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Move some methods to region_infer/mod.rs

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This commit is contained in:
Mark Mansi 2019-12-28 19:25:57 -06:00 committed by mark
parent 109c30f3d4
commit c3e74f347e
3 changed files with 440 additions and 431 deletions
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45
src/librustc_mir/borrow_check/diagnostics/explain_borrow.rs
View File

@ -2,12 +2,13 @@
use std::collections::VecDeque;
use rustc::infer::NLLRegionVariableOrigin;
use rustc::mir::{
Body, CastKind, ConstraintCategory, FakeReadCause, Local, Location, Operand, Place, Rvalue,
Statement, StatementKind, TerminatorKind,
};
use rustc::ty::adjustment::PointerCast;
use rustc::ty::{self, TyCtxt};
use rustc::ty::{self, RegionVid, TyCtxt};
use rustc_data_structures::fx::FxHashSet;
use rustc_errors::{Applicability, DiagnosticBuilder};
use rustc_index::vec::IndexVec;
@ -15,8 +16,8 @@
use rustc_span::Span;
use crate::borrow_check::{
borrow_set::BorrowData, nll::ConstraintDescription, region_infer::Cause, MirBorrowckCtxt,
WriteKind,
borrow_set::BorrowData, diagnostics::RegionErrorNamingCtx, nll::ConstraintDescription,
region_infer::Cause, MirBorrowckCtxt, WriteKind,
};
use super::{find_use, RegionName, UseSpans};
@ -254,6 +255,32 @@ pub(in crate::borrow_check) fn add_lifetime_bound_suggestion_to_diagnostic<'tcx>
}
impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
fn free_region_constraint_info(
&self,
borrow_region: RegionVid,
outlived_region: RegionVid,
) -> (ConstraintCategory, bool, Span, Option<RegionName>) {
let (category, from_closure, span) = self.nonlexical_regioncx.best_blame_constraint(
&self.body,
borrow_region,
NLLRegionVariableOrigin::FreeRegion,
|r| {
self.nonlexical_regioncx.provides_universal_region(
r,
borrow_region,
outlived_region,
)
},
);
let mut renctx = RegionErrorNamingCtx::new();
let outlived_fr_name =
self.nonlexical_regioncx.give_region_a_name(self, &mut renctx, outlived_region);
// TODO(mark-i-m): just return the region and let the caller name it
(category, from_closure, span, outlived_fr_name)
}
/// Returns structured explanation for *why* the borrow contains the
/// point from `location`. This is key for the "3-point errors"
/// [described in the NLL RFC][d].
@ -285,7 +312,8 @@ pub(in crate::borrow_check) fn explain_why_borrow_contains_point(
let borrow_region_vid = borrow.region;
debug!("explain_why_borrow_contains_point: borrow_region_vid={:?}", borrow_region_vid);
let region_sub = regioncx.find_sub_region_live_at(borrow_region_vid, location);
let region_sub =
self.nonlexical_regioncx.find_sub_region_live_at(borrow_region_vid, location);
debug!("explain_why_borrow_contains_point: region_sub={:?}", region_sub);
match find_use::find(body, regioncx, tcx, region_sub, location) {
@ -330,9 +358,8 @@ pub(in crate::borrow_check) fn explain_why_borrow_contains_point(
None => {
if let Some(region) = regioncx.to_error_region_vid(borrow_region_vid) {
let (category, from_closure, span, region_name) = self
.nonlexical_regioncx
.free_region_constraint_info(self, borrow_region_vid, region);
let (category, from_closure, span, region_name) =
self.free_region_constraint_info(borrow_region_vid, region);
if let Some(region_name) = region_name {
let opt_place_desc = self.describe_place(borrow.borrowed_place.as_ref());
BorrowExplanation::MustBeValidFor {
@ -345,14 +372,14 @@ pub(in crate::borrow_check) fn explain_why_borrow_contains_point(
} else {
debug!(
"explain_why_borrow_contains_point: \
Could not generate a region name"
Could not generate a region name"
);
BorrowExplanation::Unexplained
}
} else {
debug!(
"explain_why_borrow_contains_point: \
Could not generate an error region vid"
Could not generate an error region vid"
);
BorrowExplanation::Unexplained
}

431
src/librustc_mir/borrow_check/diagnostics/region_errors.rs
View File

@ -3,22 +3,18 @@
use rustc::infer::{
error_reporting::nice_region_error::NiceRegionError, InferCtxt, NLLRegionVariableOrigin,
};
use rustc::mir::{Body, ConstraintCategory, Location};
use rustc::mir::ConstraintCategory;
use rustc::ty::{self, RegionVid, Ty};
use rustc_errors::{Applicability, DiagnosticBuilder};
use rustc_hir::def_id::DefId;
use rustc_index::vec::IndexVec;
use rustc_span::symbol::kw;
use rustc_span::Span;
use std::collections::VecDeque;
use crate::util::borrowck_errors;
use crate::borrow_check::{
constraints::OutlivesConstraint,
nll::ConstraintDescription,
region_infer::{values::RegionElement, RegionInferenceContext, TypeTest},
type_check::Locations,
universal_regions::DefiningTy,
MirBorrowckCtxt,
};
@ -48,13 +44,6 @@ fn description(&self) -> &'static str {
}
}
#[derive(Copy, Clone, PartialEq, Eq, Debug)]
enum Trace {
StartRegion,
FromOutlivesConstraint(OutlivesConstraint),
NotVisited,
}
/// A collection of errors encountered during region inference. This is needed to efficiently
/// report errors after borrow checking.
