MIR based borrow check (opt-in).

One can either use `-Z borrowck-mir` or add the `#[rustc_mir_borrowck]` attribute
to opt into MIR based borrow checking.

Note that regardless of whether one opts in or not, AST-based borrow
check will still run as well.  The errors emitted from AST-based
borrow check will include a "(Ast)" suffix in their error message,
while the errors emitted from MIR-based borrow check will include a
"(Mir)" suffix.

post-rebase: removed check for intra-statement mutual conflict;
replaced with assertion checking that at most one borrow is generated
per statement.

post-rebase: removed dead code: `IdxSet::pairs` and supporting stuff.

7 files changed, 1422 insertions, 4 deletions

diff --git a/src/librustc/session/config.rs b/src/librustc/session/config.rs
index 6995f099677..5985dcb97c7 100644
--- a/src/librustc/session/config.rs
+++ b/src/librustc/session/config.rs
@@ -918,6 +918,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "when debug-printing compiler state, do not include spans"), // o/w tests have closure@path
     identify_regions: bool = (false, parse_bool, [UNTRACKED],
         "make unnamed regions display as '# (where # is some non-ident unique id)"),
+    borrowck_mir: bool = (false, parse_bool, [UNTRACKED],
+        "implicitly treat functions as if they have `#[rustc_mir_borrowck]` attribute"),
     time_passes: bool = (false, parse_bool, [UNTRACKED],
         "measure time of each rustc pass"),
     count_llvm_insns: bool = (false, parse_bool,
diff --git a/src/librustc_data_structures/indexed_set.rs b/src/librustc_data_structures/indexed_set.rs
index 4189089e20d..9cb6806e9ad 100644
--- a/src/librustc_data_structures/indexed_set.rs
+++ b/src/librustc_data_structures/indexed_set.rs
@@ -159,6 +159,36 @@ impl<T: Idx> IdxSet<T> {
     pub fn each_bit<F>(&self, max_bits: usize, f: F) where F: FnMut(T) {
         each_bit(self, max_bits, f)
     }
+
+    /// Removes all elements from this set.
+    pub fn reset_to_empty(&mut self) {
+        for word in self.words_mut() { *word = 0; }
+    }
+
+    pub fn elems(&self, universe_size: usize) -> Elems<T> {
+        Elems { i: 0, set: self, universe_size: universe_size }
+    }
+}
+
+pub struct Elems<'a, T: Idx> { i: usize, set: &'a IdxSet<T>, universe_size: usize }
+
+impl<'a, T: Idx> Iterator for Elems<'a, T> {
+    type Item = T;
+    fn next(&mut self) -> Option<T> {
+        if self.i >= self.universe_size { return None; }
+        let mut i = self.i;
+        loop {
+            if i >= self.universe_size {
+                self.i = i; // (mark iteration as complete.)
+                return None;
+            }
+            if self.set.contains(&T::new(i)) {
+                self.i = i + 1; // (next element to start at.)
+                return Some(T::new(i));
+            }
+            i = i + 1;
+        }
+    }
 }
 
 fn each_bit<T: Idx, F>(words: &IdxSet<T>, max_bits: usize, mut f: F) where F: FnMut(T) {
diff --git a/src/librustc_driver/driver.rs b/src/librustc_driver/driver.rs
index 8a12aa1f7ae..0e4fb075b9c 100644
--- a/src/librustc_driver/driver.rs
+++ b/src/librustc_driver/driver.rs
@@ -970,6 +970,7 @@ pub fn phase_3_run_analysis_passes<'tcx, F, R>(sess: &'tcx Session,
     // We compute "constant qualifications" between MIR_CONST and MIR_VALIDATED.
 
     // What we need to run borrowck etc.
+    passes.push_pass(MIR_CONST, mir::transform::borrow_check::BorrowckMir);
     passes.push_pass(MIR_VALIDATED, mir::transform::qualify_consts::QualifyAndPromoteConstants);
     passes.push_pass(MIR_VALIDATED,
                      mir::transform::simplify_branches::SimplifyBranches::new("initial"));
diff --git a/src/librustc_mir/dataflow/mod.rs b/src/librustc_mir/dataflow/mod.rs
index 08b8d332fa1..18a81093a43 100644
--- a/src/librustc_mir/dataflow/mod.rs
+++ b/src/librustc_mir/dataflow/mod.rs
@@ -15,7 +15,7 @@ use rustc_data_structures::indexed_vec::Idx;
 use rustc_data_structures::bitslice::{bitwise, BitwiseOperator};
 
 use rustc::ty::{self, TyCtxt};
-use rustc::mir::{self, Mir, BasicBlock, Location};
+use rustc::mir::{self, Mir, BasicBlock, BasicBlockData, Location, Statement, Terminator};
 use rustc::session::Session;
 
 use std::fmt::{self, Debug};
@@ -47,7 +47,19 @@ pub(crate) struct DataflowBuilder<'a, 'tcx: 'a, BD> where BD: BitDenotation
 }
 
 pub trait Dataflow<BD: BitDenotation> {
-    fn dataflow<P>(&mut self, p: P) where P: Fn(&BD, BD::Idx) -> &Debug;
+    /// Sets up and runs the dataflow problem, using `p` to render results if
+    /// implementation so chooses.
+    fn dataflow<P>(&mut self, p: P) where P: Fn(&BD, BD::Idx) -> &Debug {
+        let _ = p; // default implementation does not instrument process.
+        self.build_sets();
+        self.propagate();
+    }
+
+    /// Sets up the entry, gen, and kill sets for this instance of a dataflow problem.
+    fn build_sets(&mut self);
+
+    /// Finds a fixed-point solution to this instance of a dataflow problem.
+    fn propagate(&mut self);
 }
 
 impl<'a, 'tcx: 'a, BD> Dataflow<BD> for DataflowBuilder<'a, 'tcx, BD>
@@ -59,6 +71,9 @@ impl<'a, 'tcx: 'a, BD> Dataflow<BD> for DataflowBuilder<'a, 'tcx, BD>
         self.flow_state.propagate();
         self.post_dataflow_instrumentation(|c,i| p(c,i)).unwrap();
     }
+
+    fn build_sets(&mut self) { self.flow_state.build_sets(); }
+    fn propagate(&mut self) { self.flow_state.propagate(); }
 }
 
 pub(crate) fn has_rustc_mir_with(attrs: &[ast::Attribute], name: &str) -> Option<MetaItem> {
@@ -254,6 +269,93 @@ impl<E:Idx> Bits<E> {
     }
 }
 
