New upstream version 1.27.1+dfsg1

[rustc.git] / src / librustc_mir / borrow_check / nll / region_infer / values.rs

// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use std::rc::Rc;
use rustc_data_structures::bitvec::SparseBitMatrix;
use rustc_data_structures::fx::FxHashMap;
use rustc_data_structures::indexed_vec::Idx;
use rustc_data_structures::indexed_vec::IndexVec;
use rustc::mir::{BasicBlock, Location, Mir};
use rustc::ty::{self, RegionVid};
use syntax::codemap::Span;

use super::{Cause, CauseExt, TrackCauses};

/// Maps between the various kinds of elements of a region value to
/// the internal indices that w use.
pub(super) struct RegionValueElements {
    /// For each basic block, how many points are contained within?
    statements_before_block: IndexVec<BasicBlock, usize>,
    num_points: usize,
    num_universal_regions: usize,
}

impl RegionValueElements {
    pub(super) fn new(mir: &Mir<'_>, num_universal_regions: usize) -> Self {
        let mut num_points = 0;
        let statements_before_block = mir.basic_blocks()
            .iter()
            .map(|block_data| {
                let v = num_points;
                num_points += block_data.statements.len() + 1;
                v
            })
            .collect();

        debug!(
            "RegionValueElements(num_universal_regions={:?})",
            num_universal_regions
        );
        debug!(
            "RegionValueElements: statements_before_block={:#?}",
            statements_before_block
        );
        debug!("RegionValueElements: num_points={:#?}", num_points);

        Self {
            statements_before_block,
            num_universal_regions,
            num_points,
        }
    }

    /// Total number of element indices that exist.
    pub(super) fn num_elements(&self) -> usize {
        self.num_points + self.num_universal_regions
    }

    /// Converts an element of a region value into a `RegionElementIndex`.
    pub(super) fn index<T: ToElementIndex>(&self, elem: T) -> RegionElementIndex {
        elem.to_element_index(self)
    }

    /// Iterates over the `RegionElementIndex` for all points in the CFG.
    pub(super) fn all_point_indices<'a>(&'a self) -> impl Iterator<Item = RegionElementIndex> + 'a {
        (0..self.num_points).map(move |i| RegionElementIndex::new(i + self.num_universal_regions))
    }

    /// Iterates over the `RegionElementIndex` for all points in the CFG.
    pub(super) fn all_universal_region_indices(&self) -> impl Iterator<Item = RegionElementIndex> {
        (0..self.num_universal_regions).map(move |i| RegionElementIndex::new(i))
    }

    /// Converts a particular `RegionElementIndex` to the `RegionElement` it represents.
    pub(super) fn to_element(&self, i: RegionElementIndex) -> RegionElement {
        debug!("to_element(i={:?})", i);

        if let Some(r) = self.to_universal_region(i) {
            RegionElement::UniversalRegion(r)
        } else {
            let point_index = i.index() - self.num_universal_regions;

            // Find the basic block. We have a vector with the
            // starting index of the statement in each block. Imagine
            // we have statement #22, and we have a vector like:
            //
            // [0, 10, 20]
            //
            // In that case, this represents point_index 2 of
            // basic block BB2. We know this because BB0 accounts for
            // 0..10, BB1 accounts for 11..20, and BB2 accounts for
            // 20...
            //
            // To compute this, we could do a binary search, but
            // because I am lazy we instead iterate through to find
            // the last point where the "first index" (0, 10, or 20)
            // was less than the statement index (22). In our case, this will
            // be (BB2, 20).
            //
            // Nit: we could do a binary search here but I'm too lazy.
            let (block, &first_index) = self.statements_before_block
                .iter_enumerated()
                .filter(|(_, first_index)| **first_index <= point_index)
                .last()
                .unwrap();

            RegionElement::Location(Location {
                block,
                statement_index: point_index - first_index,
            })
        }
    }

    /// Converts a particular `RegionElementIndex` to a universal
    /// region, if that is what it represents. Returns `None`
    /// otherwise.
    pub(super) fn to_universal_region(&self, i: RegionElementIndex) -> Option<RegionVid> {
        if i.index() < self.num_universal_regions {
            Some(RegionVid::new(i.index()))
        } else {
            None
        }
    }
}

/// A newtype for the integers that represent one of the possible
/// elements in a region. These are the rows in the `SparseBitMatrix` that
/// is used to store the values of all regions. They have the following
/// convention:
///
/// - The first N indices represent free regions (where N = universal_regions.len()).
/// - The remainder represent the points in the CFG (see `point_indices` map).
///
/// You can convert a `RegionElementIndex` into a `RegionElement`
/// using the `to_region_elem` method.
newtype_index!(RegionElementIndex { DEBUG_FORMAT = "RegionElementIndex({})" });

/// An individual element in a region value -- the value of a
/// particular region variable consists of a set of these elements.
#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
pub(super) enum RegionElement {
    /// A point in the control-flow graph.
    Location(Location),

