rename BorrowKind::Shallow to Fake

also adds some comments

compiler/rustc_borrowck/src/borrow_set.rs

@@ -71,7 +71,7 @@ impl<'tcx> fmt::Display for BorrowData<'tcx> {
     fn fmt(&self, w: &mut fmt::Formatter<'_>) -> fmt::Result {
         let kind = match self.kind {
             mir::BorrowKind::Shared => "",
-            mir::BorrowKind::Shallow => "shallow ",
+            mir::BorrowKind::Fake => "fake ",
             mir::BorrowKind::Mut { kind: mir::MutBorrowKind::ClosureCapture } => "uniq ",
             // FIXME: differentiate `TwoPhaseBorrow`
             mir::BorrowKind::Mut {

compiler/rustc_borrowck/src/def_use.rs

@@ -49,7 +49,7 @@ pub fn categorize(context: PlaceContext) -> Option<DefUse> {
         // cross suspension points so this behavior is unproblematic.
         PlaceContext::MutatingUse(MutatingUseContext::Borrow) |
         PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow) |
-        PlaceContext::NonMutatingUse(NonMutatingUseContext::ShallowBorrow) |
+        PlaceContext::NonMutatingUse(NonMutatingUseContext::FakeBorrow) |
 
         // `PlaceMention` and `AscribeUserType` both evaluate the place, which must not
         // contain dangling references.

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -1022,7 +1022,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 self.cannot_uniquely_borrow_by_two_closures(span, &desc_place, issued_span, None)
             }
 
-            (BorrowKind::Mut { .. }, BorrowKind::Shallow) => {
+            (BorrowKind::Mut { .. }, BorrowKind::Fake) => {
                 if let Some(immutable_section_description) =
                     self.classify_immutable_section(issued_borrow.assigned_place)
                 {
@@ -1114,11 +1114,10 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 )
             }
 
-            (BorrowKind::Shared, BorrowKind::Shared | BorrowKind::Shallow)
+            (BorrowKind::Shared, BorrowKind::Shared | BorrowKind::Fake)
-            | (
+            | (BorrowKind::Fake, BorrowKind::Mut { .. } | BorrowKind::Shared | BorrowKind::Fake) => {
-                BorrowKind::Shallow,
+                unreachable!()
-                BorrowKind::Mut { .. } | BorrowKind::Shared | BorrowKind::Shallow,
+            }
-            ) => unreachable!(),
         };
 
         if issued_spans == borrow_spans {
@@ -2806,7 +2805,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         let loan_span = loan_spans.args_or_use();
 
         let descr_place = self.describe_any_place(place.as_ref());
-        if loan.kind == BorrowKind::Shallow {
+        if loan.kind == BorrowKind::Fake {
             if let Some(section) = self.classify_immutable_section(loan.assigned_place) {
                 let mut err = self.cannot_mutate_in_immutable_section(
                     span,

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -634,7 +634,7 @@ impl UseSpans<'_> {
                 err.subdiagnostic(match kind {
                     Some(kd) => match kd {
                         rustc_middle::mir::BorrowKind::Shared
-                        | rustc_middle::mir::BorrowKind::Shallow => {
+                        | rustc_middle::mir::BorrowKind::Fake => {
                             CaptureVarKind::Immut { kind_span: capture_kind_span }
                         }
 

compiler/rustc_borrowck/src/invalidation.rs

@@ -253,8 +253,8 @@ impl<'cx, 'tcx> InvalidationGenerator<'cx, 'tcx> {
         match rvalue {
             &Rvalue::Ref(_ /*rgn*/, bk, place) => {
                 let access_kind = match bk {
-                    BorrowKind::Shallow => {
+                    BorrowKind::Fake => {
-                        (Shallow(Some(ArtificialField::ShallowBorrow)), Read(ReadKind::Borrow(bk)))
+                        (Shallow(Some(ArtificialField::FakeBorrow)), Read(ReadKind::Borrow(bk)))
                     }
                     BorrowKind::Shared => (Deep, Read(ReadKind::Borrow(bk))),
                     BorrowKind::Mut { .. } => {
@@ -376,8 +376,8 @@ impl<'cx, 'tcx> InvalidationGenerator<'cx, 'tcx> {
                         // have already taken the reservation
                     }
 
