Add a MIR transform to remove fake reads

As we are now creating borrows of places that may not be valid for
borrow checking matches, these have to be removed to avoid generating
broken code.

src/librustc_mir/lib.rs

@@ -14,7 +14,7 @@ Rust MIR: a lowered representation of Rust. Also: an experiment!
 
 */
 
-#![cfg_attr(not(stage0), feature(nll))]
+#![feature(nll)]
 #![feature(in_band_lifetimes)]
 #![feature(impl_header_lifetime_elision)]
 #![feature(slice_patterns)]

src/librustc_mir/transform/cleanup_post_borrowck.rs

@@ -33,7 +33,8 @@
 use rustc_data_structures::fx::FxHashSet;
 
 use rustc::middle::region;
-use rustc::mir::{BasicBlock, Location, Mir, Rvalue, Statement, StatementKind};
+use rustc::mir::{BasicBlock, FakeReadCause, Local, Location, Mir, Place};
+use rustc::mir::{Rvalue, Statement, StatementKind};
 use rustc::mir::visit::{MutVisitor, Visitor, TyContext};
 use rustc::ty::{Ty, RegionKind, TyCtxt};
 use transform::{MirPass, MirSource};
@@ -135,3 +136,62 @@ impl<'tcx> MutVisitor<'tcx> for DeleteAscribeUserType {
         self.super_statement(block, statement, location);
     }
 }
+
+pub struct CleanFakeReadsAndBorrows;
+
+pub struct DeleteAndRecordFakeReads {
+    fake_borrow_temporaries: FxHashSet<Local>,
+}
+
+pub struct DeleteFakeBorrows {
+    fake_borrow_temporaries: FxHashSet<Local>,
+}
+
+// Removes any FakeReads from the MIR
+impl MirPass for CleanFakeReadsAndBorrows {
+    fn run_pass<'a, 'tcx>(&self,
+                          _tcx: TyCtxt<'a, 'tcx, 'tcx>,
+                          _source: MirSource,
+                          mir: &mut Mir<'tcx>) {
+        let mut delete_reads = DeleteAndRecordFakeReads {
+            fake_borrow_temporaries: FxHashSet(),
+        };
+        delete_reads.visit_mir(mir);
+        let mut delete_borrows = DeleteFakeBorrows {
+            fake_borrow_temporaries: delete_reads.fake_borrow_temporaries,
+        };
+        delete_borrows.visit_mir(mir);
+    }
+}
+
+impl<'tcx> MutVisitor<'tcx> for DeleteAndRecordFakeReads {
+    fn visit_statement(&mut self,
+                       block: BasicBlock,
+                       statement: &mut Statement<'tcx>,
+                       location: Location) {
+        if let StatementKind::FakeRead(cause, ref place) = statement.kind {
+            if let FakeReadCause::ForMatchGuard = cause {
+                match *place {
+                    Place::Local(local) => self.fake_borrow_temporaries.insert(local),
+                    _ => bug!("Fake match guard read of non-local: {:?}", place),
+                };
+            }
+            statement.make_nop();
+        }
+        self.super_statement(block, statement, location);
+    }
+}
+
+impl<'tcx> MutVisitor<'tcx> for DeleteFakeBorrows {
+    fn visit_statement(&mut self,
+                       block: BasicBlock,
+                       statement: &mut Statement<'tcx>,
+                       location: Location) {
+        if let StatementKind::Assign(Place::Local(local), _) = statement.kind {
+            if self.fake_borrow_temporaries.contains(&local) {
+                statement.make_nop();
+            }
+        }
+        self.super_statement(block, statement, location);
+    }
+}

src/librustc_mir/transform/mod.rs

@@ -237,9 +237,12 @@ fn optimized_mir<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) -> &'tcx
         no_landing_pads::NoLandingPads,
         simplify_branches::SimplifyBranches::new("initial"),
         remove_noop_landing_pads::RemoveNoopLandingPads,
-        simplify::SimplifyCfg::new("early-opt"),
         // Remove all `AscribeUserType` statements.
         cleanup_post_borrowck::CleanAscribeUserType,
+        // Remove all `FakeRead` statements and the borrows that are only
+        // used for checking matches
+        cleanup_post_borrowck::CleanFakeReadsAndBorrows,
+        simplify::SimplifyCfg::new("early-opt"),
 