///
@ -142,270 +131,18 @@ pub fn to_error_region_vid(&self, r: RegionVid) -> Option<RegionVid> {
}
}
/// Tries to find the best constraint to blame for the fact that
/// `R: from_region`, where `R` is some region that meets
/// `target_test`. This works by following the constraint graph,
/// creating a constraint path that forces `R` to outlive
/// `from_region`, and then finding the best choices within that
/// path to blame.
fn best_blame_constraint(
&self,
body: &Body<'tcx>,
from_region: RegionVid,
from_region_origin: NLLRegionVariableOrigin,
target_test: impl Fn(RegionVid) -> bool,
) -> (ConstraintCategory, bool, Span) {
debug!(
"best_blame_constraint(from_region={:?}, from_region_origin={:?})",
from_region, from_region_origin
);
// Find all paths
let (path, target_region) =
self.find_constraint_paths_between_regions(from_region, target_test).unwrap();
debug!(
"best_blame_constraint: path={:#?}",
path.iter()
.map(|&c| format!(
"{:?} ({:?}: {:?})",
c,
self.constraint_sccs.scc(c.sup),
self.constraint_sccs.scc(c.sub),
))
.collect::<Vec<_>>()
);
// Classify each of the constraints along the path.
let mut categorized_path: Vec<(ConstraintCategory, bool, Span)> = path
.iter()
.map(|constraint| {
if constraint.category == ConstraintCategory::ClosureBounds {
self.retrieve_closure_constraint_info(body, &constraint)
} else {
(constraint.category, false, constraint.locations.span(body))
/// Returns `true` if a closure is inferred to be an `FnMut` closure.
crate fn is_closure_fn_mut(&self, infcx: &InferCtxt<'_, 'tcx>, fr: RegionVid) -> bool {
if let Some(ty::ReFree(free_region)) = self.to_error_region(fr) {
if let ty::BoundRegion::BrEnv = free_region.bound_region {
if let DefiningTy::Closure(def_id, substs) = self.universal_regions.defining_ty {
let closure_kind_ty = substs.as_closure().kind_ty(def_id, infcx.tcx);
return Some(ty::ClosureKind::FnMut) == closure_kind_ty.to_opt_closure_kind();
}
})
.collect();
debug!("best_blame_constraint: categorized_path={:#?}", categorized_path);
// To find the best span to cite, we first try to look for the
// final constraint that is interesting and where the `sup` is
// not unified with the ultimate target region. The reason
// for this is that we have a chain of constraints that lead
// from the source to the target region, something like:
//
// '0: '1 ('0 is the source)
// '1: '2
// '2: '3
// '3: '4
// '4: '5
// '5: '6 ('6 is the target)
//
// Some of those regions are unified with `'6` (in the same
// SCC). We want to screen those out. After that point, the
// "closest" constraint we have to the end is going to be the
// most likely to be the point where the value escapes -- but
// we still want to screen for an "interesting" point to
// highlight (e.g., a call site or something).
let target_scc = self.constraint_sccs.scc(target_region);
let mut range = 0..path.len();
// As noted above, when reporting an error, there is typically a chain of constraints
// leading from some "source" region which must outlive some "target" region.
