support for basic (non-overlapping) 2-phase borrows

Author: RalfJung

src/lib.rs

@@ -525,6 +525,7 @@ impl<'a, 'mir, 'tcx> Machine<'a, 'mir, 'tcx> for Evaluator<'tcx> {
     fn retag(
         ecx: &mut EvalContext<'a, 'mir, 'tcx, Self>,
         fn_entry: bool,
+        two_phase: bool,
         place: PlaceTy<'tcx, Borrow>,
     ) -> EvalResult<'tcx> {
         if !ecx.tcx.sess.opts.debugging_opts.mir_emit_retag || !Self::enforce_validity(ecx) {
@@ -535,7 +536,7 @@ impl<'a, 'mir, 'tcx> Machine<'a, 'mir, 'tcx> for Evaluator<'tcx> {
             // uninitialized data.
              Ok(())
         } else {
-            ecx.retag(fn_entry, place)
+            ecx.retag(fn_entry, two_phase, place)
         }
     }
 

src/stacked_borrows.rs

@@ -8,7 +8,7 @@ use rustc::hir::{Mutability, MutMutable, MutImmutable};
 use crate::{
     EvalResult, EvalErrorKind, MiriEvalContext, HelpersEvalContextExt, Evaluator, MutValueVisitor,
     MemoryKind, MiriMemoryKind, RangeMap, AllocId, Allocation, AllocationExtra,
-    Pointer, MemPlace, Scalar, Immediate, ImmTy, PlaceTy, MPlaceTy,
+    Pointer, Immediate, ImmTy, PlaceTy, MPlaceTy,
 };
 
 pub type Timestamp = u64;
@@ -534,19 +534,21 @@ pub trait EvalContextExt<'tcx> {
         size: Size,
         fn_barrier: bool,
         new_bor: Borrow
-    ) -> EvalResult<'tcx, Pointer<Borrow>>;
+    ) -> EvalResult<'tcx>;
 
     /// Retag an indidual pointer, returning the retagged version.
     fn retag_reference(
         &mut self,
         ptr: ImmTy<'tcx, Borrow>,
         mutbl: Mutability,
         fn_barrier: bool,
+        two_phase: bool,
     ) -> EvalResult<'tcx, Immediate<Borrow>>;
 
     fn retag(
         &mut self,
         fn_entry: bool,
+        two_phase: bool,
         place: PlaceTy<'tcx, Borrow>
     ) -> EvalResult<'tcx>;
 
@@ -644,9 +646,8 @@ impl<'a, 'mir, 'tcx> EvalContextExt<'tcx> for MiriEvalContext<'a, 'mir, 'tcx> {
         size: Size,
         fn_barrier: bool,
         new_bor: Borrow
-    ) -> EvalResult<'tcx, Pointer<Borrow>> {
+    ) -> EvalResult<'tcx> {
         let ptr = place.ptr.to_ptr()?;
-        let new_ptr = Pointer::new_with_tag(ptr.alloc_id, ptr.offset, new_bor);
         let barrier = if fn_barrier { Some(self.frame().extra) } else { None };
         trace!("reborrow: Creating new reference for {:?} (pointee {}): {:?}",
             ptr, place.layout.ty, new_bor);
@@ -666,14 +667,15 @@ impl<'a, 'mir, 'tcx> EvalContextExt<'tcx> for MiriEvalContext<'a, 'mir, 'tcx> {
             let kind = if new_bor.is_unique() { RefKind::Unique } else { RefKind::Raw };
             alloc.extra.reborrow(ptr, size, barrier, new_bor, kind)?;
         }
-        Ok(new_ptr)
+        Ok(())
     }
 
     fn retag_reference(
         &mut self,
         val: ImmTy<'tcx, Borrow>,
         mutbl: Mutability,
         fn_barrier: bool,
+        two_phase: bool,
     ) -> EvalResult<'tcx, Immediate<Borrow>> {
         // We want a place for where the ptr *points to*, so we get one.
         let place = self.ref_to_mplace(val)?;
@@ -693,16 +695,24 @@ impl<'a, 'mir, 'tcx> EvalContextExt<'tcx> for MiriEvalContext<'a, 'mir, 'tcx> {
         };
 