+/// DataflowResultsConsumer abstracts over walking the MIR with some
+/// already constructed dataflow results.
+///
+/// It abstracts over the FlowState and also completely hides the
+/// underlying flow analysis results, because it needs to handle cases
+/// where we are combining the results of *multiple* flow analyses
+/// (e.g. borrows + inits + uninits).
+pub trait DataflowResultsConsumer<'a, 'tcx: 'a> {
+    type FlowState;
+
+    // Observation Hooks: override (at least one of) these to get analysis feedback.
+    fn visit_block_entry(&mut self,
+                         _bb: BasicBlock,
+                         _flow_state: &Self::FlowState) {}
+
+    fn visit_statement_entry(&mut self,
+                             _loc: Location,
+                             _stmt: &Statement<'tcx>,
+                             _flow_state: &Self::FlowState) {}
+
+    fn visit_terminator_entry(&mut self,
+                              _loc: Location,
+                              _term: &Terminator<'tcx>,
+                              _flow_state: &Self::FlowState) {}
+
+    // Main entry point: this drives the processing of results.
+
+    fn analyze_results(&mut self, flow_uninit: &mut Self::FlowState) {
+        let flow = flow_uninit;
+        for bb in self.mir().basic_blocks().indices() {
+            self.reset_to_entry_of(bb, flow);
+            self.process_basic_block(bb, flow);
+        }
+    }
+
+    fn process_basic_block(&mut self, bb: BasicBlock, flow_state: &mut Self::FlowState) {
+        let BasicBlockData { ref statements, ref terminator, is_cleanup: _ } =
+            self.mir()[bb];
+        let mut location = Location { block: bb, statement_index: 0 };
+        for stmt in statements.iter() {
+            self.reconstruct_statement_effect(location, flow_state);
+            self.visit_statement_entry(location, stmt, flow_state);
+            self.apply_local_effect(location, flow_state);
+            location.statement_index += 1;
+        }
+
+        if let Some(ref term) = *terminator {
+            self.reconstruct_terminator_effect(location, flow_state);
+            self.visit_terminator_entry(location, term, flow_state);
+
+            // We don't need to apply the effect of the terminator,
+            // since we are only visiting dataflow state on control
+            // flow entry to the various nodes. (But we still need to
+            // reconstruct the effect, because the visit method might
+            // inspect it.)
+        }
+    }
+
+    // Delegated Hooks: Provide access to the MIR and process the flow state.
+
+    fn mir(&self) -> &'a Mir<'tcx>;
+
+    // reset the state bitvector to represent the entry to block `bb`.
+    fn reset_to_entry_of(&mut self,
+                         bb: BasicBlock,
+                         flow_state: &mut Self::FlowState);
+
+    // build gen + kill sets for statement at `loc`.
+    fn reconstruct_statement_effect(&mut self,
+                                    loc: Location,
+                                    flow_state: &mut Self::FlowState);
+
+    // build gen + kill sets for terminator for `loc`.
+    fn reconstruct_terminator_effect(&mut self,
+                                     loc: Location,
+                                     flow_state: &mut Self::FlowState);
+
+    // apply current gen + kill sets to `flow_state`.
+    //
+    // (`bb` and `stmt_idx` parameters can be ignored if desired by
+    // client. For the terminator, the `stmt_idx` will be the number
+    // of statements in the block.)
+    fn apply_local_effect(&mut self,
+                          loc: Location,
+                          flow_state: &mut Self::FlowState);
+}
+
 pub struct DataflowAnalysis<'a, 'tcx: 'a, O>
     where O: BitDenotation
 {
@@ -269,6 +371,8 @@ impl<'a, 'tcx: 'a, O> DataflowAnalysis<'a, 'tcx, O>
         DataflowResults(self.flow_state)
     }
 
+    pub fn flow_state(&self) -> &DataflowState<O> { &self.flow_state }
+
     pub fn mir(&self) -> &'a Mir<'tcx> { self.mir }
 }
 
@@ -278,10 +382,14 @@ impl<O: BitDenotation> DataflowResults<O> {
     pub fn sets(&self) -> &AllSets<O::Idx> {
         &self.0.sets
     }
+
+    pub fn operator(&self) -> &O {
+        &self.0.operator
+    }
 }
 
-// FIXME: This type shouldn't be public, but the graphviz::MirWithFlowState trait
-// references it in a method signature. Look into using `pub(crate)` to address this.
+/// State of a dataflow analysis; couples a collection of bit sets
+/// with operator used to initialize and merge bits during analysis.
 pub struct DataflowState<O: BitDenotation>
 {
     /// All the sets for the analysis. (Factored into its
diff --git a/src/librustc_mir/dataflow/move_paths/mod.rs b/src/librustc_mir/dataflow/move_paths/mod.rs
index c7f8b349134..d2d80649846 100644
--- a/src/librustc_mir/dataflow/move_paths/mod.rs
+++ b/src/librustc_mir/dataflow/move_paths/mod.rs
@@ -108,6 +108,12 @@ impl<'tcx> fmt::Debug for MovePath<'tcx> {
     }
 }
 
