From fc5c2956b11e29f931cec010e3f38461ec4ac309 Mon Sep 17 00:00:00 2001
From: Dylan MacKenzie <ecstaticmorse@gmail.com>
Date: Wed, 12 Feb 2020 13:33:43 -0800
Subject: [PATCH 01/12] Impl `GenKill` for old dataflow framework's
 `GenKillSet`

This impl is temporary and will be removed along with the old dataflow
framework. It allows us to reuse the transfer function of new dataflow
analyses when defining old ones
---
 src/librustc_mir/dataflow/generic/mod.rs | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/src/librustc_mir/dataflow/generic/mod.rs b/src/librustc_mir/dataflow/generic/mod.rs
index ea643042c5f86..23b22550a3b0e 100644
--- a/src/librustc_mir/dataflow/generic/mod.rs
+++ b/src/librustc_mir/dataflow/generic/mod.rs
@@ -395,5 +395,16 @@ impl<T: Idx> GenKill<T> for BitSet<T> {
     }
 }
 
+// For compatibility with old framework
+impl<T: Idx> GenKill<T> for crate::dataflow::GenKillSet<T> {
+    fn gen(&mut self, elem: T) {
+        self.gen(elem);
+    }
+
+    fn kill(&mut self, elem: T) {
+        self.kill(elem);
+    }
+}
+
 #[cfg(test)]
 mod tests;

From 7d9dadcccca9efc63f30fa1c9adee00effb860d4 Mon Sep 17 00:00:00 2001
From: Dylan MacKenzie <ecstaticmorse@gmail.com>
Date: Wed, 12 Feb 2020 13:36:47 -0800
Subject: [PATCH 02/12] Implement `Maybe{Mut,}BorrowedLocals` analyses

---
 .../dataflow/impls/borrowed_locals.rs         | 268 ++++++++++++++----
 src/librustc_mir/dataflow/mod.rs              |   3 +-
 2 files changed, 209 insertions(+), 62 deletions(-)

diff --git a/src/librustc_mir/dataflow/impls/borrowed_locals.rs b/src/librustc_mir/dataflow/impls/borrowed_locals.rs
index 63834d0ecda00..519db963d1e7d 100644
--- a/src/librustc_mir/dataflow/impls/borrowed_locals.rs
+++ b/src/librustc_mir/dataflow/impls/borrowed_locals.rs
@@ -1,102 +1,250 @@
 pub use super::*;
 
-use crate::dataflow::{BitDenotation, GenKillSet};
+use crate::dataflow::generic::{AnalysisDomain, GenKill, GenKillAnalysis};
 use rustc::mir::visit::Visitor;
 use rustc::mir::*;
+use rustc::ty::{ParamEnv, TyCtxt};
+use rustc_span::DUMMY_SP;
+
+pub type MaybeMutBorrowedLocals<'mir, 'tcx> = MaybeBorrowedLocals<MutBorrow<'mir, 'tcx>>;
+
+/// A dataflow analysis that tracks whether a pointer or reference could possibly exist that points
+/// to a given local.
+///
+/// The `K` parameter determines what kind of borrows are tracked. By default,
+/// `MaybeBorrowedLocals` looks for *any* borrow of a local. If you are only interested in borrows
+/// that might allow mutation, use the `MaybeMutBorrowedLocals` type alias instead.
+///
+/// At present, this is used as a very limited form of alias analysis. For example,
+/// `MaybeBorrowedLocals` is used to compute which locals are live during a yield expression for
+/// immovable generators. `MaybeMutBorrowedLocals` is used during const checking to prove that a
+/// local has not been mutated via indirect assignment (e.g., `*p = 42`), the side-effects of a
+/// function call or inline assembly.
+pub struct MaybeBorrowedLocals<K = AnyBorrow> {
+    kind: K,
+}
 
-/// This calculates if any part of a MIR local could have previously been borrowed.
-/// This means that once a local has been borrowed, its bit will be set
-/// from that point and onwards, until we see a StorageDead statement for the local,
-/// at which points there is no memory associated with the local, so it cannot be borrowed.
-/// This is used to compute which locals are live during a yield expression for
-/// immovable generators.
-#[derive(Copy, Clone)]
-pub struct HaveBeenBorrowedLocals<'a, 'tcx> {
-    body: &'a Body<'tcx>,
+impl MaybeBorrowedLocals {
+    /// A dataflow analysis that records whether a pointer or reference exists that may alias the
+    /// given local.
+    pub fn new() -> Self {
+        MaybeBorrowedLocals { kind: AnyBorrow }
+    }
 }
 
-impl<'a, 'tcx> HaveBeenBorrowedLocals<'a, 'tcx> {
-    pub fn new(body: &'a Body<'tcx>) -> Self {
-        HaveBeenBorrowedLocals { body }
+impl MaybeMutBorrowedLocals<'mir, 'tcx> {
+    /// A dataflow analysis that records whether a pointer or reference exists that may *mutably*
+    /// alias the given local.
+    pub fn new_mut_only(
+        tcx: TyCtxt<'tcx>,
+        body: &'mir mir::Body<'tcx>,
+        param_env: ParamEnv<'tcx>,
+    ) -> Self {
+        MaybeBorrowedLocals { kind: MutBorrow { body, tcx, param_env } }
     }
+}
 
-    pub fn body(&self) -> &Body<'tcx> {
-        self.body
+impl<K> MaybeBorrowedLocals<K> {
+    fn transfer_function<'a, T>(&'a self, trans: &'a mut T) -> TransferFunction<'a, T, K> {
+        TransferFunction { kind: &self.kind, trans }
     }
 }
 
-impl<'a, 'tcx> BitDenotation<'tcx> for HaveBeenBorrowedLocals<'a, 'tcx> {
+impl<K> AnalysisDomain<'tcx> for MaybeBorrowedLocals<K>
+where
+    K: BorrowAnalysisKind<'tcx>,
+{
     type Idx = Local;
-    fn name() -> &'static str {
-        "has_been_borrowed_locals"
+
+    const NAME: &'static str = K::ANALYSIS_NAME;
+
+    fn bits_per_block(&self, body: &mir::Body<'tcx>) -> usize {
+        body.local_decls().len()
+    }
+
+    fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut BitSet<Self::Idx>) {
+        // No locals are aliased on function entry
+    }
+}
+
+impl<K> GenKillAnalysis<'tcx> for MaybeBorrowedLocals<K>
+where
+    K: BorrowAnalysisKind<'tcx>,
+{
+    fn statement_effect(
+        &self,
+        trans: &mut impl GenKill<Self::Idx>,
+        statement: &mir::Statement<'tcx>,
+        location: Location,
+    ) {
+        self.transfer_function(trans).visit_statement(statement, location);
     }
-    fn bits_per_block(&self) -> usize {
-        self.body.local_decls.len()
+
+    fn terminator_effect(
+        &self,
+        trans: &mut impl GenKill<Self::Idx>,
+        terminator: &mir::Terminator<'tcx>,
+        location: Location,
+    ) {
+        self.transfer_function(trans).visit_terminator(terminator, location);
     }
 
-    fn start_block_effect(&self, _on_entry: &mut BitSet<Local>) {
-        // Nothing is borrowed on function entry
+    fn call_return_effect(
+        &self,
+        _trans: &mut impl GenKill<Self::Idx>,
+        _block: mir::BasicBlock,
+        _func: &mir::Operand<'tcx>,
+        _args: &[mir::Operand<'tcx>],
+        _dest_place: &mir::Place<'tcx>,
+    ) {
     }
+}
 
