rustc_mir: generate an extra temporary during borrowed rvalue promotion.

Author: eddyb

src/librustc_mir/transform/promote_consts.rs

@@ -25,14 +25,12 @@
 use rustc::mir::*;
 use rustc::mir::visit::{PlaceContext, MutVisitor, Visitor};
 use rustc::mir::traversal::ReversePostorder;
-use rustc::ty::TyCtxt;
+use rustc::ty::{self, TyCtxt};
 use syntax_pos::Span;
 
 use rustc_data_structures::indexed_vec::{IndexVec, Idx};
 
-use std::iter;
-use std::mem;
-use std::usize;
+use std::{cmp, iter, mem, usize};
 
 /// State of a temporary during collection and promotion.
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
@@ -150,9 +148,11 @@ pub fn collect_temps(mir: &Mir, rpo: &mut ReversePostorder) -> IndexVec<Local, T
 }
 
 struct Promoter<'a, 'tcx: 'a> {
+    tcx: TyCtxt<'a, 'tcx, 'tcx>,
     source: &'a mut Mir<'tcx>,
     promoted: Mir<'tcx>,
     temps: &'a mut IndexVec<Local, TempState>,
+    extra_statements: &'a mut Vec<(Location, Statement<'tcx>)>,
 
     /// If true, all nested temps are also kept in the
     /// source MIR, not moved to the promoted MIR.
@@ -288,38 +288,78 @@ impl<'a, 'tcx> Promoter<'a, 'tcx> {
     }
 
     fn promote_candidate(mut self, candidate: Candidate) {
-        let span = self.promoted.span;
-        let new_operand = Operand::Constant(box Constant {
-            span,
-            ty: self.promoted.return_ty(),
-            literal: Literal::Promoted {
+        let mut rvalue = {
+            let promoted = &mut self.promoted;
+            let literal = Literal::Promoted {
                 index: Promoted::new(self.source.promoted.len())
-            }
-        });
-        let mut rvalue = match candidate {
-            Candidate::Ref(Location { block: bb, statement_index: stmt_idx }) => {
-                let ref mut statement = self.source[bb].statements[stmt_idx];
-                match statement.kind {
-                    StatementKind::Assign(_, ref mut rvalue) => {
-                        mem::replace(rvalue, Rvalue::Use(new_operand))
+            };
+            let operand = |ty, span| {
+                promoted.span = span;
+                promoted.local_decls[RETURN_PLACE] =
+                    LocalDecl::new_return_place(ty, span);
+                Operand::Constant(box Constant {
+                    span,
+                    ty,
+                    literal
+                })
+            };
+            let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
+            match candidate {
+                Candidate::Ref(loc) => {
+                    let ref mut statement = blocks[loc.block].statements[loc.statement_index];
+                    match statement.kind {
+                        StatementKind::Assign(_, Rvalue::Ref(r, bk, ref mut place)) => {
+                            let ty = place.ty(local_decls, self.tcx).to_ty(self.tcx);
+                            let ref_ty = self.tcx.mk_ref(r,
+                                ty::TypeAndMut {
+                                    ty,
+                                    mutbl: bk.to_mutbl_lossy()
+                                }
+                            );
+                            let span = statement.source_info.span;
+
+                            // Create a temp to hold the promoted reference.
+                            // This is because `*r` requires `r` to be a local,
+                            // otherwise we would use the `promoted` directly.
+                            let mut promoted_ref = LocalDecl::new_temp(ref_ty, span);
+                            promoted_ref.source_info = statement.source_info;
+                            let promoted_ref = local_decls.push(promoted_ref);
+                            assert_eq!(self.temps.push(TempState::Unpromotable), promoted_ref);
+                            self.extra_statements.push((loc, Statement {
+                                source_info: statement.source_info,
+                                kind: StatementKind::Assign(
+                                    Place::Local(promoted_ref),
+                                    Rvalue::Use(operand(ref_ty, span)),
+                                )
+                            }));
+                            let promoted_place = Place::Local(promoted_ref).deref();
+
+                            Rvalue::Ref(r, bk, mem::replace(place, promoted_place))
+                        }
+                        _ => bug!()
                     }
-                    _ => bug!()
                 }
-            }
-            Candidate::Argument { bb, index } => {
-                match self.source[bb].terminator_mut().kind {
-                    TerminatorKind::Call { ref mut args, .. } => {
-                        Rvalue::Use(mem::replace(&mut args[index], new_operand))
+                Candidate::Argument { bb, index } => {
+                    let terminator = blocks[bb].terminator_mut();
+                    match terminator.kind {
+                        TerminatorKind::Call { ref mut args, .. } => {
+                            let ty = args[index].ty(local_decls, self.tcx);
+                            let span = terminator.source_info.span;
+                            Rvalue::Use(mem::replace(&mut args[index], operand(ty, span)))
+                        }
+                        _ => bug!()
                     }
-                    _ => bug!()
                 }
             }
         };
+
+        assert_eq!(self.new_block(), START_BLOCK);
         self.visit_rvalue(&mut rvalue, Location {
             block: BasicBlock::new(0),
             statement_index: usize::MAX
         });
 