+impl<'tcx> fmt::Display for MovePath<'tcx> {
+    fn fmt(&self, w: &mut fmt::Formatter) -> fmt::Result {
+        write!(w, "{:?}", self.lvalue)
+    }
+}
+
 #[derive(Debug)]
 pub struct MoveData<'tcx> {
     pub move_paths: IndexVec<MovePathIndex, MovePath<'tcx>>,
diff --git a/src/librustc_mir/transform/borrow_check.rs b/src/librustc_mir/transform/borrow_check.rs
new file mode 100644
index 00000000000..2676fdbb9b4
--- /dev/null
+++ b/src/librustc_mir/transform/borrow_check.rs
@@ -0,0 +1,1270 @@
+// 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.
+
+//! This pass borrow-checks the MIR to (further) ensure it is not broken.
+
+use rustc::infer::{InferCtxt};
+use rustc::ty::{self, TyCtxt, ParamEnv};
+use rustc::mir::{AssertMessage, BasicBlock, BorrowKind, Location, Lvalue};
+use rustc::mir::{Mir, Mutability, Operand, Projection, ProjectionElem, Rvalue};
+use rustc::mir::{Statement, StatementKind, Terminator, TerminatorKind};
+use rustc::mir::transform::{MirPass, MirSource};
+
+use rustc_data_structures::indexed_set::{self, IdxSetBuf};
+use rustc_data_structures::indexed_vec::{Idx};
+
+use syntax::ast::{self};
+use syntax_pos::{DUMMY_SP, Span};
+
+use dataflow::{do_dataflow};
+use dataflow::{MoveDataParamEnv};
+use dataflow::{BitDenotation, BlockSets, DataflowResults, DataflowResultsConsumer};
+use dataflow::{MaybeInitializedLvals, MaybeUninitializedLvals};
+use dataflow::{MovingOutStatements};
+use dataflow::{Borrows, BorrowData, BorrowIndex};
+use dataflow::move_paths::{HasMoveData, MoveData, MovePathIndex, LookupResult};
+use util::borrowck_errors::{BorrowckErrors, Origin};
+
+use self::MutateMode::{JustWrite, WriteAndRead};
+use self::ConsumeKind::{Consume};
+
+pub struct BorrowckMir;
+
+impl MirPass for BorrowckMir {
+    fn run_pass<'a, 'tcx>(&self, tcx: TyCtxt<'a, 'tcx, 'tcx>, src: MirSource, mir: &mut Mir<'tcx>) {
+
+        // let err_count = tcx.sess.err_count();
+        // if err_count > 0 {
+        //     // compiling a broken program can obviously result in a
+        //     // broken MIR, so try not to report duplicate errors.
+        //     debug!("skipping BorrowckMir: {} due to {} previous errors",
+        //            tcx.node_path_str(src.item_id()), err_count);
+        //     return;
+        // }
+
+        debug!("run_pass BorrowckMir: {}", tcx.node_path_str(src.item_id()));
+
+        let def_id = tcx.hir.local_def_id(src.item_id());
+        if tcx.has_attr(def_id, "rustc_mir_borrowck") || tcx.sess.opts.debugging_opts.borrowck_mir {
+            borrowck_mir(tcx, src, mir);
+        }
+    }
+}
+
+fn borrowck_mir<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, src: MirSource, mir: &Mir<'tcx>)
+{
+    let id = src.item_id();
+    let def_id = tcx.hir.local_def_id(id);
+    debug!("borrowck_mir({}) UNIMPLEMENTED", tcx.item_path_str(def_id));
+
+    let attributes = tcx.get_attrs(def_id);
+    let param_env = tcx.param_env(def_id);
+    tcx.infer_ctxt().enter(|_infcx| {
+
+        let move_data = MoveData::gather_moves(mir, tcx, param_env);
+        let mdpe = MoveDataParamEnv { move_data: move_data, param_env: param_env };
+        let dead_unwinds = IdxSetBuf::new_empty(mir.basic_blocks().len());
+        let flow_borrows = do_dataflow(tcx, mir, id, &attributes, &dead_unwinds,
+                                       Borrows::new(tcx, mir),
+                                       |bd, i| bd.location(i));
+        let flow_inits = do_dataflow(tcx, mir, id, &attributes, &dead_unwinds,
+                                     MaybeInitializedLvals::new(tcx, mir, &mdpe),
+                                     |bd, i| &bd.move_data().move_paths[i]);
+        let flow_uninits = do_dataflow(tcx, mir, id, &attributes, &dead_unwinds,
+                                       MaybeUninitializedLvals::new(tcx, mir, &mdpe),
+                                       |bd, i| &bd.move_data().move_paths[i]);
+        let flow_move_outs = do_dataflow(tcx, mir, id, &attributes, &dead_unwinds,
+                                         MovingOutStatements::new(tcx, mir, &mdpe),
+                                         |bd, i| &bd.move_data().moves[i]);
+
+        let mut mbcx = MirBorrowckCtxt {
+            tcx: tcx,
+            mir: mir,
+            node_id: id,
+            move_data: &mdpe.move_data,
+            param_env: param_env,
+            fake_infer_ctxt: &_infcx,
+        };
+
+        let mut state = InProgress::new(flow_borrows,
+                                        flow_inits,
+                                        flow_uninits,
+                                        flow_move_outs);
+
+        mbcx.analyze_results(&mut state); // entry point for DataflowResultsConsumer
+    });
+
+    debug!("borrowck_mir done");
+}
+
+#[allow(dead_code)]
+pub struct MirBorrowckCtxt<'c, 'b, 'a: 'b+'c, 'gcx: 'a+'tcx, 'tcx: 'a> {
+    tcx: TyCtxt<'a, 'gcx, 'gcx>,
+    mir: &'b Mir<'gcx>,
+    node_id: ast::NodeId,
+    move_data: &'b MoveData<'gcx>,
+    param_env: ParamEnv<'tcx>,
+    fake_infer_ctxt: &'c InferCtxt<'c, 'gcx, 'tcx>,
+}
+
+// (forced to be `pub` due to its use as an associated type below.)
+pub struct InProgress<'b, 'tcx: 'b> {
+    borrows: FlowInProgress<Borrows<'b, 'tcx>>,
+    inits: FlowInProgress<MaybeInitializedLvals<'b, 'tcx>>,
+    uninits: FlowInProgress<MaybeUninitializedLvals<'b, 'tcx>>,
+    move_outs: FlowInProgress<MovingOutStatements<'b, 'tcx>>,
+}
+
+struct FlowInProgress<BD> where BD: BitDenotation {
+    base_results: DataflowResults<BD>,
+    curr_state: IdxSetBuf<BD::Idx>,
+    stmt_gen: IdxSetBuf<BD::Idx>,
+    stmt_kill: IdxSetBuf<BD::Idx>,
+}
+
+// Check that:
+// 1. assignments are always made to mutable locations (FIXME: does that still really go here?)
+// 2. loans made in overlapping scopes do not conflict
+// 3. assignments do not affect things loaned out as immutable
+// 4. moves do not affect things loaned out in any way
+impl<'c, 'b, 'a: 'b+'c, 'gcx, 'tcx: 'a> DataflowResultsConsumer<'b, 'gcx>
+    for MirBorrowckCtxt<'c, 'b, 'a, 'gcx, 'tcx>
+{
+    type FlowState = InProgress<'b, 'gcx>;
+
+    fn mir(&self) -> &'b Mir<'gcx> { self.mir }
+
+    fn reset_to_entry_of(&mut self, bb: BasicBlock, flow_state: &mut Self::FlowState) {
+        flow_state.each_flow(|b| b.reset_to_entry_of(bb),
+                             |i| i.reset_to_entry_of(bb),
+                             |u| u.reset_to_entry_of(bb));
+    }
+
+    fn reconstruct_statement_effect(&mut self,
+                                    location: Location,
+                                    flow_state: &mut Self::FlowState) {
+        flow_state.each_flow(|b| b.reconstruct_statement_effect(location),
+                             |i| i.reconstruct_statement_effect(location),
+                             |u| u.reconstruct_statement_effect(location));
+    }
+
+    fn apply_local_effect(&mut self,
+                          _location: Location,
+                          flow_state: &mut Self::FlowState) {
+        flow_state.each_flow(|b| b.apply_local_effect(),
+                             |i| i.apply_local_effect(),
+                             |u| u.apply_local_effect());
+    }
+
+    fn reconstruct_terminator_effect(&mut self,
+                                     location: Location,
+                                     flow_state: &mut Self::FlowState) {
+        flow_state.each_flow(|b| b.reconstruct_terminator_effect(location),
+                             |i| i.reconstruct_terminator_effect(location),
+                             |u| u.reconstruct_terminator_effect(location));
+    }
+
+    fn visit_block_entry(&mut self,
+                         bb: BasicBlock,
+                         flow_state: &Self::FlowState) {
+        let summary = flow_state.summary();
+        debug!("MirBorrowckCtxt::process_block({:?}): {}", bb, summary);
+    }
+
+    fn visit_statement_entry(&mut self,
+                             location: Location,
+                             stmt: &Statement<'gcx>,
+                             flow_state: &Self::FlowState) {
+        let summary = flow_state.summary();
+        debug!("MirBorrowckCtxt::process_statement({:?}, {:?}): {}", location, stmt, summary);
+        let span = stmt.source_info.span;
+        match stmt.kind {
+            StatementKind::Assign(ref lhs, ref rhs) => {
+                self.mutate_lvalue(ContextKind::AssignLhs.new(location),
+                                   (lhs, span), JustWrite, flow_state);
+                self.consume_rvalue(ContextKind::AssignRhs.new(location),
+                                    (rhs, span), location, flow_state);
+            }
+            StatementKind::SetDiscriminant { ref lvalue, variant_index: _ } => {
+                // FIXME: should this count as a mutate from borrowck POV?
+                self.mutate_lvalue(ContextKind::SetDiscrim.new(location),
+                                   (lvalue, span), JustWrite, flow_state);
+            }
+            StatementKind::InlineAsm { ref asm, ref outputs, ref inputs } => {
+                for (o, output) in asm.outputs.iter().zip(outputs) {
+                    if o.is_indirect {
+                        self.consume_lvalue(ContextKind::InlineAsm.new(location),
+                                            Consume,
+                                            (output, span),
+                                            flow_state);
