Add a non-shallow fake borrow

32 files changed, 189 insertions, 93 deletions

diff --git a/compiler/rustc_borrowck/src/borrow_set.rs b/compiler/rustc_borrowck/src/borrow_set.rs
index a38dd286be5..af5f7571074 100644
--- a/compiler/rustc_borrowck/src/borrow_set.rs
+++ b/compiler/rustc_borrowck/src/borrow_set.rs
@@ -69,7 +69,8 @@ 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::Fake => "fake ",
+            mir::BorrowKind::Fake(mir::FakeBorrowKind::Deep) => "fake ",
+            mir::BorrowKind::Fake(mir::FakeBorrowKind::Shallow) => "fake shallow ",
             mir::BorrowKind::Mut { kind: mir::MutBorrowKind::ClosureCapture } => "uniq ",
             // FIXME: differentiate `TwoPhaseBorrow`
             mir::BorrowKind::Mut {
diff --git a/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs b/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
index ec0d4af599e..f6d6b486c89 100644
--- a/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
@@ -17,9 +17,9 @@ use rustc_middle::hir::nested_filter::OnlyBodies;
 use rustc_middle::mir::tcx::PlaceTy;
 use rustc_middle::mir::{
     self, AggregateKind, BindingForm, BorrowKind, CallSource, ClearCrossCrate, ConstraintCategory,
-    FakeReadCause, LocalDecl, LocalInfo, LocalKind, Location, MutBorrowKind, Operand, Place,
-    PlaceRef, ProjectionElem, Rvalue, Statement, StatementKind, Terminator, TerminatorKind,
-    VarBindingForm,
+    FakeBorrowKind, FakeReadCause, LocalDecl, LocalInfo, LocalKind, Location, MutBorrowKind,
+    Operand, Place, PlaceRef, ProjectionElem, Rvalue, Statement, StatementKind, Terminator,
+    TerminatorKind, VarBindingForm,
 };
 use rustc_middle::ty::{
     self, suggest_constraining_type_params, PredicateKind, ToPredicate, Ty, TyCtxt,
@@ -1486,7 +1486,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         let first_borrow_desc;
         let mut err = match (gen_borrow_kind, issued_borrow.kind) {
             (
-                BorrowKind::Shared,
+                BorrowKind::Shared | BorrowKind::Fake(FakeBorrowKind::Deep),
                 BorrowKind::Mut { kind: MutBorrowKind::Default | MutBorrowKind::TwoPhaseBorrow },
             ) => {
                 first_borrow_desc = "mutable ";
@@ -1504,7 +1504,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             }
             (
                 BorrowKind::Mut { kind: MutBorrowKind::Default | MutBorrowKind::TwoPhaseBorrow },
-                BorrowKind::Shared,
+                BorrowKind::Shared | BorrowKind::Fake(FakeBorrowKind::Deep),
             ) => {
                 first_borrow_desc = "immutable ";
                 let mut err = self.cannot_reborrow_already_borrowed(
@@ -1566,7 +1566,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 self.cannot_uniquely_borrow_by_two_closures(span, &desc_place, issued_span, None)
             }
 
-            (BorrowKind::Mut { .. }, BorrowKind::Fake) => {
+            (BorrowKind::Mut { .. }, BorrowKind::Fake(FakeBorrowKind::Shallow)) => {
                 if let Some(immutable_section_description) =
                     self.classify_immutable_section(issued_borrow.assigned_place)
                 {
@@ -1629,7 +1629,10 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 )
             }
 
-            (BorrowKind::Shared, BorrowKind::Mut { kind: MutBorrowKind::ClosureCapture }) => {
+            (
+                BorrowKind::Shared | BorrowKind::Fake(FakeBorrowKind::Deep),
+                BorrowKind::Mut { kind: MutBorrowKind::ClosureCapture },
+            ) => {
                 first_borrow_desc = "first ";
                 self.cannot_reborrow_already_uniquely_borrowed(
                     span,
@@ -1659,8 +1662,14 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 )
             }
 
-            (BorrowKind::Shared, BorrowKind::Shared | BorrowKind::Fake)
-            | (BorrowKind::Fake, BorrowKind::Mut { .. } | BorrowKind::Shared | BorrowKind::Fake) => {
+            (
+                BorrowKind::Shared | BorrowKind::Fake(FakeBorrowKind::Deep),
+                BorrowKind::Shared | BorrowKind::Fake(_),
+            )
+            | (
+                BorrowKind::Fake(FakeBorrowKind::Shallow),
+                BorrowKind::Mut { .. } | BorrowKind::Shared | BorrowKind::Fake(_),
+            ) => {
                 unreachable!()
             }
         };
@@ -3572,7 +3581,7 @@ 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::Fake {
+        if let BorrowKind::Fake(_) = loan.kind {
             if let Some(section) = self.classify_immutable_section(loan.assigned_place) {
                 let mut err = self.cannot_mutate_in_immutable_section(
                     span,
diff --git a/compiler/rustc_borrowck/src/diagnostics/mod.rs b/compiler/rustc_borrowck/src/diagnostics/mod.rs
index dbea317e7bb..f88b506eb3e 100644
--- a/compiler/rustc_borrowck/src/diagnostics/mod.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/mod.rs
@@ -654,7 +654,7 @@ impl UseSpans<'_> {
                     match kind {
                         Some(kd) => match kd {
                             rustc_middle::mir::BorrowKind::Shared
-                            | rustc_middle::mir::BorrowKind::Fake => {
+                            | rustc_middle::mir::BorrowKind::Fake(_) => {
                                 CaptureVarKind::Immut { kind_span: capture_kind_span }
                             }
 
diff --git a/compiler/rustc_borrowck/src/lib.rs b/compiler/rustc_borrowck/src/lib.rs
index 7fbf4c47ec8..e839255c34f 100644
--- a/compiler/rustc_borrowck/src/lib.rs
+++ b/compiler/rustc_borrowck/src/lib.rs
@@ -1056,18 +1056,19 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     Control::Continue
                 }
 
