every match arm reads the match's borrowed input

src/librustc/ich/impls_mir.rs

@@ -241,6 +241,9 @@ for mir::StatementKind<'gcx> {
                 place.hash_stable(hcx, hasher);
                 rvalue.hash_stable(hcx, hasher);
             }
+            mir::StatementKind::ReadForMatch(ref place) => {
+                place.hash_stable(hcx, hasher);
+            }
             mir::StatementKind::SetDiscriminant { ref place, variant_index } => {
                 place.hash_stable(hcx, hasher);
                 variant_index.hash_stable(hcx, hasher);

src/librustc/mir/mod.rs

@@ -1225,6 +1225,10 @@ pub enum StatementKind<'tcx> {
     /// Write the RHS Rvalue to the LHS Place.
     Assign(Place<'tcx>, Rvalue<'tcx>),
 
+    /// This represents all the reading that a pattern match may do
+    /// (e.g. inspecting constants and discriminant values).
+    ReadForMatch(Place<'tcx>),
+
     /// Write the discriminant for a variant to the enum Place.
     SetDiscriminant { place: Place<'tcx>, variant_index: usize },
 
@@ -1327,6 +1331,7 @@ impl<'tcx> Debug for Statement<'tcx> {
         use self::StatementKind::*;
         match self.kind {
             Assign(ref place, ref rv) => write!(fmt, "{:?} = {:?}", place, rv),
+            ReadForMatch(ref place) => write!(fmt, "ReadForMatch({:?})", place),
             // (reuse lifetime rendering policy from ppaux.)
             EndRegion(ref ce) => write!(fmt, "EndRegion({})", ty::ReScope(*ce)),
             Validate(ref op, ref places) => write!(fmt, "Validate({:?}, {:?})", op, places),
@@ -2212,6 +2217,7 @@ BraceStructTypeFoldableImpl! {
 EnumTypeFoldableImpl! {
     impl<'tcx> TypeFoldable<'tcx> for StatementKind<'tcx> {
         (StatementKind::Assign)(a, b),
+        (StatementKind::ReadForMatch)(place),
         (StatementKind::SetDiscriminant) { place, variant_index },
         (StatementKind::StorageLive)(a),
         (StatementKind::StorageDead)(a),

src/librustc/mir/visit.rs

@@ -355,6 +355,11 @@ macro_rules! make_mir_visitor {
                                           ref $($mutability)* rvalue) => {
                         self.visit_assign(block, place, rvalue, location);
                     }
+                    StatementKind::ReadForMatch(ref $($mutability)* place) => {
+                        self.visit_place(place,
+                                         PlaceContext::Inspect,
+                                         location);
+                    }
                     StatementKind::EndRegion(_) => {}
                     StatementKind::Validate(_, ref $($mutability)* places) => {
                         for operand in places {

src/librustc/session/config.rs

@@ -1290,16 +1290,22 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         useful for profiling / PGO."),
     relro_level: Option<RelroLevel> = (None, parse_relro_level, [TRACKED],
         "choose which RELRO level to use"),
+    disable_ast_check_for_mutation_in_guard: bool = (false, parse_bool, [UNTRACKED],
+        "skip AST-based mutation-in-guard check (mir-borrowck provides more precise check)"),
     nll_subminimal_causes: bool = (false, parse_bool, [UNTRACKED],
         "when tracking region error causes, accept subminimal results for faster execution."),
     nll_facts: bool = (false, parse_bool, [UNTRACKED],
                        "dump facts from NLL analysis into side files"),
     disable_nll_user_type_assert: bool = (false, parse_bool, [UNTRACKED],
         "disable user provided type assertion in NLL"),
+    nll_dont_emit_read_for_match: bool = (false, parse_bool, [UNTRACKED],
+        "in match codegen, do not include ReadForMatch statements (used by mir-borrowck)"),
     polonius: bool = (false, parse_bool, [UNTRACKED],
         "enable polonius-based borrow-checker"),
     codegen_time_graph: bool = (false, parse_bool, [UNTRACKED],
         "generate a graphical HTML report of time spent in codegen and LLVM"),
+    trans_time_graph: bool = (false, parse_bool, [UNTRACKED],
+        "generate a graphical HTML report of time spent in trans and LLVM"),
     thinlto: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "enable ThinLTO when possible"),
     inline_in_all_cgus: Option<bool> = (None, parse_opt_bool, [TRACKED],

src/librustc/ty/context.rs

@@ -1344,12 +1344,31 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         self.on_disk_query_result_cache.serialize(self.global_tcx(), encoder)
     }
 
+    /// If true, we should use a naive AST walk to determine if match
+    /// guard could perform bad mutations (or mutable-borrows).
+    pub fn check_for_mutation_in_guard_via_ast_walk(self) -> bool {
+        !self.sess.opts.debugging_opts.disable_ast_check_for_mutation_in_guard
+    }
+
     /// If true, we should use the MIR-based borrowck (we may *also* use
     /// the AST-based borrowck).
     pub fn use_mir_borrowck(self) -> bool {
         self.borrowck_mode().use_mir()
     }
 