-    fn statement_effect(&self, trans: &mut GenKillSet<Local>, loc: Location) {
-        let stmt = &self.body[loc.block].statements[loc.statement_index];
+impl<K> BottomValue for MaybeBorrowedLocals<K> {
+    // bottom = unborrowed
+    const BOTTOM_VALUE: bool = false;
+}
 
-        BorrowedLocalsVisitor { trans }.visit_statement(stmt, loc);
+/// A `Visitor` that defines the transfer function for `MaybeBorrowedLocals`.
+struct TransferFunction<'a, T, K> {
+    trans: &'a mut T,
+    kind: &'a K,
+}
 
-        // StorageDead invalidates all borrows and raw pointers to a local
-        match stmt.kind {
-            StatementKind::StorageDead(l) => trans.kill(l),
-            _ => (),
+impl<T, K> Visitor<'tcx> for TransferFunction<'a, T, K>
+where
+    T: GenKill<Local>,
+    K: BorrowAnalysisKind<'tcx>,
+{
+    fn visit_statement(&mut self, stmt: &Statement<'tcx>, location: Location) {
+        self.super_statement(stmt, location);
+
+        // When we reach a `StorageDead` statement, we can assume that any pointers to this memory
+        // are now invalid.
+        if let StatementKind::StorageDead(local) = stmt.kind {
+            self.trans.kill(local);
         }
     }
 
-    fn terminator_effect(&self, trans: &mut GenKillSet<Local>, loc: Location) {
-        let terminator = self.body[loc.block].terminator();
-        BorrowedLocalsVisitor { trans }.visit_terminator(terminator, loc);
-        match &terminator.kind {
-            // Drop terminators borrows the location
-            TerminatorKind::Drop { location, .. }
-            | TerminatorKind::DropAndReplace { location, .. } => {
-                if let Some(local) = find_local(location) {
-                    trans.gen(local);
+    fn visit_rvalue(&mut self, rvalue: &mir::Rvalue<'tcx>, location: Location) {
+        self.super_rvalue(rvalue, location);
+
+        match rvalue {
+            mir::Rvalue::AddressOf(mt, borrowed_place) => {
+                if !borrowed_place.is_indirect() && self.kind.in_address_of(*mt, borrowed_place) {
+                    self.trans.gen(borrowed_place.local);
                 }
             }
-            _ => (),
+
+            mir::Rvalue::Ref(_, kind, borrowed_place) => {
+                if !borrowed_place.is_indirect() && self.kind.in_ref(*kind, borrowed_place) {
+                    self.trans.gen(borrowed_place.local);
+                }
+            }
+
+            mir::Rvalue::Cast(..)
+            | mir::Rvalue::Use(..)
+            | mir::Rvalue::Repeat(..)
+            | mir::Rvalue::Len(..)
+            | mir::Rvalue::BinaryOp(..)
+            | mir::Rvalue::CheckedBinaryOp(..)
+            | mir::Rvalue::NullaryOp(..)
+            | mir::Rvalue::UnaryOp(..)
+            | mir::Rvalue::Discriminant(..)
+            | mir::Rvalue::Aggregate(..) => {}
         }
     }
 
-    fn propagate_call_return(
-        &self,
-        _in_out: &mut BitSet<Local>,
-        _call_bb: mir::BasicBlock,
-        _dest_bb: mir::BasicBlock,
-        _dest_place: &mir::Place<'tcx>,
-    ) {
-        // Nothing to do when a call returns successfully
+    fn visit_terminator(&mut self, terminator: &mir::Terminator<'tcx>, location: Location) {
+        self.super_terminator(terminator, location);
+
+        match terminator.kind {
+            // Drop terminators may call custom drop glue (`Drop::drop`), which takes `&mut self`
+            // as a parameter. Hypothetically, a drop impl could launder that reference into the
+            // surrounding environment through a raw pointer, thus creating a valid `*mut` pointing
+            // to the dropped local. We are not yet willing to declare this particular case UB, so
+            // we must treat all dropped locals as mutably borrowed for now. See discussion on
+            // [#61069].
+            //
+            // [#61069]: https://github.com/rust-lang/rust/pull/61069
+            mir::TerminatorKind::Drop { location: dropped_place, .. }
+            | mir::TerminatorKind::DropAndReplace { location: dropped_place, .. } => {
+                self.trans.gen(dropped_place.local);
+            }
+
+            TerminatorKind::Abort
+            | TerminatorKind::Assert { .. }
+            | TerminatorKind::Call { .. }
+            | TerminatorKind::FalseEdges { .. }
+            | TerminatorKind::FalseUnwind { .. }
+            | TerminatorKind::GeneratorDrop
+            | TerminatorKind::Goto { .. }
+            | TerminatorKind::Resume
+            | TerminatorKind::Return
+            | TerminatorKind::SwitchInt { .. }
+            | TerminatorKind::Unreachable
+            | TerminatorKind::Yield { .. } => {}
+        }
     }
 }
 
-impl<'a, 'tcx> BottomValue for HaveBeenBorrowedLocals<'a, 'tcx> {
-    // bottom = unborrowed
-    const BOTTOM_VALUE: bool = false;
+pub struct AnyBorrow;
+
+pub struct MutBorrow<'mir, 'tcx> {
+    tcx: TyCtxt<'tcx>,
+    body: &'mir Body<'tcx>,
+    param_env: ParamEnv<'tcx>,
 }
 
-struct BorrowedLocalsVisitor<'gk> {
-    trans: &'gk mut GenKillSet<Local>,
+impl MutBorrow<'mir, 'tcx> {
+    // `&` and `&raw` only allow mutation if the borrowed place is `!Freeze`.
+    //
+    // This assumes that it is UB to take the address of a struct field whose type is
+    // `Freeze`, then use pointer arithmetic to derive a pointer to a *different* field of
+    // that same struct whose type is `!Freeze`. If we decide that this is not UB, we will
+    // have to check the type of the borrowed **local** instead of the borrowed **place**
+    // below. See [rust-lang/unsafe-code-guidelines#134].
+    //
+    // [rust-lang/unsafe-code-guidelines#134]: https://github.com/rust-lang/unsafe-code-guidelines/issues/134
+    fn shared_borrow_allows_mutation(&self, place: &Place<'tcx>) -> bool {
+        !place.ty(self.body, self.tcx).ty.is_freeze(self.tcx, self.param_env, DUMMY_SP)
+    }
 }
 
-fn find_local(place: &Place<'_>) -> Option<Local> {
-    if !place.is_indirect() { Some(place.local) } else { None }
+pub trait BorrowAnalysisKind<'tcx> {
+    const ANALYSIS_NAME: &'static str;
+
+    fn in_address_of(&self, mt: Mutability, place: &Place<'tcx>) -> bool;
+    fn in_ref(&self, kind: mir::BorrowKind, place: &Place<'tcx>) -> bool;
 }
 
-impl<'tcx> Visitor<'tcx> for BorrowedLocalsVisitor<'_> {
-    fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
-        if let Rvalue::Ref(_, _, ref place) = *rvalue {
-            if let Some(local) = find_local(place) {
-                self.trans.gen(local);
+impl BorrowAnalysisKind<'tcx> for AnyBorrow {
+    const ANALYSIS_NAME: &'static str = "maybe_borrowed_locals";
+
+    fn in_ref(&self, _: mir::BorrowKind, _: &Place<'_>) -> bool {
+        true
+    }
+    fn in_address_of(&self, _: Mutability, _: &Place<'_>) -> bool {
+        true
+    }
+}
+
+impl BorrowAnalysisKind<'tcx> for MutBorrow<'mir, 'tcx> {
+    const ANALYSIS_NAME: &'static str = "maybe_mut_borrowed_locals";
+
+    fn in_ref(&self, kind: mir::BorrowKind, place: &Place<'tcx>) -> bool {
+        match kind {
+            mir::BorrowKind::Mut { .. } => true,
+            mir::BorrowKind::Shared | mir::BorrowKind::Shallow | mir::BorrowKind::Unique => {
+                self.shared_borrow_allows_mutation(place)
             }
         }
+    }
 