-                (Read(_), BorrowKind::Shared | BorrowKind::Fake)
-                | (Read(ReadKind::Borrow(BorrowKind::Fake)), BorrowKind::Mut { .. }) => {
-                    Control::Continue
-                }
+                (Read(_), BorrowKind::Shared | BorrowKind::Fake(_))
+                | (
+                    Read(ReadKind::Borrow(BorrowKind::Fake(FakeBorrowKind::Shallow))),
+                    BorrowKind::Mut { .. },
+                ) => Control::Continue,
 
-                (Reservation(_), BorrowKind::Fake | 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::Fake) => {
+                (Write(WriteKind::Move), BorrowKind::Fake(FakeBorrowKind::Shallow)) => {
                     // Handled by initialization checks.
                     Control::Continue
                 }
@@ -1175,10 +1176,12 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         match rvalue {
             &Rvalue::Ref(_ /*rgn*/, bk, place) => {
                 let access_kind = match bk {
-                    BorrowKind::Fake => {
+                    BorrowKind::Fake(FakeBorrowKind::Shallow) => {
                         (Shallow(Some(ArtificialField::FakeBorrow)), Read(ReadKind::Borrow(bk)))
                     }
-                    BorrowKind::Shared => (Deep, Read(ReadKind::Borrow(bk))),
+                    BorrowKind::Shared | BorrowKind::Fake(FakeBorrowKind::Deep) => {
+                        (Deep, Read(ReadKind::Borrow(bk)))
+                    }
                     BorrowKind::Mut { .. } => {
                         let wk = WriteKind::MutableBorrow(bk);
                         if allow_two_phase_borrow(bk) {
@@ -1197,7 +1200,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     flow_state,
                 );
 
-                let action = if bk == BorrowKind::Fake {
+                let action = if bk == BorrowKind::Fake(FakeBorrowKind::Shallow) {
                     InitializationRequiringAction::MatchOn
                 } else {
                     InitializationRequiringAction::Borrow
@@ -1556,7 +1559,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
             // only mutable borrows should be 2-phase
             assert!(match borrow.kind {
-                BorrowKind::Shared | BorrowKind::Fake => false,
+                BorrowKind::Shared | BorrowKind::Fake(_) => false,
                 BorrowKind::Mut { .. } => true,
             });
 
