NLL: disallow creation of immediately unusable variables

src/librustc/ich/impls_mir.rs

@@ -238,7 +238,8 @@ for mir::StatementKind<'gcx> {
                 place.hash_stable(hcx, hasher);
                 rvalue.hash_stable(hcx, hasher);
             }
-            mir::StatementKind::ReadForMatch(ref place) => {
+            mir::StatementKind::FakeRead(ref cause, ref place) => {
+                cause.hash_stable(hcx, hasher);
                 place.hash_stable(hcx, hasher);
             }
             mir::StatementKind::SetDiscriminant { ref place, variant_index } => {
@@ -271,6 +272,8 @@ for mir::StatementKind<'gcx> {
     }
 }
 
+impl_stable_hash_for!(enum mir::FakeReadCause { ForMatch, ForLet });
+
 impl<'a, 'gcx, T> HashStable<StableHashingContext<'a>>
     for mir::ValidationOperand<'gcx, T>
     where T: HashStable<StableHashingContext<'a>>

src/librustc/mir/mod.rs

@@ -1613,8 +1613,10 @@ pub enum StatementKind<'tcx> {
     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>),
+    /// (e.g. inspecting constants and discriminant values), and the
+    /// kind of pattern it comes from. This is in order to adapt potential
+    /// error messages to these specific patterns.
+    FakeRead(FakeReadCause, Place<'tcx>),
 
     /// Write the discriminant for a variant to the enum Place.
     SetDiscriminant {
@@ -1662,6 +1664,31 @@ pub enum StatementKind<'tcx> {
     Nop,
 }
 
+/// The `FakeReadCause` describes the type of pattern why a `FakeRead` statement exists.
+#[derive(Copy, Clone, RustcEncodable, RustcDecodable, Debug)]
+pub enum FakeReadCause {
+    /// Inject a fake read of the borrowed input at the start of each arm's
+    /// pattern testing code.
+    ///
+    /// This should ensure that you cannot change the variant for an enum
+    /// while you are in the midst of matching on it.
+    ForMatch,
+
+    /// Officially, the semantics of
+    ///
+    /// `let pattern = <expr>;`
+    ///
+    /// is that `<expr>` is evaluated into a temporary and then this temporary is
+    /// into the pattern.
+    ///
+    /// However, if we see the simple pattern `let var = <expr>`, we optimize this to
+    /// evaluate `<expr>` directly into the variable `var`. This is mostly unobservable,
+    /// but in some cases it can affect the borrow checker, as in #53695.
+    /// Therefore, we insert a "fake read" here to ensure that we get
+    /// appropriate errors.
+    ForLet,
+}
+
 /// The `ValidationOp` describes what happens with each of the operands of a
 /// `Validate` statement.
 #[derive(Copy, Clone, RustcEncodable, RustcDecodable, PartialEq, Eq)]
@@ -1718,7 +1745,7 @@ 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),
+            FakeRead(ref cause, ref place) => write!(fmt, "FakeRead({:?}, {:?})", cause, 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),
@@ -2585,6 +2612,7 @@ CloneTypeFoldableAndLiftImpls! {
     Mutability,
     SourceInfo,
     UpvarDecl,
+    FakeReadCause,
     ValidationOp,
     SourceScope,
     SourceScopeData,
@@ -2651,7 +2679,7 @@ BraceStructTypeFoldableImpl! {
 EnumTypeFoldableImpl! {
     impl<'tcx> TypeFoldable<'tcx> for StatementKind<'tcx> {
         (StatementKind::Assign)(a, b),
-        (StatementKind::ReadForMatch)(place),
+        (StatementKind::FakeRead)(cause, place),
         (StatementKind::SetDiscriminant) { place, variant_index },
         (StatementKind::StorageLive)(a),
         (StatementKind::StorageDead)(a),

src/librustc/mir/visit.rs

@@ -354,7 +354,7 @@ macro_rules! make_mir_visitor {
                                           ref $($mutability)* rvalue) => {
                         self.visit_assign(block, place, rvalue, location);
                     }
-                    StatementKind::ReadForMatch(ref $($mutability)* place) => {
+                    StatementKind::FakeRead(_, ref $($mutability)* place) => {
                         self.visit_place(place,
                                          PlaceContext::Inspect,
                                          location);

src/librustc/session/config.rs

@@ -1330,7 +1330,7 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
     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)"),
+        "in match codegen, do not include FakeRead statements (used by mir-borrowck)"),
     dont_buffer_diagnostics: bool = (false, parse_bool, [UNTRACKED],
         "emit diagnostics rather than buffering (breaks NLL error downgrading, sorting)."),
     polonius: bool = (false, parse_bool, [UNTRACKED],

src/librustc/ty/context.rs

@@ -1478,7 +1478,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         self.emit_read_for_match()
     }
 