-        self.super_rvalue(rvalue, location)
+    fn in_address_of(&self, mt: Mutability, place: &Place<'tcx>) -> bool {
+        match mt {
+            Mutability::Mut => true,
+            Mutability::Not => self.shared_borrow_allows_mutation(place),
+        }
     }
 }
diff --git a/src/librustc_mir/dataflow/mod.rs b/src/librustc_mir/dataflow/mod.rs
index 7cd7fc309b6b9..41bac894e48ce 100644
--- a/src/librustc_mir/dataflow/mod.rs
+++ b/src/librustc_mir/dataflow/mod.rs
@@ -23,8 +23,7 @@ pub(crate) use self::drop_flag_effects::*;
 pub use self::impls::borrows::Borrows;
 pub use self::impls::DefinitelyInitializedPlaces;
 pub use self::impls::EverInitializedPlaces;
-pub use self::impls::HaveBeenBorrowedLocals;
-pub use self::impls::IndirectlyMutableLocals;
+pub use self::impls::{MaybeBorrowedLocals, MaybeMutBorrowedLocals};
 pub use self::impls::{MaybeInitializedPlaces, MaybeUninitializedPlaces};
 pub use self::impls::{MaybeStorageLive, RequiresStorage};
 

From 34783b73bd891a66fb2af613fef7492376fc7c8a Mon Sep 17 00:00:00 2001
From: Dylan MacKenzie <ecstaticmorse@gmail.com>
Date: Wed, 12 Feb 2020 13:37:19 -0800
Subject: [PATCH 03/12] Remove outdated `IndirectlyMutableLocals`

`MaybeMutBorrowedLocals` serves the same purpose and has a better name.
---
 .../dataflow/impls/indirect_mutation.rs       | 136 ------------------
 src/librustc_mir/dataflow/impls/mod.rs        |   2 -
 2 files changed, 138 deletions(-)
 delete mode 100644 src/librustc_mir/dataflow/impls/indirect_mutation.rs

diff --git a/src/librustc_mir/dataflow/impls/indirect_mutation.rs b/src/librustc_mir/dataflow/impls/indirect_mutation.rs
deleted file mode 100644
index 85bf342c8a39a..0000000000000
--- a/src/librustc_mir/dataflow/impls/indirect_mutation.rs
+++ /dev/null
@@ -1,136 +0,0 @@
-use rustc::mir::visit::Visitor;
-use rustc::mir::{self, Local, Location};
-use rustc::ty::{self, TyCtxt};
-use rustc_index::bit_set::BitSet;
-use rustc_span::DUMMY_SP;
-
-use crate::dataflow::{self, GenKillSet};
-
-/// Whether a borrow to a `Local` has been created that could allow that `Local` to be mutated
-/// indirectly. This could either be a mutable reference (`&mut`) or a shared borrow if the type of
-/// that `Local` allows interior mutability. Operations that can mutate local's indirectly include:
-/// assignments through a pointer (`*p = 42`), function calls, drop terminators and inline assembly.
-///
-/// If this returns false for a `Local` at a given statement (or terminator), that `Local` could
-/// not possibly have been mutated indirectly prior to that statement.
-#[derive(Copy, Clone)]
-pub struct IndirectlyMutableLocals<'mir, 'tcx> {
-    body: &'mir mir::Body<'tcx>,
-    tcx: TyCtxt<'tcx>,
-    param_env: ty::ParamEnv<'tcx>,
-}
-
-impl<'mir, 'tcx> IndirectlyMutableLocals<'mir, 'tcx> {
-    pub fn new(
-        tcx: TyCtxt<'tcx>,
-        body: &'mir mir::Body<'tcx>,
-        param_env: ty::ParamEnv<'tcx>,
-    ) -> Self {
-        IndirectlyMutableLocals { body, tcx, param_env }
-    }
-
-    fn transfer_function<'a>(
-        &self,
-        trans: &'a mut GenKillSet<Local>,
-    ) -> TransferFunction<'a, 'mir, 'tcx> {
-        TransferFunction { body: self.body, tcx: self.tcx, param_env: self.param_env, trans }
-    }
-}
-
-impl<'mir, 'tcx> dataflow::BitDenotation<'tcx> for IndirectlyMutableLocals<'mir, 'tcx> {
-    type Idx = Local;
-
-    fn name() -> &'static str {
-        "mut_borrowed_locals"
-    }
-
-    fn bits_per_block(&self) -> usize {
-        self.body.local_decls.len()
-    }
-
-    fn start_block_effect(&self, _entry_set: &mut BitSet<Local>) {
-        // Nothing is borrowed on function entry
-    }
-
-    fn statement_effect(&self, trans: &mut GenKillSet<Local>, loc: Location) {
-        let stmt = &self.body[loc.block].statements[loc.statement_index];
-        self.transfer_function(trans).visit_statement(stmt, loc);
-    }
-
-    fn terminator_effect(&self, trans: &mut GenKillSet<Local>, loc: Location) {
-        let terminator = self.body[loc.block].terminator();
-        self.transfer_function(trans).visit_terminator(terminator, loc);
-    }
-
-    fn propagate_call_return(
-        &self,
-        _in_out: &mut BitSet<Local>,
-        _call_bb: mir::BasicBlock,
-        _dest_bb: mir::BasicBlock,
-        _dest_place: &mir::Place<'tcx>,
-    ) {
-        // Nothing to do when a call returns successfully
-    }
-}
-
-impl<'mir, 'tcx> dataflow::BottomValue for IndirectlyMutableLocals<'mir, 'tcx> {
-    // bottom = unborrowed
-    const BOTTOM_VALUE: bool = false;
-}
-
-/// A `Visitor` that defines the transfer function for `IndirectlyMutableLocals`.
-struct TransferFunction<'a, 'mir, 'tcx> {
-    trans: &'a mut GenKillSet<Local>,
-    body: &'mir mir::Body<'tcx>,
-    tcx: TyCtxt<'tcx>,
-    param_env: ty::ParamEnv<'tcx>,
-}
-
-impl<'tcx> TransferFunction<'_, '_, 'tcx> {
-    /// Returns `true` if this borrow would allow mutation of the `borrowed_place`.
-    fn borrow_allows_mutation(
-        &self,
-        kind: mir::BorrowKind,
-        borrowed_place: &mir::Place<'tcx>,
-    ) -> bool {
-        match kind {
-            mir::BorrowKind::Mut { .. } => true,
-
-            mir::BorrowKind::Shared | mir::BorrowKind::Shallow | mir::BorrowKind::Unique => {
-                !borrowed_place.ty(self.body, self.tcx).ty.is_freeze(
-                    self.tcx,
-                    self.param_env,
-                    DUMMY_SP,
-                )
-            }
-        }
-    }
-}
-
-impl<'tcx> Visitor<'tcx> for TransferFunction<'_, '_, 'tcx> {
-    fn visit_rvalue(&mut self, rvalue: &mir::Rvalue<'tcx>, location: Location) {
-        if let mir::Rvalue::Ref(_, kind, ref borrowed_place) = *rvalue {
-            if self.borrow_allows_mutation(kind, borrowed_place) {
-                if !borrowed_place.is_indirect() {
-                    self.trans.gen(borrowed_place.local);
-                }
-            }
-        }
-
-        self.super_rvalue(rvalue, location);
-    }
-
-    fn visit_terminator(&mut self, terminator: &mir::Terminator<'tcx>, location: Location) {
-        // This method purposely does nothing except call `super_terminator`. It exists solely to
-        // document the subtleties around drop terminators.
-
-        self.super_terminator(terminator, location);
-
-        if let mir::TerminatorKind::Drop { location: _, .. }
-        | mir::TerminatorKind::DropAndReplace { location: _, .. } = &terminator.kind
-        {
-            // Although drop terminators mutably borrow the location being dropped, that borrow
-            // cannot live beyond the drop terminator because the dropped location is invalidated.
-        }
-    }
-}
diff --git a/src/librustc_mir/dataflow/impls/mod.rs b/src/librustc_mir/dataflow/impls/mod.rs
index 5b2264c2a6526..acea31857818b 100644
--- a/src/librustc_mir/dataflow/impls/mod.rs
+++ b/src/librustc_mir/dataflow/impls/mod.rs
@@ -20,11 +20,9 @@ use super::drop_flag_effects_for_location;
 use super::on_lookup_result_bits;
 
