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Fix missing explanation of where the borrowed reference is used when the same borrow occurs multiple times due to loop iterations #102080

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Merged
merged 1 commit into from
Oct 16, 2022
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Fix missing explanation of where the borrowed reference is used when the same borrow occurs multiple times due to loop iterations #102080

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156 changes: 23 additions & 133 deletions compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs
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Original file line number Diff line number Diff line change
@@ -1,8 +1,5 @@
//! Print diagnostics to explain why values are borrowed.
use std::collections::VecDeque;

use rustc_data_structures::fx::FxHashSet;
use rustc_errors::{Applicability, Diagnostic};
use rustc_index::vec::IndexVec;
use rustc_infer::infer::NllRegionVariableOrigin;
Expand Down Expand Up @@ -359,19 +356,37 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
let borrow_region_vid = borrow.region;
debug!(?borrow_region_vid);

let region_sub = self.regioncx.find_sub_region_live_at(borrow_region_vid, location);
let mut region_sub = self.regioncx.find_sub_region_live_at(borrow_region_vid, location);
debug!(?region_sub);

match find_use::find(body, regioncx, tcx, region_sub, location) {
let mut use_location = location;
let mut use_in_later_iteration_of_loop = false;

if region_sub == borrow_region_vid {
// When `region_sub` is the same as `borrow_region_vid` (the location where the borrow is
// issued is the same location that invalidates the reference), this is likely a loop iteration
// - in this case, try using the loop terminator location in `find_sub_region_live_at`.
if let Some(loop_terminator_location) =
regioncx.find_loop_terminator_location(borrow.region, body)
{
region_sub = self
.regioncx
.find_sub_region_live_at(borrow_region_vid, loop_terminator_location);
debug!("explain_why_borrow_contains_point: region_sub in loop={:?}", region_sub);
use_location = loop_terminator_location;
use_in_later_iteration_of_loop = true;
}
}

match find_use::find(body, regioncx, tcx, region_sub, use_location) {
Some(Cause::LiveVar(local, location)) => {
let span = body.source_info(location).span;
let spans = self
.move_spans(Place::from(local).as_ref(), location)
.or_else(|| self.borrow_spans(span, location));

let borrow_location = location;
if self.is_use_in_later_iteration_of_loop(borrow_location, location) {
let later_use = self.later_use_kind(borrow, spans, location);
if use_in_later_iteration_of_loop {
let later_use = self.later_use_kind(borrow, spans, use_location);
BorrowExplanation::UsedLaterInLoop(later_use.0, later_use.1, later_use.2)
} else {
// Check if the location represents a `FakeRead`, and adapt the error
Expand Down Expand Up @@ -425,131 +440,6 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
}
}

/// true if `borrow_location` can reach `use_location` by going through a loop and
/// `use_location` is also inside of that loop
fn is_use_in_later_iteration_of_loop(
&self,
borrow_location: Location,
use_location: Location,
) -> bool {
let back_edge = self.reach_through_backedge(borrow_location, use_location);
back_edge.map_or(false, |back_edge| self.can_reach_head_of_loop(use_location, back_edge))
}

/// Returns the outmost back edge if `from` location can reach `to` location passing through
/// that back edge
fn reach_through_backedge(&self, from: Location, to: Location) -> Option<Location> {
let mut visited_locations = FxHashSet::default();
let mut pending_locations = VecDeque::new();
visited_locations.insert(from);
pending_locations.push_back(from);
debug!("reach_through_backedge: from={:?} to={:?}", from, to,);

let mut outmost_back_edge = None;
while let Some(location) = pending_locations.pop_front() {
debug!(
"reach_through_backedge: location={:?} outmost_back_edge={:?}
pending_locations={:?} visited_locations={:?}",
location, outmost_back_edge, pending_locations, visited_locations
);

if location == to && outmost_back_edge.is_some() {
// We've managed to reach the use location
debug!("reach_through_backedge: found!");
return outmost_back_edge;
}

let block = &self.body.basic_blocks[location.block];

if location.statement_index < block.statements.len() {
let successor = location.successor_within_block();
if visited_locations.insert(successor) {
pending_locations.push_back(successor);
}
} else {
pending_locations.extend(
block
.terminator()
.successors()
.map(|bb| Location { statement_index: 0, block: bb })
.filter(|s| visited_locations.insert(*s))
.map(|s| {
if self.is_back_edge(location, s) {
match outmost_back_edge {
None => {
outmost_back_edge = Some(location);
}

Some(back_edge)
if location.dominates(back_edge, &self.dominators) =>
{
outmost_back_edge = Some(location);
}