+                    } else {
+                        self.mutate_lvalue(ContextKind::InlineAsm.new(location),
+                                           (output, span),
+                                           if o.is_rw { WriteAndRead } else { JustWrite },
+                                           flow_state);
+                    }
+                }
+                for input in inputs {
+                    self.consume_operand(ContextKind::InlineAsm.new(location),
+                                         Consume,
+                                         (input, span), flow_state);
+                }
+            }
+            StatementKind::EndRegion(ref _rgn) => {
+                // ignored when consuming results (update to
+                // flow_state already handled).
+            }
+            StatementKind::Nop |
+            StatementKind::Validate(..) |
+            StatementKind::StorageLive(..) |
+            StatementKind::StorageDead(..) => {
+                // ignored by borrowck
+            }
+        }
+    }
+
+    fn visit_terminator_entry(&mut self,
+                              location: Location,
+                              term: &Terminator<'gcx>,
+                              flow_state: &Self::FlowState) {
+        let loc = location;
+        let summary = flow_state.summary();
+        debug!("MirBorrowckCtxt::process_terminator({:?}, {:?}): {}", location, term, summary);
+        let span = term.source_info.span;
+        match term.kind {
+            TerminatorKind::SwitchInt { ref discr, switch_ty: _, values: _, targets: _ } => {
+                self.consume_operand(ContextKind::SwitchInt.new(loc),
+                                     Consume,
+                                     (discr, span), flow_state);
+            }
+            TerminatorKind::Drop { location: ref drop_lvalue, target: _, unwind: _ } => {
+                self.consume_lvalue(ContextKind::Drop.new(loc),
+                                    ConsumeKind::Drop,
+                                    (drop_lvalue, span), flow_state);
+            }
+            TerminatorKind::DropAndReplace { location: ref drop_lvalue,
+                                             value: ref new_value,
+                                             target: _,
+                                             unwind: _ } => {
+                self.mutate_lvalue(ContextKind::DropAndReplace.new(loc),
+                                   (drop_lvalue, span), JustWrite, flow_state);
+                self.consume_operand(ContextKind::DropAndReplace.new(loc),
+                                     ConsumeKind::Drop,
+                                     (new_value, span), flow_state);
+            }
+            TerminatorKind::Call { ref func, ref args, ref destination, cleanup: _ } => {
+                self.consume_operand(ContextKind::CallOperator.new(loc),
+                                     Consume,
+                                     (func, span), flow_state);
+                for arg in args {
+                    self.consume_operand(ContextKind::CallOperand.new(loc),
+                                         Consume,
+                                         (arg, span), flow_state);
+                }
+                if let Some((ref dest, _/*bb*/)) = *destination {
+                    self.mutate_lvalue(ContextKind::CallDest.new(loc),
+                                       (dest, span), JustWrite, flow_state);
+                }
+            }
+            TerminatorKind::Assert { ref cond, expected: _, ref msg, target: _, cleanup: _ } => {
+                self.consume_operand(ContextKind::Assert.new(loc),
+                                     Consume,
+                                     (cond, span), flow_state);
+                match *msg {
+                    AssertMessage::BoundsCheck { ref len, ref index } => {
+                        self.consume_operand(ContextKind::Assert.new(loc),
+                                             Consume,
+                                             (len, span), flow_state);
+                        self.consume_operand(ContextKind::Assert.new(loc),
+                                             Consume,
+                                             (index, span), flow_state);
+                    }
+                    AssertMessage::Math(_/*const_math_err*/) => {}
+                }
+            }
+
+            TerminatorKind::Goto { target: _ } |
+            TerminatorKind::Resume |
+            TerminatorKind::Return |
+            TerminatorKind::Unreachable => {
+                // no data used, thus irrelevant to borrowck
+            }
+        }
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+enum MutateMode { JustWrite, WriteAndRead }
+
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+enum ConsumeKind { Drop, Consume }
+
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+enum Control { Continue, Break }
+
+impl<'c, 'b, 'a: 'b+'c, 'gcx, 'tcx: 'a> MirBorrowckCtxt<'c, 'b, 'a, 'gcx, 'tcx> {
+    fn mutate_lvalue(&mut self,
+                     context: Context,
+                     lvalue_span: (&Lvalue<'gcx>, Span),
+                     mode: MutateMode,
+                     flow_state: &InProgress<'b, 'gcx>) {
+        // Write of P[i] or *P, or WriteAndRead of any P, requires P init'd.
+        match mode {
+            MutateMode::WriteAndRead => {
+                self.check_if_path_is_moved(context, lvalue_span, flow_state);
+            }
+            MutateMode::JustWrite => {
+                self.check_if_assigned_path_is_moved(context, lvalue_span, flow_state);
+            }
+        }
+
+        // check we don't invalidate any outstanding loans
+        self.each_borrow_involving_path(context,
+                                        lvalue_span.0, flow_state, |this, _index, _data| {
+                                            this.report_illegal_mutation_of_borrowed(context,
+                                                                                     lvalue_span);
+                                            Control::Break
+                                        });
+
+        // check for reassignments to immutable local variables
+        self.check_if_reassignment_to_immutable_state(context, lvalue_span, flow_state);
+    }
+
+    fn consume_rvalue(&mut self,
+                      context: Context,
+                      (rvalue, span): (&Rvalue<'gcx>, Span),
+                      location: Location,
+                      flow_state: &InProgress<'b, 'gcx>) {
+        match *rvalue {
+            Rvalue::Ref(_/*rgn*/, bk, ref lvalue) => {
+                self.borrow(context, location, bk, (lvalue, span), flow_state)
+            }
+
+            Rvalue::Use(ref operand) |
+            Rvalue::Repeat(ref operand, _) |
+            Rvalue::UnaryOp(_/*un_op*/, ref operand) |
+            Rvalue::Cast(_/*cast_kind*/, ref operand, _/*ty*/) => {
+                self.consume_operand(context, Consume, (operand, span), flow_state)
+            }
+
+            Rvalue::Len(ref lvalue) |
+            Rvalue::Discriminant(ref lvalue) => {
+                // len(_)/discriminant(_) merely read, not consume.
+                self.check_if_path_is_moved(context, (lvalue, span), flow_state);
+            }
+
+            Rvalue::BinaryOp(_bin_op, ref operand1, ref operand2) |
+            Rvalue::CheckedBinaryOp(_bin_op, ref operand1, ref operand2) => {
+                self.consume_operand(context, Consume, (operand1, span), flow_state);
+                self.consume_operand(context, Consume, (operand2, span), flow_state);
+            }
+
+            Rvalue::NullaryOp(_op, _ty) => {
+                // nullary ops take no dynamic input; no borrowck effect.
+                //
+                // FIXME: is above actually true? Do we want to track
+                // the fact that uninitialized data can be created via
+                // `NullOp::Box`?
+            }
+
+            Rvalue::Aggregate(ref _aggregate_kind, ref operands) => {
+                for operand in operands {
+                    self.consume_operand(context, Consume, (operand, span), flow_state);
+                }
+            }
+        }
+    }
+
+    fn consume_operand(&mut self,
+                       context: Context,
+                       consume_via_drop: ConsumeKind,
+                       (operand, span): (&Operand<'gcx>, Span),
+                       flow_state: &InProgress<'b, 'gcx>) {
+        match *operand {
+            Operand::Consume(ref lvalue) =>
+                self.consume_lvalue(context, consume_via_drop, (lvalue, span), flow_state),
+            Operand::Constant(_) => {}
+        }
+    }
+
+    fn consume_lvalue(&mut self,
+                      context: Context,
+                      consume_via_drop: ConsumeKind,
+                      lvalue_span: (&Lvalue<'gcx>, Span),
+                      flow_state: &InProgress<'b, 'gcx>) {
+        let lvalue = lvalue_span.0;
+        let ty = lvalue.ty(self.mir, self.tcx).to_ty(self.tcx);
+        let moves_by_default =
+            self.fake_infer_ctxt.type_moves_by_default(self.param_env, ty, DUMMY_SP);
+        if moves_by_default {
+            // move of lvalue: check if this is move of already borrowed path
