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``````use rustc_index::IndexVec;
use rustc_middle::mir::{BasicBlock, Body, Location};

/// Maps between a MIR Location, which identifies a particular
/// statement within a basic block, to a "rich location", which
/// identifies at a finer granularity. In particular, we distinguish
/// the *start* of a statement and the *mid-point*. The mid-point is
/// the point *just* before the statement takes effect; in particular,
/// for an assignment `A = B`, it is the point where B is about to be
/// written into A. This mid-point is a kind of hack to work around
/// our inability to track the position information at sufficient
/// granularity through outlives relations; however, the rich location
/// table serves another purpose: it compresses locations from
/// multiple words into a single u32.
pub struct LocationTable {
num_points: usize,
statements_before_block: IndexVec<BasicBlock, usize>,
}

rustc_index::newtype_index! {
#[orderable]
#[debug_format = "LocationIndex({})"]
pub struct LocationIndex {}
}

#[derive(Copy, Clone, Debug)]
pub enum RichLocation {
Start(Location),
Mid(Location),
}

impl LocationTable {
pub(crate) fn new(body: &Body<'_>) -> Self {
let mut num_points = 0;
let statements_before_block = body
.basic_blocks
.iter()
.map(|block_data| {
let v = num_points;
num_points += (block_data.statements.len() + 1) * 2;
v
})
.collect();

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

Self { num_points, statements_before_block }
}

pub fn all_points(&self) -> impl Iterator<Item = LocationIndex> {
(0..self.num_points).map(LocationIndex::from_usize)
}

pub fn start_index(&self, location: Location) -> LocationIndex {
let Location { block, statement_index } = location;
let start_index = self.statements_before_block[block];
LocationIndex::from_usize(start_index + statement_index * 2)
}

pub fn mid_index(&self, location: Location) -> LocationIndex {
let Location { block, statement_index } = location;
let start_index = self.statements_before_block[block];
LocationIndex::from_usize(start_index + statement_index * 2 + 1)
}

pub fn to_location(&self, index: LocationIndex) -> RichLocation {
let point_index = index.index();

// 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).
let (block, &first_index) = self
.statements_before_block
.iter_enumerated()
.rfind(|&(_, &first_index)| first_index <= point_index)
.unwrap();

let statement_index = (point_index - first_index) / 2;
if index.is_start() {
RichLocation::Start(Location { block, statement_index })
} else {
RichLocation::Mid(Location { block, statement_index })
}
}
}

impl LocationIndex {
fn is_start(self) -> bool {
// even indices are start points; odd indices are mid points
(self.index() % 2) == 0
}
}
``````