@@ -2121,14 +2124,14 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 | WriteKind::Replace
                 | WriteKind::StorageDeadOrDrop
                 | WriteKind::MutableBorrow(BorrowKind::Shared)
-                | WriteKind::MutableBorrow(BorrowKind::Fake),
+                | WriteKind::MutableBorrow(BorrowKind::Fake(_)),
             )
             | Write(
                 WriteKind::Move
                 | WriteKind::Replace
                 | WriteKind::StorageDeadOrDrop
                 | WriteKind::MutableBorrow(BorrowKind::Shared)
-                | WriteKind::MutableBorrow(BorrowKind::Fake),
+                | WriteKind::MutableBorrow(BorrowKind::Fake(_)),
             ) => {
                 if self.is_mutable(place.as_ref(), is_local_mutation_allowed).is_err()
                     && !self.has_buffered_diags()
@@ -2152,7 +2155,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 return false;
             }
             Read(
-                ReadKind::Borrow(BorrowKind::Mut { .. } | BorrowKind::Shared | BorrowKind::Fake)
+                ReadKind::Borrow(BorrowKind::Mut { .. } | BorrowKind::Shared | BorrowKind::Fake(_))
                 | ReadKind::Copy,
             ) => {
                 // Access authorized
diff --git a/compiler/rustc_borrowck/src/places_conflict.rs b/compiler/rustc_borrowck/src/places_conflict.rs
index 7e8dba43b71..c559f9d5748 100644
--- a/compiler/rustc_borrowck/src/places_conflict.rs
+++ b/compiler/rustc_borrowck/src/places_conflict.rs
@@ -55,7 +55,7 @@ use crate::Overlap;
 use crate::{AccessDepth, Deep, Shallow};
 use rustc_hir as hir;
 use rustc_middle::mir::{
-    Body, BorrowKind, MutBorrowKind, Place, PlaceElem, PlaceRef, ProjectionElem,
+    Body, BorrowKind, FakeBorrowKind, MutBorrowKind, Place, PlaceElem, PlaceRef, ProjectionElem,
 };
 use rustc_middle::ty::{self, TyCtxt};
 use std::cmp::max;
@@ -271,10 +271,10 @@ 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::Fake
+    if borrow_kind == BorrowKind::Fake(FakeBorrowKind::Shallow)
         && borrow_place.projection.len() < access_place.projection.len()
     {
-        debug!("borrow_conflicts_with_place: fake borrow");
+        debug!("borrow_conflicts_with_place: shallow borrow");
         false
     } else {
         debug!("borrow_conflicts_with_place: full borrow, CONFLICT");
diff --git a/compiler/rustc_borrowck/src/polonius/loan_invalidations.rs b/compiler/rustc_borrowck/src/polonius/loan_invalidations.rs
index 956de1dec9b..de469080d14 100644
--- a/compiler/rustc_borrowck/src/polonius/loan_invalidations.rs
+++ b/compiler/rustc_borrowck/src/polonius/loan_invalidations.rs
@@ -1,6 +1,8 @@
 use rustc_data_structures::graph::dominators::Dominators;
 use rustc_middle::mir::visit::Visitor;
-use rustc_middle::mir::{self, BasicBlock, Body, Location, NonDivergingIntrinsic, Place, Rvalue};
+use rustc_middle::mir::{
+    self, BasicBlock, Body, FakeBorrowKind, Location, NonDivergingIntrinsic, Place, Rvalue,
+};
 use rustc_middle::mir::{BorrowKind, Mutability, Operand};
 use rustc_middle::mir::{InlineAsmOperand, Terminator, TerminatorKind};
 use rustc_middle::mir::{Statement, StatementKind};
@@ -239,10 +241,12 @@ impl<'cx, 'tcx> LoanInvalidationsGenerator<'cx, 'tcx> {
         match rvalue {
             &Rvalue::Ref(_ /*rgn*/, bk, place) => {
                 let access_kind = match bk {
-                    BorrowKind::Fake => {
+                    BorrowKind::Fake(FakeBorrowKind::Shallow) => {
                         (Shallow(Some(ArtificialField::FakeBorrow)), Read(ReadKind::Borrow(bk)))
                     }
-                    BorrowKind::Shared => (Deep, Read(ReadKind::Borrow(bk))),
+                    BorrowKind::Shared | BorrowKind::Fake(FakeBorrowKind::Deep) => {
+                        (Deep, Read(ReadKind::Borrow(bk)))
+                    }
                     BorrowKind::Mut { .. } => {
                         let wk = WriteKind::MutableBorrow(bk);
                         if allow_two_phase_borrow(bk) {
@@ -357,8 +361,11 @@ impl<'cx, 'tcx> LoanInvalidationsGenerator<'cx, 'tcx> {
                         // have already taken the reservation
                     }
 
-                    (Read(_), BorrowKind::Fake | BorrowKind::Shared)
-                    | (Read(ReadKind::Borrow(BorrowKind::Fake)), BorrowKind::Mut { .. }) => {
+                    (Read(_), BorrowKind::Fake(_) | BorrowKind::Shared)
+                    | (
+                        Read(ReadKind::Borrow(BorrowKind::Fake(FakeBorrowKind::Shallow))),
+                        BorrowKind::Mut { .. },
+                    ) => {
                         // Reads don't invalidate shared or shallow borrows
                     }
 
@@ -403,7 +410,7 @@ impl<'cx, 'tcx> LoanInvalidationsGenerator<'cx, 'tcx> {
 
             // only mutable borrows should be 2-phase
             assert!(match borrow.kind {
-                BorrowKind::Shared | BorrowKind::Fake => false,
+                BorrowKind::Shared | BorrowKind::Fake(_) => false,
                 BorrowKind::Mut { .. } => true,
             });
 
diff --git a/compiler/rustc_const_eval/src/transform/check_consts/check.rs b/compiler/rustc_const_eval/src/transform/check_consts/check.rs
index a506d10c1d0..f9786acfc6c 100644
--- a/compiler/rustc_const_eval/src/transform/check_consts/check.rs
+++ b/compiler/rustc_const_eval/src/transform/check_consts/check.rs
@@ -414,7 +414,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                         BorrowKind::Shared => {
                             PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow)
                         }
-                        BorrowKind::Fake => {
+                        BorrowKind::Fake(_) => {
                             PlaceContext::NonMutatingUse(NonMutatingUseContext::FakeBorrow)
                         }
                         BorrowKind::Mut { .. } => {
@@ -487,7 +487,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                 }
             }
 
-            Rvalue::Ref(_, BorrowKind::Shared | BorrowKind::Fake, 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,
diff --git a/compiler/rustc_const_eval/src/transform/check_consts/resolver.rs b/compiler/rustc_const_eval/src/transform/check_consts/resolver.rs
index 2c835f6750f..5ae3ffaaec2 100644
--- a/compiler/rustc_const_eval/src/transform/check_consts/resolver.rs
+++ b/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::Fake => {
+            mir::BorrowKind::Shared | mir::BorrowKind::Fake(_) => {
                 self.shared_borrow_allows_mutation(place)
             }
         }
diff --git a/compiler/rustc_const_eval/src/transform/validate.rs b/compiler/rustc_const_eval/src/transform/validate.rs
index a499e4b980f..69274f962cc 100644
--- a/compiler/rustc_const_eval/src/transform/validate.rs
+++ b/compiler/rustc_const_eval/src/transform/validate.rs
@@ -924,7 +924,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                     }
                 }
             },
-            Rvalue::Ref(_, BorrowKind::Fake, _) => {
+            Rvalue::Ref(_, BorrowKind::Fake(_), _) => {
                 if self.mir_phase >= MirPhase::Runtime(RuntimePhase::Initial) {
                     self.fail(
                         location,
diff --git a/compiler/rustc_middle/src/mir/pretty.rs b/compiler/rustc_middle/src/mir/pretty.rs
index 15bd5c08965..fead4a16dab 100644
--- a/compiler/rustc_middle/src/mir/pretty.rs
+++ b/compiler/rustc_middle/src/mir/pretty.rs
@@ -954,7 +954,8 @@ impl<'tcx> Debug for Rvalue<'tcx> {
             Ref(region, borrow_kind, ref place) => {
                 let kind_str = match borrow_kind {
                     BorrowKind::Shared => "",
-                    BorrowKind::Fake => "fake ",
+                    BorrowKind::Fake(FakeBorrowKind::Deep) => "fake ",
+                    BorrowKind::Fake(FakeBorrowKind::Shallow) => "fake shallow ",
                     BorrowKind::Mut { .. } => "mut ",
                 };
 