+            self.each_borrow_involving_path(
+                context, lvalue_span.0, flow_state, |this, _idx, borrow| {
+                    if !borrow.compatible_with(BorrowKind::Mut) {
+                        this.report_move_out_while_borrowed(context, lvalue_span);
+                        Control::Break
+                    } else {
+                        Control::Continue
+                    }
+                });
+        } else {
+            // copy of lvalue: check if this is "copy of frozen path" (FIXME: see check_loans.rs)
+            self.each_borrow_involving_path(
+                context, lvalue_span.0, flow_state, |this, _idx, borrow| {
+                    if !borrow.compatible_with(BorrowKind::Shared) {
+                        this.report_use_while_mutably_borrowed(context, lvalue_span);
+                        Control::Break
+                    } else {
+                        Control::Continue
+                    }
+                });
+        }
+
+        // Finally, check if path was already moved.
+        match consume_via_drop {
+            ConsumeKind::Drop => {
+                // If path is merely being dropped, then we'll already
+                // check the drop flag to see if it is moved (thus we
+                // skip this check in that case).
+            }
+            ConsumeKind::Consume => {
+                self.check_if_path_is_moved(context, lvalue_span, flow_state);
+            }
+        }
+    }
+
+    fn borrow(&mut self,
+              context: Context,
+              location: Location,
+              bk: BorrowKind,
+              lvalue_span: (&Lvalue<'gcx>, Span),
+              flow_state: &InProgress<'b, 'gcx>) {
+        debug!("borrow location: {:?} lvalue: {:?} span: {:?}",
+               location, lvalue_span.0, lvalue_span.1);
+        self.check_if_path_is_moved(context, lvalue_span, flow_state);
+        self.check_for_conflicting_loans(context, location, bk, lvalue_span, flow_state);
+    }
+}
+
+impl<'c, 'b, 'a: 'b+'c, 'gcx, 'tcx: 'a> MirBorrowckCtxt<'c, 'b, 'a, 'gcx, 'tcx> {
+    fn check_if_reassignment_to_immutable_state(&mut self,
+                                                context: Context,
+                                                (lvalue, span): (&Lvalue<'gcx>, Span),
+                                                flow_state: &InProgress<'b, 'gcx>) {
+        let move_data = flow_state.inits.base_results.operator().move_data();
+
+        // determine if this path has a non-mut owner (and thus needs checking).
+        let mut l = lvalue;
+        loop {
+            match *l {
+                Lvalue::Projection(ref proj) => {
+                    l = &proj.base;
+                    continue;
+                }
+                Lvalue::Local(local) => {
+                    match self.mir.local_decls[local].mutability {
+                        Mutability::Not => break, // needs check
+                        Mutability::Mut => return,
+                    }
+                }
+                Lvalue::Static(_) => {
+                    // mutation of non-mut static is always illegal,
+                    // independent of dataflow.
+                    self.report_assignment_to_static(context, (lvalue, span));
+                    return;
+                }
+            }
+        }
+
+        if let Some(mpi) = self.move_path_for_lvalue(context, move_data, lvalue) {
+            if flow_state.inits.curr_state.contains(&mpi) {
+                // may already be assigned before reaching this statement;
+                // report error.
+                self.report_illegal_reassignment(context, (lvalue, span));
+            }
+        }
+    }
+
+    fn check_if_path_is_moved(&mut self,
+                              context: Context,
+                              lvalue_span: (&Lvalue<'gcx>, Span),
+                              flow_state: &InProgress<'b, 'gcx>) {
+        // FIXME: analogous code in check_loans first maps `lvalue` to
+        // its base_path ... but is that what we want here?
+        let lvalue = self.base_path(lvalue_span.0);
+
+        let maybe_uninits = &flow_state.uninits;
+        let move_data = maybe_uninits.base_results.operator().move_data();
+        if let Some(mpi) = self.move_path_for_lvalue(context, move_data, lvalue) {
+            if maybe_uninits.curr_state.contains(&mpi) {
+                // find and report move(s) that could cause this to be uninitialized
+
+                // FIXME: for each move in flow_state.move_outs ...
+                &flow_state.move_outs;
+
+                self.report_use_of_moved(context, lvalue_span);
+            } else {
+                // sanity check: initialized on *some* path, right?
+                assert!(flow_state.inits.curr_state.contains(&mpi));
+            }
+        }
+    }
+
+    fn move_path_for_lvalue(&mut self,
+                            _context: Context,
+                            move_data: &MoveData<'gcx>,
+                            lvalue: &Lvalue<'gcx>)
+                            -> Option<MovePathIndex>
+    {
+        // If returns None, then there is no move path corresponding
+        // to a direct owner of `lvalue` (which means there is nothing
+        // that borrowck tracks for its analysis).
+
+        match move_data.rev_lookup.find(lvalue) {
+            LookupResult::Parent(_) => None,
+            LookupResult::Exact(mpi) => Some(mpi),
+        }
+    }
+
+    fn check_if_assigned_path_is_moved(&mut self,
+                                       context: Context,
+                                       (lvalue, span): (&Lvalue<'gcx>, Span),
+                                       flow_state: &InProgress<'b, 'gcx>) {
+        // recur down lvalue; dispatch to check_if_path_is_moved when necessary
+        let mut lvalue = lvalue;
+        loop {
+            match *lvalue {
+                Lvalue::Local(_) | Lvalue::Static(_) => {
+                    // assigning to `x` does not require `x` be initialized.
+                    break;
+                }
+                Lvalue::Projection(ref proj) => {
+                    let Projection { ref base, ref elem } = **proj;
+                    match *elem {
+                        ProjectionElem::Deref |
+                        // assigning to *P requires `P` initialized.
+                        ProjectionElem::Index(_/*operand*/) |
+                        ProjectionElem::ConstantIndex { .. } |
+                        // assigning to P[i] requires `P` initialized.
+                        ProjectionElem::Downcast(_/*adt_def*/, _/*variant_idx*/) =>
+                        // assigning to (P->variant) is okay if assigning to `P` is okay
+                        //
+                        // FIXME: is this true even if P is a adt with a dtor?
+                        { }
+
+                        ProjectionElem::Subslice { .. } => {
+                            panic!("we dont allow assignments to subslices, context: {:?}",
+                                   context);
+                        }
+
+                        ProjectionElem::Field(..) => {
+                            // if type of `P` has a dtor, then
+                            // assigning to `P.f` requires `P` itself
+                            // be already initialized
+                            let tcx = self.tcx;
+                            match base.ty(self.mir, tcx).to_ty(tcx).sty {
+                                ty::TyAdt(def, _) if def.has_dtor(tcx) => {
+
+                                    // FIXME: analogous code in
+                                    // check_loans.rs first maps
+                                    // `base` to its base_path.
+
+                                    self.check_if_path_is_moved(context,
+                                                                (base, span), flow_state);
+
+                                    // (base initialized; no need to
+                                    // recur further)
+                                    break;
+                                }
+                                _ => {}
+                            }
+                        }
+                    }
+
+                    lvalue = base;
+                    continue;
+                }
+            }
+        }
+    }
+
+    fn check_for_conflicting_loans(&mut self,
+                                   context: Context,
+                                   _location: Location,
+                                   _bk: BorrowKind,
+                                   lvalue_span: (&Lvalue<'gcx>, Span),
+                                   flow_state: &InProgress<'b, 'gcx>) {
+        // NOTE FIXME: The analogous code in old borrowck
+        // check_loans.rs is careful to iterate over every *issued*
+        // loan, as opposed to just the in scope ones.
+        //
+        // (Or if you prefer, all the *other* iterations over loans
+        // only consider loans that are in scope of some given
+        // CodeExtent)
+        //
+        // The (currently skeletal) code here does not encode such a
+        // distinction, which means it is almost certainly over
+        // looking something.
+        //
+        // (It is probably going to reject code that should be
+        // accepted, I suspect, by treated issued-but-out-of-scope
