Struct rustc_mir::borrow_check::type_check::liveness::local_use_map::LocalUseMap

pub(crate) struct LocalUseMap {
    first_def_at: IndexVec<Local, Option<AppearanceIndex>>,
    first_use_at: IndexVec<Local, Option<AppearanceIndex>>,
    first_drop_at: IndexVec<Local, Option<AppearanceIndex>>,
    appearances: IndexVec<AppearanceIndex, Appearance>,
}

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 Locals we’ll do the liveness analysis later, this means that our internal IndexVecs 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 IndexVecs that we only use a fraction of, time (and code simplicity) was favored. The rationale is that we only keep a small number of IndexVecs throughout the entire analysis while, in contrast, we’re accessing each Local many times.

Fields

first_def_at: IndexVec<Local, Option<AppearanceIndex>>

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_use_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_drop_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.

appearances: IndexVec<AppearanceIndex, Appearance>

Implementations

impl LocalUseMap

pub(crate) fn build(
    live_locals: &[Local],
    elements: &RegionValueElements,
    body: &Body<'_>
) -> Self

pub(crate) fn defs(&self, local: Local) -> impl Iterator<Item = PointIndex> + '_

pub(crate) fn uses(&self, local: Local) -> impl Iterator<Item = PointIndex> + '_

pub(crate) fn drops(
    &self,
    local: Local
) -> impl Iterator<Item = PointIndex> + '_