// In most cases, we prefer to "blame" the constraints closer to the target --
// but there is one exception. When constraints arise from higher-ranked subtyping,
// we generally prefer to blame the source value,
// as the "target" in this case tends to be some type annotation that the user gave.
// Therefore, if we find that the region origin is some instantiation
// of a higher-ranked region, we start our search from the "source" point
// rather than the "target", and we also tweak a few other things.
//
// An example might be this bit of Rust code:
//
// ```rust
// let x: fn(&'static ()) = |_| {};
// let y: for<'a> fn(&'a ()) = x;
// ```
//
// In MIR, this will be converted into a combination of assignments and type ascriptions.
// In particular, the 'static is imposed through a type ascription:
//
// ```rust
// x = ...;
// AscribeUserType(x, fn(&'static ())
// y = x;
// ```
//
// We wind up ultimately with constraints like
//
// ```rust
// !a: 'temp1 // from the `y = x` statement
// 'temp1: 'temp2
// 'temp2: 'static // from the AscribeUserType
// ```
//
// and here we prefer to blame the source (the y = x statement).
let blame_source = match from_region_origin {
NLLRegionVariableOrigin::FreeRegion
| NLLRegionVariableOrigin::Existential { from_forall: false } => true,
NLLRegionVariableOrigin::Placeholder(_)
| NLLRegionVariableOrigin::Existential { from_forall: true } => false,
};
let find_region = |i: &usize| {
let constraint = path[*i];
let constraint_sup_scc = self.constraint_sccs.scc(constraint.sup);
if blame_source {
match categorized_path[*i].0 {
ConstraintCategory::OpaqueType
| ConstraintCategory::Boring
| ConstraintCategory::BoringNoLocation
| ConstraintCategory::Internal => false,
ConstraintCategory::TypeAnnotation
| ConstraintCategory::Return
| ConstraintCategory::Yield => true,
_ => constraint_sup_scc != target_scc,
}
} else {
match categorized_path[*i].0 {
ConstraintCategory::OpaqueType
| ConstraintCategory::Boring
| ConstraintCategory::BoringNoLocation
| ConstraintCategory::Internal => false,
_ => true,
}
}
};
let best_choice =
if blame_source { range.rev().find(find_region) } else { range.find(find_region) };
debug!(
"best_blame_constraint: best_choice={:?} blame_source={}",
best_choice, blame_source
);
if let Some(i) = best_choice {
if let Some(next) = categorized_path.get(i + 1) {
if categorized_path[i].0 == ConstraintCategory::Return
&& next.0 == ConstraintCategory::OpaqueType
{
// The return expression is being influenced by the return type being
// impl Trait, point at the return type and not the return expr.
return *next;
}
}
return categorized_path[i];
}
// If that search fails, that is.. unusual. Maybe everything
// is in the same SCC or something. In that case, find what
// appears to be the most interesting point to report to the
// user via an even more ad-hoc guess.
categorized_path.sort_by(|p0, p1| p0.0.cmp(&p1.0));
debug!("`: sorted_path={:#?}", categorized_path);
*categorized_path.first().unwrap()
}
/// Walks the graph of constraints (where `'a: 'b` is considered
/// an edge `'a -> 'b`) to find all paths from `from_region` to
/// `to_region`. The paths are accumulated into the vector
/// `results`. The paths are stored as a series of
/// `ConstraintIndex` values -- in other words, a list of *edges*.
///
/// Returns: a series of constraints as well as the region `R`
/// that passed the target test.
fn find_constraint_paths_between_regions(
&self,
from_region: RegionVid,
target_test: impl Fn(RegionVid) -> bool,
) -> Option<(Vec<OutlivesConstraint>, RegionVid)> {
let mut context = IndexVec::from_elem(Trace::NotVisited, &self.definitions);
context[from_region] = Trace::StartRegion;
// Use a deque so that we do a breadth-first search. We will
// stop at the first match, which ought to be the shortest
// path (fewest constraints).
let mut deque = VecDeque::new();
deque.push_back(from_region);
while let Some(r) = deque.pop_front() {
debug!(
"find_constraint_paths_between_regions: from_region={:?} r={:?} value={}",
from_region,
r,
self.region_value_str(r),
);
// Check if we reached the region we were looking for. If so,
// we can reconstruct the path that led to it and return it.
if target_test(r) {
let mut result = vec![];
let mut p = r;
loop {
match context[p] {
Trace::NotVisited => {
bug!("found unvisited region {:?} on path to {:?}", p, r)
}
Trace::FromOutlivesConstraint(c) => {
result.push(c);
p = c.sup;
}
Trace::StartRegion => {
result.reverse();
return Some((result, r));
}
}
}
}
// Otherwise, walk over the outgoing constraints and
// enqueue any regions we find, keeping track of how we
// reached them.