+        // loans as issued-and-in-scope, and thus causing them to
+        // interfere with other loans.)
+        //
+        // However, I just want to get something running, especially
+        // since I am trying to move into new territory with NLL, so
+        // lets get this going first, and then address the issued vs
+        // in-scope distinction later.
+
+        let state = &flow_state.borrows;
+        let data = &state.base_results.operator().borrows();
+
+        debug!("check_for_conflicting_loans location: {:?}", _location);
+
+        // does any loan generated here conflict with a previously issued loan?
+        let mut loans_generated = 0;
+        for (g, gen) in state.elems_generated().map(|g| (g, &data[g])) {
+            loans_generated += 1;
+            for (i, issued) in state.elems_incoming().map(|i| (i, &data[i])) {
+                debug!("check_for_conflicting_loans gen: {:?} issued: {:?} conflicts: {}",
+                       (g, gen, self.base_path(&gen.lvalue),
+                        self.restrictions(&gen.lvalue).collect::<Vec<_>>()),
+                       (i, issued, self.base_path(&issued.lvalue),
+                        self.restrictions(&issued.lvalue).collect::<Vec<_>>()),
+                       self.conflicts_with(gen, issued));
+                if self.conflicts_with(gen, issued) {
+                    self.report_conflicting_borrow(context, lvalue_span, gen, issued);
+                }
+            }
+        }
+
+        // MIR statically ensures each statement gens *at most one*
+        // loan; mutual conflict (within a statement) can't arise.
+        //
+        // As safe-guard, assert that above property actually holds.
+        assert!(loans_generated <= 1);
+    } }
+
+impl<'c, 'b, 'a: 'b+'c, 'gcx, 'tcx: 'a> MirBorrowckCtxt<'c, 'b, 'a, 'gcx, 'tcx> {
+    fn each_borrow_involving_path<F>(&mut self,
+                                     _context: Context,
+                                     lvalue: &Lvalue<'gcx>,
+                                     flow_state: &InProgress<'b, 'gcx>,
+                                     mut op: F)
+        where F: FnMut(&mut Self, BorrowIndex, &BorrowData<'gcx>) -> Control
+    {
+        // FIXME: analogous code in check_loans first maps `lvalue` to
+        // its base_path.
+
+        let domain = flow_state.borrows.base_results.operator();
+        let data = domain.borrows();
+
+        // check for loan restricting path P being used. Accounts for
+        // borrows of P, P.a.b, etc.
+        for i in flow_state.borrows.elems_incoming() {
+            // FIXME: check_loans.rs filtered this to "in scope"
+            // loans; i.e. it took a scope S and checked that each
+            // restriction's kill_scope was a superscope of S.
+            let borrowed = &data[i];
+            for restricted in self.restrictions(&borrowed.lvalue) {
+                if restricted == lvalue {
+                    let ctrl = op(self, i, borrowed);
+                    if ctrl == Control::Break { return; }
+                }
+            }
+        }
+
+        // check for loans (not restrictions) on any base path.
+        // e.g. Rejects `{ let x = &mut a.b; let y = a.b.c; }`,
+        // since that moves out of borrowed path `a.b`.
+        //
+        // Limiting to loans (not restrictions) keeps this one
+        // working: `{ let x = &mut a.b; let y = a.c; }`
+        let mut cursor = lvalue;
+        loop {
+            // FIXME: check_loans.rs invoked `op` *before* cursor
+            // shift here.  Might just work (and even avoid redundant
+            // errors?) given code above?  But for now, I want to try
+            // doing what I think is more "natural" check.
+            for i in flow_state.borrows.elems_incoming() {
+                let borrowed = &data[i];
+                if borrowed.lvalue == *cursor {
+                    let ctrl = op(self, i, borrowed);
+                    if ctrl == Control::Break { return; }
+                }
+            }
+
+            match *cursor {
+                Lvalue::Local(_) | Lvalue::Static(_) => break,
+                Lvalue::Projection(ref proj) => cursor = &proj.base,
+            }
+        }
+    }
+}
+
+mod restrictions {
+    use super::MirBorrowckCtxt;
+
+    use rustc::hir;
+    use rustc::ty::{self, TyCtxt};
+    use rustc::mir::{Lvalue, Mir, Operand, ProjectionElem};
+
+    pub(super) struct Restrictions<'c, 'tcx: 'c> {
+        mir: &'c Mir<'tcx>,
+        tcx: TyCtxt<'c, 'tcx, 'tcx>,
+        lvalue_stack: Vec<&'c Lvalue<'tcx>>,
+    }
+
+    impl<'c, 'b, 'a: 'b+'c, 'gcx, 'tcx: 'a> MirBorrowckCtxt<'c, 'b, 'a, 'gcx, 'tcx> {
+        pub(super) fn restrictions<'d>(&self,
+                                       lvalue: &'d Lvalue<'gcx>)
+                                       -> Restrictions<'d, 'gcx> where 'b: 'd
+        {
+            let lvalue_stack = if self.has_restrictions(lvalue) { vec![lvalue] } else { vec![] };
+            Restrictions { lvalue_stack: lvalue_stack, mir: self.mir, tcx: self.tcx }
+        }
+
+        fn has_restrictions(&self, lvalue: &Lvalue<'gcx>) -> bool {
+            let mut cursor = lvalue;
+            loop {
+                let proj = match *cursor {
+                    Lvalue::Local(_) => return true,
+                    Lvalue::Static(_) => return false,
+                    Lvalue::Projection(ref proj) => proj,
+                };
+                match proj.elem {
+                    ProjectionElem::Index(..) |
+                    ProjectionElem::ConstantIndex { .. } |
+                    ProjectionElem::Downcast(..) |
+                    ProjectionElem::Subslice { .. } |
+                    ProjectionElem::Field(_/*field*/, _/*ty*/) => {
+                        cursor = &proj.base;
+                        continue;
+                    }
+                    ProjectionElem::Deref => {
+                        let ty = proj.base.ty(self.mir, self.tcx).to_ty(self.tcx);
+                        match ty.sty {
+                            ty::TyRawPtr(_) => {
+                                return false;
+                            }
+                            ty::TyRef(_, ty::TypeAndMut { ty: _, mutbl: hir::MutImmutable }) => {
+                                // FIXME: do I need to check validity of
+                                // region here though? (I think the original
+                                // check_loans code did, like readme says)
+                                return false;
+                            }
+                            ty::TyRef(_, ty::TypeAndMut { ty: _, mutbl: hir::MutMutable }) => {
+                                cursor = &proj.base;
+                                continue;
+                            }
+                            ty::TyAdt(..) if ty.is_box() => {
+                                cursor = &proj.base;
+                                continue;
+                            }
+                            _ => {
+                                panic!("unknown type fed to Projection Deref.");
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    impl<'c, 'tcx> Iterator for Restrictions<'c, 'tcx> {
+        type Item = &'c Lvalue<'tcx>;
+        fn next(&mut self) -> Option<Self::Item> {
+            'pop: loop {
+                let lvalue = match self.lvalue_stack.pop() {
+                    None => return None,
+                    Some(lvalue) => lvalue,
+                };
+
+                // `lvalue` may not be a restriction itself, but may
+                // hold one further down (e.g. we never return
+                // downcasts here, but may return a base of a
+                // downcast).
+                //
+                // Also, we need to enqueue any additional
+                // subrestrictions that it implies, since we can only
+                // return from from this call alone.
+
+                let mut cursor = lvalue;
+                'cursor: loop {
+                    let proj = match *cursor {
+                        Lvalue::Local(_) => return Some(cursor), // search yielded this leaf
+                        Lvalue::Static(_) => continue 'pop, // fruitless leaf; try next on stack
+                        Lvalue::Projection(ref proj) => proj,
+                    };
+
+                    match proj.elem {
+                        ProjectionElem::Field(_/*field*/, _/*ty*/) => {
+                            // FIXME: add union handling
+                            self.lvalue_stack.push(&proj.base);
+                            return Some(cursor);
+                        }
+                        ProjectionElem::Downcast(..) |
+                        ProjectionElem::Subslice { .. } |
+                        ProjectionElem::ConstantIndex { .. } |
+                        ProjectionElem::Index(Operand::Constant(..)) => {
+                            cursor = &proj.base;
+                            continue 'cursor;
+                        }
+                        ProjectionElem::Index(Operand::Consume(ref index)) => {
+                            self.lvalue_stack.push(index); // FIXME: did old borrowck do this?
+                            cursor = &proj.base;
+                            continue 'cursor;
+                        }
+                        ProjectionElem::Deref => {