Some(_) => {}
}
}

s
}),
);
}
}

None
}

/// true if `from` location can reach `loop_head` location and `loop_head` dominates all the
/// intermediate nodes
fn can_reach_head_of_loop(&self, from: Location, loop_head: Location) -> bool {
self.find_loop_head_dfs(from, loop_head, &mut FxHashSet::default())
}

fn find_loop_head_dfs(
&self,
from: Location,
loop_head: Location,
visited_locations: &mut FxHashSet<Location>,
) -> bool {
visited_locations.insert(from);

if from == loop_head {
return true;
}

if loop_head.dominates(from, &self.dominators) {
let block = &self.body.basic_blocks[from.block];

if from.statement_index < block.statements.len() {
let successor = from.successor_within_block();

if !visited_locations.contains(&successor)
&& self.find_loop_head_dfs(successor, loop_head, visited_locations)
{
return true;
}
} else {
for bb in block.terminator().successors() {
let successor = Location { statement_index: 0, block: bb };

if !visited_locations.contains(&successor)
&& self.find_loop_head_dfs(successor, loop_head, visited_locations)
{
return true;
}
}
}
}

false
}

/// True if an edge `source -> target` is a backedge -- in other words, if the target
/// dominates the source.
fn is_back_edge(&self, source: Location, target: Location) -> bool {
target.dominates(source, &self.dominators)
}

/// Determine how the borrow was later used.
/// First span returned points to the location of the conflicting use
/// Second span if `Some` is returned in the case of closures and points
Expand Down
23 changes: 22 additions & 1 deletion compiler/rustc_borrowck/src/region_infer/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -15,7 +15,7 @@ use rustc_infer::infer::region_constraints::{GenericKind, VarInfos, VerifyBound,
use rustc_infer::infer::{InferCtxt, NllRegionVariableOrigin, RegionVariableOrigin};
use rustc_middle::mir::{
Body, ClosureOutlivesRequirement, ClosureOutlivesSubject, ClosureRegionRequirements,
ConstraintCategory, Local, Location, ReturnConstraint,
ConstraintCategory, Local, Location, ReturnConstraint, TerminatorKind,
};
use rustc_middle::traits::ObligationCause;
use rustc_middle::traits::ObligationCauseCode;
Expand Down Expand Up @@ -2236,6 +2236,27 @@ impl<'tcx> RegionInferenceContext<'tcx> {
pub(crate) fn universe_info(&self, universe: ty::UniverseIndex) -> UniverseInfo<'tcx> {
self.universe_causes[&universe].clone()
}

/// Tries to find the terminator of the loop in which the region 'r' resides.
/// Returns the location of the terminator if found.
pub(crate) fn find_loop_terminator_location(
&self,
r: RegionVid,
body: &Body<'_>,
) -> Option<Location> {
let scc = self.constraint_sccs.scc(r.to_region_vid());
let locations = self.scc_values.locations_outlived_by(scc);
for location in locations {
let bb = &body[location.block];
if let Some(terminator) = &bb.terminator {
// terminator of a loop should be TerminatorKind::FalseUnwind
if let TerminatorKind::FalseUnwind { .. } = terminator.kind {
return Some(location);
}
}
}
None
}
}

impl<'tcx> RegionDefinition<'tcx> {
Expand Down
5 changes: 4 additions & 1 deletion src/test/ui/borrowck/borrowck-mut-borrow-linear-errors.stderr
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Expand Up @@ -25,7 +25,10 @@ error[E0499]: cannot borrow `x` as mutable more than once at a time
--> $DIR/borrowck-mut-borrow-linear-errors.rs:12:30
|
LL | _ => { addr.push(&mut x); }
| ^^^^^^ `x` was mutably borrowed here in the previous iteration of the loop
| ----------^^^^^^-
| | |
| | `x` was mutably borrowed here in the previous iteration of the loop
| first borrow used here, in later iteration of loop

error: aborting due to 3 previous errors

Expand Down
5 changes: 4 additions & 1 deletion src/test/ui/borrowck/two-phase-across-loop.stderr
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Expand Up @@ -2,7 +2,10 @@ error[E0499]: cannot borrow `foo` as mutable more than once at a time
--> $DIR/two-phase-across-loop.rs:17:22
|
LL | strings.push(foo.get_string());
| ^^^^^^^^^^^^^^^^ `foo` was mutably borrowed here in the previous iteration of the loop
| -------------^^^^^^^^^^^^^^^^-
| | |
| | `foo` was mutably borrowed here in the previous iteration of the loop
| first borrow used here, in later iteration of loop

error: aborting due to previous error

Expand Down
16 changes: 10 additions & 6 deletions src/test/ui/nll/closures-in-loops.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -13,17 +13,21 @@ error[E0499]: cannot borrow `x` as mutable more than once at a time
--> $DIR/closures-in-loops.rs:13:16
|
LL | v.push(|| x = String::new());
| ^^ - borrows occur due to use of `x` in closure
| |
| `x` was mutably borrowed here in the previous iteration of the loop
| -------^^-------------------
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The spans are a bit unfortunate. Is there anything to do here?

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You mean the format of using "-------^^-------------------" is not clear?

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I just realised: the longer --- is meant to point to loop_terminator_location. It should point to some } below, shouldn't it?

| | | |
| | | borrows occur due to use of `x` in closure
| | `x` was mutably borrowed here in the previous iteration of the loop
| first borrow used here, in later iteration of loop

error[E0524]: two closures require unique access to `x` at the same time
--> $DIR/closures-in-loops.rs:20:16
|
LL | v.push(|| *x = String::new());
| ^^ -- borrows occur due to use of `x` in closure
| |
| closures are constructed here in different iterations of loop
| -------^^--------------------
| | | |
| | | borrows occur due to use of `x` in closure
| | closures are constructed here in different iterations of loop
| first borrow used here, in later iteration of loop

error: aborting due to 3 previous errors

Expand Down