// A constraint like `'r: 'x` can come from our constraint
// graph.
let fr_static = self.universal_regions.fr_static;
let outgoing_edges_from_graph =
self.constraint_graph.outgoing_edges(r, &self.constraints, fr_static);
// Always inline this closure because it can be hot.
let mut handle_constraint = #[inline(always)]
|constraint: OutlivesConstraint| {
debug_assert_eq!(constraint.sup, r);
let sub_region = constraint.sub;
if let Trace::NotVisited = context[sub_region] {
context[sub_region] = Trace::FromOutlivesConstraint(constraint);
deque.push_back(sub_region);
}
};
// This loop can be hot.
for constraint in outgoing_edges_from_graph {
handle_constraint(constraint);
}
// Member constraints can also give rise to `'r: 'x` edges that
// were not part of the graph initially, so watch out for those.
// (But they are extremely rare; this loop is very cold.)
for constraint in self.applied_member_constraints(r) {
let p_c = &self.member_constraints[constraint.member_constraint_index];
let constraint = OutlivesConstraint {
sup: r,
sub: constraint.min_choice,
locations: Locations::All(p_c.definition_span),
category: ConstraintCategory::OpaqueType,
};
handle_constraint(constraint);
}
}
None
false
}
/// Report an error because the universal region `fr` was required to outlive
@ -484,30 +221,6 @@ pub(in crate::borrow_check) fn report_error<'a>(
}
}
/// We have a constraint `fr1: fr2` that is not satisfied, where
/// `fr2` represents some universal region. Here, `r` is some
/// region where we know that `fr1: r` and this function has the
/// job of determining whether `r` is "to blame" for the fact that
/// `fr1: fr2` is required.
///
/// This is true under two conditions:
///
/// - `r == fr2`
/// - `fr2` is `'static` and `r` is some placeholder in a universe
/// that cannot be named by `fr1`; in that case, we will require
/// that `fr1: 'static` because it is the only way to `fr1: r` to
/// be satisfied. (See `add_incompatible_universe`.)
fn provides_universal_region(&self, r: RegionVid, fr1: RegionVid, fr2: RegionVid) -> bool {
debug!("provides_universal_region(r={:?}, fr1={:?}, fr2={:?})", r, fr1, fr2);
let result = {
r == fr2 || {
fr2 == self.universal_regions.fr_static && self.cannot_name_placeholder(fr1, r)
}
};
debug!("provides_universal_region: result = {:?}", result);
result
}
/// Report a specialized error when `FnMut` closures return a reference to a captured variable.
/// This function expects `fr` to be local and `outlived_fr` to not be local.
///
@ -817,130 +530,4 @@ fn add_static_impl_trait_suggestion(
}
}
}
crate fn free_region_constraint_info(
&self,
mbcx: &MirBorrowckCtxt<'_, 'tcx>,
borrow_region: RegionVid,
outlived_region: RegionVid,
) -> (ConstraintCategory, bool, Span, Option<RegionName>) {
let (category, from_closure, span) = self.best_blame_constraint(
&mbcx.body,
borrow_region,
NLLRegionVariableOrigin::FreeRegion,
|r| self.provides_universal_region(r, borrow_region, outlived_region),
);
let mut renctx = RegionErrorNamingCtx::new();
let outlived_fr_name = self.give_region_a_name(mbcx, &mut renctx, outlived_region);
(category, from_closure, span, outlived_fr_name)
}
// Finds some region R such that `fr1: R` and `R` is live at
// `elem`.