 mod borrowed_locals;
-mod indirect_mutation;
 mod storage_liveness;
 
 pub use self::borrowed_locals::*;
-pub use self::indirect_mutation::IndirectlyMutableLocals;
 pub use self::storage_liveness::*;
 
 pub(super) mod borrows;

From 9972502bafab062b06ef04c02c653f1b868937bd Mon Sep 17 00:00:00 2001
From: Dylan MacKenzie <ecstaticmorse@gmail.com>
Date: Wed, 12 Feb 2020 13:38:11 -0800
Subject: [PATCH 04/12] Reenable peek test for indirect mutation analysis

This uses the new `MaybeMutBorrowedLocals` pass but we keep the
`rustc_peek_indirectly_mutable` since the two are interchangable except
when inspecting a local after it has been marked `StorageDead`.
---
 src/librustc_mir/transform/rustc_peek.rs | 26 ++++++------------------
 1 file changed, 6 insertions(+), 20 deletions(-)

diff --git a/src/librustc_mir/transform/rustc_peek.rs b/src/librustc_mir/transform/rustc_peek.rs
index 7d8506eb28105..16edb7d552b24 100644
--- a/src/librustc_mir/transform/rustc_peek.rs
+++ b/src/librustc_mir/transform/rustc_peek.rs
@@ -12,9 +12,8 @@ use rustc_index::bit_set::BitSet;
 use crate::dataflow::generic::{Analysis, Results, ResultsCursor};
 use crate::dataflow::move_paths::{HasMoveData, MoveData};
 use crate::dataflow::move_paths::{LookupResult, MovePathIndex};
-use crate::dataflow::IndirectlyMutableLocals;
+use crate::dataflow::MaybeMutBorrowedLocals;
 use crate::dataflow::MoveDataParamEnv;
-use crate::dataflow::{do_dataflow, DebugFormatted};
 use crate::dataflow::{
     DefinitelyInitializedPlaces, MaybeInitializedPlaces, MaybeUninitializedPlaces,
 };
@@ -24,7 +23,6 @@ pub struct SanityCheck;
 impl<'tcx> MirPass<'tcx> for SanityCheck {
     fn run_pass(&self, tcx: TyCtxt<'tcx>, src: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>) {
         use crate::dataflow::has_rustc_mir_with;
-
         let def_id = src.def_id();
         if !tcx.has_attr(def_id, sym::rustc_mir) {
             debug!("skipping rustc_peek::SanityCheck on {}", tcx.def_path_str(def_id));
@@ -37,7 +35,6 @@ impl<'tcx> MirPass<'tcx> for SanityCheck {
         let param_env = tcx.param_env(def_id);
         let move_data = MoveData::gather_moves(body, tcx, param_env).unwrap();
         let mdpe = MoveDataParamEnv { move_data: move_data, param_env: param_env };
-        let dead_unwinds = BitSet::new_empty(body.basic_blocks().len());
 
         let flow_inits = MaybeInitializedPlaces::new(tcx, body, &mdpe)
             .into_engine(tcx, body, def_id)
@@ -48,15 +45,9 @@ impl<'tcx> MirPass<'tcx> for SanityCheck {
         let flow_def_inits = DefinitelyInitializedPlaces::new(tcx, body, &mdpe)
             .into_engine(tcx, body, def_id)
             .iterate_to_fixpoint();
-        let _flow_indirectly_mut = do_dataflow(
-            tcx,
-            body,
-            def_id,
-            &attributes,
-            &dead_unwinds,
-            IndirectlyMutableLocals::new(tcx, body, param_env),
-            |_, i| DebugFormatted::new(&i),
-        );
+        let flow_mut_borrowed = MaybeMutBorrowedLocals::new_mut_only(tcx, body, param_env)
+            .into_engine(tcx, body, def_id)
+            .iterate_to_fixpoint();
 
         if has_rustc_mir_with(&attributes, sym::rustc_peek_maybe_init).is_some() {
             sanity_check_via_rustc_peek(tcx, body, def_id, &attributes, &flow_inits);
@@ -67,12 +58,9 @@ impl<'tcx> MirPass<'tcx> for SanityCheck {
         if has_rustc_mir_with(&attributes, sym::rustc_peek_definite_init).is_some() {
             sanity_check_via_rustc_peek(tcx, body, def_id, &attributes, &flow_def_inits);
         }
-        // FIXME: Uncomment these as analyses are migrated to the new framework
-        /*
         if has_rustc_mir_with(&attributes, sym::rustc_peek_indirectly_mutable).is_some() {
-            sanity_check_via_rustc_peek(tcx, body, def_id, &attributes, &flow_indirectly_mut);
+            sanity_check_via_rustc_peek(tcx, body, def_id, &attributes, &flow_mut_borrowed);
         }
-        */
         if has_rustc_mir_with(&attributes, sym::stop_after_dataflow).is_some() {
             tcx.sess.fatal("stop_after_dataflow ended compilation");
         }
@@ -276,8 +264,7 @@ where
     }
 }
 
-/* FIXME: Add this back once `IndirectlyMutableLocals` uses the new dataflow framework.
-impl<'tcx> RustcPeekAt<'tcx> for IndirectlyMutableLocals<'_, 'tcx> {
+impl<'tcx> RustcPeekAt<'tcx> for MaybeMutBorrowedLocals<'_, 'tcx> {
     fn peek_at(
         &self,
         tcx: TyCtxt<'tcx>,
@@ -298,4 +285,3 @@ impl<'tcx> RustcPeekAt<'tcx> for IndirectlyMutableLocals<'_, 'tcx> {
         }
     }
 }
-*/

From 1d737fb032b762e69e8d809c0e042d45e08b457d Mon Sep 17 00:00:00 2001
From: Dylan MacKenzie <ecstaticmorse@gmail.com>
Date: Wed, 12 Feb 2020 13:41:26 -0800
Subject: [PATCH 05/12] Use `MaybeBorrowedLocals` for generator analyses

It should have the same semantics as `HaveBeenBorrowedLocals`
---
 .../dataflow/impls/storage_liveness.rs        | 46 ++++++++-----------
 src/librustc_mir/transform/generator.rs       | 20 +++-----
 2 files changed, 27 insertions(+), 39 deletions(-)

diff --git a/src/librustc_mir/dataflow/impls/storage_liveness.rs b/src/librustc_mir/dataflow/impls/storage_liveness.rs
index 040c13e8210ea..7508d71945e5f 100644
--- a/src/librustc_mir/dataflow/impls/storage_liveness.rs
+++ b/src/librustc_mir/dataflow/impls/storage_liveness.rs
@@ -1,8 +1,8 @@
 pub use super::*;
 