+                            // (handled below)
+                        }
+                    }
+
+                    assert_eq!(proj.elem, ProjectionElem::Deref);
+
+                    let ty = proj.base.ty(self.mir, self.tcx).to_ty(self.tcx);
+                    match ty.sty {
+                        ty::TyRawPtr(_) => {
+                            // borrowck ignores raw ptrs; treat analogous to imm borrow
+                            continue 'pop;
+                        }
+                        // R-Deref-Imm-Borrowed
+                        ty::TyRef(_/*rgn*/, ty::TypeAndMut { ty: _, mutbl: hir::MutImmutable }) => {
+                            // immutably-borrowed referents do not
+                            // have recursively-implied restrictions
+                            // (because preventing actions on `*LV`
+                            // does nothing about aliases like `*LV1`)
+
+                            // FIXME: do I need to check validity of
+                            // `_r` here though? (I think the original
+                            // check_loans code did, like the readme
+                            // says)
+
+                            // (And do I *really* not have to
+                            // recursively process the `base` as a
+                            // further search here? Leaving this `if
+                            // false` here as a hint to look at this
+                            // again later.
+                            //
+                            // Ah, it might be because the
+                            // restrictions are distinct from the path
+                            // substructure. Note that there is a
+                            // separate loop over the path
+                            // substructure in fn
+                            // each_borrow_involving_path, for better
+                            // or for worse.
+
+                            if false {
+                                cursor = &proj.base;
+                                continue 'cursor;
+                            } else {
+                                continue 'pop;
+                            }
+                        }
+
+                        // R-Deref-Mut-Borrowed
+                        ty::TyRef(_/*rgn*/, ty::TypeAndMut { ty: _, mutbl: hir::MutMutable }) => {
+                            // mutably-borrowed referents are
+                            // themselves restricted.
+
+                            // FIXME: do I need to check validity of
+                            // `_r` here though? (I think the original
+                            // check_loans code did, like the readme
+                            // says)
+
+                            // schedule base for future iteration.
+                            self.lvalue_stack.push(&proj.base);
+                            return Some(cursor); // search yielded interior node
+                        }
+
+                        // R-Deref-Send-Pointer
+                        ty::TyAdt(..) if ty.is_box() => {
+                            // borrowing interior of a box implies that
+                            // its base can no longer be mutated (o/w box
+                            // storage would be freed)
+                            self.lvalue_stack.push(&proj.base);
+                            return Some(cursor); // search yielded interior node
+                        }
+
+                        _ => panic!("unknown type fed to Projection Deref."),
+                    }
+                }
+            }
+        }
+    }
+}
+
+impl<'c, 'b, 'a: 'b+'c, 'gcx, 'tcx: 'a> MirBorrowckCtxt<'c, 'b, 'a, 'gcx, 'tcx> {
+    fn report_use_of_moved(&mut self,
+                           _context: Context,
+                           (lvalue, span): (&Lvalue, Span)) {
+        let mut err = self.tcx.cannot_act_on_uninitialized_variable(
+            span, "use", &self.describe_lvalue(lvalue), Origin::Mir);
+        // FIXME: add span_label for use of uninitialized variable
+        err.emit();
+    }
+
+    fn report_move_out_while_borrowed(&mut self,
+                                      _context: Context,
+                                      (lvalue, span): (&Lvalue, Span)) {
+        let mut err = self.tcx.cannot_move_when_borrowed(
+            span, &self.describe_lvalue(lvalue), Origin::Mir);
+        // FIXME 1: add span_label for "borrow of `()` occurs here"
+        // FIXME 2: add span_label for "move out of `{}` occurs here"
+        err.emit();
+    }
+
+    fn report_use_while_mutably_borrowed(&mut self,
+                                         _context: Context,
+                                         (lvalue, span): (&Lvalue, Span)) {
+        let mut err = self.tcx.cannot_use_when_mutably_borrowed(
+            span, &self.describe_lvalue(lvalue), Origin::Mir);
+        // FIXME 1: add span_label for "borrow of `()` occurs here"
+        // FIXME 2: add span_label for "use of `{}` occurs here"
+        err.emit();
+    }
+
+    fn report_conflicting_borrow(&mut self,
+                                 _context: Context,
+                                 (lvalue, span): (&Lvalue, Span),
+                                 loan1: &BorrowData,
+                                 loan2: &BorrowData) {
+        // FIXME: obviously falsifiable. Generalize for non-eq lvalues later.
+        assert_eq!(loan1.lvalue, loan2.lvalue);
+
+        // FIXME: supply non-"" `opt_via` when appropriate
+        let mut err = match (loan1.kind, "immutable", "mutable",
+                             loan2.kind, "immutable", "mutable") {
+            (BorrowKind::Shared, lft, _, BorrowKind::Mut, _, rgt) |
+            (BorrowKind::Mut, _, lft, BorrowKind::Shared, rgt, _) |
+            (BorrowKind::Mut, _, lft, BorrowKind::Mut, _, rgt) =>
+                self.tcx.cannot_reborrow_already_borrowed(
+                    span, &self.describe_lvalue(lvalue),
+                    "", lft, "it", rgt, "", Origin::Mir),
+
+            _ =>  self.tcx.cannot_mutably_borrow_multiply(
+                span, &self.describe_lvalue(lvalue), "", Origin::Mir),
+            // FIXME: add span labels for first and second mutable borrows, as well as
+            // end point for first.
+        };
+        err.emit();
+    }
+
+    fn report_illegal_mutation_of_borrowed(&mut self, _: Context, (lvalue, span): (&Lvalue, Span)) {
+        let mut err = self.tcx.cannot_assign_to_borrowed(
+            span, &self.describe_lvalue(lvalue), Origin::Mir);
+        // FIXME: add span labels for borrow and assignment points
+        err.emit();
+    }
+
+    fn report_illegal_reassignment(&mut self, _context: Context, (lvalue, span): (&Lvalue, Span)) {
+        let mut err = self.tcx.cannot_reassign_immutable(
+            span, &self.describe_lvalue(lvalue), Origin::Mir);
+        // FIXME: add span labels for borrow and assignment points
+        err.emit();
+    }
+
+    fn report_assignment_to_static(&mut self, _context: Context, (lvalue, span): (&Lvalue, Span)) {
+        let mut err = self.tcx.cannot_assign_static(
+            span, &self.describe_lvalue(lvalue), Origin::Mir);
+        // FIXME: add span labels for borrow and assignment points
+        err.emit();
+    }
+}
+
+impl<'c, 'b, 'a: 'b+'c, 'gcx, 'tcx: 'a> MirBorrowckCtxt<'c, 'b, 'a, 'gcx, 'tcx> {
+    // End-user visible description of `lvalue`
+    fn describe_lvalue(&self, lvalue: &Lvalue) -> String {
+        let mut buf = String::new();
+        self.append_lvalue_to_string(lvalue, &mut buf);
+        buf
+    }
+
+    // Appends end-user visible description of `lvalue` to `buf`.
+    fn append_lvalue_to_string(&self, lvalue: &Lvalue, buf: &mut String) {
+        match *lvalue {
+            Lvalue::Local(local) => {
+                let local = &self.mir.local_decls[local];
+                match local.name {
+                    Some(name) => buf.push_str(&format!("{}", name)),
+                    None => buf.push_str("_"),
+                }
+            }
+            Lvalue::Static(ref static_) => {
+                buf.push_str(&format!("{}", &self.tcx.item_name(static_.def_id)));
+            }
+            Lvalue::Projection(ref proj) => {
+                let (prefix, suffix, index_operand) = match proj.elem {
+                    ProjectionElem::Deref =>
+                        ("(*", format!(")"), None),
+                    ProjectionElem::Downcast(..) =>
+                        ("",   format!(""), None), // (dont emit downcast info)
+                    ProjectionElem::Field(field, _ty) =>
+                        ("",   format!(".{}", field.index()), None),
+                    ProjectionElem::Index(ref index) =>
+                        ("",   format!(""), Some(index)),
+                    ProjectionElem::ConstantIndex { offset, min_length, from_end: true } =>
+                        ("",   format!("[{} of {}]", offset, min_length), None),
+                    ProjectionElem::ConstantIndex { offset, min_length, from_end: false } =>
+                        ("",   format!("[-{} of {}]", offset, min_length), None),
+                    ProjectionElem::Subslice { from, to: 0 } =>
+                        ("",   format!("[{}:]", from), None),
+                    ProjectionElem::Subslice { from: 0, to } =>
