src/librustc_mir/borrow_check/nll/type_check/liveness/local_use_map.rs

use crate::borrow_check::nll::region_infer::values::{PointIndex, RegionValueElements};
use crate::util::liveness::{categorize, DefUse};
use rustc::mir::visit::{PlaceContext, Visitor};
use rustc::mir::{Local, Location, ReadOnlyBodyCache};
use rustc_index::vec::{Idx, IndexVec};
use rustc_data_structures::vec_linked_list as vll;

/// A map that cross references each local with the locations where it
/// is defined (assigned), used, or dropped. Used during liveness
/// computation.
///
/// We keep track only of `Local`s we'll do the liveness analysis later,
/// this means that our internal `IndexVec`s will only be sparsely populated.
/// In the time-memory trade-off between keeping compact vectors with new
/// indexes (and needing to continuously map the `Local` index to its compact
/// counterpart) and having `IndexVec`s that we only use a fraction of, time
/// (and code simplicity) was favored. The rationale is that we only keep
/// a small number of `IndexVec`s throughout the entire analysis while, in
/// contrast, we're accessing each `Local` *many* times.
crate struct LocalUseMap {
    /// Head of a linked list of **definitions** of each variable --
    /// definition in this context means assignment, e.g., `x` is
    /// defined in `x = y` but not `y`; that first def is the head of
    /// a linked list that lets you enumerate all places the variable
    /// is assigned.
    first_def_at: IndexVec<Local, Option<AppearanceIndex>>,

    /// Head of a linked list of **uses** of each variable -- use in
    /// this context means that the existing value of the variable is
    /// read or modified. e.g., `y` is used in `x = y` but not `x`.
    /// Note that `DROP(x)` terminators are excluded from this list.
    first_use_at: IndexVec<Local, Option<AppearanceIndex>>,

    /// Head of a linked list of **drops** of each variable -- these
    /// are a special category of uses corresponding to the drop that
    /// we add for each local variable.
    first_drop_at: IndexVec<Local, Option<AppearanceIndex>>,

    appearances: IndexVec<AppearanceIndex, Appearance>,
}

struct Appearance {
    point_index: PointIndex,
    next: Option<AppearanceIndex>,
}

rustc_index::newtype_index! {
    pub struct AppearanceIndex { .. }
}

impl vll::LinkElem for Appearance {
    type LinkIndex = AppearanceIndex;

    fn next(elem: &Self) -> Option<AppearanceIndex> {
        elem.next
    }
}

impl LocalUseMap {
    crate fn build(
        live_locals: &Vec<Local>,
        elements: &RegionValueElements,
        body: ReadOnlyBodyCache<'_, '_>,
    ) -> Self {
        let nones = IndexVec::from_elem_n(None, body.local_decls.len());
        let mut local_use_map = LocalUseMap {
            first_def_at: nones.clone(),
            first_use_at: nones.clone(),
            first_drop_at: nones,
            appearances: IndexVec::new(),
        };

        if live_locals.is_empty() {
            return local_use_map;
        }

        let mut locals_with_use_data: IndexVec<Local, bool> =
            IndexVec::from_elem_n(false, body.local_decls.len());
        live_locals.iter().for_each(|&local| locals_with_use_data[local] = true);

        LocalUseMapBuild { local_use_map: &mut local_use_map, elements, locals_with_use_data }
            .visit_body(body);

        local_use_map
    }

    crate fn defs(&self, local: Local) -> impl Iterator<Item = PointIndex> + '_ {
        vll::iter(self.first_def_at[local], &self.appearances)
            .map(move |aa| self.appearances[aa].point_index)
    }

    crate fn uses(&self, local: Local) -> impl Iterator<Item = PointIndex> + '_ {
        vll::iter(self.first_use_at[local], &self.appearances)
            .map(move |aa| self.appearances[aa].point_index)
    }

    crate fn drops(&self, local: Local) -> impl Iterator<Item = PointIndex> + '_ {
        vll::iter(self.first_drop_at[local], &self.appearances)
            .map(move |aa| self.appearances[aa].point_index)
    }
}

struct LocalUseMapBuild<'me> {
    local_use_map: &'me mut LocalUseMap,
    elements: &'me RegionValueElements,

    // Vector used in `visit_local` to signal which `Local`s do we need
    // def/use/drop information on, constructed from `live_locals` (that
    // contains the variables we'll do the liveness analysis for).
    // This vector serves optimization purposes only: we could have
    // obtained the same information from `live_locals` but we want to
    // avoid repeatedly calling `Vec::contains()` (see `LocalUseMap` for
    // the rationale on the time-memory trade-off we're favoring here).
    locals_with_use_data: IndexVec<Local, bool>,
}

impl LocalUseMapBuild<'_> {
    fn insert_def(&mut self, local: Local, location: Location) {
        Self::insert(
            self.elements,
            &mut self.local_use_map.first_def_at[local],
            &mut self.local_use_map.appearances,
            location,
        );
    }

    fn insert_use(&mut self, local: Local, location: Location) {
        Self::insert(
            self.elements,
            &mut self.local_use_map.first_use_at[local],
            &mut self.local_use_map.appearances,
            location,
        );
    }

    fn insert_drop(&mut self, local: Local, location: Location) {
        Self::insert(
            self.elements,
            &mut self.local_use_map.first_drop_at[local],
            &mut self.local_use_map.appearances,
            location,
        );
    }

    fn insert(
        elements: &RegionValueElements,
        first_appearance: &mut Option<AppearanceIndex>,
        appearances: &mut IndexVec<AppearanceIndex, Appearance>,
        location: Location,
    ) {
        let point_index = elements.point_from_location(location);
        let appearance_index =
            appearances.push(Appearance { point_index, next: *first_appearance });
        *first_appearance = Some(appearance_index);
    }
}

impl Visitor<'tcx> for LocalUseMapBuild<'_> {
    fn visit_local(&mut self, &local: &Local, context: PlaceContext, location: Location) {
        if self.locals_with_use_data[local] {
            match categorize(context) {
                Some(DefUse::Def) => self.insert_def(local, location),
                Some(DefUse::Use) => self.insert_use(local, location),
                Some(DefUse::Drop) => self.insert_drop(local, location),
                _ => (),
            }
        }
    }
}