Added dataflow analysis for `Borrows`.

post-rebase: addressed review comment: rename `loc_map`/`location_map` and `rgn_map`/`region_map`.

post-rebase: remove now unnecessary `mut` decl.

post-rebase: address comments: bind iterator expr, and alpha-rename `loc`/`location` and `idx`/`index`.

src/librustc_mir/dataflow/impls/borrows.rs

+// Copyright 2012-2017 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::mir::{self, Location, Mir};
+use rustc::mir::visit::Visitor;
+use rustc::ty::{Region, TyCtxt};
+use rustc::ty::RegionKind::ReScope;
+use rustc::util::nodemap::{FxHashMap, FxHashSet};
+
+use rustc_data_structures::bitslice::{BitwiseOperator};
+use rustc_data_structures::indexed_set::{IdxSet};
+use rustc_data_structures::indexed_vec::{IndexVec};
+
+use dataflow::{BitDenotation, BlockSets, DataflowOperator};
+pub use dataflow::indexes::BorrowIndex;
+
+use std::fmt;
+
+// `Borrows` maps each dataflow bit to an `Rvalue::Ref`, which can be
+// uniquely identified in the MIR by the `Location` of the assigment
+// statement in which it appears on the right hand side.
+pub struct Borrows<'a, 'tcx: 'a> {
+    tcx: TyCtxt<'a, 'tcx, 'tcx>,
+    mir: &'a Mir<'tcx>,
+    borrows: IndexVec<BorrowIndex, BorrowData<'tcx>>,
+    location_map: FxHashMap<Location, BorrowIndex>,
+    region_map: FxHashMap<Region<'tcx>, FxHashSet<BorrowIndex>>,
+}
+
+// temporarily allow some dead fields: `kind` and `region` will be
+// needed by borrowck; `lvalue` will probably be a MovePathIndex when
+// that is extended to include borrowed data paths.
+#[allow(dead_code)]
+#[derive(Debug)]
+pub struct BorrowData<'tcx> {
+    pub(crate) location: Location,
+    pub(crate) kind: mir::BorrowKind,
+    pub(crate) region: Region<'tcx>,
+    pub(crate) lvalue: mir::Lvalue<'tcx>,
+}
+
+impl<'tcx> fmt::Display for BorrowData<'tcx> {
+    fn fmt(&self, w: &mut fmt::Formatter) -> fmt::Result {
+        let kind = match self.kind {
+            mir::BorrowKind::Shared => "",
+            mir::BorrowKind::Unique => "uniq ",
+            mir::BorrowKind::Mut => "mut ",
+        };
+        let region = format!("{}", self.region);
+        let region = if region.len() > 0 { format!("{} ", region) } else { region };
+        write!(w, "&{}{}{:?}", region, kind, self.lvalue)
+    }
+}
+
+impl<'a, 'tcx> Borrows<'a, 'tcx> {
+    pub fn new(tcx: TyCtxt<'a, 'tcx, 'tcx>, mir: &'a Mir<'tcx>) -> Self {
+        let mut visitor = GatherBorrows { idx_vec: IndexVec::new(),
+                                          location_map: FxHashMap(),
+                                          region_map: FxHashMap(), };
+        visitor.visit_mir(mir);
+        return Borrows { tcx: tcx,
+                         mir: mir,
+                         borrows: visitor.idx_vec,
+                         location_map: visitor.location_map,
+                         region_map: visitor.region_map, };
+
+        struct GatherBorrows<'tcx> {
+            idx_vec: IndexVec<BorrowIndex, BorrowData<'tcx>>,
+            location_map: FxHashMap<Location, BorrowIndex>,
+            region_map: FxHashMap<Region<'tcx>, FxHashSet<BorrowIndex>>,
+        }
+        impl<'tcx> Visitor<'tcx> for GatherBorrows<'tcx> {
+            fn visit_rvalue(&mut self,
+                            rvalue: &mir::Rvalue<'tcx>,
+                            location: mir::Location) {
+                if let mir::Rvalue::Ref(region, kind, ref lvalue) = *rvalue {
+                    let borrow = BorrowData {
+                        location: location, kind: kind, region: region, lvalue: lvalue.clone(),
+                    };
+                    let idx = self.idx_vec.push(borrow);
+                    self.location_map.insert(location, idx);
+                    let borrows = self.region_map.entry(region).or_insert(FxHashSet());
+                    borrows.insert(idx);
+                }
+            }
+        }
+    }
+
+    pub fn borrows(&self) -> &IndexVec<BorrowIndex, BorrowData<'tcx>> { &self.borrows }
+
+    pub fn location(&self, idx: BorrowIndex) -> &Location {
+        &self.borrows[idx].location
+    }
+}
+
+impl<'a, 'tcx> BitDenotation for Borrows<'a, 'tcx> {