-                    (Read(_), BorrowKind::Shallow | BorrowKind::Shared)
+                    (Read(_), BorrowKind::Fake | BorrowKind::Shared)
-                    | (Read(ReadKind::Borrow(BorrowKind::Shallow)), BorrowKind::Mut { .. }) => {
+                    | (Read(ReadKind::Borrow(BorrowKind::Fake)), BorrowKind::Mut { .. }) => {
                         // Reads don't invalidate shared or shallow borrows
                     }
 
@@ -422,7 +422,7 @@ impl<'cx, 'tcx> InvalidationGenerator<'cx, 'tcx> {
 
             // only mutable borrows should be 2-phase
             assert!(match borrow.kind {
-                BorrowKind::Shared | BorrowKind::Shallow => false,
+                BorrowKind::Shared | BorrowKind::Fake => false,
                 BorrowKind::Mut { .. } => true,
             });
 

compiler/rustc_borrowck/src/lib.rs

@@ -846,7 +846,7 @@ use self::ReadOrWrite::{Activation, Read, Reservation, Write};
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 enum ArtificialField {
     ArrayLength,
-    ShallowBorrow,
+    FakeBorrow,
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
@@ -1085,18 +1085,18 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     Control::Continue
                 }
 
-                (Read(_), BorrowKind::Shared | BorrowKind::Shallow)
+                (Read(_), BorrowKind::Shared | BorrowKind::Fake)
-                | (Read(ReadKind::Borrow(BorrowKind::Shallow)), BorrowKind::Mut { .. }) => {
+                | (Read(ReadKind::Borrow(BorrowKind::Fake)), BorrowKind::Mut { .. }) => {
                     Control::Continue
                 }
 
-                (Reservation(_), BorrowKind::Shallow | BorrowKind::Shared) => {
+                (Reservation(_), BorrowKind::Fake | BorrowKind::Shared) => {
                     // This used to be a future compatibility warning (to be
                     // disallowed on NLL). See rust-lang/rust#56254
                     Control::Continue
                 }
 
-                (Write(WriteKind::Move), BorrowKind::Shallow) => {
+                (Write(WriteKind::Move), BorrowKind::Fake) => {
                     // Handled by initialization checks.
                     Control::Continue
                 }
@@ -1204,8 +1204,8 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         match rvalue {
             &Rvalue::Ref(_ /*rgn*/, bk, place) => {
                 let access_kind = match bk {
-                    BorrowKind::Shallow => {
+                    BorrowKind::Fake => {
-                        (Shallow(Some(ArtificialField::ShallowBorrow)), Read(ReadKind::Borrow(bk)))
+                        (Shallow(Some(ArtificialField::FakeBorrow)), Read(ReadKind::Borrow(bk)))
                     }
                     BorrowKind::Shared => (Deep, Read(ReadKind::Borrow(bk))),
                     BorrowKind::Mut { .. } => {
@@ -1226,7 +1226,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     flow_state,
                 );
 
-                let action = if bk == BorrowKind::Shallow {
+                let action = if bk == BorrowKind::Fake {
                     InitializationRequiringAction::MatchOn
                 } else {
                     InitializationRequiringAction::Borrow
@@ -1583,7 +1583,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
             // only mutable borrows should be 2-phase
             assert!(match borrow.kind {
-                BorrowKind::Shared | BorrowKind::Shallow => false,
+                BorrowKind::Shared | BorrowKind::Fake => false,
                 BorrowKind::Mut { .. } => true,
             });
 