crate fn find_sub_region_live_at(&self, fr1: RegionVid, elem: Location) -> RegionVid {
debug!("find_sub_region_live_at(fr1={:?}, elem={:?})", fr1, elem);
self.find_constraint_paths_between_regions(fr1, |r| {
// First look for some `r` such that `fr1: r` and `r` is live at `elem`
debug!(
"find_sub_region_live_at: liveness_constraints for {:?} are {:?}",
r,
self.liveness_constraints.region_value_str(r),
);
self.liveness_constraints.contains(r, elem)
})
.or_else(|| {
// If we fail to find that, we may find some `r` such that
// `fr1: r` and `r` is a placeholder from some universe
// `fr1` cannot name. This would force `fr1` to be
// `'static`.
self.find_constraint_paths_between_regions(fr1, |r| {
self.cannot_name_placeholder(fr1, r)
})
})
.or_else(|| {
// If we fail to find THAT, it may be that `fr1` is a
// placeholder that cannot "fit" into its SCC. In that
// case, there should be some `r` where `fr1: r`, both
// `fr1` and `r` are in the same SCC, and `fr1` is a
// placeholder that `r` cannot name. We can blame that
// edge.
self.find_constraint_paths_between_regions(fr1, |r| {
self.constraint_sccs.scc(fr1) == self.constraint_sccs.scc(r)
&& self.cannot_name_placeholder(r, fr1)
})
})
.map(|(_path, r)| r)
.unwrap()
}
// Finds a good span to blame for the fact that `fr1` outlives `fr2`.
crate fn find_outlives_blame_span(
&self,
body: &Body<'tcx>,
fr1: RegionVid,
fr1_origin: NLLRegionVariableOrigin,
fr2: RegionVid,
) -> (ConstraintCategory, Span) {
let (category, _, span) = self.best_blame_constraint(body, fr1, fr1_origin, |r| {
self.provides_universal_region(r, fr1, fr2)
});
(category, span)
}
fn retrieve_closure_constraint_info(
&self,
body: &Body<'tcx>,
constraint: &OutlivesConstraint,
) -> (ConstraintCategory, bool, Span) {
let loc = match constraint.locations {
Locations::All(span) => return (constraint.category, false, span),
Locations::Single(loc) => loc,
};
let opt_span_category =
self.closure_bounds_mapping[&loc].get(&(constraint.sup, constraint.sub));
opt_span_category.map(|&(category, span)| (category, true, span)).unwrap_or((
constraint.category,
false,
body.source_info(loc).span,
))
}
/// Returns `true` if a closure is inferred to be an `FnMut` closure.
crate fn is_closure_fn_mut(&self, infcx: &InferCtxt<'_, 'tcx>, fr: RegionVid) -> bool {
if let Some(ty::ReFree(free_region)) = self.to_error_region(fr) {
if let ty::BoundRegion::BrEnv = free_region.bound_region {
if let DefiningTy::Closure(def_id, substs) = self.universal_regions.defining_ty {
let closure_kind_ty = substs.as_closure().kind_ty(def_id, infcx.tcx);
return Some(ty::ClosureKind::FnMut) == closure_kind_ty.to_opt_closure_kind();
}
}
}
false
}
/// If `r2` represents a placeholder region, then this returns
/// `true` if `r1` cannot name that placeholder in its
/// value; otherwise, returns `false`.
fn cannot_name_placeholder(&self, r1: RegionVid, r2: RegionVid) -> bool {
debug!("cannot_name_value_of(r1={:?}, r2={:?})", r1, r2);
match self.definitions[r2].origin {
NLLRegionVariableOrigin::Placeholder(placeholder) => {
let universe1 = self.definitions[r1].universe;
debug!(
"cannot_name_value_of: universe1={:?} placeholder={:?}",
universe1, placeholder
);
universe1.cannot_name(placeholder.universe)
}
NLLRegionVariableOrigin::FreeRegion | NLLRegionVariableOrigin::Existential { .. } => {
false
}
}
}
}

395
src/librustc_mir/borrow_check/region_infer/mod.rs
View File

@ -1,3 +1,4 @@
use std::collections::VecDeque;
use std::rc::Rc;
use rustc::infer::canonical::QueryOutlivesConstraint;
@ -225,6 +226,13 @@ enum RegionRelationCheckResult {
Error,
}
#[derive(Copy, Clone, PartialEq, Eq, Debug)]
enum Trace {
StartRegion,
FromOutlivesConstraint(OutlivesConstraint),
NotVisited,
}
impl<'tcx> RegionInferenceContext<'tcx> {
/// Creates a new region inference context with a total of
/// `num_region_variables` valid inference variables; the first N
@ -1612,6 +1620,393 @@ fn check_member_constraints(
.unwrap(),
}
}
/// We have a constraint `fr1: fr2` that is not satisfied, where
/// `fr2` represents some universal region. Here, `r` is some
/// region where we know that `fr1: r` and this function has the
/// job of determining whether `r` is "to blame" for the fact that
/// `fr1: fr2` is required.