+use crate::dataflow::generic::{Results, ResultsRefCursor};
 use crate::dataflow::BitDenotation;
-use crate::dataflow::HaveBeenBorrowedLocals;
-use crate::dataflow::{DataflowResults, DataflowResultsCursor, DataflowResultsRefCursor};
+use crate::dataflow::MaybeBorrowedLocals;
 use rustc::mir::visit::{NonMutatingUseContext, PlaceContext, Visitor};
 use rustc::mir::*;
 use std::cell::RefCell;
@@ -69,22 +69,23 @@ impl<'a, 'tcx> BottomValue for MaybeStorageLive<'a, 'tcx> {
     const BOTTOM_VALUE: bool = false;
 }
 
+type BorrowedLocalsResults<'a, 'tcx> = ResultsRefCursor<'a, 'a, 'tcx, MaybeBorrowedLocals>;
+
 /// Dataflow analysis that determines whether each local requires storage at a
 /// given location; i.e. whether its storage can go away without being observed.
 pub struct RequiresStorage<'mir, 'tcx> {
     body: ReadOnlyBodyAndCache<'mir, 'tcx>,
-    borrowed_locals:
-        RefCell<DataflowResultsRefCursor<'mir, 'tcx, HaveBeenBorrowedLocals<'mir, 'tcx>>>,
+    borrowed_locals: RefCell<BorrowedLocalsResults<'mir, 'tcx>>,
 }
 
 impl<'mir, 'tcx: 'mir> RequiresStorage<'mir, 'tcx> {
     pub fn new(
         body: ReadOnlyBodyAndCache<'mir, 'tcx>,
-        borrowed_locals: &'mir DataflowResults<'tcx, HaveBeenBorrowedLocals<'mir, 'tcx>>,
+        borrowed_locals: &'mir Results<'tcx, MaybeBorrowedLocals>,
     ) -> Self {
         RequiresStorage {
             body,
-            borrowed_locals: RefCell::new(DataflowResultsCursor::new(borrowed_locals, *body)),
+            borrowed_locals: RefCell::new(ResultsRefCursor::new(*body, borrowed_locals)),
         }
     }
 
@@ -111,11 +112,12 @@ impl<'mir, 'tcx> BitDenotation<'tcx> for RequiresStorage<'mir, 'tcx> {
     }
 
     fn before_statement_effect(&self, sets: &mut GenKillSet<Self::Idx>, loc: Location) {
-        // If we borrow or assign to a place then it needs storage for that
-        // statement.
-        self.check_for_borrow(sets, loc);
-
         let stmt = &self.body[loc.block].statements[loc.statement_index];
+
+        // If a place is borrowed in a statement, it needs storage for that statement.
+        self.borrowed_locals.borrow().analysis().statement_effect(sets, stmt, loc);
+
+        // If a place is assigned to in a statement, it needs storage for that statement.
         match stmt.kind {
             StatementKind::StorageDead(l) => sets.kill(l),
             StatementKind::Assign(box (ref place, _))
@@ -138,12 +140,13 @@ impl<'mir, 'tcx> BitDenotation<'tcx> for RequiresStorage<'mir, 'tcx> {
     }
 
     fn before_terminator_effect(&self, sets: &mut GenKillSet<Local>, loc: Location) {
-        self.check_for_borrow(sets, loc);
+        let terminator = self.body[loc.block].terminator();
 
-        if let TerminatorKind::Call { destination: Some((Place { local, .. }, _)), .. } =
-            self.body[loc.block].terminator().kind
-        {
-            sets.gen(local);
+        // If a place is borrowed in a terminator, it needs storage for that terminator.
+        self.borrowed_locals.borrow().analysis().terminator_effect(sets, terminator, loc);
+
+        if let TerminatorKind::Call { destination: Some((place, _)), .. } = terminator.kind {
+            sets.gen(place.local);
         }
     }
 
@@ -179,14 +182,6 @@ impl<'mir, 'tcx> RequiresStorage<'mir, 'tcx> {
         let mut visitor = MoveVisitor { sets, borrowed_locals: &self.borrowed_locals };
         visitor.visit_location(self.body, loc);
     }
-
-    /// Gen locals that are newly borrowed. This includes borrowing any part of
-    /// a local (we rely on this behavior of `HaveBeenBorrowedLocals`).
-    fn check_for_borrow(&self, sets: &mut GenKillSet<Local>, loc: Location) {
-        let mut borrowed_locals = self.borrowed_locals.borrow_mut();
-        borrowed_locals.seek(loc);
-        borrowed_locals.each_gen_bit(|l| sets.gen(l));
-    }
 }
 
 impl<'mir, 'tcx> BottomValue for RequiresStorage<'mir, 'tcx> {
@@ -195,8 +190,7 @@ impl<'mir, 'tcx> BottomValue for RequiresStorage<'mir, 'tcx> {
 }
 
 struct MoveVisitor<'a, 'mir, 'tcx> {
-    borrowed_locals:
-        &'a RefCell<DataflowResultsRefCursor<'mir, 'tcx, HaveBeenBorrowedLocals<'mir, 'tcx>>>,
+    borrowed_locals: &'a RefCell<BorrowedLocalsResults<'mir, 'tcx>>,
     sets: &'a mut GenKillSet<Local>,
 }
 
@@ -204,7 +198,7 @@ impl<'a, 'mir: 'a, 'tcx> Visitor<'tcx> for MoveVisitor<'a, 'mir, 'tcx> {
     fn visit_local(&mut self, local: &Local, context: PlaceContext, loc: Location) {
         if PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) == context {
             let mut borrowed_locals = self.borrowed_locals.borrow_mut();
-            borrowed_locals.seek(loc);
+            borrowed_locals.seek_before(loc);
             if !borrowed_locals.contains(*local) {
                 self.sets.kill(*local);
             }
diff --git a/src/librustc_mir/transform/generator.rs b/src/librustc_mir/transform/generator.rs
index a6fc65731780a..cd00274afcc00 100644
--- a/src/librustc_mir/transform/generator.rs
+++ b/src/librustc_mir/transform/generator.rs
@@ -49,9 +49,10 @@
 //! For generators with state 1 (returned) and state 2 (poisoned) it does nothing.
 //! Otherwise it drops all the values in scope at the last suspension point.
 
+use crate::dataflow::generic::{Analysis, ResultsCursor};
 use crate::dataflow::{do_dataflow, DataflowResultsCursor, DebugFormatted};
 use crate::dataflow::{DataflowResults, DataflowResultsConsumer, FlowAtLocation};
-use crate::dataflow::{HaveBeenBorrowedLocals, MaybeStorageLive, RequiresStorage};
+use crate::dataflow::{MaybeBorrowedLocals, MaybeStorageLive, RequiresStorage};
 use crate::transform::no_landing_pads::no_landing_pads;
 use crate::transform::simplify;
 use crate::transform::{MirPass, MirSource};
@@ -471,17 +472,10 @@ fn locals_live_across_suspend_points(
 
     // Calculate the MIR locals which have been previously
     // borrowed (even if they are still active).
-    let borrowed_locals_analysis = HaveBeenBorrowedLocals::new(body_ref);
-    let borrowed_locals_results = do_dataflow(
-        tcx,
-        body_ref,
-        def_id,
-        &[],
-        &dead_unwinds,
-        borrowed_locals_analysis,
-        |bd, p| DebugFormatted::new(&bd.body().local_decls[p]),
-    );
-    let mut borrowed_locals_cursor = DataflowResultsCursor::new(&borrowed_locals_results, body_ref);
+    let borrowed_locals_results =
+        MaybeBorrowedLocals::new().into_engine(tcx, body_ref, def_id).iterate_to_fixpoint();
+
+    let mut borrowed_locals_cursor = ResultsCursor::new(body_ref, &borrowed_locals_results);
 