@@ -2142,14 +2142,14 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 | WriteKind::Replace
                 | WriteKind::StorageDeadOrDrop
                 | WriteKind::MutableBorrow(BorrowKind::Shared)
-                | WriteKind::MutableBorrow(BorrowKind::Shallow),
+                | WriteKind::MutableBorrow(BorrowKind::Fake),
             )
             | Write(
                 WriteKind::Move
                 | WriteKind::Replace
                 | WriteKind::StorageDeadOrDrop
                 | WriteKind::MutableBorrow(BorrowKind::Shared)
-                | WriteKind::MutableBorrow(BorrowKind::Shallow),
+                | WriteKind::MutableBorrow(BorrowKind::Fake),
             ) => {
                 if self.is_mutable(place.as_ref(), is_local_mutation_allowed).is_err()
                     && !self.has_buffered_errors()
@@ -2173,7 +2173,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 return false;
             }
             Read(
-                ReadKind::Borrow(BorrowKind::Mut { .. } | BorrowKind::Shared | BorrowKind::Shallow)
+                ReadKind::Borrow(BorrowKind::Mut { .. } | BorrowKind::Shared | BorrowKind::Fake)
                 | ReadKind::Copy,
             ) => {
                 // Access authorized

compiler/rustc_borrowck/src/places_conflict.rs

@@ -204,7 +204,7 @@ fn place_components_conflict<'tcx>(
 
             match (elem, &base_ty.kind(), access) {
                 (_, _, Shallow(Some(ArtificialField::ArrayLength)))
-                | (_, _, Shallow(Some(ArtificialField::ShallowBorrow))) => {
+                | (_, _, Shallow(Some(ArtificialField::FakeBorrow))) => {
                     // The array length is like additional fields on the
                     // type; it does not overlap any existing data there.
                     // Furthermore, if cannot actually be a prefix of any
@@ -273,10 +273,10 @@ fn place_components_conflict<'tcx>(
     // If the second example, where we did, then we still know
     // that the borrow can access a *part* of our place that
     // our access cares about, so we still have a conflict.
-    if borrow_kind == BorrowKind::Shallow
+    if borrow_kind == BorrowKind::Fake
         && borrow_place.projection.len() < access_place.projection.len()
     {
-        debug!("borrow_conflicts_with_place: shallow borrow");
+        debug!("borrow_conflicts_with_place: fake borrow");
         false
     } else {
         debug!("borrow_conflicts_with_place: full borrow, CONFLICT");

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -751,7 +751,7 @@ impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
             PlaceContext::MutatingUse(_) => ty::Invariant,
             PlaceContext::NonUse(StorageDead | StorageLive | VarDebugInfo) => ty::Invariant,
             PlaceContext::NonMutatingUse(
-                Inspect | Copy | Move | PlaceMention | SharedBorrow | ShallowBorrow | AddressOf
+                Inspect | Copy | Move | PlaceMention | SharedBorrow | FakeBorrow | AddressOf
                 | Projection,
             ) => ty::Covariant,
             PlaceContext::NonUse(AscribeUserTy(variance)) => variance,

compiler/rustc_codegen_ssa/src/mir/analyze.rs

@@ -219,7 +219,7 @@ impl<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> Visitor<'tcx>
             | PlaceContext::NonMutatingUse(
                 NonMutatingUseContext::Inspect
                 | NonMutatingUseContext::SharedBorrow
-                | NonMutatingUseContext::ShallowBorrow
+                | NonMutatingUseContext::FakeBorrow
                 | NonMutatingUseContext::AddressOf
                 | NonMutatingUseContext::Projection,
             ) => {