         // Reborrow.
-        let new_ptr = self.reborrow(place, size, fn_barrier, new_bor)?;
+        self.reborrow(place, size, fn_barrier, new_bor)?;
+        let new_place = place.with_tag(new_bor);
+        // Handle two-phase borrows.
+        if two_phase {
+            // We immediately share it, to allow read accesses
+            let two_phase_time = self.machine.stacked_borrows.increment_clock();
+            let two_phase_bor = Borrow::Shr(Some(two_phase_time));
+            self.reborrow(new_place, size, /*fn_barrier*/false, two_phase_bor)?;
+        }
 
-        // Return new ptr
-        let new_place = MemPlace { ptr: Scalar::Ptr(new_ptr), ..*place };
+        // Return new ptr.
         Ok(new_place.to_ref())
     }
 
     fn retag(
         &mut self,
         fn_entry: bool,
+        two_phase: bool,
         place: PlaceTy<'tcx, Borrow>
     ) -> EvalResult<'tcx> {
         // Determine mutability and whether to add a barrier.
@@ -725,19 +735,20 @@ impl<'a, 'mir, 'tcx> EvalContextExt<'tcx> for MiriEvalContext<'a, 'mir, 'tcx> {
         if let Some((mutbl, barrier)) = qualify(place.layout.ty, fn_entry) {
             // fast path
             let val = self.read_immediate(self.place_to_op(place)?)?;
-            let val = self.retag_reference(val, mutbl, barrier)?;
+            let val = self.retag_reference(val, mutbl, barrier, two_phase)?;
             self.write_immediate(val, place)?;
             return Ok(());
         }
         let place = self.force_allocation(place)?;
 
-        let mut visitor = RetagVisitor { ecx: self, fn_entry };
+        let mut visitor = RetagVisitor { ecx: self, fn_entry, two_phase };
         visitor.visit_value(place)?;
 
         // The actual visitor
         struct RetagVisitor<'ecx, 'a, 'mir, 'tcx> {
             ecx: &'ecx mut MiriEvalContext<'a, 'mir, 'tcx>,
             fn_entry: bool,
+            two_phase: bool,
         }
         impl<'ecx, 'a, 'mir, 'tcx>
             MutValueVisitor<'a, 'mir, 'tcx, Evaluator<'tcx>>
@@ -758,7 +769,7 @@ impl<'a, 'mir, 'tcx> EvalContextExt<'tcx> for MiriEvalContext<'a, 'mir, 'tcx> {
                 // making it useless.
                 if let Some((mutbl, barrier)) = qualify(place.layout.ty, self.fn_entry) {
                     let val = self.ecx.read_immediate(place.into())?;
-                    let val = self.ecx.retag_reference(val, mutbl, barrier)?;
+                    let val = self.ecx.retag_reference(val, mutbl, barrier, self.two_phase)?;
                     self.ecx.write_immediate(val, place.into())?;
                 }
                 Ok(())

tests/run-pass/2phase.rs

@@ -0,0 +1,69 @@
+#![feature(nll)]
+
+trait S: Sized {
+    fn tpb(&mut self, _s: Self) {}
+}
+
+impl S for i32 {}
+
+fn two_phase1() {
+    let mut x = 3;
+    x.tpb(x);
+}
+
+fn two_phase2() {
+    let mut v = vec![];
+    v.push(v.len());
+}
+
+/*
+fn two_phase_overlapping1() {
+    let mut x = vec![];
+    let p = &x;
+    x.push(p.len());
+}
+
+fn two_phase_overlapping2() {
+    use std::ops::AddAssign;
+    let mut x = 1;
+    let l = &x;
+    x.add_assign(x + *l);
+}
+*/
+
+fn match_two_phase() {
+    let mut x = 3;
+    match x {
+        ref mut y if { let _val = x; let _val = *y; true } => {},
+        _ => (),
+    }
+}
+
+fn with_interior_mutability() {
+    use std::cell::Cell;
+
+    trait Thing: Sized {
+        fn do_the_thing(&mut self, _s: i32) {}
+    }
+
+    impl<T> Thing for Cell<T> {}
+
+    let mut x = Cell::new(1);
+    let l = &x;
+    x
+        .do_the_thing({
+            x.set(3);
+            l.set(4);
+            x.get() + l.get()
+        })
+    ;
+}
+
+fn main() {
+    two_phase1();
+    two_phase2();
+    //two_phase_overlapping1();
+    //two_phase_overlapping2();
+    match_two_phase();
+    with_interior_mutability();
+}