     // Calculate the MIR locals that we actually need to keep storage around
     // for.
@@ -521,7 +515,7 @@ fn locals_live_across_suspend_points(
                 // If a borrow is converted to a raw reference, we must also assume that it lives
                 // forever. Note that the final liveness is still bounded by the storage liveness
                 // of the local, which happens using the `intersect` operation below.
-                borrowed_locals_cursor.seek(loc);
+                borrowed_locals_cursor.seek_before(loc);
                 liveness.outs[block].union(borrowed_locals_cursor.get());
             }
 

From d045a17c4b032a858323f6c9bea698ee2e5920b7 Mon Sep 17 00:00:00 2001
From: Dylan MacKenzie <ecstaticmorse@gmail.com>
Date: Wed, 12 Feb 2020 13:42:31 -0800
Subject: [PATCH 06/12] Use `MaybeMutBorrowedLocals` for const-checking

---
 .../transform/check_consts/resolver.rs        |  2 +-
 .../transform/check_consts/validation.rs      | 31 ++++++++-----------
 2 files changed, 14 insertions(+), 19 deletions(-)

diff --git a/src/librustc_mir/transform/check_consts/resolver.rs b/src/librustc_mir/transform/check_consts/resolver.rs
index b804dc4b5b678..3e14cc6d32a67 100644
--- a/src/librustc_mir/transform/check_consts/resolver.rs
+++ b/src/librustc_mir/transform/check_consts/resolver.rs
@@ -15,7 +15,7 @@ use crate::dataflow::{self as old_dataflow, generic as dataflow};
 /// `FlowSensitiveAnalysis`.
 ///
 /// This transfer does nothing when encountering an indirect assignment. Consumers should rely on
-/// the `IndirectlyMutableLocals` dataflow pass to see if a `Local` may have become qualified via
+/// the `MaybeMutBorrowedLocals` dataflow pass to see if a `Local` may have become qualified via
 /// an indirect assignment or function call.
 struct TransferFunction<'a, 'mir, 'tcx, Q> {
     item: &'a Item<'mir, 'tcx>,
diff --git a/src/librustc_mir/transform/check_consts/validation.rs b/src/librustc_mir/transform/check_consts/validation.rs
index e9715f682b04a..9fef62325f281 100644
--- a/src/librustc_mir/transform/check_consts/validation.rs
+++ b/src/librustc_mir/transform/check_consts/validation.rs
@@ -15,17 +15,19 @@ use rustc_span::Span;
 use std::borrow::Cow;
 use std::ops::Deref;
 
-use self::old_dataflow::IndirectlyMutableLocals;
 use super::ops::{self, NonConstOp};
 use super::qualifs::{self, HasMutInterior, NeedsDrop};
 use super::resolver::FlowSensitiveAnalysis;
 use super::{is_lang_panic_fn, ConstKind, Item, Qualif};
 use crate::const_eval::{is_const_fn, is_unstable_const_fn};
-use crate::dataflow::{self as old_dataflow, generic as dataflow};
-use dataflow::Analysis;
+use crate::dataflow::generic::{self as dataflow, Analysis};
+use crate::dataflow::MaybeMutBorrowedLocals;
 
+// We are using `MaybeMutBorrowedLocals` as a proxy for whether an item may have been mutated
+// through a pointer prior to the given point. This is okay even though `MaybeMutBorrowedLocals`
+// kills locals upon `StorageDead` because a local will never be used after a `StorageDead`.
 pub type IndirectlyMutableResults<'mir, 'tcx> =
-    old_dataflow::DataflowResultsCursor<'mir, 'tcx, IndirectlyMutableLocals<'mir, 'tcx>>;
+    dataflow::ResultsCursor<'mir, 'tcx, MaybeMutBorrowedLocals<'mir, 'tcx>>;
 
 struct QualifCursor<'a, 'mir, 'tcx, Q: Qualif> {
     cursor: dataflow::ResultsCursor<'mir, 'tcx, FlowSensitiveAnalysis<'a, 'mir, 'tcx, Q>>,
@@ -58,7 +60,7 @@ pub struct Qualifs<'a, 'mir, 'tcx> {
 
 impl Qualifs<'a, 'mir, 'tcx> {
     fn indirectly_mutable(&mut self, local: Local, location: Location) -> bool {
-        self.indirectly_mutable.seek(location);
+        self.indirectly_mutable.seek_before(location);
         self.indirectly_mutable.get().contains(local)
     }
 
@@ -134,22 +136,15 @@ impl Deref for Validator<'_, 'mir, 'tcx> {
 
 impl Validator<'a, 'mir, 'tcx> {
     pub fn new(item: &'a Item<'mir, 'tcx>) -> Self {
+        let Item { tcx, body, def_id, param_env, .. } = *item;
+
         let needs_drop = QualifCursor::new(NeedsDrop, item);
         let has_mut_interior = QualifCursor::new(HasMutInterior, item);
 
-        let dead_unwinds = BitSet::new_empty(item.body.basic_blocks().len());
-        let indirectly_mutable = old_dataflow::do_dataflow(
-            item.tcx,
-            &*item.body,
-            item.def_id,
-            &item.tcx.get_attrs(item.def_id),
-            &dead_unwinds,
-            old_dataflow::IndirectlyMutableLocals::new(item.tcx, *item.body, item.param_env),
-            |_, local| old_dataflow::DebugFormatted::new(&local),
-        );
-
-        let indirectly_mutable =
-            old_dataflow::DataflowResultsCursor::new(indirectly_mutable, *item.body);
+        let indirectly_mutable = MaybeMutBorrowedLocals::new_mut_only(tcx, *body, param_env)
+            .into_engine(tcx, *body, def_id)
+            .iterate_to_fixpoint()
+            .into_results_cursor(*body);
 
         let qualifs = Qualifs { needs_drop, has_mut_interior, indirectly_mutable };
 

From 6f167e9c5f421613ff3de37771b1352cd98dd4f7 Mon Sep 17 00:00:00 2001
From: Dylan MacKenzie <ecstaticmorse@gmail.com>
Date: Wed, 12 Feb 2020 13:42:56 -0800
Subject: [PATCH 07/12] Remove ignore and add explanation of indirect mutation
 peek test

---
 src/test/ui/mir-dataflow/indirect-mutation-offset.rs | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/test/ui/mir-dataflow/indirect-mutation-offset.rs b/src/test/ui/mir-dataflow/indirect-mutation-offset.rs
index 884c83b66163c..caa307e269fe7 100644
--- a/src/test/ui/mir-dataflow/indirect-mutation-offset.rs
+++ b/src/test/ui/mir-dataflow/indirect-mutation-offset.rs
@@ -1,6 +1,11 @@
 // compile-flags: -Zunleash-the-miri-inside-of-you
 
-// ignore-test Temporarily ignored while this analysis is migrated to the new framework.
+// This test demonstrates a shortcoming of the `MaybeMutBorrowedLocals` analysis. It does not
+// handle code that takes a reference to one field of a struct, then use pointer arithmetic to
+// transform it to another field of that same struct that may have interior mutability. For now,
+// this is UB, but this may change in the future. See [rust-lang/unsafe-code-guidelines#134].
+//
+// [rust-lang/unsafe-code-guidelines#134]: https://github.com/rust-lang/unsafe-code-guidelines/issues/134
 
 #![feature(core_intrinsics, rustc_attrs, const_raw_ptr_deref)]
 

From 15a5382ef1115ff11c1357fd21ab4aa12626efee Mon Sep 17 00:00:00 2001
From: Dylan MacKenzie <ecstaticmorse@gmail.com>
Date: Thu, 13 Feb 2020 13:56:19 -0800
Subject: [PATCH 08/12] Rename `MaybeBorrowedLocals` constructors