///
/// This is true under two conditions:
///
/// - `r == fr2`
/// - `fr2` is `'static` and `r` is some placeholder in a universe
/// that cannot be named by `fr1`; in that case, we will require
/// that `fr1: 'static` because it is the only way to `fr1: r` to
/// be satisfied. (See `add_incompatible_universe`.)
crate fn provides_universal_region(
&self,
r: RegionVid,
fr1: RegionVid,
fr2: RegionVid,
) -> bool {
debug!("provides_universal_region(r={:?}, fr1={:?}, fr2={:?})", r, fr1, fr2);
let result = {
r == fr2 || {
fr2 == self.universal_regions.fr_static && self.cannot_name_placeholder(fr1, r)
}
};
debug!("provides_universal_region: result = {:?}", result);
result
}
/// If `r2` represents a placeholder region, then this returns
/// `true` if `r1` cannot name that placeholder in its
/// value; otherwise, returns `false`.
crate fn cannot_name_placeholder(&self, r1: RegionVid, r2: RegionVid) -> bool {
debug!("cannot_name_value_of(r1={:?}, r2={:?})", r1, r2);
match self.definitions[r2].origin {
NLLRegionVariableOrigin::Placeholder(placeholder) => {
let universe1 = self.definitions[r1].universe;
debug!(
"cannot_name_value_of: universe1={:?} placeholder={:?}",
universe1, placeholder
);
universe1.cannot_name(placeholder.universe)
}
NLLRegionVariableOrigin::FreeRegion | NLLRegionVariableOrigin::Existential { .. } => {
false
}
}
}
crate fn retrieve_closure_constraint_info(
&self,
body: &Body<'tcx>,
constraint: &OutlivesConstraint,
) -> (ConstraintCategory, bool, Span) {
let loc = match constraint.locations {
Locations::All(span) => return (constraint.category, false, span),
Locations::Single(loc) => loc,
};
let opt_span_category =
self.closure_bounds_mapping[&loc].get(&(constraint.sup, constraint.sub));
opt_span_category.map(|&(category, span)| (category, true, span)).unwrap_or((
constraint.category,
false,
body.source_info(loc).span,
))
}
/// Finds a good span to blame for the fact that `fr1` outlives `fr2`.
crate fn find_outlives_blame_span(
&self,
body: &Body<'tcx>,
fr1: RegionVid,
fr1_origin: NLLRegionVariableOrigin,
fr2: RegionVid,
) -> (ConstraintCategory, Span) {
let (category, _, span) = self.best_blame_constraint(body, fr1, fr1_origin, |r| {
self.provides_universal_region(r, fr1, fr2)
});
(category, span)
}
/// Walks the graph of constraints (where `'a: 'b` is considered
/// an edge `'a -> 'b`) to find all paths from `from_region` to
/// `to_region`. The paths are accumulated into the vector
/// `results`. The paths are stored as a series of
/// `ConstraintIndex` values -- in other words, a list of *edges*.
///
/// Returns: a series of constraints as well as the region `R`
/// that passed the target test.
crate fn find_constraint_paths_between_regions(
&self,
from_region: RegionVid,
target_test: impl Fn(RegionVid) -> bool,
) -> Option<(Vec<OutlivesConstraint>, RegionVid)> {
let mut context = IndexVec::from_elem(Trace::NotVisited, &self.definitions);
context[from_region] = Trace::StartRegion;
// Use a deque so that we do a breadth-first search. We will
// stop at the first match, which ought to be the shortest
// path (fewest constraints).
let mut deque = VecDeque::new();
deque.push_back(from_region);
while let Some(r) = deque.pop_front() {
debug!(
"find_constraint_paths_between_regions: from_region={:?} r={:?} value={}",
from_region,
r,
self.region_value_str(r),
);
// Check if we reached the region we were looking for. If so,
// we can reconstruct the path that led to it and return it.
if target_test(r) {
let mut result = vec![];
let mut p = r;
loop {
match context[p] {
Trace::NotVisited => {
bug!("found unvisited region {:?} on path to {:?}", p, r)
}
Trace::FromOutlivesConstraint(c) => {
result.push(c);
p = c.sup;
}
Trace::StartRegion => {
result.reverse();
return Some((result, r));
}
}
}
}
// Otherwise, walk over the outgoing constraints and
// enqueue any regions we find, keeping track of how we
// reached them.