-    /// If true, make MIR codegen for `match` emit ReadForMatch
+    /// If true, make MIR codegen for `match` emit FakeRead
     /// statements (which simulate the maximal effect of executing the
     /// patterns in a match arm).
     pub fn emit_read_for_match(&self) -> bool {

src/librustc_codegen_llvm/mir/statement.rs

@@ -89,7 +89,7 @@ impl FunctionCx<'a, 'll, 'tcx> {
                 asm::codegen_inline_asm(&bx, asm, outputs, input_vals);
                 bx
             }
-            mir::StatementKind::ReadForMatch(_) |
+            mir::StatementKind::FakeRead(..) |
             mir::StatementKind::EndRegion(_) |
             mir::StatementKind::Validate(..) |
             mir::StatementKind::AscribeUserType(..) |

src/librustc_mir/borrow_check/error_reporting.rs

@@ -12,7 +12,7 @@ use borrow_check::WriteKind;
 use rustc::middle::region::ScopeTree;
 use rustc::mir::VarBindingForm;
 use rustc::mir::{BindingForm, BorrowKind, ClearCrossCrate, Field, Local};
-use rustc::mir::{LocalDecl, LocalKind, Location, Operand, Place};
+use rustc::mir::{FakeReadCause, LocalDecl, LocalKind, Location, Operand, Place};
 use rustc::mir::{ProjectionElem, Rvalue, Statement, StatementKind};
 use rustc::ty;
 use rustc_data_structures::fx::FxHashSet;
@@ -989,6 +989,21 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
             false
         }
     }
+
+    /// Returns the `FakeReadCause` at this location if it is a `FakeRead` statement.
+    pub(super) fn retrieve_fake_read_cause_for_location(
+        &self,
+        location: &Location,
+    ) -> Option<FakeReadCause> {
+        let stmt = self.mir.basic_blocks()[location.block]
+            .statements
+            .get(location.statement_index)?;
+        if let StatementKind::FakeRead(cause, _) = stmt.kind {
+            Some(cause)
+        } else {
+            None
+        }
+    }
 }
 
 // The span(s) associated to a use of a place.

src/librustc_mir/borrow_check/mod.rs

@@ -478,9 +478,9 @@ impl<'cx, 'gcx, 'tcx> DataflowResultsConsumer<'cx, 'tcx> for MirBorrowckCtxt<'cx
                     flow_state,
                 );
             }
-            StatementKind::ReadForMatch(ref place) => {
+            StatementKind::FakeRead(_, ref place) => {
                 self.access_place(
-                    ContextKind::ReadForMatch.new(location),
+                    ContextKind::FakeRead.new(location),
                     (place, span),
                     (Deep, Read(ReadKind::Borrow(BorrowKind::Shared))),
                     LocalMutationIsAllowed::No,
@@ -2206,7 +2206,7 @@ enum ContextKind {
     CallDest,
     Assert,
     Yield,
-    ReadForMatch,
+    FakeRead,
     StorageDead,
 }
 

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

@@ -11,7 +11,7 @@
 use borrow_check::borrow_set::BorrowData;
 use borrow_check::nll::region_infer::Cause;
 use borrow_check::{Context, MirBorrowckCtxt, WriteKind};
-use rustc::mir::{Local, Location, Place, TerminatorKind};
+use rustc::mir::{FakeReadCause, Local, Location, Place, TerminatorKind};
 use rustc_errors::DiagnosticBuilder;
 use rustc::ty::Region;
 