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -423,8 +423,8 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                         BorrowKind::Shared => {
                             PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow)
                         }
-                        BorrowKind::Shallow => {
+                        BorrowKind::Fake => {
-                            PlaceContext::NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
+                            PlaceContext::NonMutatingUse(NonMutatingUseContext::FakeBorrow)
                         }
                         BorrowKind::Mut { .. } => {
                             PlaceContext::MutatingUse(MutatingUseContext::Borrow)
@@ -500,7 +500,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                 self.check_mut_borrow(place.local, hir::BorrowKind::Raw)
             }
 
-            Rvalue::Ref(_, BorrowKind::Shared | BorrowKind::Shallow, place)
+            Rvalue::Ref(_, BorrowKind::Shared | BorrowKind::Fake, place)
             | Rvalue::AddressOf(Mutability::Not, place) => {
                 let borrowed_place_has_mut_interior = qualifs::in_place::<HasMutInterior, _>(
                     &self.ccx,

compiler/rustc_const_eval/src/transform/check_consts/resolver.rs

@@ -105,7 +105,7 @@ where
     fn ref_allows_mutation(&self, kind: mir::BorrowKind, place: mir::Place<'tcx>) -> bool {
         match kind {
             mir::BorrowKind::Mut { .. } => true,
-            mir::BorrowKind::Shared | mir::BorrowKind::Shallow => {
+            mir::BorrowKind::Shared | mir::BorrowKind::Fake => {
                 self.shared_borrow_allows_mutation(place)
             }
         }

compiler/rustc_const_eval/src/transform/promote_consts.rs

@@ -456,7 +456,7 @@ impl<'tcx> Validator<'_, 'tcx> {
         match kind {
             // Reject these borrow types just to be safe.
             // FIXME(RalfJung): could we allow them? Should we? No point in it until we have a usecase.
-            BorrowKind::Shallow | BorrowKind::Mut { kind: MutBorrowKind::ClosureCapture } => {
+            BorrowKind::Fake | BorrowKind::Mut { kind: MutBorrowKind::ClosureCapture } => {
                 return Err(Unpromotable);
             }
 

compiler/rustc_const_eval/src/transform/validate.rs

@@ -848,11 +848,11 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
         }
         match rvalue {
             Rvalue::Use(_) | Rvalue::CopyForDeref(_) | Rvalue::Aggregate(..) => {}
-            Rvalue::Ref(_, BorrowKind::Shallow, _) => {
+            Rvalue::Ref(_, BorrowKind::Fake, _) => {
                 if self.mir_phase >= MirPhase::Runtime(RuntimePhase::Initial) {
                     self.fail(
                         location,
-                        "`Assign` statement with a `Shallow` borrow should have been removed in runtime MIR",
+                        "`Assign` statement with a `Fake` borrow should have been removed in runtime MIR",
                     );
                 }
             }

compiler/rustc_middle/src/mir/pretty.rs

@@ -942,7 +942,7 @@ impl<'tcx> Debug for Rvalue<'tcx> {
             Ref(region, borrow_kind, ref place) => {
                 let kind_str = match borrow_kind {
                     BorrowKind::Shared => "",
-                    BorrowKind::Shallow => "shallow ",
+                    BorrowKind::Fake => "fake ",
                     BorrowKind::Mut { .. } => "mut ",
                 };
 

compiler/rustc_middle/src/mir/statement.rs

@@ -446,7 +446,7 @@ impl<'tcx> Rvalue<'tcx> {
 impl BorrowKind {
     pub fn mutability(&self) -> Mutability {
         match *self {
-            BorrowKind::Shared | BorrowKind::Shallow => Mutability::Not,
+            BorrowKind::Shared | BorrowKind::Fake => Mutability::Not,
             BorrowKind::Mut { .. } => Mutability::Mut,
         }
     }
@@ -454,7 +454,7 @@ impl BorrowKind {
     pub fn allows_two_phase_borrow(&self) -> bool {
         match *self {
             BorrowKind::Shared
-            | BorrowKind::Shallow
+            | BorrowKind::Fake
             | BorrowKind::Mut { kind: MutBorrowKind::Default | MutBorrowKind::ClosureCapture } => {
                 false
             }

