Auto merge of #46537 - pnkfelix:two-phase-borrows, r=arielb1

[MIR-borrowck] Two phase borrows

This adds limited support for two-phase borrows as described in
  http://smallcultfollowing.com/babysteps/blog/2017/03/01/nested-method-calls-via-two-phase-borrowing/

The support is off by default; you opt into it via the flag `-Z two-phase-borrows`

I have written "*limited* support" above because there are simple variants of the simple `v.push(v.len())` example that one would think should work but currently do not, such as the one documented in the test compile-fail/borrowck/two-phase-reservation-sharing-interference-2.rs

(To be clear, that test is not describing something that is unsound. It is just providing an explicit example of a limitation in the implementation given in this PR. I have ideas on how to fix, but I want to land the work that is in this PR first, so that I can stop repeatedly rebasing this branch.)

src/librustc/mir/mod.rs

@@ -1137,7 +1137,7 @@ impl<'tcx> Debug for Statement<'tcx> {
 
 /// A path to a value; something that can be evaluated without
 /// changing or disturbing program state.
-#[derive(Clone, PartialEq, RustcEncodable, RustcDecodable)]
+#[derive(Clone, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable)]
 pub enum Place<'tcx> {
     /// local variable
     Local(Local),
@@ -1151,7 +1151,7 @@ pub enum Place<'tcx> {
 
 /// The def-id of a static, along with its normalized type (which is
 /// stored to avoid requiring normalization when reading MIR).
-#[derive(Clone, PartialEq, RustcEncodable, RustcDecodable)]
+#[derive(Clone, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable)]
 pub struct Static<'tcx> {
     pub def_id: DefId,
     pub ty: Ty<'tcx>,

src/librustc/session/config.rs

@@ -1028,6 +1028,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "emit EndRegion as part of MIR; enable transforms that solely process EndRegion"),
     borrowck: Option<String> = (None, parse_opt_string, [UNTRACKED],
         "select which borrowck is used (`ast`, `mir`, or `compare`)"),
+    two_phase_borrows: bool = (false, parse_bool, [UNTRACKED],
+        "use two-phase reserved/active distinction for `&mut` borrows in MIR borrowck"),
     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

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use std::borrow::{Borrow, BorrowMut, ToOwned};
 use std::fmt;
 use std::iter;
 use std::marker::PhantomData;
@@ -73,6 +74,25 @@ pub struct IdxSet<T: Idx> {
     bits: [Word],
 }
 
+impl<T: Idx> Borrow<IdxSet<T>> for IdxSetBuf<T> {
+    fn borrow(&self) -> &IdxSet<T> {
+        &*self
+    }
+}
+
+impl<T: Idx> BorrowMut<IdxSet<T>> for IdxSetBuf<T> {
+    fn borrow_mut(&mut self) -> &mut IdxSet<T> {
+        &mut *self
+    }
+}
+
+impl<T: Idx> ToOwned for IdxSet<T> {
+    type Owned = IdxSetBuf<T>;
+    fn to_owned(&self) -> Self::Owned {
+        IdxSet::to_owned(self)
+    }
+}
+
 impl<T: Idx> fmt::Debug for IdxSetBuf<T> {
     fn fmt(&self, w: &mut fmt::Formatter) -> fmt::Result {
         w.debug_list()

src/librustc_driver/pretty.rs

@@ -423,24 +423,28 @@ impl<'hir> pprust_hir::PpAnn for IdentifiedAnnotation<'hir> {
             pprust_hir::NodeName(_) => Ok(()),
             pprust_hir::NodeItem(item) => {
                 s.s.space()?;
-                s.synth_comment(item.id.to_string())
+                s.synth_comment(format!("node_id: {} hir local_id: {}",
+                                        item.id, item.hir_id.local_id.0))
             }
             pprust_hir::NodeSubItem(id) => {
                 s.s.space()?;
                 s.synth_comment(id.to_string())
             }
             pprust_hir::NodeBlock(blk) => {
                 s.s.space()?;
-                s.synth_comment(format!("block {}", blk.id))
+                s.synth_comment(format!("block node_id: {} hir local_id: {}",
+                                        blk.id, blk.hir_id.local_id.0))
             }
             pprust_hir::NodeExpr(expr) => {
                 s.s.space()?;
-                s.synth_comment(expr.id.to_string())?;
+                s.synth_comment(format!("node_id: {} hir local_id: {}",
+                                        expr.id, expr.hir_id.local_id.0))?;
                 s.pclose()
             }
             pprust_hir::NodePat(pat) => {
                 s.s.space()?;
-                s.synth_comment(format!("pat {}", pat.id))
+                s.synth_comment(format!("pat node_id: {} hir local_id: {}",
+                                        pat.id, pat.hir_id.local_id.0))
             }
         }
     }

src/librustc_mir/borrow_check/error_reporting.rs

@@ -11,15 +11,15 @@
 use syntax_pos::Span;
 use rustc::middle::region::ScopeTree;
 use rustc::mir::{BorrowKind, Field, Local, Location, Operand};
-use rustc::mir::{Place, ProjectionElem, Rvalue, StatementKind};
+use rustc::mir::{Place, ProjectionElem, Rvalue, Statement, StatementKind};
 use rustc::ty::{self, RegionKind};
 use rustc_data_structures::indexed_vec::Idx;
 
 use std::rc::Rc;
 
 use super::{MirBorrowckCtxt, Context};
 use super::{InitializationRequiringAction, PrefixSet};
-use dataflow::{BorrowData, Borrows, FlowAtLocation, MovingOutStatements};
+use dataflow::{ActiveBorrows, BorrowData, FlowAtLocation, MovingOutStatements};
 use dataflow::move_paths::MovePathIndex;
 use util::borrowck_errors::{BorrowckErrors, Origin};
 