    /// An in-scope, universally quantified region (e.g., a lifetime parameter).
    UniversalRegion(RegionVid),
}

pub(super) trait ToElementIndex {
    fn to_element_index(self, elements: &RegionValueElements) -> RegionElementIndex;
}

impl ToElementIndex for Location {
    fn to_element_index(self, elements: &RegionValueElements) -> RegionElementIndex {
        let Location {
            block,
            statement_index,
        } = self;
        let start_index = elements.statements_before_block[block];
        RegionElementIndex::new(elements.num_universal_regions + start_index + statement_index)
    }
}

impl ToElementIndex for RegionVid {
    fn to_element_index(self, elements: &RegionValueElements) -> RegionElementIndex {
        assert!(self.index() < elements.num_universal_regions);
        RegionElementIndex::new(self.index())
    }
}

impl ToElementIndex for RegionElementIndex {
    fn to_element_index(self, _elements: &RegionValueElements) -> RegionElementIndex {
        self
    }
}

/// Stores the values for a set of regions. These are stored in a
/// compact `SparseBitMatrix` representation, with one row per region
/// variable. The columns consist of either universal regions or
/// points in the CFG.
pub(super) struct RegionValues {
    elements: Rc<RegionValueElements>,
    matrix: SparseBitMatrix<RegionVid, RegionElementIndex>,

    /// If cause tracking is enabled, maps from a pair (r, e)
    /// consisting of a region `r` that contains some element `e` to
    /// the reason that the element is contained. There should be an
    /// entry for every bit set to 1 in `SparseBitMatrix`.
    causes: Option<CauseMap>,
}

type CauseMap = FxHashMap<(RegionVid, RegionElementIndex), Rc<Cause>>;

impl RegionValues {
    /// Creates a new set of "region values" that tracks causal information.
    /// Each of the regions in num_region_variables will be initialized with an
    /// empty set of points and no causal information.
    pub(super) fn new(elements: &Rc<RegionValueElements>, num_region_variables: usize) -> Self {
        assert!(
            elements.num_universal_regions <= num_region_variables,
            "universal regions are a subset of the region variables"
        );

        Self {
            elements: elements.clone(),
            matrix: SparseBitMatrix::new(
                RegionVid::new(num_region_variables),
                RegionElementIndex::new(elements.num_elements()),
            ),
            causes: Some(CauseMap::default()),
        }
    }

    /// Duplicates the region values. If track_causes is false, then the
    /// resulting value will not track causal information (and any existing
    /// causal information is dropped). Otherwise, the causal information is
    /// preserved and maintained. Tracking the causal information makes region
    /// propagation significantly slower, so we prefer not to do it until an
    /// error is reported.
    pub(super) fn duplicate(&self, track_causes: TrackCauses) -> Self {
        Self {
            elements: self.elements.clone(),
            matrix: self.matrix.clone(),
            causes: if track_causes.0 {
                self.causes.clone()
            } else {
                None
            },
        }
    }

    /// Adds the given element to the value for the given region. Returns true if
    /// the element is newly added (i.e., was not already present).
    pub(super) fn add<E: ToElementIndex>(&mut self, r: RegionVid, elem: E, cause: &Cause) -> bool {
        let i = self.elements.index(elem);
        self.add_internal(r, i, |_| cause.clone())
    }

    /// Internal method to add an element to a region.
    ///
    /// Takes a "lazy" cause -- this function will return the cause, but it will only
    /// be invoked if cause tracking is enabled.
    fn add_internal<F>(&mut self, r: RegionVid, i: RegionElementIndex, make_cause: F) -> bool
    where
        F: FnOnce(&CauseMap) -> Cause,
    {
        if self.matrix.add(r, i) {
            debug!("add(r={:?}, i={:?})", r, self.elements.to_element(i));

            if let Some(causes) = &mut self.causes {
                let cause = Rc::new(make_cause(causes));
                causes.insert((r, i), cause);
            }

            true
        } else {
            if let Some(causes) = &mut self.causes {
                let cause = make_cause(causes);
                let old_cause = causes.get_mut(&(r, i)).unwrap();
                // #49998: compare using root cause alone to avoid
                // useless traffic from similar outlives chains.

                let overwrite = if ty::tls::with(|tcx| {
                    tcx.sess.opts.debugging_opts.nll_subminimal_causes
                }) {
                    cause.root_cause() < old_cause.root_cause()
                } else {
                    cause < **old_cause
                };
                if overwrite {
                    *old_cause = Rc::new(cause);
                    return true;
                }
            }

            false
        }
    }

    /// Adds `elem` to `to_region` because of a relation:
    ///
    ///     to_region: from_region @ constraint_location
    ///
    /// that was added by the cod at `constraint_span`.
    pub(super) fn add_due_to_outlives<T: ToElementIndex>(
        &mut self,
        from_region: RegionVid,
        to_region: RegionVid,
        elem: T,
        constraint_location: Location,
        constraint_span: Span,
    ) -> bool {
        let elem = self.elements.index(elem);
        self.add_internal(to_region, elem, |causes| {
            causes[&(from_region, elem)].outlives(constraint_location, constraint_span)
        })
    }