// A constraint like `'r: 'x` can come from our constraint
// graph.
let fr_static = self.universal_regions.fr_static;
let outgoing_edges_from_graph =
self.constraint_graph.outgoing_edges(r, &self.constraints, fr_static);
// Always inline this closure because it can be hot.
let mut handle_constraint = #[inline(always)]
|constraint: OutlivesConstraint| {
debug_assert_eq!(constraint.sup, r);
let sub_region = constraint.sub;
if let Trace::NotVisited = context[sub_region] {
context[sub_region] = Trace::FromOutlivesConstraint(constraint);
deque.push_back(sub_region);
}
};
// This loop can be hot.
for constraint in outgoing_edges_from_graph {
handle_constraint(constraint);
}
// Member constraints can also give rise to `'r: 'x` edges that
// were not part of the graph initially, so watch out for those.
// (But they are extremely rare; this loop is very cold.)
for constraint in self.applied_member_constraints(r) {
let p_c = &self.member_constraints[constraint.member_constraint_index];
let constraint = OutlivesConstraint {
sup: r,
sub: constraint.min_choice,
locations: Locations::All(p_c.definition_span),
category: ConstraintCategory::OpaqueType,
};
handle_constraint(constraint);
}
}
None
}
/// Finds some region R such that `fr1: R` and `R` is live at `elem`.
crate fn find_sub_region_live_at(&self, fr1: RegionVid, elem: Location) -> RegionVid {
debug!("find_sub_region_live_at(fr1={:?}, elem={:?})", fr1, elem);
self.find_constraint_paths_between_regions(fr1, |r| {
// First look for some `r` such that `fr1: r` and `r` is live at `elem`
debug!(
"find_sub_region_live_at: liveness_constraints for {:?} are {:?}",
r,
self.liveness_constraints.region_value_str(r),
);
self.liveness_constraints.contains(r, elem)
})
.or_else(|| {
// If we fail to find that, we may find some `r` such that
// `fr1: r` and `r` is a placeholder from some universe
// `fr1` cannot name. This would force `fr1` to be
// `'static`.
self.find_constraint_paths_between_regions(fr1, |r| {
self.cannot_name_placeholder(fr1, r)
})
})
.or_else(|| {
// If we fail to find THAT, it may be that `fr1` is a
// placeholder that cannot "fit" into its SCC. In that
// case, there should be some `r` where `fr1: r`, both
// `fr1` and `r` are in the same SCC, and `fr1` is a
// placeholder that `r` cannot name. We can blame that
// edge.
self.find_constraint_paths_between_regions(fr1, |r| {
self.constraint_sccs.scc(fr1) == self.constraint_sccs.scc(r)
&& self.cannot_name_placeholder(r, fr1)
})
})
.map(|(_path, r)| r)
.unwrap()
}
/// Tries to find the best constraint to blame for the fact that
/// `R: from_region`, where `R` is some region that meets
/// `target_test`. This works by following the constraint graph,
/// creating a constraint path that forces `R` to outlive
/// `from_region`, and then finding the best choices within that
/// path to blame.
crate fn best_blame_constraint(
&self,
body: &Body<'tcx>,
from_region: RegionVid,
from_region_origin: NLLRegionVariableOrigin,
target_test: impl Fn(RegionVid) -> bool,
) -> (ConstraintCategory, bool, Span) {
debug!(
"best_blame_constraint(from_region={:?}, from_region_origin={:?})",
from_region, from_region_origin
);
// Find all paths
let (path, target_region) =
self.find_constraint_paths_between_regions(from_region, target_test).unwrap();
debug!(
"best_blame_constraint: path={:#?}",
path.iter()
.map(|&c| format!(
"{:?} ({:?}: {:?})",
c,
self.constraint_sccs.scc(c.sup),
self.constraint_sccs.scc(c.sub),
))
.collect::<Vec<_>>()
);
// Classify each of the constraints along the path.