diff --git a/compiler/rustc_middle/src/mir/statement.rs b/compiler/rustc_middle/src/mir/statement.rs
index 235298ffcb8..375f1f15a39 100644
--- a/compiler/rustc_middle/src/mir/statement.rs
+++ b/compiler/rustc_middle/src/mir/statement.rs
@@ -451,7 +451,7 @@ impl<'tcx> Rvalue<'tcx> {
 impl BorrowKind {
     pub fn mutability(&self) -> Mutability {
         match *self {
-            BorrowKind::Shared | BorrowKind::Fake => Mutability::Not,
+            BorrowKind::Shared | BorrowKind::Fake(_) => Mutability::Not,
             BorrowKind::Mut { .. } => Mutability::Mut,
         }
     }
@@ -459,7 +459,7 @@ impl BorrowKind {
     pub fn allows_two_phase_borrow(&self) -> bool {
         match *self {
             BorrowKind::Shared
-            | BorrowKind::Fake
+            | BorrowKind::Fake(_)
             | BorrowKind::Mut { kind: MutBorrowKind::Default | MutBorrowKind::ClosureCapture } => {
                 false
             }
diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs
index 97c3eb55638..6292633f65c 100644
--- a/compiler/rustc_middle/src/mir/syntax.rs
+++ b/compiler/rustc_middle/src/mir/syntax.rs
@@ -165,13 +165,16 @@ pub enum BorrowKind {
     /// Data must be immutable and is aliasable.
     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
-    /// conflict with a mutable borrow of `a.b.c`.
-    ///
-    /// This is used when lowering matches: when matching on a place we want to
-    /// ensure that place have the same value from the start of the match until
-    /// an arm is selected. This prevents this code from compiling:
+    /// An immutable, aliasable borrow that is discarded after borrow-checking. Can behave either
+    /// like a normal shared borrow or like a special shallow borrow (see [`FakeBorrowKind`]).
+    ///
+    /// This is used when lowering index expressions and matches. This is used to prevent code like
+    /// the following from compiling:
+    /// ```compile_fail,E0506
+    /// let mut x = vec![vec![0, 1]];
+    /// let y = vec![];
+    /// let _ = x[0][{x = y; 1}];
+    /// ```
     /// ```compile_fail,E0510
     /// let mut x = &Some(0);
     /// match *x {
@@ -180,11 +183,8 @@ pub enum BorrowKind {
     ///     Some(_) => (),
     /// }
     /// ```
-    /// This can't be a shared borrow because mutably borrowing (*x as Some).0
-    /// 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.
-    Fake,
+    Fake(FakeBorrowKind),
 
     /// Data is mutable and not aliasable.
     Mut { kind: MutBorrowKind },
@@ -240,6 +240,57 @@ pub enum MutBorrowKind {
     ClosureCapture,
 }
 
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, TyEncodable, TyDecodable)]
+#[derive(Hash, HashStable)]
+pub enum FakeBorrowKind {
+    /// A shared shallow borrow. 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 conflict with a
+    /// mutable borrow of `a.b.c`.
+    ///
+    /// This is used when lowering matches: when matching on a place we want to ensure that place
+    /// have the same value from the start of the match until an arm is selected. This prevents this
+    /// code from compiling:
+    /// ```compile_fail,E0510
+    /// let mut x = &Some(0);
+    /// match *x {
+    ///     None => (),
+    ///     Some(_) if { x = &None; false } => (),
+    ///     Some(_) => (),
+    /// }
+    /// ```
+    /// This can't be a shared borrow because mutably borrowing `(*x as Some).0` should not checking
+    /// the discriminant or accessing other variants, because the mutating `(*x as Some).0` can't
+    /// affect the discriminant of `x`. E.g. the following is allowed:
+    /// ```rust
+    /// let mut x = Some(0);
+    /// match x {
+    ///     Some(_)
+    ///         if {
+    ///             if let Some(ref mut y) = x {
+    ///                 *y += 1;
+    ///             };
+    ///             true
+    ///         } => {}
+    ///     _ => {}
+    /// }
+    /// ```
+    Shallow,
+    /// A shared (deep) borrow. Data must be immutable and is aliasable.
+    ///
+    /// This is used when lowering deref patterns, where shallow borrows wouldn't prevent something
+    /// like:
+    // ```compile_fail
+    // let mut b = Box::new(false);
+    // match b {
+    //     deref!(true) => {} // not reached because `*b == false`
+    //     _ if { *b = true; false } => {} // not reached because the guard is `false`
+    //     deref!(false) => {} // not reached because the guard changed it
+    //     // UB because we reached the unreachable.
+    // }
+    // ```
+    Deep,
+}
+
 ///////////////////////////////////////////////////////////////////////////
 // Statements
 
diff --git a/compiler/rustc_middle/src/mir/tcx.rs b/compiler/rustc_middle/src/mir/tcx.rs
index 506003ff7c0..4c2bf9abe97 100644
--- a/compiler/rustc_middle/src/mir/tcx.rs
+++ b/compiler/rustc_middle/src/mir/tcx.rs
@@ -293,7 +293,7 @@ impl BorrowKind {
 