+                        ("",   format!("[:-{}]", to), None),
+                    ProjectionElem::Subslice { from, to } =>
+                        ("",   format!("[{}:-{}]", from, to), None),
+                };
+                buf.push_str(prefix);
+                self.append_lvalue_to_string(&proj.base, buf);
+                if let Some(index) = index_operand {
+                    buf.push_str("[");
+                    self.append_operand_to_string(index, buf);
+                    buf.push_str("]");
+                } else {
+                    buf.push_str(&suffix);
+                }
+
+            }
+        }
+    }
+
+    fn append_operand_to_string(&self, operand: &Operand, buf: &mut String) {
+        match *operand {
+            Operand::Consume(ref lvalue) => {
+                self.append_lvalue_to_string(lvalue, buf);
+            }
+            Operand::Constant(ref constant) => {
+                buf.push_str(&format!("{:?}", constant));
+            }
+        }
+    }
+}
+
+impl<'c, 'b, 'a: 'b+'c, 'gcx, 'tcx: 'a> MirBorrowckCtxt<'c, 'b, 'a, 'gcx, 'tcx> {
+    // FIXME: needs to be able to express errors analogous to check_loans.rs
+    fn conflicts_with(&self, loan1: &BorrowData<'gcx>, loan2: &BorrowData<'gcx>) -> bool {
+        if loan1.compatible_with(loan2.kind) { return false; }
+
+        let loan2_base_path = self.base_path(&loan2.lvalue);
+        for restricted in self.restrictions(&loan1.lvalue) {
+            if restricted != loan2_base_path { continue; }
+            return true;
+        }
+
+        let loan1_base_path = self.base_path(&loan1.lvalue);
+        for restricted in self.restrictions(&loan2.lvalue) {
+            if restricted != loan1_base_path { continue; }
+            return true;
+        }
+
+        return false;
+    }
+
+    // FIXME (#16118): function intended to allow the borrow checker
+    // to be less precise in its handling of Box while still allowing
+    // moves out of a Box. They should be removed when/if we stop
+    // treating Box specially (e.g. when/if DerefMove is added...)
+
+    fn base_path<'d>(&self, lvalue: &'d Lvalue<'gcx>) -> &'d Lvalue<'gcx> {
+        //! Returns the base of the leftmost (deepest) dereference of an
+        //! Box in `lvalue`. If there is no dereference of an Box
+        //! in `lvalue`, then it just returns `lvalue` itself.
+
+        let mut cursor = lvalue;
+        let mut deepest = lvalue;
+        loop {
+            let proj = match *cursor {
+                Lvalue::Local(..) | Lvalue::Static(..) => return deepest,
+                Lvalue::Projection(ref proj) => proj,
+            };
+            if proj.elem == ProjectionElem::Deref &&
+                lvalue.ty(self.mir, self.tcx).to_ty(self.tcx).is_box()
+            {
+                deepest = &proj.base;
+            }
+            cursor = &proj.base;
+        }
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+struct Context {
+    kind: ContextKind,
+    loc: Location,
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+enum ContextKind {
+    AssignLhs,
+    AssignRhs,
+    SetDiscrim,
+    InlineAsm,
+    SwitchInt,
+    Drop,
+    DropAndReplace,
+    CallOperator,
+    CallOperand,
+    CallDest,
+    Assert,
+}
+
+impl ContextKind {
+    fn new(self, loc: Location) -> Context { Context { kind: self, loc: loc } }
+}
+
+impl<'b, 'tcx: 'b> InProgress<'b, 'tcx> {
+    pub(super) fn new(borrows: DataflowResults<Borrows<'b, 'tcx>>,
+                      inits: DataflowResults<MaybeInitializedLvals<'b, 'tcx>>,
+                      uninits: DataflowResults<MaybeUninitializedLvals<'b, 'tcx>>,
+                      move_outs: DataflowResults<MovingOutStatements<'b, 'tcx>>) -> Self {
+        InProgress {
+            borrows: FlowInProgress::new(borrows),
+            inits: FlowInProgress::new(inits),
+            uninits: FlowInProgress::new(uninits),
+            move_outs: FlowInProgress::new(move_outs),
+        }
+    }
+
+    fn each_flow<XB, XI, XU>(&mut self,
+                             mut xform_borrows: XB,
+                             mut xform_inits: XI,
+                             mut xform_uninits: XU) where
+        XB: FnMut(&mut FlowInProgress<Borrows<'b, 'tcx>>),
+        XI: FnMut(&mut FlowInProgress<MaybeInitializedLvals<'b, 'tcx>>),
+        XU: FnMut(&mut FlowInProgress<MaybeUninitializedLvals<'b, 'tcx>>),
+    {
+        xform_borrows(&mut self.borrows);
+        xform_inits(&mut self.inits);
+        xform_uninits(&mut self.uninits);
+    }
+
+    fn summary(&self) -> String {
+        let mut s = String::new();
+
+        s.push_str("borrows in effect: [");
+        let mut saw_one = false;
+        self.borrows.each_state_bit(|borrow| {
+            if saw_one { s.push_str(", "); };
+            saw_one = true;
+            let borrow_data = &self.borrows.base_results.operator().borrows()[borrow];
+            s.push_str(&format!("{}", borrow_data));
+        });
+        s.push_str("] ");
+
+        s.push_str("borrows generated: [");
+        let mut saw_one = false;
+        self.borrows.each_gen_bit(|borrow| {
+            if saw_one { s.push_str(", "); };
+            saw_one = true;
+            let borrow_data = &self.borrows.base_results.operator().borrows()[borrow];
+            s.push_str(&format!("{}", borrow_data));
+        });
+        s.push_str("] ");
+
+        s.push_str("inits: [");
+        let mut saw_one = false;
+        self.inits.each_state_bit(|mpi_init| {
+            if saw_one { s.push_str(", "); };
+            saw_one = true;
+            let move_path =
+                &self.inits.base_results.operator().move_data().move_paths[mpi_init];
+            s.push_str(&format!("{}", move_path));
+        });
+        s.push_str("] ");
+
+        s.push_str("uninits: [");
+        let mut saw_one = false;
+        self.uninits.each_state_bit(|mpi_uninit| {
+            if saw_one { s.push_str(", "); };
+            saw_one = true;
+            let move_path =
+                &self.uninits.base_results.operator().move_data().move_paths[mpi_uninit];
+            s.push_str(&format!("{}", move_path));
+        });
+        s.push_str("]");
+
+        return s;
+    }
+}
+
+impl<BD> FlowInProgress<BD> where BD: BitDenotation {
+    fn each_state_bit<F>(&self, f: F) where F: FnMut(BD::Idx) {
+        self.curr_state.each_bit(self.base_results.operator().bits_per_block(), f)
+    }
+
+    fn each_gen_bit<F>(&self, f: F) where F: FnMut(BD::Idx) {
+        self.stmt_gen.each_bit(self.base_results.operator().bits_per_block(), f)
+    }
+
+    fn new(results: DataflowResults<BD>) -> Self {
+        let bits_per_block = results.sets().bits_per_block();
+        let curr_state = IdxSetBuf::new_empty(bits_per_block);
+        let stmt_gen = IdxSetBuf::new_empty(bits_per_block);
+        let stmt_kill = IdxSetBuf::new_empty(bits_per_block);
+        FlowInProgress {
+            base_results: results,
+            curr_state: curr_state,
+            stmt_gen: stmt_gen,
+            stmt_kill: stmt_kill,
+        }
+    }
+
+    fn reset_to_entry_of(&mut self, bb: BasicBlock) {
+        (*self.curr_state).clone_from(self.base_results.sets().on_entry_set_for(bb.index()));
+    }
+
+    fn reconstruct_statement_effect(&mut self, loc: Location) {
+        self.stmt_gen.reset_to_empty();
+        self.stmt_kill.reset_to_empty();
+        let mut ignored = IdxSetBuf::new_empty(0);
+        let mut sets = BlockSets {
+            on_entry: &mut ignored, gen_set: &mut self.stmt_gen, kill_set: &mut self.stmt_kill,
+        };
+        self.base_results.operator().statement_effect(&mut sets, loc);
+    }
+
+    fn reconstruct_terminator_effect(&mut self, loc: Location) {
+        self.stmt_gen.reset_to_empty();
+        self.stmt_kill.reset_to_empty();
+        let mut ignored = IdxSetBuf::new_empty(0);
+        let mut sets = BlockSets {
+            on_entry: &mut ignored, gen_set: &mut self.stmt_gen, kill_set: &mut self.stmt_kill,
+        };
+        self.base_results.operator().terminator_effect(&mut sets, loc);
+    }
+
+    fn apply_local_effect(&mut self) {
+        self.curr_state.union(&self.stmt_gen);
+        self.curr_state.subtract(&self.stmt_kill);
+    }
+
+    fn elems_generated(&self) -> indexed_set::Elems<BD::Idx> {
+        let univ = self.base_results.sets().bits_per_block();
+        self.stmt_gen.elems(univ)
+    }
+
+    fn elems_incoming(&self) -> indexed_set::Elems<BD::Idx> {
+        let univ = self.base_results.sets().bits_per_block();
+        self.curr_state.elems(univ)
+    }
+}
+
+impl<'tcx> BorrowData<'tcx> {
+    fn compatible_with(&self, bk: BorrowKind) -> bool {
+        match (self.kind, bk) {
+            (BorrowKind::Shared, BorrowKind::Shared) => true,
+
+            (BorrowKind::Mut, _) |
+            (BorrowKind::Unique, _) |
+            (_, BorrowKind::Mut) |
+            (_, BorrowKind::Unique) => false,
+        }
+    }
+}
diff --git a/src/librustc_mir/transform/mod.rs b/src/librustc_mir/transform/mod.rs
index e3e4265a7db..38070a4b485 100644
--- a/src/librustc_mir/transform/mod.rs
+++ b/src/librustc_mir/transform/mod.rs
@@ -31,6 +31,7 @@ pub mod simplify;
 pub mod erase_regions;
 pub mod no_landing_pads;
 pub mod type_check;
+pub mod borrow_check;
 pub mod rustc_peek;
 pub mod elaborate_drops;
 pub mod add_call_guards;