    /// Adds all the universal regions outlived by `from_region` to
    /// `to_region`.
    pub(super) fn add_universal_regions_outlived_by(
        &mut self,
        from_region: RegionVid,
        to_region: RegionVid,
        constraint_location: Location,
        constraint_span: Span,
    ) -> bool {
        // We could optimize this by improving `SparseBitMatrix::merge` so
        // it does not always merge an entire row. That would
        // complicate causal tracking though.
        debug!(
            "add_universal_regions_outlived_by(from_region={:?}, to_region={:?})",
            from_region, to_region
        );
        let mut changed = false;
        for elem in self.elements.all_universal_region_indices() {
            if self.contains(from_region, elem) {
                changed |= self.add_due_to_outlives(
                    from_region,
                    to_region,
                    elem,
                    constraint_location,
                    constraint_span,
                );
            }
        }
        changed
    }

    /// True if the region `r` contains the given element.
    pub(super) fn contains<E: ToElementIndex>(&self, r: RegionVid, elem: E) -> bool {
        let i = self.elements.index(elem);
        self.matrix.contains(r, i)
    }

    /// Iterate over the value of the region `r`, yielding up element
    /// indices. You may prefer `universal_regions_outlived_by` or
    /// `elements_contained_in`.
    pub(super) fn element_indices_contained_in<'a>(
        &'a self,
        r: RegionVid,
    ) -> impl Iterator<Item = RegionElementIndex> + 'a {
        self.matrix.iter(r).map(move |i| i)
    }

    /// Returns just the universal regions that are contained in a given region's value.
    pub(super) fn universal_regions_outlived_by<'a>(
        &'a self,
        r: RegionVid,
    ) -> impl Iterator<Item = RegionVid> + 'a {
        self.element_indices_contained_in(r)
            .map(move |i| self.elements.to_universal_region(i))
            .take_while(move |v| v.is_some()) // universal regions are a prefix
            .map(move |v| v.unwrap())
    }

    /// Returns all the elements contained in a given region's value.
    pub(super) fn elements_contained_in<'a>(
        &'a self,
        r: RegionVid,
    ) -> impl Iterator<Item = RegionElement> + 'a {
        self.element_indices_contained_in(r)
            .map(move |r| self.elements.to_element(r))
    }

    /// Returns a "pretty" string value of the region. Meant for debugging.
    pub(super) fn region_value_str(&self, r: RegionVid) -> String {
        let mut result = String::new();
        result.push_str("{");

        // Set to Some(l1, l2) when we have observed all the locations
        // from l1..=l2 (inclusive) but not yet printed them. This
        // gets extended if we then see l3 where l3 is the successor
        // to l2.
        let mut open_location: Option<(Location, Location)> = None;

        let mut sep = "";
        let mut push_sep = |s: &mut String| {
            s.push_str(sep);
            sep = ", ";
        };

        for element in self.elements_contained_in(r) {
            match element {
                RegionElement::Location(l) => {
                    if let Some((location1, location2)) = open_location {
                        if location2.block == l.block
                            && location2.statement_index == l.statement_index - 1
                        {
                            open_location = Some((location1, l));
                            continue;
                        }

                        push_sep(&mut result);
                        Self::push_location_range(&mut result, location1, location2);
                    }

                    open_location = Some((l, l));
                }

                RegionElement::UniversalRegion(fr) => {
                    if let Some((location1, location2)) = open_location {
                        push_sep(&mut result);
                        Self::push_location_range(&mut result, location1, location2);
                        open_location = None;
                    }

                    push_sep(&mut result);
                    result.push_str(&format!("{:?}", fr));
                }
            }
        }

        if let Some((location1, location2)) = open_location {
            push_sep(&mut result);
            Self::push_location_range(&mut result, location1, location2);
        }

        result.push_str("}");

        result
    }

    fn push_location_range(str: &mut String, location1: Location, location2: Location) {
        if location1 == location2 {
            str.push_str(&format!("{:?}", location1));
        } else {
            assert_eq!(location1.block, location2.block);
            str.push_str(&format!(
                "{:?}[{}..={}]",
                location1.block, location1.statement_index, location2.statement_index
            ));
        }
    }

    /// Given a region `r` that contains the element `elem`, returns the `Cause`
    /// that tells us *why* `elem` is found in that region.
    ///
    /// Returns None if cause tracking is disabled or `elem` is not
    /// actually found in `r`.
    pub(super) fn cause<T: ToElementIndex>(&self, r: RegionVid, elem: T) -> Option<Rc<Cause>> {
        let index = self.elements.index(elem);
        if let Some(causes) = &self.causes {
            causes.get(&(r, index)).cloned()
        } else {
            None
        }
    }
}