             // We have no type corresponding to a shallow borrow, so use
             // `&` as an approximation.
-            BorrowKind::Fake => hir::Mutability::Not,
+            BorrowKind::Fake(_) => hir::Mutability::Not,
         }
     }
 }
diff --git a/compiler/rustc_middle/src/mir/visit.rs b/compiler/rustc_middle/src/mir/visit.rs
index 4f7b2f7cbe4..0f97bcded3e 100644
--- a/compiler/rustc_middle/src/mir/visit.rs
+++ b/compiler/rustc_middle/src/mir/visit.rs
@@ -653,7 +653,7 @@ macro_rules! make_mir_visitor {
                             BorrowKind::Shared => PlaceContext::NonMutatingUse(
                                 NonMutatingUseContext::SharedBorrow
                             ),
-                            BorrowKind::Fake => PlaceContext::NonMutatingUse(
+                            BorrowKind::Fake(_) => PlaceContext::NonMutatingUse(
                                 NonMutatingUseContext::FakeBorrow
                             ),
                             BorrowKind::Mut { .. } =>
@@ -1279,6 +1279,8 @@ pub enum NonMutatingUseContext {
     /// Shared borrow.
     SharedBorrow,
     /// A fake borrow.
+    /// FIXME: do we need to distinguish shallow and deep fake borrows? In fact, do we need to
+    /// distinguish fake and normal deep borrows?
     FakeBorrow,
     /// AddressOf for *const pointer.
     AddressOf,
diff --git a/compiler/rustc_mir_build/src/build/expr/as_place.rs b/compiler/rustc_mir_build/src/build/expr/as_place.rs
index f12e25db6fc..060b328ef48 100644
--- a/compiler/rustc_mir_build/src/build/expr/as_place.rs
+++ b/compiler/rustc_mir_build/src/build/expr/as_place.rs
@@ -685,7 +685,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                             fake_borrow_temp.into(),
                             Rvalue::Ref(
                                 tcx.lifetimes.re_erased,
-                                BorrowKind::Fake,
+                                BorrowKind::Fake(FakeBorrowKind::Shallow),
                                 Place { local: base_place.local, projection },
                             ),
                         );
diff --git a/compiler/rustc_mir_build/src/build/matches/mod.rs b/compiler/rustc_mir_build/src/build/matches/mod.rs
index 648d0b52b32..b41523cb9c2 100644
--- a/compiler/rustc_mir_build/src/build/matches/mod.rs
+++ b/compiler/rustc_mir_build/src/build/matches/mod.rs
@@ -2106,7 +2106,8 @@ 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::Fake, place);
+                let borrow =
+                    Rvalue::Ref(re_erased, BorrowKind::Fake(FakeBorrowKind::Shallow), place);
                 self.cfg.push_assign(block, scrutinee_source_info, Place::from(temp), borrow);
             }
 
diff --git a/compiler/rustc_mir_build/src/check_unsafety.rs b/compiler/rustc_mir_build/src/check_unsafety.rs
index 0c1e1d59c4f..227d19c3e43 100644
--- a/compiler/rustc_mir_build/src/check_unsafety.rs
+++ b/compiler/rustc_mir_build/src/check_unsafety.rs
@@ -513,7 +513,7 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                 visit::walk_expr(&mut visitor, expr);
                 if visitor.found {
                     match borrow_kind {
-                        BorrowKind::Fake | BorrowKind::Shared
+                        BorrowKind::Fake(_) | BorrowKind::Shared
                             if !self.thir[arg].ty.is_freeze(self.tcx, self.param_env) =>
                         {
                             self.requires_unsafe(expr.span, BorrowOfLayoutConstrainedField)
@@ -521,7 +521,7 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                         BorrowKind::Mut { .. } => {
                             self.requires_unsafe(expr.span, MutationOfLayoutConstrainedField)
                         }
-                        BorrowKind::Fake | BorrowKind::Shared => {}
+                        BorrowKind::Fake(_) | BorrowKind::Shared => {}
                     }
                 }
             }
diff --git a/compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs b/compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs
index 693994b5da7..bdc70de58e8 100644
--- a/compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs
+++ b/compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs
@@ -102,7 +102,7 @@ where
             }
 