@@ -96,7 +96,7 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
             Some(name) => format!("`{}`", name),
             None => "value".to_owned(),
         };
-        let borrow_msg = match self.describe_place(&borrow.place) {
+        let borrow_msg = match self.describe_place(&borrow.borrowed_place) {
             Some(name) => format!("`{}`", name),
             None => "value".to_owned(),
         };
@@ -124,7 +124,7 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
             span,
             &self.describe_place(place).unwrap_or("_".to_owned()),
             self.retrieve_borrow_span(borrow),
-            &self.describe_place(&borrow.place).unwrap_or("_".to_owned()),
+            &self.describe_place(&borrow.borrowed_place).unwrap_or("_".to_owned()),
             Origin::Mir,
         );
 
@@ -143,12 +143,9 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
         use rustc::hir::ExprClosure;
         use rustc::mir::AggregateKind;
 
-        let local = if let StatementKind::Assign(Place::Local(local), _) =
-            self.mir[location.block].statements[location.statement_index].kind
-        {
-            local
-        } else {
-            return None;
+        let local = match self.mir[location.block].statements.get(location.statement_index) {
+            Some(&Statement { kind: StatementKind::Assign(Place::Local(local), _), .. }) => local,
+            _ => return None,
         };
 
         for stmt in &self.mir[location.block].statements[location.statement_index + 1..] {
@@ -324,11 +321,11 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
         _: Context,
         borrow: &BorrowData<'tcx>,
         drop_span: Span,
-        borrows: &Borrows<'cx, 'gcx, 'tcx>
+        borrows: &ActiveBorrows<'cx, 'gcx, 'tcx>
     ) {
         let end_span = borrows.opt_region_end_span(&borrow.region);
-        let scope_tree = borrows.scope_tree();
-        let root_place = self.prefixes(&borrow.place, PrefixSet::All).last().unwrap();
+        let scope_tree = borrows.0.scope_tree();
+        let root_place = self.prefixes(&borrow.borrowed_place, PrefixSet::All).last().unwrap();
 
         match root_place {
             &Place::Local(local) => {
@@ -357,7 +354,7 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
             _ => drop_span,
         };
 
-        match (borrow.region, &self.describe_place(&borrow.place)) {
+        match (borrow.region, &self.describe_place(&borrow.borrowed_place)) {
             (RegionKind::ReScope(_), Some(name)) => {
                 self.report_scoped_local_value_does_not_live_long_enough(
                     name, &scope_tree, &borrow, drop_span, borrow_span, proper_span, end_span);

src/librustc_mir/borrow_check/flows.rs

@@ -17,13 +17,13 @@ use rustc::mir::{BasicBlock, Location};
 
 use dataflow::{MaybeInitializedLvals, MaybeUninitializedLvals};
 use dataflow::{EverInitializedLvals, MovingOutStatements};
-use dataflow::{Borrows, FlowAtLocation, FlowsAtLocation};
+use dataflow::{ActiveBorrows, FlowAtLocation, FlowsAtLocation};
 use dataflow::move_paths::HasMoveData;
 use std::fmt;
 
 // (forced to be `pub` due to its use as an associated type below.)
-pub struct Flows<'b, 'gcx: 'tcx, 'tcx: 'b> {
-    pub borrows: FlowAtLocation<Borrows<'b, 'gcx, 'tcx>>,
+pub(crate) struct Flows<'b, 'gcx: 'tcx, 'tcx: 'b> {
+    pub borrows: FlowAtLocation<ActiveBorrows<'b, 'gcx, 'tcx>>,
     pub inits: FlowAtLocation<MaybeInitializedLvals<'b, 'gcx, 'tcx>>,
     pub uninits: FlowAtLocation<MaybeUninitializedLvals<'b, 'gcx, 'tcx>>,
     pub move_outs: FlowAtLocation<MovingOutStatements<'b, 'gcx, 'tcx>>,
@@ -32,7 +32,7 @@ pub struct Flows<'b, 'gcx: 'tcx, 'tcx: 'b> {
 
 impl<'b, 'gcx, 'tcx> Flows<'b, 'gcx, 'tcx> {
     pub fn new(
-        borrows: FlowAtLocation<Borrows<'b, 'gcx, 'tcx>>,
+        borrows: FlowAtLocation<ActiveBorrows<'b, 'gcx, 'tcx>>,
         inits: FlowAtLocation<MaybeInitializedLvals<'b, 'gcx, 'tcx>>,
         uninits: FlowAtLocation<MaybeUninitializedLvals<'b, 'gcx, 'tcx>>,
         move_outs: FlowAtLocation<MovingOutStatements<'b, 'gcx, 'tcx>>,
@@ -87,7 +87,7 @@ impl<'b, 'gcx, 'tcx> fmt::Display for Flows<'b, 'gcx, 'tcx> {
                 s.push_str(", ");
             };
             saw_one = true;
-            let borrow_data = &self.borrows.operator().borrows()[borrow];
+            let borrow_data = &self.borrows.operator().borrows()[borrow.borrow_index()];
             s.push_str(&format!("{}", borrow_data));
         });
         s.push_str("] ");
@@ -99,7 +99,7 @@ impl<'b, 'gcx, 'tcx> fmt::Display for Flows<'b, 'gcx, 'tcx> {
                 s.push_str(", ");
             };
             saw_one = true;
-            let borrow_data = &self.borrows.operator().borrows()[borrow];
+            let borrow_data = &self.borrows.operator().borrows()[borrow.borrow_index()];
             s.push_str(&format!("{}", borrow_data));
         });
         s.push_str("] ");