@@ -142,7 +142,13 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
                     if spans.for_closure() {
                         "borrow later captured here by closure"
                     } else {
-                        "borrow later used here"
+                        // Check if the location represents a `FakeRead`, and adapt the error
+                        // message to the `FakeReadCause` it is from: in particular,
+                        // the ones inserted in optimized `let var = <expr>` patterns.
+                        match self.retrieve_fake_read_cause_for_location(&location) {
+                            Some(FakeReadCause::ForLet) => "borrow later stored here",
+                            _ => "borrow later used here"
+                        }
                     }
                 };
                 err.span_label(spans.var_or_use(), message);

src/librustc_mir/borrow_check/nll/invalidation.rs

@@ -93,9 +93,9 @@ impl<'cg, 'cx, 'tcx, 'gcx> Visitor<'tcx> for InvalidationGenerator<'cg, 'cx, 'tc
                     JustWrite
                 );
             }
-            StatementKind::ReadForMatch(ref place) => {
+            StatementKind::FakeRead(_, ref place) => {
                 self.access_place(
-                    ContextKind::ReadForMatch.new(location),
+                    ContextKind::FakeRead.new(location),
                     place,
                     (Deep, Read(ReadKind::Borrow(BorrowKind::Shared))),
                     LocalMutationIsAllowed::No,

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

@@ -995,7 +995,7 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
                     );
                 }
             }
-            StatementKind::ReadForMatch(_)
+            StatementKind::FakeRead(..)
             | StatementKind::StorageLive(_)
             | StatementKind::StorageDead(_)
             | StatementKind::InlineAsm { .. }

src/librustc_mir/build/matches/mod.rs

@@ -145,19 +145,16 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                     if let (true, Some(borrow_temp)) =
                         (tcx.emit_read_for_match(), borrowed_input_temp.clone())
                     {
-                        // inject a fake read of the borrowed input at
-                        // the start of each arm's pattern testing
-                        // code.
-                        //
-                        // This should ensure that you cannot change
-                        // the variant for an enum while you are in
-                        // the midst of matching on it.
+                        // Inject a fake read, see comments on `FakeReadCause::ForMatch`.
                         let pattern_source_info = self.source_info(pattern.span);
                         self.cfg.push(
                             *pre_binding_block,
                             Statement {
                                 source_info: pattern_source_info,
-                                kind: StatementKind::ReadForMatch(borrow_temp.clone()),
+                                kind: StatementKind::FakeRead(
+                                    FakeReadCause::ForMatch,
+                                    borrow_temp.clone(),
+                                ),
                             },
                         );
                     }
@@ -264,6 +261,18 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                 let place =
                     self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard);
                 unpack!(block = self.into(&place, block, initializer));
+
+
+                // Inject a fake read, see comments on `FakeReadCause::ForLet`.
+                let source_info = self.source_info(irrefutable_pat.span);
+                self.cfg.push(
+                    block,
+                    Statement {
+                        source_info,
+                        kind: StatementKind::FakeRead(FakeReadCause::ForLet, place.clone()),
+                    },
+                );
+
                 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
                 block.unit()
             }
@@ -305,6 +314,15 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                     },
                 );
 
+                // Inject a fake read, see comments on `FakeReadCause::ForLet`.
+                self.cfg.push(
+                    block,
+                    Statement {
+                        source_info,
+                        kind: StatementKind::FakeRead(FakeReadCause::ForLet, place.clone()),
+                    },
+                );
+
                 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
                 block.unit()
             }

src/librustc_mir/dataflow/impls/borrows.rs

@@ -336,7 +336,7 @@ impl<'a, 'gcx, 'tcx> BitDenotation for Borrows<'a, 'gcx, 'tcx> {
                 }
             }
 
-            mir::StatementKind::ReadForMatch(..) |
+            mir::StatementKind::FakeRead(..) |
             mir::StatementKind::SetDiscriminant { .. } |
             mir::StatementKind::StorageLive(..) |
             mir::StatementKind::Validate(..) |

src/librustc_mir/dataflow/move_paths/builder.rs

@@ -281,7 +281,7 @@ impl<'b, 'a, 'gcx, 'tcx> Gatherer<'b, 'a, 'gcx, 'tcx> {
                 }
                 self.gather_rvalue(rval);
             }
-            StatementKind::ReadForMatch(ref place) => {
+            StatementKind::FakeRead(_, ref place) => {
                 self.create_move_path(place);
             }
             StatementKind::InlineAsm { ref outputs, ref inputs, ref asm } => {

src/librustc_mir/interpret/step.rs

@@ -150,9 +150,9 @@ impl<'a, 'mir, 'tcx, M: Machine<'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M> {
                 self.deallocate_local(old_val)?;
             }
 
-            // No dynamic semantics attached to `ReadForMatch`; MIR
+            // No dynamic semantics attached to `FakeRead`; MIR
             // interpreter is solely intended for borrowck'ed code.
-            ReadForMatch(..) => {}
+            FakeRead(..) => {}
 