             Rvalue::Cast(..)
-            | Rvalue::Ref(_, BorrowKind::Fake, _)
+            | Rvalue::Ref(_, BorrowKind::Fake(_), _)
             | Rvalue::ShallowInitBox(..)
             | Rvalue::Use(..)
             | Rvalue::ThreadLocalRef(..)
diff --git a/compiler/rustc_mir_transform/src/cleanup_post_borrowck.rs b/compiler/rustc_mir_transform/src/cleanup_post_borrowck.rs
index da82f8de781..48a6a83e146 100644
--- a/compiler/rustc_mir_transform/src/cleanup_post_borrowck.rs
+++ b/compiler/rustc_mir_transform/src/cleanup_post_borrowck.rs
@@ -29,7 +29,7 @@ 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::Fake, _)))
+                    | StatementKind::Assign(box (_, Rvalue::Ref(_, BorrowKind::Fake(_), _)))
                     | StatementKind::Coverage(
                         // These kinds of coverage statements are markers inserted during
                         // MIR building, and are not needed after InstrumentCoverage.
diff --git a/compiler/rustc_mir_transform/src/promote_consts.rs b/compiler/rustc_mir_transform/src/promote_consts.rs
index 1f4af0ec63d..c14d4ceb091 100644
--- a/compiler/rustc_mir_transform/src/promote_consts.rs
+++ b/compiler/rustc_mir_transform/src/promote_consts.rs
@@ -384,7 +384,7 @@ 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::Fake | BorrowKind::Mut { kind: MutBorrowKind::ClosureCapture } => {
+            BorrowKind::Fake(_) | BorrowKind::Mut { kind: MutBorrowKind::ClosureCapture } => {
                 return Err(Unpromotable);
             }
 
diff --git a/compiler/rustc_smir/src/rustc_smir/convert/mir.rs b/compiler/rustc_smir/src/rustc_smir/convert/mir.rs
index c9f66612590..a701dfe7046 100644
--- a/compiler/rustc_smir/src/rustc_smir/convert/mir.rs
+++ b/compiler/rustc_smir/src/rustc_smir/convert/mir.rs
@@ -229,7 +229,7 @@ impl<'tcx> Stable<'tcx> for mir::BorrowKind {
         use rustc_middle::mir::BorrowKind::*;
         match *self {
             Shared => stable_mir::mir::BorrowKind::Shared,
-            Fake => stable_mir::mir::BorrowKind::Fake,
+            Fake(kind) => stable_mir::mir::BorrowKind::Fake(kind.stable(tables)),
             Mut { kind } => stable_mir::mir::BorrowKind::Mut { kind: kind.stable(tables) },
         }
     }
@@ -247,6 +247,17 @@ impl<'tcx> Stable<'tcx> for mir::MutBorrowKind {
     }
 }
 
+impl<'tcx> Stable<'tcx> for mir::FakeBorrowKind {
+    type T = stable_mir::mir::FakeBorrowKind;
+    fn stable(&self, _: &mut Tables<'_>) -> Self::T {
+        use rustc_middle::mir::FakeBorrowKind::*;
+        match *self {
+            Deep => stable_mir::mir::FakeBorrowKind::Deep,
+            Shallow => stable_mir::mir::FakeBorrowKind::Shallow,
+        }
+    }
+}
+
 impl<'tcx> Stable<'tcx> for mir::NullOp<'tcx> {
     type T = stable_mir::mir::NullOp;
     fn stable(&self, tables: &mut Tables<'_>) -> Self::T {
diff --git a/compiler/stable_mir/src/mir/body.rs b/compiler/stable_mir/src/mir/body.rs
index 1ad05633d62..e928a6f3dcd 100644
--- a/compiler/stable_mir/src/mir/body.rs
+++ b/compiler/stable_mir/src/mir/body.rs
@@ -865,11 +865,9 @@ pub enum BorrowKind {
     /// Data must be immutable and is aliasable.
     Shared,
 
-    /// The immediately borrowed place must be immutable, but projections from
-    /// 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`.
-    Fake,
+    /// An immutable, aliasable borrow that is discarded after borrow-checking. Can behave either
+    /// like a normal shared borrow or like a special shallow borrow (see [`FakeBorrowKind`]).
+    Fake(FakeBorrowKind),
 
     /// Data is mutable and not aliasable.
     Mut {
@@ -884,7 +882,7 @@ impl BorrowKind {
             BorrowKind::Mut { .. } => Mutability::Mut,
             BorrowKind::Shared => Mutability::Not,
             // FIXME: There's no type corresponding to a shallow borrow, so use `&` as an approximation.
-            BorrowKind::Fake => Mutability::Not,
+            BorrowKind::Fake(_) => Mutability::Not,
         }
     }
 }
@@ -896,6 +894,17 @@ pub enum MutBorrowKind {
     ClosureCapture,
 }
 
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub enum FakeBorrowKind {
+    /// A shared (deep) borrow. Data must be immutable and is aliasable.
+    Deep,
+    /// The immediately borrowed place must be immutable, but projections from
+    /// 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,
+}
+
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 pub enum Mutability {
     Not,
diff --git a/compiler/stable_mir/src/mir/pretty.rs b/compiler/stable_mir/src/mir/pretty.rs
index 4ac4833add7..bbca3965852 100644
--- a/compiler/stable_mir/src/mir/pretty.rs
+++ b/compiler/stable_mir/src/mir/pretty.rs
@@ -8,7 +8,7 @@ use std::{fmt, io, iter};
 
 use super::{AssertMessage, BinOp, TerminatorKind};
 