---
 src/librustc_mir/dataflow/impls/borrowed_locals.rs    | 7 +++++--
 src/librustc_mir/transform/check_consts/validation.rs | 2 +-
 src/librustc_mir/transform/generator.rs               | 2 +-
 src/librustc_mir/transform/rustc_peek.rs              | 2 +-
 4 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/src/librustc_mir/dataflow/impls/borrowed_locals.rs b/src/librustc_mir/dataflow/impls/borrowed_locals.rs
index 519db963d1e7d..20aab8b32a51d 100644
--- a/src/librustc_mir/dataflow/impls/borrowed_locals.rs
+++ b/src/librustc_mir/dataflow/impls/borrowed_locals.rs
@@ -27,7 +27,7 @@ pub struct MaybeBorrowedLocals<K = AnyBorrow> {
 impl MaybeBorrowedLocals {
     /// A dataflow analysis that records whether a pointer or reference exists that may alias the
     /// given local.
-    pub fn new() -> Self {
+    pub fn all_borrows() -> Self {
         MaybeBorrowedLocals { kind: AnyBorrow }
     }
 }
@@ -35,7 +35,10 @@ impl MaybeBorrowedLocals {
 impl MaybeMutBorrowedLocals<'mir, 'tcx> {
     /// A dataflow analysis that records whether a pointer or reference exists that may *mutably*
     /// alias the given local.
-    pub fn new_mut_only(
+    ///
+    /// This includes `&mut` and pointers derived from an `&mut`, as well as shared borrows of
+    /// types with interior mutability.
+    pub fn mut_borrows_only(
         tcx: TyCtxt<'tcx>,
         body: &'mir mir::Body<'tcx>,
         param_env: ParamEnv<'tcx>,
diff --git a/src/librustc_mir/transform/check_consts/validation.rs b/src/librustc_mir/transform/check_consts/validation.rs
index 9fef62325f281..d9e179ad42a68 100644
--- a/src/librustc_mir/transform/check_consts/validation.rs
+++ b/src/librustc_mir/transform/check_consts/validation.rs
@@ -141,7 +141,7 @@ impl Validator<'a, 'mir, 'tcx> {
         let needs_drop = QualifCursor::new(NeedsDrop, item);
         let has_mut_interior = QualifCursor::new(HasMutInterior, item);
 
-        let indirectly_mutable = MaybeMutBorrowedLocals::new_mut_only(tcx, *body, param_env)
+        let indirectly_mutable = MaybeMutBorrowedLocals::mut_borrows_only(tcx, *body, param_env)
             .into_engine(tcx, *body, def_id)
             .iterate_to_fixpoint()
             .into_results_cursor(*body);
diff --git a/src/librustc_mir/transform/generator.rs b/src/librustc_mir/transform/generator.rs
index cd00274afcc00..4b3cd2be66d1d 100644
--- a/src/librustc_mir/transform/generator.rs
+++ b/src/librustc_mir/transform/generator.rs
@@ -473,7 +473,7 @@ fn locals_live_across_suspend_points(
     // Calculate the MIR locals which have been previously
     // borrowed (even if they are still active).
     let borrowed_locals_results =
-        MaybeBorrowedLocals::new().into_engine(tcx, body_ref, def_id).iterate_to_fixpoint();
+        MaybeBorrowedLocals::all_borrows().into_engine(tcx, body_ref, def_id).iterate_to_fixpoint();
 
     let mut borrowed_locals_cursor = ResultsCursor::new(body_ref, &borrowed_locals_results);
 
diff --git a/src/librustc_mir/transform/rustc_peek.rs b/src/librustc_mir/transform/rustc_peek.rs
index 16edb7d552b24..6176cf8bc0fa6 100644
--- a/src/librustc_mir/transform/rustc_peek.rs
+++ b/src/librustc_mir/transform/rustc_peek.rs
@@ -45,7 +45,7 @@ impl<'tcx> MirPass<'tcx> for SanityCheck {
         let flow_def_inits = DefinitelyInitializedPlaces::new(tcx, body, &mdpe)
             .into_engine(tcx, body, def_id)
             .iterate_to_fixpoint();
-        let flow_mut_borrowed = MaybeMutBorrowedLocals::new_mut_only(tcx, body, param_env)
+        let flow_mut_borrowed = MaybeMutBorrowedLocals::mut_borrows_only(tcx, body, param_env)
             .into_engine(tcx, body, def_id)
             .iterate_to_fixpoint();
 

From 0984639348c2fc98389746f6815e576cfcaacda8 Mon Sep 17 00:00:00 2001
From: Dylan MacKenzie <ecstaticmorse@gmail.com>
Date: Thu, 13 Feb 2020 13:57:01 -0800
Subject: [PATCH 09/12] Ignore mut borrow from drop terminator in const-eval

---
 .../dataflow/impls/borrowed_locals.rs         | 45 ++++++++++++++-----
 .../transform/check_consts/validation.rs      |  6 +++
 2 files changed, 39 insertions(+), 12 deletions(-)

diff --git a/src/librustc_mir/dataflow/impls/borrowed_locals.rs b/src/librustc_mir/dataflow/impls/borrowed_locals.rs
index 20aab8b32a51d..0ce37aea69df3 100644
--- a/src/librustc_mir/dataflow/impls/borrowed_locals.rs
+++ b/src/librustc_mir/dataflow/impls/borrowed_locals.rs
@@ -22,13 +22,14 @@ pub type MaybeMutBorrowedLocals<'mir, 'tcx> = MaybeBorrowedLocals<MutBorrow<'mir
 /// function call or inline assembly.
 pub struct MaybeBorrowedLocals<K = AnyBorrow> {
     kind: K,
+    ignore_borrow_on_drop: bool,
 }
 
 impl MaybeBorrowedLocals {
     /// A dataflow analysis that records whether a pointer or reference exists that may alias the
     /// given local.
     pub fn all_borrows() -> Self {
-        MaybeBorrowedLocals { kind: AnyBorrow }
+        MaybeBorrowedLocals { kind: AnyBorrow, ignore_borrow_on_drop: false }
     }
 }
 
@@ -43,13 +44,37 @@ impl MaybeMutBorrowedLocals<'mir, 'tcx> {
         body: &'mir mir::Body<'tcx>,
         param_env: ParamEnv<'tcx>,
     ) -> Self {
-        MaybeBorrowedLocals { kind: MutBorrow { body, tcx, param_env } }
+        MaybeBorrowedLocals {
+            kind: MutBorrow { body, tcx, param_env },
+            ignore_borrow_on_drop: false,
+        }
     }
 }
 
 impl<K> MaybeBorrowedLocals<K> {
+    /// During dataflow analysis, ignore the borrow that may occur when a place is dropped.
+    ///
+    /// Drop terminators may call custom drop glue (`Drop::drop`), which takes `&mut self` as a
+    /// parameter. In the general case, a drop impl could launder that reference into the
+    /// surrounding environment through a raw pointer, thus creating a valid `*mut` pointing to the
+    /// dropped local. We are not yet willing to declare this particular case UB, so we must treat
+    /// all dropped locals as mutably borrowed for now. See discussion on [#61069].
+    ///
+    /// In some contexts, we know that this borrow will never occur. For example, during
+    /// const-eval, custom drop glue cannot be run. Code that calls this should document the
+    /// assumptions that justify `Drop` terminators in this way.
+    ///
+    /// [#61069]: https://github.com/rust-lang/rust/pull/61069
+    pub fn unsound_ignore_borrow_on_drop(self) -> Self {
+        MaybeBorrowedLocals { ignore_borrow_on_drop: true, ..self }
+    }
+
     fn transfer_function<'a, T>(&'a self, trans: &'a mut T) -> TransferFunction<'a, T, K> {
-        TransferFunction { kind: &self.kind, trans }
+        TransferFunction {
+            kind: &self.kind,
+            trans,
+            ignore_borrow_on_drop: self.ignore_borrow_on_drop,
+        }
     }
 }
 