             // Validity checks.
             Validate(op, ref places) => {

src/librustc_mir/transform/check_unsafety.rs

@@ -108,7 +108,7 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
         self.source_info = statement.source_info;
         match statement.kind {
             StatementKind::Assign(..) |
-            StatementKind::ReadForMatch(..) |
+            StatementKind::FakeRead(..) |
             StatementKind::SetDiscriminant { .. } |
             StatementKind::StorageLive(..) |
             StatementKind::StorageDead(..) |

src/librustc_mir/transform/qualify_consts.rs

@@ -1090,7 +1090,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Qualifier<'a, 'tcx, 'tcx> {
                 StatementKind::Assign(ref place, ref rvalue) => {
                     this.visit_assign(bb, place, rvalue, location);
                 }
-                StatementKind::ReadForMatch(..) |
+                StatementKind::FakeRead(..) |
                 StatementKind::SetDiscriminant { .. } |
                 StatementKind::StorageLive(_) |
                 StatementKind::StorageDead(_) |

src/librustc_mir/transform/qualify_min_const_fn.rs

@@ -216,7 +216,7 @@ fn check_statement(
             check_rvalue(tcx, mir, rval, span)
         }
 
-        StatementKind::ReadForMatch(_) => Err((span, "match in const fn is unstable".into())),
+        StatementKind::FakeRead(..) => Err((span, "match in const fn is unstable".into())),
 
         // just an assignment
         StatementKind::SetDiscriminant { .. } => Ok(()),

src/librustc_mir/transform/remove_noop_landing_pads.rs

@@ -49,7 +49,7 @@ impl RemoveNoopLandingPads {
     ) -> bool {
         for stmt in &mir[bb].statements {
             match stmt.kind {
-                StatementKind::ReadForMatch(_) |
+                StatementKind::FakeRead(..) |
                 StatementKind::StorageLive(_) |
                 StatementKind::StorageDead(_) |
                 StatementKind::EndRegion(_) |

src/librustc_mir/transform/rustc_peek.rs

@@ -157,7 +157,7 @@ fn each_block<'a, 'tcx, O>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
             mir::StatementKind::Assign(ref place, ref rvalue) => {
                 (place, rvalue)
             }
-            mir::StatementKind::ReadForMatch(_) |
+            mir::StatementKind::FakeRead(..) |
             mir::StatementKind::StorageLive(_) |
             mir::StatementKind::StorageDead(_) |
             mir::StatementKind::InlineAsm { .. } |

src/librustc_passes/mir_stats.rs

@@ -82,7 +82,7 @@ impl<'a, 'tcx> mir_visit::Visitor<'tcx> for StatCollector<'a, 'tcx> {
         self.record("Statement", statement);
         self.record(match statement.kind {
             StatementKind::Assign(..) => "StatementKind::Assign",
-            StatementKind::ReadForMatch(..) => "StatementKind::ReadForMatch",
+            StatementKind::FakeRead(..) => "StatementKind::FakeRead",
             StatementKind::EndRegion(..) => "StatementKind::EndRegion",
             StatementKind::Validate(..) => "StatementKind::Validate",
             StatementKind::SetDiscriminant { .. } => "StatementKind::SetDiscriminant",

src/test/mir-opt/basic_assignment.rs

@@ -47,6 +47,7 @@ fn main() {
 //    bb0: {
 //        StorageLive(_1);
 //        _1 = const false;
+//        FakeRead(ForLet, _1);
 //        StorageLive(_2);
 //        StorageLive(_3);
 //        _3 = _1;
@@ -55,6 +56,7 @@ fn main() {
 //        StorageLive(_4);
 //        _4 = std::option::Option<std::boxed::Box<u32>>::None;
 //        AscribeUserType(_4, o, Canonical { variables: [], value: std::option::Option<std::boxed::Box<u32>> });
+//        FakeRead(ForLet, _4);
 //        StorageLive(_5);
 //        StorageLive(_6);
 //        _6 = move _4;

src/test/mir-opt/box_expr.rs

@@ -63,6 +63,7 @@ impl Drop for S {
 //
 //     bb4: {
 //         StorageDead(_2);
+//         FakeRead(ForLet, _1);
 //         StorageLive(_4);
 //         _4 = move _1;
 //         _3 = const std::mem::drop(move _4) -> [return: bb5, unwind: bb7];