compiler/rustc_middle/src/mir/syntax.rs

@@ -123,7 +123,7 @@ pub enum AnalysisPhase {
     /// * [`TerminatorKind::FalseEdge`]
     /// * [`StatementKind::FakeRead`]
     /// * [`StatementKind::AscribeUserType`]
-    /// * [`Rvalue::Ref`] with `BorrowKind::Shallow`
+    /// * [`Rvalue::Ref`] with `BorrowKind::Fake`
     ///
     /// Furthermore, `Deref` projections must be the first projection within any place (if they
     /// appear at all)
@@ -182,7 +182,7 @@ pub enum BorrowKind {
     /// should not prevent `if let None = x { ... }`, for example, because the
     /// mutating `(*x as Some).0` can't affect the discriminant of `x`.
     /// We can also report errors with this kind of borrow differently.
-    Shallow,
+    Fake,
 
     /// Data is mutable and not aliasable.
     Mut { kind: MutBorrowKind },

compiler/rustc_middle/src/mir/tcx.rs

@@ -278,7 +278,7 @@ impl BorrowKind {
 
             // We have no type corresponding to a shallow borrow, so use
             // `&` as an approximation.
-            BorrowKind::Shallow => hir::Mutability::Not,
+            BorrowKind::Fake => hir::Mutability::Not,
         }
     }
 }

compiler/rustc_middle/src/mir/visit.rs

@@ -649,8 +649,8 @@ macro_rules! make_mir_visitor {
                             BorrowKind::Shared => PlaceContext::NonMutatingUse(
                                 NonMutatingUseContext::SharedBorrow
                             ),
-                            BorrowKind::Shallow => PlaceContext::NonMutatingUse(
+                            BorrowKind::Fake => PlaceContext::NonMutatingUse(
-                                NonMutatingUseContext::ShallowBorrow
+                                NonMutatingUseContext::FakeBorrow
                             ),
                             BorrowKind::Mut { .. } =>
                                 PlaceContext::MutatingUse(MutatingUseContext::Borrow),
@@ -1261,8 +1261,8 @@ pub enum NonMutatingUseContext {
     Move,
     /// Shared borrow.
     SharedBorrow,
-    /// Shallow borrow.
+    /// A fake borrow.
-    ShallowBorrow,
+    FakeBorrow,
     /// AddressOf for *const pointer.
     AddressOf,
     /// PlaceMention statement.
@@ -1341,7 +1341,7 @@ impl PlaceContext {
         matches!(
             self,
             PlaceContext::NonMutatingUse(
-                NonMutatingUseContext::SharedBorrow | NonMutatingUseContext::ShallowBorrow
+                NonMutatingUseContext::SharedBorrow | NonMutatingUseContext::FakeBorrow
             ) | PlaceContext::MutatingUse(MutatingUseContext::Borrow)
         )
     }

compiler/rustc_mir_build/src/build/expr/as_place.rs

@@ -690,7 +690,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                             fake_borrow_temp.into(),
                             Rvalue::Ref(
                                 tcx.lifetimes.re_erased,
-                                BorrowKind::Shallow,
+                                BorrowKind::Fake,
                                 Place { local: base_place.local, projection },
                             ),
                         );

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -2021,7 +2021,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             let re_erased = tcx.lifetimes.re_erased;
             let scrutinee_source_info = self.source_info(scrutinee_span);
             for &(place, temp) in fake_borrows {
-                let borrow = Rvalue::Ref(re_erased, BorrowKind::Shallow, place);
+                let borrow = Rvalue::Ref(re_erased, BorrowKind::Fake, place);
                 self.cfg.push_assign(block, scrutinee_source_info, Place::from(temp), borrow);
             }
 