+    /// If true, make MIR codegen for `match` emit a temp that holds a
+    /// borrow of the input to the match expression.
+    pub fn generate_borrow_of_any_match_input(&self) -> bool {
+        self.emit_read_for_match()
+    }
+
+    /// If true, make MIR codegen for `match` emit ReadForMatch
+    /// statements (which simulate the maximal effect of executing the
+    /// patterns in a match arm).
+    pub fn emit_read_for_match(&self) -> bool {
+        self.use_mir_borrowck() && !self.sess.opts.debugging_opts.nll_dont_emit_read_for_match
+    }
+
     /// If true, pattern variables for use in guards on match arms
     /// will be bound as references to the data, and occurrences of
     /// those variables in the guard expression will implicitly

src/librustc_codegen_llvm/mir/statement.rs

@@ -82,6 +82,7 @@ impl<'a, 'tcx> FunctionCx<'a, 'tcx> {
                 asm::codegen_inline_asm(&bx, asm, outputs, input_vals);
                 bx
             }
+            mir::StatementKind::ReadForMatch(_) |
             mir::StatementKind::EndRegion(_) |
             mir::StatementKind::Validate(..) |
             mir::StatementKind::UserAssertTy(..) |

src/librustc_mir/borrow_check/borrow_set.rs

@@ -53,14 +53,25 @@ impl<'tcx> Index<BorrowIndex> for BorrowSet<'tcx> {
     }
 }
 
+/// Every two-phase borrow has *exactly one* use (or else it is not a
+/// proper two-phase borrow under our current definition). However, not
+/// all uses are actually ones that activate the reservation.. In
+/// particular, a shared borrow of a `&mut` does not activate the
+/// reservation.
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+crate enum TwoPhaseUse {
+    MutActivate,
+    SharedUse,
+}
+
 #[derive(Debug)]
 crate struct BorrowData<'tcx> {
     /// Location where the borrow reservation starts.
     /// In many cases, this will be equal to the activation location but not always.
     crate reserve_location: Location,
     /// Location where the borrow is activated. None if this is not a
     /// 2-phase borrow.
-    crate activation_location: Option<Location>,
+    crate activation_location: Option<(TwoPhaseUse, Location)>,
     /// What kind of borrow this is
     crate kind: mir::BorrowKind,
     /// The region for which this borrow is live
@@ -215,17 +226,16 @@ impl<'a, 'gcx, 'tcx> Visitor<'tcx> for GatherBorrows<'a, 'gcx, 'tcx> {
                 Some(&borrow_index) => {
                     let borrow_data = &mut self.idx_vec[borrow_index];
 
-                    // Watch out: the use of TMP in the borrow
-                    // itself doesn't count as an
-                    // activation. =)
+                    // Watch out: the use of TMP in the borrow itself
+                    // doesn't count as an activation. =)
                     if borrow_data.reserve_location == location && context == PlaceContext::Store {
                         return;
                     }
 
                     if let Some(other_activation) = borrow_data.activation_location {
                         span_bug!(
                             self.mir.source_info(location).span,
-                            "found two activations for 2-phase borrow temporary {:?}: \
+                            "found two uses for 2-phase borrow temporary {:?}: \
                              {:?} and {:?}",
                             temp,
                             location,
@@ -235,11 +245,25 @@ impl<'a, 'gcx, 'tcx> Visitor<'tcx> for GatherBorrows<'a, 'gcx, 'tcx> {
 
                     // Otherwise, this is the unique later use
                     // that we expect.
-                    borrow_data.activation_location = Some(location);
-                    self.activation_map
-                        .entry(location)
-                        .or_insert(Vec::new())
-                        .push(borrow_index);
+
+                    let two_phase_use;
+
+                    match context {
+                        // The use of TMP in a shared borrow does not
+                        // count as an actual activation.
+                        PlaceContext::Borrow { kind: mir::BorrowKind::Shared, .. } => {
+                            two_phase_use = TwoPhaseUse::SharedUse;
+                        }
+                        _ => {
+                            two_phase_use = TwoPhaseUse::MutActivate;
+                            self.activation_map
+                                .entry(location)
+                                .or_insert(Vec::new())
+                                .push(borrow_index);
+                        }
+                    }
+
+                    borrow_data.activation_location = Some((two_phase_use, location));
                 }
 
