needed for correctness, since an SCC upstream of another with
// a universe violation will "infect" its downstream SCCs to also
// outlive static.
added_constraints = true;
let scc_representative_outlives_static = OutlivesConstraint {
sup: annotation.representative,
sub: fr_static,
category: ConstraintCategory::IllegalUniverse,
locations: Locations::All(rustc_span::DUMMY_SP),
span: rustc_span::DUMMY_SP,
variance_info: VarianceDiagInfo::None,
from_closure: false,
};
self.push(scc_representative_outlives_static);
}
}
if added_constraints {
// We changed the constraint set and so must recompute SCCs.
self.compute_sccs(fr_static, definitions)
} else {
// If we didn't add any back-edges; no more work needs doing
sccs
}
}
}
impl<'tcx> Index<OutlivesConstraintIndex> for OutlivesConstraintSet<'tcx> {
type Output = OutlivesConstraint<'tcx>;
fn index(&self, i: OutlivesConstraintIndex) -> &Self::Output {
&self.outlives[i]
}
}
#[derive(Copy, Clone, PartialEq, Eq)]
pub struct OutlivesConstraint<'tcx> {
// NB. The ordering here is not significant for correctness, but
// it is for convenience. Before we dump the constraints in the
// debugging logs, we sort them, and we'd like the "super region"
// to be first, etc. (In particular, span should remain last.)
/// The region SUP must outlive SUB...
pub sup: RegionVid,
/// Region that must be outlived.
pub sub: RegionVid,
/// Where did this constraint arise?
pub locations: Locations,
/// The `Span` associated with the creation of this constraint.
/// This should be used in preference to obtaining the span from
/// `locations`, since the `locations` may give a poor span
/// in some cases (e.g. converting a constraint from a promoted).
pub span: Span,
/// What caused this constraint?
pub category: ConstraintCategory<'tcx>,
/// Variance diagnostic information
pub variance_info: VarianceDiagInfo<TyCtxt<'tcx>>,
/// If this constraint is promoted from closure requirements.
pub from_closure: bool,
}
impl<'tcx> fmt::Debug for OutlivesConstraint<'tcx> {
fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(
formatter,
"({:?}: {:?}) due to {:?} ({:?}) ({:?})",
self.sup, self.sub, self.locations, self.variance_info, self.category,
)
}
}
rustc_index::newtype_index! {
#[debug_format = "OutlivesConstraintIndex({})"]
pub(crate) struct OutlivesConstraintIndex {}
}
rustc_index::newtype_index! {
#[orderable]
#[debug_format = "ConstraintSccIndex({})"]
pub struct ConstraintSccIndex {}
}
```