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -288,7 +288,7 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                         );
                     };
                     match borrow_kind {
-                        BorrowKind::Shallow | BorrowKind::Shared => {
+                        BorrowKind::Fake | BorrowKind::Shared => {
                             if !ty.is_freeze(self.tcx, self.param_env) {
                                 self.requires_unsafe(pat.span, BorrowOfLayoutConstrainedField);
                             }
@@ -483,7 +483,7 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                 visit::walk_expr(&mut visitor, expr);
                 if visitor.found {
                     match borrow_kind {
-                        BorrowKind::Shallow | BorrowKind::Shared
+                        BorrowKind::Fake | BorrowKind::Shared
                             if !self.thir[arg].ty.is_freeze(self.tcx, self.param_env) =>
                         {
                             self.requires_unsafe(expr.span, BorrowOfLayoutConstrainedField)
@@ -491,7 +491,7 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                         BorrowKind::Mut { .. } => {
                             self.requires_unsafe(expr.span, MutationOfLayoutConstrainedField)
                         }
-                        BorrowKind::Shallow | BorrowKind::Shared => {}
+                        BorrowKind::Fake | BorrowKind::Shared => {}
                     }
                 }
             }

compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs

@@ -102,7 +102,7 @@ where
             }
 
             Rvalue::Cast(..)
-            | Rvalue::Ref(_, BorrowKind::Shallow, _)
+            | Rvalue::Ref(_, BorrowKind::Fake, _)
             | Rvalue::ShallowInitBox(..)
             | Rvalue::Use(..)
             | Rvalue::ThreadLocalRef(..)

compiler/rustc_mir_dataflow/src/impls/liveness.rs

@@ -201,7 +201,7 @@ impl DefUse {
                 | NonMutatingUseContext::Inspect
                 | NonMutatingUseContext::Move
                 | NonMutatingUseContext::PlaceMention
-                | NonMutatingUseContext::ShallowBorrow
+                | NonMutatingUseContext::FakeBorrow
                 | NonMutatingUseContext::SharedBorrow,
             ) => Some(DefUse::Use),
 

compiler/rustc_mir_transform/src/cleanup_post_borrowck.rs

@@ -10,7 +10,7 @@
 //! [`Assign`]: rustc_middle::mir::StatementKind::Assign
 //! [`FakeRead`]: rustc_middle::mir::StatementKind::FakeRead
 //! [`Nop`]: rustc_middle::mir::StatementKind::Nop
-//! [`Shallow`]: rustc_middle::mir::BorrowKind::Shallow
+//! [`Fake`]: rustc_middle::mir::BorrowKind::Fake
 
 use crate::MirPass;
 use rustc_middle::mir::{Body, BorrowKind, Rvalue, StatementKind, TerminatorKind};
@@ -24,7 +24,7 @@ impl<'tcx> MirPass<'tcx> for CleanupPostBorrowck {
             for statement in basic_block.statements.iter_mut() {
                 match statement.kind {
                     StatementKind::AscribeUserType(..)
-                    | StatementKind::Assign(box (_, Rvalue::Ref(_, BorrowKind::Shallow, _)))
+                    | StatementKind::Assign(box (_, Rvalue::Ref(_, BorrowKind::Fake, _)))
                     | StatementKind::FakeRead(..) => statement.make_nop(),
                     _ => (),
                 }

compiler/rustc_mir_transform/src/const_prop.rs

@@ -668,7 +668,7 @@ impl<'tcx> Visitor<'tcx> for CanConstProp {
             // These can't ever be propagated under any scheme, as we can't reason about indirect
             // mutation.
             | NonMutatingUse(NonMutatingUseContext::SharedBorrow)
-            | NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
+            | NonMutatingUse(NonMutatingUseContext::FakeBorrow)
             | NonMutatingUse(NonMutatingUseContext::AddressOf)
             | MutatingUse(MutatingUseContext::Borrow)
             | MutatingUse(MutatingUseContext::AddressOf) => {

compiler/rustc_mir_transform/src/copy_prop.rs

@@ -131,7 +131,7 @@ impl<'tcx> MutVisitor<'tcx> for Replacer<'_, 'tcx> {
         let observes_address = match ctxt {
             PlaceContext::NonMutatingUse(
                 NonMutatingUseContext::SharedBorrow
-                | NonMutatingUseContext::ShallowBorrow
+                | NonMutatingUseContext::FakeBorrow
                 | NonMutatingUseContext::AddressOf,
             ) => true,
             // For debuginfo, merging locals is ok.