-use super::BorrowKind;
+use super::{BorrowKind, FakeBorrowKind};
 
 impl Display for Ty {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
@@ -352,7 +352,8 @@ fn pretty_rvalue<W: Write>(writer: &mut W, rval: &Rvalue) -> io::Result<()> {
         Rvalue::Ref(_, borrowkind, place) => {
             let kind = match borrowkind {
                 BorrowKind::Shared => "&",
-                BorrowKind::Fake => "&fake ",
+                BorrowKind::Fake(FakeBorrowKind::Deep) => "&fake ",
+                BorrowKind::Fake(FakeBorrowKind::Shallow) => "&fake shallow ",
                 BorrowKind::Mut { .. } => "&mut ",
             };
             write!(writer, "{kind}{:?}", place)
diff --git a/tests/mir-opt/building/match/match_false_edges.full_tested_match.built.after.mir b/tests/mir-opt/building/match/match_false_edges.full_tested_match.built.after.mir
index 0d381ce70f1..bade0fa4b45 100644
--- a/tests/mir-opt/building/match/match_false_edges.full_tested_match.built.after.mir
+++ b/tests/mir-opt/building/match/match_false_edges.full_tested_match.built.after.mir
@@ -60,7 +60,7 @@ fn full_tested_match() -> () {
     bb7: {
         StorageLive(_6);
         _6 = &((_2 as Some).0: i32);
-        _3 = &fake _2;
+        _3 = &fake shallow _2;
         StorageLive(_7);
         _7 = guard() -> [return: bb8, unwind: bb16];
     }
diff --git a/tests/mir-opt/building/match/match_false_edges.full_tested_match2.built.after.mir b/tests/mir-opt/building/match/match_false_edges.full_tested_match2.built.after.mir
index d397de66fc4..0d78bb8b235 100644
--- a/tests/mir-opt/building/match/match_false_edges.full_tested_match2.built.after.mir
+++ b/tests/mir-opt/building/match/match_false_edges.full_tested_match2.built.after.mir
@@ -66,7 +66,7 @@ fn full_tested_match2() -> () {
     bb7: {
         StorageLive(_6);
         _6 = &((_2 as Some).0: i32);
-        _3 = &fake _2;
+        _3 = &fake shallow _2;
         StorageLive(_7);
         _7 = guard() -> [return: bb8, unwind: bb16];
     }
diff --git a/tests/mir-opt/building/match/match_false_edges.main.built.after.mir b/tests/mir-opt/building/match/match_false_edges.main.built.after.mir
index 63920cec885..ebb75ae141a 100644
--- a/tests/mir-opt/building/match/match_false_edges.main.built.after.mir
+++ b/tests/mir-opt/building/match/match_false_edges.main.built.after.mir
@@ -87,7 +87,7 @@ fn main() -> () {
     bb10: {
         StorageLive(_7);
         _7 = &((_2 as Some).0: i32);
-        _3 = &fake _2;
+        _3 = &fake shallow _2;
         StorageLive(_8);
         _8 = guard() -> [return: bb11, unwind: bb24];
     }
@@ -129,7 +129,7 @@ fn main() -> () {
     bb16: {
         StorageLive(_11);
         _11 = &((_2 as Some).0: i32);
-        _3 = &fake _2;
+        _3 = &fake shallow _2;
         StorageLive(_12);
         StorageLive(_13);
         _13 = (*_11);
diff --git a/tests/mir-opt/building/match/sort_candidates.constant_eq.SimplifyCfg-initial.after.mir b/tests/mir-opt/building/match/sort_candidates.constant_eq.SimplifyCfg-initial.after.mir
index 21ddd39137f..060cd6132e3 100644
--- a/tests/mir-opt/building/match/sort_candidates.constant_eq.SimplifyCfg-initial.after.mir
+++ b/tests/mir-opt/building/match/sort_candidates.constant_eq.SimplifyCfg-initial.after.mir
@@ -72,8 +72,8 @@ fn constant_eq(_1: &str, _2: bool) -> u32 {
     }
 
     bb12: {
-        _6 = &fake (_3.0: &str);
-        _7 = &fake (_3.1: bool);
+        _6 = &fake shallow (_3.0: &str);
+        _7 = &fake shallow (_3.1: bool);
         StorageLive(_10);
         _10 = const true;
         switchInt(move _10) -> [0: bb14, otherwise: bb13];
diff --git a/tests/mir-opt/building/match/sort_candidates.disjoint_ranges.SimplifyCfg-initial.after.mir b/tests/mir-opt/building/match/sort_candidates.disjoint_ranges.SimplifyCfg-initial.after.mir
index 6068cef8fbc..07daa3eddfa 100644
--- a/tests/mir-opt/building/match/sort_candidates.disjoint_ranges.SimplifyCfg-initial.after.mir
+++ b/tests/mir-opt/building/match/sort_candidates.disjoint_ranges.SimplifyCfg-initial.after.mir
@@ -54,7 +54,7 @@ fn disjoint_ranges(_1: i32, _2: bool) -> u32 {
     }
 
