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.

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,

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) {

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"));

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

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>>,

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;