constraint_set.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 rustc::ty::RegionVid;
use rustc_data_structures::indexed_vec::{Idx, IndexVec};
use borrow_check::nll::type_check::Locations;

use std::fmt;
use std::cmp::Ordering;
use std::ops::Deref;

#[derive(Clone, Default)]
crate struct ConstraintSet {
    constraints: IndexVec<ConstraintIndex, OutlivesConstraint>,
}

impl ConstraintSet {
    pub fn push(&mut self, constraint: OutlivesConstraint) {
        debug!(
            "add_outlives({:?}: {:?} @ {:?})",
            constraint.sup, constraint.sub, constraint.locations
        );
        if constraint.sup == constraint.sub {
            // 'a: 'a is pretty uninteresting
            return;
        }
        self.constraints.push(constraint);
    }

    /// Once all constraints have been added, `link()` is used to thread together the constraints
    /// based on which would be affected when a particular region changes. See the next field of
    /// `OutlivesContraint` for more details.
    /// link returns a map that is needed later by `each_affected_by_dirty`.
    pub fn link(&mut self, len: usize) -> IndexVec<RegionVid, Option<ConstraintIndex>> {
        let mut map = IndexVec::from_elem_n(None, len);

        for (idx, constraint) in self.constraints.iter_enumerated_mut().rev() {
            let mut head = &mut map[constraint.sub];
            debug_assert!(constraint.next.is_none());
            constraint.next = *head;
            *head = Some(idx);
        }

        map
    }

    /// When a region R1 changes, we need to reprocess all constraints R2: R1 to take into account
    /// any new elements that R1 now has. This method will quickly enumerate all such constraints
    /// (that is, constraints where R1 is in the "subregion" position).
    /// To use it, invoke with `map[R1]` where map is the map returned by `link`;
    /// the callback op will be invoked for each affected constraint.
    pub fn each_affected_by_dirty(
        &self,
        mut opt_dep_idx: Option<ConstraintIndex>,
        mut op: impl FnMut(ConstraintIndex),
    ) {
        while let Some(dep_idx) = opt_dep_idx {
            op(dep_idx);
            opt_dep_idx = self.constraints[dep_idx].next;
        }
    }
}

impl Deref for ConstraintSet {
    type Target = IndexVec<ConstraintIndex, OutlivesConstraint>;

    fn deref(&self) -> &Self::Target { &self.constraints }
}

#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct OutlivesConstraint {
    // 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,

    /// Later on, we thread the constraints onto a linked list
    /// grouped by their `sub` field. So if you had:
    ///
    /// Index | Constraint | Next Field
    /// ----- | ---------- | ----------
    /// 0     | `'a: 'b`   | Some(2)
    /// 1     | `'b: 'c`   | None
    /// 2     | `'c: 'b`   | None
    pub next: Option<ConstraintIndex>,

    /// Where did this constraint arise?
    pub locations: Locations,
}

impl fmt::Debug for OutlivesConstraint {
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        write!(
            formatter,
            "({:?}: {:?}) due to {:?}",
            self.sup, self.sub, self.locations
        )
    }
}

/// Constraints that are considered interesting can be categorized to
/// determine why they are interesting.
#[derive(Debug, Eq, PartialEq)]
crate enum ConstraintCategory {
    Assignment,
    CallArgument,
    Cast,
    Other,
}

impl Ord for ConstraintCategory {
    fn cmp(&self, other: &Self) -> Ordering {
        if self == other {
            return Ordering::Equal;
        }

        match (self, other) {
            (ConstraintCategory::Assignment, _) => Ordering::Greater,
            (_, ConstraintCategory::Assignment) => Ordering::Less,
            (ConstraintCategory::CallArgument, _) => Ordering::Greater,
            (_, ConstraintCategory::CallArgument) => Ordering::Less,
            (ConstraintCategory::Cast, _) => Ordering::Greater,
            (_, ConstraintCategory::Cast) => Ordering::Less,
            (ConstraintCategory::Other, _) => Ordering::Greater,
        }
    }
}

impl PartialOrd for ConstraintCategory {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        Some(self.cmp(other))
    }
}


newtype_index!(ConstraintIndex { DEBUG_FORMAT = "ConstraintIndex({})" });