compiler/rustc_mir_transform/src/coroutine.rs

@@ -637,6 +637,14 @@ struct LivenessInfo {
     storage_liveness: IndexVec<BasicBlock, Option<BitSet<Local>>>,
 }
 
+/// Computes which locals have to be stored in the state-machine for the
+/// given coroutine.
+///
+/// The basic idea is as follows:
+/// - a local is live until we encounter a `StorageDead` statement. In
+///   case none exist, the local is considered to be always live.
+/// - a local has to be stored if it is either directly used after the
+///   the suspend point, or if it is live and has been previously borrowed.
 fn locals_live_across_suspend_points<'tcx>(
     tcx: TyCtxt<'tcx>,
     body: &Body<'tcx>,
@@ -1449,16 +1457,15 @@ pub(crate) fn mir_coroutine_witnesses<'tcx>(
     // The first argument is the coroutine type passed by value
     let coroutine_ty = body.local_decls[ty::CAPTURE_STRUCT_LOCAL].ty;
 
-    // Get the interior types and args which typeck computed
     let movable = match *coroutine_ty.kind() {
         ty::Coroutine(_, _, movability) => movability == hir::Movability::Movable,
         ty::Error(_) => return None,
         _ => span_bug!(body.span, "unexpected coroutine type {}", coroutine_ty),
     };
 
-    // When first entering the coroutine, move the resume argument into its new local.
+    // The witness simply contains all locals live across suspend points.
-    let always_live_locals = always_storage_live_locals(&body);
 
+    let always_live_locals = always_storage_live_locals(&body);
     let liveness_info = locals_live_across_suspend_points(tcx, body, &always_live_locals, movable);
 
     // Extract locals which are live across suspension point into `layout`

compiler/rustc_mir_transform/src/ssa.rs

@@ -234,7 +234,7 @@ impl<'tcx> Visitor<'tcx> for SsaVisitor<'_> {
             // so we have to remove them too.
             PlaceContext::NonMutatingUse(
                 NonMutatingUseContext::SharedBorrow
-                | NonMutatingUseContext::ShallowBorrow
+                | NonMutatingUseContext::FakeBorrow
                 | NonMutatingUseContext::AddressOf,
             )
             | PlaceContext::MutatingUse(_) => {

compiler/rustc_smir/src/rustc_smir/mod.rs

@@ -455,7 +455,7 @@ impl<'tcx> Stable<'tcx> for mir::BorrowKind {
         use mir::BorrowKind::*;
         match *self {
             Shared => stable_mir::mir::BorrowKind::Shared,
-            Shallow => stable_mir::mir::BorrowKind::Shallow,
+            Fake => stable_mir::mir::BorrowKind::Fake,
             Mut { kind } => stable_mir::mir::BorrowKind::Mut { kind: kind.stable(tables) },
         }
     }

compiler/stable_mir/src/mir/body.rs

@@ -437,9 +437,10 @@ pub enum BorrowKind {
     Shared,
 
     /// The immediately borrowed place must be immutable, but projections from
-    /// it don't need to be. For example, a shallow borrow of `a.b` doesn't
+    /// it don't need to be. This is used to prevent match guards from replacing
+    /// the scrutinee. For example, a fake borrow of `a.b` doesn't
     /// conflict with a mutable borrow of `a.b.c`.
-    Shallow,
+    Fake,
 
     /// Data is mutable and not aliasable.
     Mut {