From 810f309ff30fe7a75917f9e5359074dc991b4590 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sat, 30 May 2020 23:46:21 +0200
Subject: [PATCH 01/12] MIR sanity check: validate types on assignment

---
 src/librustc_mir/transform/validate.rs | 54 ++++++++++++++++++++++++--
 1 file changed, 51 insertions(+), 3 deletions(-)

diff --git a/src/librustc_mir/transform/validate.rs b/src/librustc_mir/transform/validate.rs
index 625f40cd792..3d48a2387a8 100644
--- a/src/librustc_mir/transform/validate.rs
+++ b/src/librustc_mir/transform/validate.rs
@@ -7,7 +7,7 @@
         BasicBlock, Body, Location, Operand, Rvalue, Statement, StatementKind, Terminator,
         TerminatorKind,
     },
-    ty::{self, ParamEnv, TyCtxt},
+    ty::{self, fold::BottomUpFolder, ParamEnv, Ty, TyCtxt, TypeFoldable},
 };
 
 #[derive(Copy, Clone, Debug)]
@@ -83,6 +83,40 @@ fn check_edge(&self, location: Location, bb: BasicBlock, edge_kind: EdgeKind) {
     }
 }
 
+/// Check if src can be assigned into dest.
+/// This is not precise, it will accept some incorrect assignments.
+fn mir_assign_valid_types<'tcx>(tcx: TyCtxt<'tcx>, src: Ty<'tcx>, dest: Ty<'tcx>) -> bool {
+    if src == dest {
+        // Equal types, all is good.
+        return true;
+    }
+
+    // Type-changing assignments can happen for (at least) two reasons:
+    // 1. `&mut T` -> `&T` gets optimized from a reborrow to a mere assignment.
+    // 2. Subtyping is used. While all normal lifetimes are erased, higher-ranked types
+    //    with their late-bound lifetimes are still around and can lead to type differences.
+    // Normalize both of them away.
+    // FIXME: Share this code with `interpret/eval_context.rs`.
+    let normalize = |ty: Ty<'tcx>| {
+        ty.fold_with(&mut BottomUpFolder {
+            tcx,
+            // Normalize all references to immutable.
+            ty_op: |ty| match ty.kind {
+                ty::Ref(_, pointee, _) => tcx.mk_imm_ref(tcx.lifetimes.re_erased, pointee),
+                _ => ty,
+            },
+            // We just erase all late-bound lifetimes, but this is not fully correct (FIXME):
+            // lifetimes in invariant positions could matter (e.g. through associated types).
+            // But that just means we miss some potential incompatible types, it will not
+            // lead to wrong errors.
+            lt_op: |_| tcx.lifetimes.re_erased,
+            // Leave consts unchanged.
+            ct_op: |ct| ct,
+        })
+    };
+    normalize(src) == normalize(dest)
+}
+
 impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
     fn visit_operand(&mut self, operand: &Operand<'tcx>, location: Location) {
         // `Operand::Copy` is only supposed to be used with `Copy` types.
@@ -99,9 +133,23 @@ fn visit_operand(&mut self, operand: &Operand<'tcx>, location: Location) {
     }
 
     fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
-        // The sides of an assignment must not alias. Currently this just checks whether the places
-        // are identical.
         if let StatementKind::Assign(box (dest, rvalue)) = &statement.kind {
+            // LHS and RHS of the assignment must have the same type.
+            let left_ty = dest.ty(&self.body.local_decls, self.tcx).ty;
+            let right_ty = rvalue.ty(&self.body.local_decls, self.tcx);
+            if !mir_assign_valid_types(self.tcx, right_ty, left_ty) {
+                self.fail(
+                    location,
+                    format!(
+                        "encountered `Assign` statement with incompatible types:\n\
+                        left-hand side has type: {}\n\
+                        right-hand side has type: {}",
+                        left_ty, right_ty,
+                    ),
+                );
+            }
+            // The sides of an assignment must not alias. Currently this just checks whether the places
+            // are identical.
             match rvalue {
                 Rvalue::Use(Operand::Copy(src) | Operand::Move(src)) => {
                     if dest == src {