+        let span = self.promoted.span;
         self.assign(RETURN_PLACE, rvalue, span);
         self.source.promoted.push(self.promoted);
     }
@@ -343,43 +383,29 @@ pub fn promote_candidates<'a, 'tcx>(mir: &mut Mir<'tcx>,
                                     candidates: Vec<Candidate>) {
     // Visit candidates in reverse, in case they're nested.
     debug!("promote_candidates({:?})", candidates);
+
+    let mut extra_statements = vec![];
     for candidate in candidates.into_iter().rev() {
-        let (span, ty) = match candidate {
-            Candidate::Ref(Location { block: bb, statement_index: stmt_idx }) => {
-                let statement = &mir[bb].statements[stmt_idx];
-                let dest = match statement.kind {
-                    StatementKind::Assign(ref dest, _) => dest,
-                    _ => {
-                        span_bug!(statement.source_info.span,
-                                  "expected assignment to promote");
-                    }
-                };
-                if let Place::Local(index) = *dest {
-                    if temps[index] == TempState::PromotedOut {
-                        // Already promoted.
-                        continue;
+        match candidate {
+            Candidate::Ref(Location { block, statement_index }) => {
+                match mir[block].statements[statement_index].kind {
+                    StatementKind::Assign(Place::Local(local), _) => {
+                        if temps[local] == TempState::PromotedOut {
+                            // Already promoted.
+                            continue;
+                        }
                     }
+                    _ => {}
                 }
-                (statement.source_info.span, dest.ty(mir, tcx).to_ty(tcx))
-            }
-            Candidate::Argument { bb, index } => {
-                let terminator = mir[bb].terminator();
-                let ty = match terminator.kind {
-                    TerminatorKind::Call { ref args, .. } => {
-                        args[index].ty(mir, tcx)
-                    }
-                    _ => {
-                        span_bug!(terminator.source_info.span,
-                                  "expected call argument to promote");
-                    }
-                };
-                (terminator.source_info.span, ty)
             }
-        };
+            Candidate::Argument { .. } => {}
+        }
+
 
-        // Declare return place local
-        let initial_locals = iter::once(LocalDecl::new_return_place(ty, span))
-            .collect();
+        // Declare return place local so that `Mir::new` doesn't complain.
+        let initial_locals = iter::once(
+            LocalDecl::new_return_place(tcx.types.never, mir.span)
+        ).collect();
 
         let mut promoter = Promoter {
             promoted: Mir::new(
@@ -393,16 +419,24 @@ pub fn promote_candidates<'a, 'tcx>(mir: &mut Mir<'tcx>,
                 initial_locals,
                 0,
                 vec![],
-                span
+                mir.span
             ),
+            tcx,
             source: mir,
             temps: &mut temps,
+            extra_statements: &mut extra_statements,
             keep_original: false
         };
-        assert_eq!(promoter.new_block(), START_BLOCK);
         promoter.promote_candidate(candidate);
     }
 
+    // Insert each of `extra_statements` before its indicated location, which
+    // has to be done in reverse location order, to not invalidate the rest.
+    extra_statements.sort_by_key(|&(loc, _)| cmp::Reverse(loc));
+    for (loc, statement) in extra_statements {
+        mir[loc.block].statements.insert(loc.statement_index, statement);
+    }
+
     // Eliminate assignments to, and drops of promoted temps.
     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
     for block in mir.basic_blocks_mut() {

src/test/mir-opt/end_region_destruction_extents_1.rs

@@ -130,17 +130,21 @@ unsafe impl<'a, #[may_dangle] 'b> Drop for D1<'a, 'b> {
 //     let mut _7: &'10s S1;
 //     let mut _8: &'10s S1;
 //     let mut _9: S1;
+//     let mut _10: &'10s S1;
+//     let mut _11: &'12ds S1;
 //
 //     bb0: {
 //         StorageLive(_2);
 //         StorageLive(_3);
 //         StorageLive(_4);
 //         StorageLive(_5);
-//         _5 = promoted[1];
+//         _11 = promoted[1];
+//         _5 = &'12ds (*_11);
 //         _4 = &'12ds (*_5);
 //         StorageLive(_7);
 //         StorageLive(_8);
-//         _8 = promoted[0];
+//         _10 = promoted[0];
+//         _8 = &'10s (*_10);
 //         _7 = &'10s (*_8);
 //         _3 = D1<'12ds, '10s>::{{constructor}}(move _4, move _7);
 //         EndRegion('10s);