@@ -112,6 +137,7 @@ impl<K> BottomValue for MaybeBorrowedLocals<K> {
 struct TransferFunction<'a, T, K> {
     trans: &'a mut T,
     kind: &'a K,
+    ignore_borrow_on_drop: bool,
 }
 
 impl<T, K> Visitor<'tcx> for TransferFunction<'a, T, K>
@@ -162,17 +188,12 @@ where
         self.super_terminator(terminator, location);
 
         match terminator.kind {
-            // Drop terminators may call custom drop glue (`Drop::drop`), which takes `&mut self`
-            // as a parameter. Hypothetically, a drop impl could launder that reference into the
-            // surrounding environment through a raw pointer, thus creating a valid `*mut` pointing
-            // to the dropped local. We are not yet willing to declare this particular case UB, so
-            // we must treat all dropped locals as mutably borrowed for now. See discussion on
-            // [#61069].
-            //
-            // [#61069]: https://github.com/rust-lang/rust/pull/61069
             mir::TerminatorKind::Drop { location: dropped_place, .. }
             | mir::TerminatorKind::DropAndReplace { location: dropped_place, .. } => {
-                self.trans.gen(dropped_place.local);
+                // See documentation for `unsound_ignore_borrow_on_drop` for an explanation.
+                if !self.ignore_borrow_on_drop {
+                    self.trans.gen(dropped_place.local);
+                }
             }
 
             TerminatorKind::Abort
diff --git a/src/librustc_mir/transform/check_consts/validation.rs b/src/librustc_mir/transform/check_consts/validation.rs
index d9e179ad42a68..d4fa2b10152fe 100644
--- a/src/librustc_mir/transform/check_consts/validation.rs
+++ b/src/librustc_mir/transform/check_consts/validation.rs
@@ -141,7 +141,13 @@ impl Validator<'a, 'mir, 'tcx> {
         let needs_drop = QualifCursor::new(NeedsDrop, item);
         let has_mut_interior = QualifCursor::new(HasMutInterior, item);
 
+        // We can use `unsound_ignore_borrow_on_drop` here because custom drop impls are not
+        // allowed in a const.
+        //
+        // FIXME(ecstaticmorse): Someday we want to allow custom drop impls. How do we do this
+        // without breaking stable code?
         let indirectly_mutable = MaybeMutBorrowedLocals::mut_borrows_only(tcx, *body, param_env)
+            .unsound_ignore_borrow_on_drop()
             .into_engine(tcx, *body, def_id)
             .iterate_to_fixpoint()
             .into_results_cursor(*body);

From 668d2fe807068073647f9c92d81ff0e7b9ab8486 Mon Sep 17 00:00:00 2001
From: Dylan MacKenzie <ecstaticmorse@gmail.com>
Date: Thu, 13 Feb 2020 15:58:56 -0800
Subject: [PATCH 10/12] Update line # in test output

---
 src/test/ui/mir-dataflow/indirect-mutation-offset.stderr | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/test/ui/mir-dataflow/indirect-mutation-offset.stderr b/src/test/ui/mir-dataflow/indirect-mutation-offset.stderr
index 0ae9a40c96aaa..8d3548ececdd9 100644
--- a/src/test/ui/mir-dataflow/indirect-mutation-offset.stderr
+++ b/src/test/ui/mir-dataflow/indirect-mutation-offset.stderr
@@ -1,5 +1,5 @@
 error: rustc_peek: bit not set
-  --> $DIR/indirect-mutation-offset.rs:34:14
+  --> $DIR/indirect-mutation-offset.rs:41:14
    |
 LL |     unsafe { rustc_peek(x) };
    |              ^^^^^^^^^^^^^

From 9d423950cc141e110242ff1973f2d7b84c719a86 Mon Sep 17 00:00:00 2001
From: Dylan MacKenzie <ecstaticmorse@gmail.com>
Date: Mon, 17 Feb 2020 13:39:50 -0800
Subject: [PATCH 11/12] Use doc comment for explanation of
 `shared_borrow_allows_mutation`

---
 .../dataflow/impls/borrowed_locals.rs          | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/src/librustc_mir/dataflow/impls/borrowed_locals.rs b/src/librustc_mir/dataflow/impls/borrowed_locals.rs
index 0ce37aea69df3..44693c50eac9c 100644
--- a/src/librustc_mir/dataflow/impls/borrowed_locals.rs
+++ b/src/librustc_mir/dataflow/impls/borrowed_locals.rs
@@ -221,15 +221,15 @@ pub struct MutBorrow<'mir, 'tcx> {
 }
 
 impl MutBorrow<'mir, 'tcx> {
-    // `&` and `&raw` only allow mutation if the borrowed place is `!Freeze`.
-    //
-    // This assumes that it is UB to take the address of a struct field whose type is
-    // `Freeze`, then use pointer arithmetic to derive a pointer to a *different* field of
-    // that same struct whose type is `!Freeze`. If we decide that this is not UB, we will
-    // have to check the type of the borrowed **local** instead of the borrowed **place**
-    // below. See [rust-lang/unsafe-code-guidelines#134].
-    //
-    // [rust-lang/unsafe-code-guidelines#134]: https://github.com/rust-lang/unsafe-code-guidelines/issues/134
+    /// `&` and `&raw` only allow mutation if the borrowed place is `!Freeze`.
+    ///
+    /// This assumes that it is UB to take the address of a struct field whose type is
+    /// `Freeze`, then use pointer arithmetic to derive a pointer to a *different* field of
+    /// that same struct whose type is `!Freeze`. If we decide that this is not UB, we will
+    /// have to check the type of the borrowed **local** instead of the borrowed **place**
+    /// below. See [rust-lang/unsafe-code-guidelines#134].
+    ///
+    /// [rust-lang/unsafe-code-guidelines#134]: https://github.com/rust-lang/unsafe-code-guidelines/issues/134
     fn shared_borrow_allows_mutation(&self, place: &Place<'tcx>) -> bool {
         !place.ty(self.body, self.tcx).ty.is_freeze(self.tcx, self.param_env, DUMMY_SP)
     }

From 077a93c6a9b1c8ee0e541ea484f7e13c207d50d0 Mon Sep 17 00:00:00 2001
From: Dylan MacKenzie <ecstaticmorse@gmail.com>
Date: Mon, 17 Feb 2020 13:43:13 -0800
Subject: [PATCH 12/12] Fix typo in comment

---
 src/librustc_mir/dataflow/impls/borrowed_locals.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/librustc_mir/dataflow/impls/borrowed_locals.rs b/src/librustc_mir/dataflow/impls/borrowed_locals.rs
index 44693c50eac9c..95a676c0892c5 100644
--- a/src/librustc_mir/dataflow/impls/borrowed_locals.rs
+++ b/src/librustc_mir/dataflow/impls/borrowed_locals.rs
@@ -62,7 +62,7 @@ impl<K> MaybeBorrowedLocals<K> {
     ///
     /// In some contexts, we know that this borrow will never occur. For example, during
     /// const-eval, custom drop glue cannot be run. Code that calls this should document the
-    /// assumptions that justify `Drop` terminators in this way.
+    /// assumptions that justify ignoring `Drop` terminators in this way.
     ///
     /// [#61069]: https://github.com/rust-lang/rust/pull/61069
     pub fn unsound_ignore_borrow_on_drop(self) -> Self {