From 50d7deac4de3bfde44a634ff4dabf3115f694c79 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sun, 31 May 2020 09:54:25 +0200
Subject: [PATCH 02/12] prepare visit_statement for checking more kinds of
 statements

---
 src/librustc_mir/transform/validate.rs | 55 ++++++++++++++------------
 1 file changed, 29 insertions(+), 26 deletions(-)

diff --git a/src/librustc_mir/transform/validate.rs b/src/librustc_mir/transform/validate.rs
index 3d48a2387a8..051ce9e6b1e 100644
--- a/src/librustc_mir/transform/validate.rs
+++ b/src/librustc_mir/transform/validate.rs
@@ -133,34 +133,37 @@ fn visit_operand(&mut self, operand: &Operand<'tcx>, location: Location) {
     }
 
     fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
-        if let StatementKind::Assign(box (dest, rvalue)) = &statement.kind {
-            // LHS and RHS of the assignment must have the same type.
-            let left_ty = dest.ty(&self.body.local_decls, self.tcx).ty;
-            let right_ty = rvalue.ty(&self.body.local_decls, self.tcx);
-            if !mir_assign_valid_types(self.tcx, right_ty, left_ty) {
-                self.fail(
-                    location,
-                    format!(
-                        "encountered `Assign` statement with incompatible types:\n\
-                        left-hand side has type: {}\n\
-                        right-hand side has type: {}",
-                        left_ty, right_ty,
-                    ),
-                );
-            }
-            // The sides of an assignment must not alias. Currently this just checks whether the places
-            // are identical.
-            match rvalue {
-                Rvalue::Use(Operand::Copy(src) | Operand::Move(src)) => {
-                    if dest == src {
-                        self.fail(
-                            location,
-                            "encountered `Assign` statement with overlapping memory",
-                        );
-                    }
+        match &statement.kind {
+            StatementKind::Assign(box (dest, rvalue)) => {
+                // LHS and RHS of the assignment must have the same type.
+                let left_ty = dest.ty(&self.body.local_decls, self.tcx).ty;
+                let right_ty = rvalue.ty(&self.body.local_decls, self.tcx);
+                if !mir_assign_valid_types(self.tcx, right_ty, left_ty) {
+                    self.fail(
+                        location,
+                        format!(
+                            "encountered `Assign` statement with incompatible types:\n\
+                            left-hand side has type: {}\n\
+                            right-hand side has type: {}",
+                            left_ty, right_ty,
+                        ),
+                    );
+                }
+                // The sides of an assignment must not alias. Currently this just checks whether the places
+                // are identical.
+                match rvalue {
+                    Rvalue::Use(Operand::Copy(src) | Operand::Move(src)) => {
+                        if dest == src {
+                            self.fail(
+                                location,
+                                "encountered `Assign` statement with overlapping memory",
+                            );
+                        }
+                    }
+                    _ => {}
                 }
-                _ => {}
             }
+            _ => {}
         }
     }
 

From 93e3552d040edfa67cdedfe2fe4a44fe0c4ead59 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sun, 31 May 2020 15:02:33 +0200
Subject: [PATCH 03/12] also normalize constants when comparing types

---
 src/librustc_mir/interpret/eval_context.rs |  1 +
 src/librustc_mir/transform/validate.rs     | 68 +++++++++++-----------
 2 files changed, 36 insertions(+), 33 deletions(-)