let mut categorized_path: Vec<(ConstraintCategory, bool, Span)> = path
.iter()
.map(|constraint| {
if constraint.category == ConstraintCategory::ClosureBounds {
self.retrieve_closure_constraint_info(body, &constraint)
} else {
(constraint.category, false, constraint.locations.span(body))
}
})
.collect();
debug!("best_blame_constraint: categorized_path={:#?}", categorized_path);
// To find the best span to cite, we first try to look for the
// final constraint that is interesting and where the `sup` is
// not unified with the ultimate target region. The reason
// for this is that we have a chain of constraints that lead
// from the source to the target region, something like:
//
// '0: '1 ('0 is the source)
// '1: '2
// '2: '3
// '3: '4
// '4: '5
// '5: '6 ('6 is the target)
//
// Some of those regions are unified with `'6` (in the same
// SCC). We want to screen those out. After that point, the
// "closest" constraint we have to the end is going to be the
// most likely to be the point where the value escapes -- but
// we still want to screen for an "interesting" point to
// highlight (e.g., a call site or something).
let target_scc = self.constraint_sccs.scc(target_region);
let mut range = 0..path.len();
// As noted above, when reporting an error, there is typically a chain of constraints
// leading from some "source" region which must outlive some "target" region.
// In most cases, we prefer to "blame" the constraints closer to the target --
// but there is one exception. When constraints arise from higher-ranked subtyping,
// we generally prefer to blame the source value,
// as the "target" in this case tends to be some type annotation that the user gave.
// Therefore, if we find that the region origin is some instantiation
// of a higher-ranked region, we start our search from the "source" point
// rather than the "target", and we also tweak a few other things.
//
// An example might be this bit of Rust code:
//
// ```rust
// let x: fn(&'static ()) = |_| {};
// let y: for<'a> fn(&'a ()) = x;
// ```
//
// In MIR, this will be converted into a combination of assignments and type ascriptions.
// In particular, the 'static is imposed through a type ascription:
//
// ```rust
// x = ...;
// AscribeUserType(x, fn(&'static ())
// y = x;
// ```
//
// We wind up ultimately with constraints like
//
// ```rust
// !a: 'temp1 // from the `y = x` statement
// 'temp1: 'temp2
// 'temp2: 'static // from the AscribeUserType
// ```
//
// and here we prefer to blame the source (the y = x statement).
let blame_source = match from_region_origin {
NLLRegionVariableOrigin::FreeRegion
| NLLRegionVariableOrigin::Existential { from_forall: false } => true,
NLLRegionVariableOrigin::Placeholder(_)
| NLLRegionVariableOrigin::Existential { from_forall: true } => false,
};
let find_region = |i: &usize| {
let constraint = path[*i];
let constraint_sup_scc = self.constraint_sccs.scc(constraint.sup);
if blame_source {
match categorized_path[*i].0 {
ConstraintCategory::OpaqueType
| ConstraintCategory::Boring
| ConstraintCategory::BoringNoLocation
| ConstraintCategory::Internal => false,
ConstraintCategory::TypeAnnotation
| ConstraintCategory::Return
| ConstraintCategory::Yield => true,
_ => constraint_sup_scc != target_scc,
}
} else {
match categorized_path[*i].0 {
ConstraintCategory::OpaqueType
| ConstraintCategory::Boring
| ConstraintCategory::BoringNoLocation
| ConstraintCategory::Internal => false,
_ => true,
}
}
};
let best_choice =
if blame_source { range.rev().find(find_region) } else { range.find(find_region) };
debug!(
"best_blame_constraint: best_choice={:?} blame_source={}",
best_choice, blame_source
);
if let Some(i) = best_choice {
if let Some(next) = categorized_path.get(i + 1) {
if categorized_path[i].0 == ConstraintCategory::Return
&& next.0 == ConstraintCategory::OpaqueType
{
// The return expression is being influenced by the return type being
// impl Trait, point at the return type and not the return expr.
return *next;
}
}
return categorized_path[i];
}
// If that search fails, that is.. unusual. Maybe everything
// is in the same SCC or something. In that case, find what
// appears to be the most interesting point to report to the
// user via an even more ad-hoc guess.
categorized_path.sort_by(|p0, p1| p0.0.cmp(&p1.0));
debug!("`: sorted_path={:#?}", categorized_path);
*categorized_path.first().unwrap()
}
}
impl<'tcx> RegionDefinition<'tcx> {