From 0a5211e80951eeb89e6f8e9e10b31398904d8d76 Mon Sep 17 00:00:00 2001
From: "Felix S. Klock II" <pnkfelix@pnkfx.org>
Date: Mon, 22 May 2017 13:06:51 +0200
Subject: [PATCH] Add post-pass to remove EndRegions of unborrowed extents.

---
 src/librustc_driver/driver.rs                 |  3 +
 .../transform/clean_end_regions.rs            | 84 +++++++++++++++++++
 src/librustc_mir/transform/mod.rs             |  1 +
 3 files changed, 88 insertions(+)
 create mode 100644 src/librustc_mir/transform/clean_end_regions.rs

diff --git a/src/librustc_driver/driver.rs b/src/librustc_driver/driver.rs
index bca82ff9a46d..b1620ed53c71 100644
--- a/src/librustc_driver/driver.rs
+++ b/src/librustc_driver/driver.rs
@@ -912,6 +912,9 @@ pub fn phase_3_run_analysis_passes<'tcx, F, R>(sess: &'tcx Session,
     let mut passes = Passes::new();
     passes.push_hook(mir::transform::dump_mir::DumpMir);
 
+    // Remove all `EndRegion` statements that are not involved in borrows.
+    passes.push_pass(MIR_CONST, mir::transform::clean_end_regions::CleanEndRegions);
+
     // What we need to do constant evaluation.
     passes.push_pass(MIR_CONST, mir::transform::simplify::SimplifyCfg::new("initial"));
     passes.push_pass(MIR_CONST, mir::transform::type_check::TypeckMir);
diff --git a/src/librustc_mir/transform/clean_end_regions.rs b/src/librustc_mir/transform/clean_end_regions.rs
new file mode 100644
index 000000000000..36125f945436
--- /dev/null
+++ b/src/librustc_mir/transform/clean_end_regions.rs
@@ -0,0 +1,84 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! This module provides one pass, `CleanEndRegions`, that reduces the
+//! set of `EndRegion` statements in the MIR.
+//!
+//! The "pass" is actually implemented as two traversals (aka visits)
+//! of the input MIR. The first traversal, `GatherBorrowedRegions`,
+//! finds all of the regions in the MIR that are involved in a borrow.
+//!
+//! The second traversal, `DeleteTrivialEndRegions`, walks over the
+//! MIR and removes any `EndRegion` that is applied to a region that
+//! was not seen in the previous pass.
+
+use rustc_data_structures::fx::FxHashSet;
+
+use rustc::middle::region::CodeExtent;
+use rustc::mir::transform::{MirPass, MirSource};
+use rustc::mir::{BasicBlock, Location, Mir, Rvalue, Statement, StatementKind};
+use rustc::mir::visit::{MutVisitor, Visitor};
+use rustc::ty::{RegionKind, TyCtxt};
+
+pub struct CleanEndRegions;
+
+struct GatherBorrowedRegions {
+    seen_regions: FxHashSet<CodeExtent>,
+}
+
+struct DeleteTrivialEndRegions<'a> {
+    seen_regions: &'a FxHashSet<CodeExtent>,
+}
+
+impl MirPass for CleanEndRegions {
+    fn run_pass<'a, 'tcx>(&self,
+                          _tcx: TyCtxt<'a, 'tcx, 'tcx>,
+                          _source: MirSource,
+                          mir: &mut Mir<'tcx>) {
+        let mut gather = GatherBorrowedRegions { seen_regions: FxHashSet() };
+        gather.visit_mir(mir);
+
+        let mut delete = DeleteTrivialEndRegions { seen_regions: &mut gather.seen_regions };
+        delete.visit_mir(mir);
+    }
+}
+
+impl<'tcx> Visitor<'tcx> for GatherBorrowedRegions {
+    fn visit_rvalue(&mut self,
+                    rvalue: &Rvalue<'tcx>,
+                    location: Location) {
+        if let Rvalue::Ref(r, _, _) = *rvalue {
+            if let RegionKind::ReScope(ce) = *r {
+                self.seen_regions.insert(ce);
+            }
+        }
+        self.super_rvalue(rvalue, location);
+    }
+}
+
+impl<'a, 'tcx> MutVisitor<'tcx> for DeleteTrivialEndRegions<'a> {
+    fn visit_statement(&mut self,
+                       block: BasicBlock,
+                       statement: &mut Statement<'tcx>,
+                       location: Location) {
+        let mut delete_it = false;
+
+        if let StatementKind::EndRegion(ref extent) = statement.kind {
+            if !self.seen_regions.contains(extent) {
+                delete_it = true;
+            }
+        }
+
+        if delete_it {
+            statement.kind = StatementKind::Nop;
+        }
+        self.super_statement(block, statement, location);
+    }
+}
diff --git a/src/librustc_mir/transform/mod.rs b/src/librustc_mir/transform/mod.rs
index fcea5d4c8604..4594c611d596 100644
--- a/src/librustc_mir/transform/mod.rs
+++ b/src/librustc_mir/transform/mod.rs
@@ -24,6 +24,7 @@ use syntax::ast;
 use syntax_pos::{DUMMY_SP, Span};
 use transform;
 
+pub mod clean_end_regions;
 pub mod simplify_branches;
 pub mod simplify;
 pub mod erase_regions;