                 None => {}

src/librustc_mir/borrow_check/mod.rs

@@ -423,6 +423,14 @@ impl<'cx, 'gcx, 'tcx> DataflowResultsConsumer<'cx, 'tcx> for MirBorrowckCtxt<'cx
                     flow_state,
                 );
             }
+            StatementKind::ReadForMatch(ref place) => {
+                self.access_place(ContextKind::ReadForMatch.new(location),
+                                  (place, span),
+                                  (Deep, Read(ReadKind::Borrow(BorrowKind::Shared))),
+                                  LocalMutationIsAllowed::No,
+                                  flow_state,
+                                  );
+            }
             StatementKind::SetDiscriminant {
                 ref place,
                 variant_index: _,
@@ -1689,14 +1697,16 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
         );
         let mut error_reported = false;
         match kind {
-            Reservation(WriteKind::MutableBorrow(BorrowKind::Unique))
-            | Write(WriteKind::MutableBorrow(BorrowKind::Unique)) => {
-                if let Err(_place_err) = self.is_mutable(place, LocalMutationIsAllowed::Yes) {
-                    span_bug!(span, "&unique borrow for {:?} should not fail", place);
-                }
-            }
-            Reservation(WriteKind::MutableBorrow(BorrowKind::Mut { .. }))
-            | Write(WriteKind::MutableBorrow(BorrowKind::Mut { .. })) => {
+            Reservation(WriteKind::MutableBorrow(borrow_kind @ BorrowKind::Unique))
+            | Reservation(WriteKind::MutableBorrow(borrow_kind @ BorrowKind::Mut { .. }))
+            | Write(WriteKind::MutableBorrow(borrow_kind @ BorrowKind::Unique))
+            | Write(WriteKind::MutableBorrow(borrow_kind @ BorrowKind::Mut { .. })) =>
+            {
+                let is_local_mutation_allowed = match borrow_kind {
+                    BorrowKind::Unique => LocalMutationIsAllowed::Yes,
+                    BorrowKind::Mut { .. } => is_local_mutation_allowed,
+                    BorrowKind::Shared => unreachable!(),
+                };
                 match self.is_mutable(place, is_local_mutation_allowed) {
                     Ok(root_place) => self.add_used_mut(root_place, flow_state),
                     Err(place_err) => {
@@ -2090,6 +2100,7 @@ enum ContextKind {
     CallDest,
     Assert,
     Yield,
+    ReadForMatch,
     StorageDead,
 }
 

src/librustc_mir/borrow_check/nll/invalidation.rs

@@ -93,6 +93,14 @@ impl<'cg, 'cx, 'tcx, 'gcx> Visitor<'tcx> for InvalidationGenerator<'cg, 'cx, 'tc
                     JustWrite
                 );
             }
+            StatementKind::ReadForMatch(ref place) => {
+                self.access_place(
+                    ContextKind::ReadForMatch.new(location),
+                    place,
+                    (Deep, Read(ReadKind::Borrow(BorrowKind::Shared))),
+                    LocalMutationIsAllowed::No,
+                );
+            }
             StatementKind::SetDiscriminant {
                 ref place,
                 variant_index: _,

src/librustc_mir/borrow_check/nll/type_check/mod.rs

@@ -836,7 +836,8 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
                     );
                 }
             }
-            StatementKind::StorageLive(_)
+            StatementKind::ReadForMatch(_)
+            | StatementKind::StorageLive(_)
             | StatementKind::StorageDead(_)
             | StatementKind::InlineAsm { .. }
             | StatementKind::EndRegion(_)

src/librustc_mir/borrow_check/path_utils.rs

@@ -12,7 +12,7 @@
 /// allowed to be split into separate Reservation and
 /// Activation phases.
 use borrow_check::ArtificialField;
-use borrow_check::borrow_set::{BorrowSet, BorrowData};
+use borrow_check::borrow_set::{BorrowSet, BorrowData, TwoPhaseUse};
 use borrow_check::{Context, Overlap};
 use borrow_check::{ShallowOrDeep, Deep, Shallow};
 use dataflow::indexes::BorrowIndex;
@@ -431,10 +431,13 @@ pub(super) fn is_active<'tcx>(
 ) -> bool {
     debug!("is_active(borrow_data={:?}, location={:?})", borrow_data, location);
 
-    // If this is not a 2-phase borrow, it is always active.
     let activation_location = match borrow_data.activation_location {
-        Some(v) => v,
+        // If this is not a 2-phase borrow, it is always active.
         None => return true,
+        // And if the unique 2-phase use is not an activation, then it is *never* active.
+        Some((TwoPhaseUse::SharedUse, _)) => return false,
+        // Otherwise, we derive info from the activation point `v`:
+        Some((TwoPhaseUse::MutActivate, v)) => v,
     };
 
     // Otherwise, it is active for every location *except* in between

src/librustc_mir/build/expr/as_place.rs

@@ -11,7 +11,7 @@
 //! See docs in build/expr/mod.rs
 
 use build::{BlockAnd, BlockAndExtension, Builder};
-use build::ForGuard::{OutsideGuard, WithinGuard};
+use build::ForGuard::{OutsideGuard, RefWithinGuard, ValWithinGuard};
 use build::expr::category::Category;
 use hair::*;
 use rustc::mir::*;
@@ -88,10 +88,11 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
             }
             ExprKind::VarRef { id } => {
                 let place = if this.is_bound_var_in_guard(id) {
-                    let index = this.var_local_id(id, WithinGuard);
                     if this.hir.tcx().all_pat_vars_are_implicit_refs_within_guards() {
+                        let index = this.var_local_id(id, RefWithinGuard);
                         Place::Local(index).deref()
                     } else {
+                        let index = this.var_local_id(id, ValWithinGuard);
                         Place::Local(index)
                     }
                 } else {