         // These next passes must be executed together
         add_call_guards::CriticalCallEdges,

src/test/mir-opt/remove_fake_borrows.rs

@@ -0,0 +1,122 @@
+// Test that the fake borrows for matches are removed after borrow checking.
+
+// ignore-wasm32-bare
+
+#![feature(nll)]
+
+fn match_guard(x: Option<&&i32>) -> i32 {
+    match x {
+        Some(0) if true => 0,
+        _ => 1,
+    }
+}
+
+fn main() {
+    match_guard(None);
+}
+
+// END RUST SOURCE
+
+// START rustc.match_guard.CleanFakeReadsAndBorrows.before.mir
+// bb0: {
+//     FakeRead(ForMatchedPlace, _1);
+//     _2 = discriminant(_1);
+//     _3 = &shallow _1;
+//     _4 = &shallow ((_1 as Some).0: &'<empty> &'<empty> i32);
+//     _5 = &shallow (*((_1 as Some).0: &'<empty> &'<empty> i32));
+//     _6 = &shallow (*(*((_1 as Some).0: &'<empty> &'<empty> i32)));
+//     switchInt(move _2) -> [1isize: bb6, otherwise: bb4];
+// }
+// bb1: {
+//     _0 = const 0i32;
+//     goto -> bb9;
+// }
+// bb2: {
+//     _0 = const 1i32;
+//     goto -> bb9;
+// }
+// bb3: {
+//     FakeRead(ForMatchGuard, _3);
+//     FakeRead(ForMatchGuard, _4);
+//     FakeRead(ForMatchGuard, _5);
+//     FakeRead(ForMatchGuard, _6);
+//     goto -> bb7;
+// }
+// bb4: {
+//     FakeRead(ForMatchGuard, _3);
+//     FakeRead(ForMatchGuard, _4);
+//     FakeRead(ForMatchGuard, _5);
+//     FakeRead(ForMatchGuard, _6);
+//     goto -> bb2;
+// }
+// bb5: {
+//     unreachable;
+// }
+// bb6: {
+//     switchInt((*(*((_1 as Some).0: &'<empty> &'<empty> i32)))) -> [0i32: bb3, otherwise: bb4];
+// }
+// bb7: {
+//     goto -> bb1;
+// }
+// bb8: {
+//     goto -> bb4;
+// }
+// bb9: {
+//     return;
+// }
+// bb10: {
+//     resume;
+// }
+// END rustc.match_guard.CleanFakeReadsAndBorrows.before.mir
+
+// START rustc.match_guard.CleanFakeReadsAndBorrows.after.mir
+// bb0: {
+//     nop;
+//     _2 = discriminant(_1);
+//     nop;
+//     nop;
+//     nop;
+//     nop;
+//     switchInt(move _2) -> [1isize: bb6, otherwise: bb4];
+// }
+// bb1: {
+//     _0 = const 0i32;
+//     goto -> bb9;
+// }
+// bb2: {
+//     _0 = const 1i32;
+//     goto -> bb9;
+// }
+// bb3: {
+//     nop;
+//     nop;
+//     nop;
+//     nop;
+//     goto -> bb7;
+// }
+// bb4: {
+//     nop;
+//     nop;
+//     nop;
+//     nop;
+//     goto -> bb2;
+// }
+// bb5: {
+//     unreachable;
+// }
+// bb6: {
+//     switchInt((*(*((_1 as Some).0: &'<empty> &'<empty> i32)))) -> [0i32: bb3, otherwise: bb4];
+// }
+// bb7: {
+//     goto -> bb1;
+// }
+// bb8: {
+//     goto -> bb4;
+// }
+// bb9: {
+//     return;
+// }
+// bb10: {
+//     resume;
+// }
+// END rustc.match_guard.CleanFakeReadsAndBorrows.after.mir