     bb9: {
-        _3 = &fake _1;
+        _3 = &fake shallow _1;
         StorageLive(_8);
         _8 = _2;
         switchInt(move _8) -> [0: bb11, otherwise: bb10];
diff --git a/tests/mir-opt/match_arm_scopes.complicated_match.panic-abort.SimplifyCfg-initial.after-ElaborateDrops.after.diff b/tests/mir-opt/match_arm_scopes.complicated_match.panic-abort.SimplifyCfg-initial.after-ElaborateDrops.after.diff
index ba333ba1a58..209f0d09c29 100644
--- a/tests/mir-opt/match_arm_scopes.complicated_match.panic-abort.SimplifyCfg-initial.after-ElaborateDrops.after.diff
+++ b/tests/mir-opt/match_arm_scopes.complicated_match.panic-abort.SimplifyCfg-initial.after-ElaborateDrops.after.diff
@@ -80,8 +80,8 @@
           _6 = &(_2.1: bool);
           StorageLive(_8);
           _8 = &(_2.2: std::string::String);
--         _3 = &fake (_2.0: bool);
--         _4 = &fake (_2.1: bool);
+-         _3 = &fake shallow (_2.0: bool);
+-         _4 = &fake shallow (_2.1: bool);
           StorageLive(_9);
           StorageLive(_10);
           _10 = _1;
@@ -137,8 +137,8 @@
           _6 = &(_2.0: bool);
           StorageLive(_8);
           _8 = &(_2.2: std::string::String);
--         _3 = &fake (_2.0: bool);
--         _4 = &fake (_2.1: bool);
+-         _3 = &fake shallow (_2.0: bool);
+-         _4 = &fake shallow (_2.1: bool);
           StorageLive(_12);
           StorageLive(_13);
           _13 = _1;
diff --git a/tests/mir-opt/match_arm_scopes.complicated_match.panic-unwind.SimplifyCfg-initial.after-ElaborateDrops.after.diff b/tests/mir-opt/match_arm_scopes.complicated_match.panic-unwind.SimplifyCfg-initial.after-ElaborateDrops.after.diff
index ba333ba1a58..209f0d09c29 100644
--- a/tests/mir-opt/match_arm_scopes.complicated_match.panic-unwind.SimplifyCfg-initial.after-ElaborateDrops.after.diff
+++ b/tests/mir-opt/match_arm_scopes.complicated_match.panic-unwind.SimplifyCfg-initial.after-ElaborateDrops.after.diff
@@ -80,8 +80,8 @@
           _6 = &(_2.1: bool);
           StorageLive(_8);
           _8 = &(_2.2: std::string::String);
--         _3 = &fake (_2.0: bool);
--         _4 = &fake (_2.1: bool);
+-         _3 = &fake shallow (_2.0: bool);
+-         _4 = &fake shallow (_2.1: bool);
           StorageLive(_9);
           StorageLive(_10);
           _10 = _1;
@@ -137,8 +137,8 @@
           _6 = &(_2.0: bool);
           StorageLive(_8);
           _8 = &(_2.2: std::string::String);
--         _3 = &fake (_2.0: bool);
--         _4 = &fake (_2.1: bool);
+-         _3 = &fake shallow (_2.0: bool);
+-         _4 = &fake shallow (_2.1: bool);
           StorageLive(_12);
           StorageLive(_13);
           _13 = _1;
diff --git a/tests/mir-opt/remove_fake_borrows.match_guard.CleanupPostBorrowck.panic-abort.diff b/tests/mir-opt/remove_fake_borrows.match_guard.CleanupPostBorrowck.panic-abort.diff
index 8feb59ec874..d76d65a18a7 100644
--- a/tests/mir-opt/remove_fake_borrows.match_guard.CleanupPostBorrowck.panic-abort.diff
+++ b/tests/mir-opt/remove_fake_borrows.match_guard.CleanupPostBorrowck.panic-abort.diff
@@ -33,10 +33,10 @@
       }
   
       bb4: {
--         _3 = &fake _1;
--         _4 = &fake (*(*((_1 as Some).0: &&i32)));
--         _5 = &fake (*((_1 as Some).0: &&i32));
--         _6 = &fake ((_1 as Some).0: &&i32);
+-         _3 = &fake shallow _1;
+-         _4 = &fake shallow (*(*((_1 as Some).0: &&i32)));
+-         _5 = &fake shallow (*((_1 as Some).0: &&i32));
+-         _6 = &fake shallow ((_1 as Some).0: &&i32);
 +         nop;
 +         nop;
 +         nop;
diff --git a/tests/mir-opt/remove_fake_borrows.match_guard.CleanupPostBorrowck.panic-unwind.diff b/tests/mir-opt/remove_fake_borrows.match_guard.CleanupPostBorrowck.panic-unwind.diff
index 8feb59ec874..d76d65a18a7 100644
--- a/tests/mir-opt/remove_fake_borrows.match_guard.CleanupPostBorrowck.panic-unwind.diff
+++ b/tests/mir-opt/remove_fake_borrows.match_guard.CleanupPostBorrowck.panic-unwind.diff
@@ -33,10 +33,10 @@
       }
   
       bb4: {
--         _3 = &fake _1;
--         _4 = &fake (*(*((_1 as Some).0: &&i32)));
--         _5 = &fake (*((_1 as Some).0: &&i32));
--         _6 = &fake ((_1 as Some).0: &&i32);
+-         _3 = &fake shallow _1;
+-         _4 = &fake shallow (*(*((_1 as Some).0: &&i32)));
+-         _5 = &fake shallow (*((_1 as Some).0: &&i32));
+-         _6 = &fake shallow ((_1 as Some).0: &&i32);
 +         nop;
 +         nop;
 +         nop;