+    type Idx = BorrowIndex;
+    fn name() -> &'static str { "borrows" }
+    fn bits_per_block(&self) -> usize {
+        self.borrows.len()
+    }
+    fn start_block_effect(&self, _sets: &mut BlockSets<BorrowIndex>)  {
+        // no borrows of code extents have been taken prior to
+        // function execution, so this method has no effect on
+        // `_sets`.
+    }
+    fn statement_effect(&self,
+                        sets: &mut BlockSets<BorrowIndex>,
+                        location: Location) {
+        let block = &self.mir.basic_blocks().get(location.block).unwrap_or_else(|| {
+            panic!("could not find block at location {:?}", location);
+        });
+        let stmt = block.statements.get(location.statement_index).unwrap_or_else(|| {
+            panic!("could not find statement at location {:?}");
+        });
+        match stmt.kind {
+            mir::StatementKind::EndRegion(extent) => {
+                let borrow_indexes = self.region_map.get(&ReScope(extent)).unwrap_or_else(|| {
+                    panic!("could not find BorrowIndexs for code-extent {:?}", extent);
+                });
+
+                for idx in borrow_indexes { sets.kill(&idx); }
+            }
+
+            mir::StatementKind::Assign(_, ref rhs) => {
+                if let mir::Rvalue::Ref(region, _, _) = *rhs {
+                    let index = self.location_map.get(&location).unwrap_or_else(|| {
+                        panic!("could not find BorrowIndex for location {:?}", location);
+                    });
+                    assert!(self.region_map.get(region).unwrap_or_else(|| {
+                        panic!("could not find BorrowIndexs for region {:?}", region);
+                    }).contains(&index));
+                    sets.gen(&index);
+                }
+            }
+
+            mir::StatementKind::InlineAsm { .. } |
+            mir::StatementKind::SetDiscriminant { .. } |
+            mir::StatementKind::StorageLive(..) |
+            mir::StatementKind::StorageDead(..) |
+            mir::StatementKind::Validate(..) |
+            mir::StatementKind::Nop => {}
+
+        }
+    }
+    fn terminator_effect(&self,
+                         _sets: &mut BlockSets<BorrowIndex>,
+                         _location: Location) {
+        // no terminators start nor end code extents.
+    }
+
+    fn propagate_call_return(&self,
+                             _in_out: &mut IdxSet<BorrowIndex>,
+                             _call_bb: mir::BasicBlock,
+                             _dest_bb: mir::BasicBlock,
+                             _dest_lval: &mir::Lvalue) {
+        // there are no effects on the extents from method calls.
+    }
+}
+
+impl<'a, 'tcx> BitwiseOperator for Borrows<'a, 'tcx> {
+    #[inline]
+    fn join(&self, pred1: usize, pred2: usize) -> usize {
+        pred1 | pred2 // union effects of preds when computing borrows
+    }
+}
+
+impl<'a, 'tcx> DataflowOperator for Borrows<'a, 'tcx> {
+    #[inline]
+    fn bottom_value() -> bool {
+        false // bottom = no Rvalue::Refs are active by default
+    }
+}

src/librustc_mir/dataflow/impls/mod.rs

@@ -30,6 +30,9 @@
 use super::drop_flag_effects_for_location;
 use super::on_lookup_result_bits;
 
+#[allow(dead_code)]
+pub(super) mod borrows;
+
 /// `MaybeInitializedLvals` tracks all l-values that might be
 /// initialized upon reaching a particular point in the control flow
 /// for a function.

src/librustc_mir/dataflow/mod.rs

@@ -25,7 +25,7 @@
 
 pub use self::impls::{MaybeInitializedLvals, MaybeUninitializedLvals};
 pub use self::impls::{DefinitelyInitializedLvals, MovingOutStatements};
-
+pub use self::impls::borrows::{Borrows, BorrowData, BorrowIndex};
 pub(crate) use self::drop_flag_effects::*;
 
 mod drop_flag_effects;

src/librustc_mir/dataflow/move_paths/mod.rs

@@ -63,6 +63,9 @@ fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
 
     /// Index into MoveData.moves.
     new_index!(MoveOutIndex, "mo");
+
+    /// Index into Borrows.locations
+    new_index!(BorrowIndex, "bw");
 }
 
 pub use self::indexes::MovePathIndex;