src/librustc_mir/transform/clean_end_regions.rs

   1 // Copyright 2017 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! This module provides one pass, `CleanEndRegions`, that reduces the
  12 //! set of `EndRegion` statements in the MIR.
  13 //!
  14 //! The "pass" is actually implemented as two traversals (aka visits)
  15 //! of the input MIR. The first traversal, `GatherBorrowedRegions`,
  16 //! finds all of the regions in the MIR that are involved in a borrow.
  17 //!
  18 //! The second traversal, `DeleteTrivialEndRegions`, walks over the
  19 //! MIR and removes any `EndRegion` that is applied to a region that
  20 //! was not seen in the previous pass.
  21
  22 use rustc_data_structures::fx::FxHashSet;
  23
  24 use rustc::middle::region;
  25 use rustc::mir::{BasicBlock, Location, Mir, Rvalue, Statement, StatementKind};
  26 use rustc::mir::visit::{MutVisitor, Visitor, TyContext};
  27 use rustc::ty::{Ty, RegionKind, TyCtxt};
  28 use transform::{MirPass, MirSource};
  29
  30 pub struct CleanEndRegions;
  31
  32 struct GatherBorrowedRegions {
  33     seen_regions: FxHashSet<region::Scope>,
  34 }
  35
  36 struct DeleteTrivialEndRegions<'a> {
  37     seen_regions: &'a FxHashSet<region::Scope>,
  38 }
  39
  40 impl MirPass for CleanEndRegions {
  41     fn run_pass<'a, 'tcx>(&self,
  42                           tcx: TyCtxt<'a, 'tcx, 'tcx>,
  43                           _source: MirSource,
  44                           mir: &mut Mir<'tcx>) {
  45         if !tcx.sess.emit_end_regions() { return; }
  46
  47         let mut gather = GatherBorrowedRegions {
  48             seen_regions: FxHashSet()
  49         };
  50         gather.visit_mir(mir);
  51
  52         let mut delete = DeleteTrivialEndRegions { seen_regions: &mut gather.seen_regions };
  53         delete.visit_mir(mir);
  54     }
  55 }
  56
  57 impl<'tcx> Visitor<'tcx> for GatherBorrowedRegions {
  58     fn visit_rvalue(&mut self,
  59                     rvalue: &Rvalue<'tcx>,
  60                     location: Location) {
  61         // Gather regions that are used for borrows
  62         if let Rvalue::Ref(r, _, _) = *rvalue {
  63             if let RegionKind::ReScope(ce) = *r {
  64                 self.seen_regions.insert(ce);
  65             }
  66         }
  67         self.super_rvalue(rvalue, location);
  68     }
  69
  70     fn visit_ty(&mut self, ty: &Ty<'tcx>, _: TyContext) {
  71         // Gather regions that occur in types
  72         for re in ty.walk().flat_map(|t| t.regions()) {
  73             match *re {
  74                 RegionKind::ReScope(ce) => { self.seen_regions.insert(ce); }
  75                 _ => {},
  76             }
  77         }
  78         self.super_ty(ty);
  79     }
  80 }
  81
  82 impl<'a, 'tcx> MutVisitor<'tcx> for DeleteTrivialEndRegions<'a> {
  83     fn visit_statement(&mut self,
  84                        block: BasicBlock,
  85                        statement: &mut Statement<'tcx>,
  86                        location: Location) {
  87         let mut delete_it = false;
  88
  89         if let StatementKind::EndRegion(ref region_scope) = statement.kind {
  90             if !self.seen_regions.contains(region_scope) {
  91                 delete_it = true;
  92             }
  93         }
  94
  95         if delete_it {
  96             statement.kind = StatementKind::Nop;
  97         }
  98         self.super_statement(block, statement, location);
  99     }
 100 }