diff --git a/src/librustc_mir/interpret/eval_context.rs b/src/librustc_mir/interpret/eval_context.rs
index 22f4691c22b..1a6ed41ba47 100644
--- a/src/librustc_mir/interpret/eval_context.rs
+++ b/src/librustc_mir/interpret/eval_context.rs
@@ -239,6 +239,7 @@ pub(super) fn mir_assign_valid_types<'tcx>(
     // 2. Subtyping is used. While all normal lifetimes are erased, higher-ranked types
     //    with their late-bound lifetimes are still around and can lead to type differences.
     // Normalize both of them away.
+    // Also see the related but slightly different pre-monomorphization method in `transform/validate.rs`.
     let normalize = |ty: Ty<'tcx>| {
         ty.fold_with(&mut BottomUpFolder {
             tcx,
diff --git a/src/librustc_mir/transform/validate.rs b/src/librustc_mir/transform/validate.rs
index 051ce9e6b1e..14c67c2372c 100644
--- a/src/librustc_mir/transform/validate.rs
+++ b/src/librustc_mir/transform/validate.rs
@@ -81,40 +81,42 @@ fn check_edge(&self, location: Location, bb: BasicBlock, edge_kind: EdgeKind) {
             self.fail(location, format!("encountered jump to invalid basic block {:?}", bb))
         }
     }
-}
 
-/// Check if src can be assigned into dest.
-/// This is not precise, it will accept some incorrect assignments.
-fn mir_assign_valid_types<'tcx>(tcx: TyCtxt<'tcx>, src: Ty<'tcx>, dest: Ty<'tcx>) -> bool {
-    if src == dest {
-        // Equal types, all is good.
-        return true;
+    /// Check if src can be assigned into dest.
+    /// This is not precise, it will accept some incorrect assignments.
+    fn mir_assign_valid_types(&self, src: Ty<'tcx>, dest: Ty<'tcx>) -> bool {
+        if src == dest {
+            // Equal types, all is good.
+            return true;
+        }
+
+        // Type-changing assignments can happen for (at least) two reasons:
+        // 1. `&mut T` -> `&T` gets optimized from a reborrow to a mere assignment.
+        // 2. Subtyping is used. While all normal lifetimes are erased, higher-ranked types
+        //    with their late-bound lifetimes are still around and can lead to type differences.
+        // Normalize both of them away.
+        // Also see the related but slightly different post-monomorphization method in `interpret/eval_context.rs`.
+        let normalize = |ty: Ty<'tcx>| {
+            ty.fold_with(&mut BottomUpFolder {
+                tcx: self.tcx,
+                // Normalize all references to immutable.
+                ty_op: |ty| match ty.kind {
+                    ty::Ref(_, pointee, _) => {
+                        self.tcx.mk_imm_ref(self.tcx.lifetimes.re_erased, pointee)
+                    }
+                    _ => ty,
+                },
+                // We just erase all late-bound lifetimes, but this is not fully correct (FIXME):
+                // lifetimes in invariant positions could matter (e.g. through associated types).
+                // But that just means we miss some potential incompatible types, it will not
+                // lead to wrong errors.
+                lt_op: |_| self.tcx.lifetimes.re_erased,
+                // Evaluate consts.
+                ct_op: |ct| ct.eval(self.tcx, self.param_env),
+            })
+        };
+        normalize(src) == normalize(dest)
     }
-
-    // Type-changing assignments can happen for (at least) two reasons:
-    // 1. `&mut T` -> `&T` gets optimized from a reborrow to a mere assignment.
-    // 2. Subtyping is used. While all normal lifetimes are erased, higher-ranked types
-    //    with their late-bound lifetimes are still around and can lead to type differences.
-    // Normalize both of them away.
-    // FIXME: Share this code with `interpret/eval_context.rs`.
-    let normalize = |ty: Ty<'tcx>| {
-        ty.fold_with(&mut BottomUpFolder {
-            tcx,
-            // Normalize all references to immutable.
-            ty_op: |ty| match ty.kind {
-                ty::Ref(_, pointee, _) => tcx.mk_imm_ref(tcx.lifetimes.re_erased, pointee),
-                _ => ty,
-            },
-            // We just erase all late-bound lifetimes, but this is not fully correct (FIXME):
-            // lifetimes in invariant positions could matter (e.g. through associated types).
-            // But that just means we miss some potential incompatible types, it will not
-            // lead to wrong errors.
-            lt_op: |_| tcx.lifetimes.re_erased,
-            // Leave consts unchanged.
-            ct_op: |ct| ct,
-        })
-    };
-    normalize(src) == normalize(dest)
 }
 
 impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
@@ -138,7 +140,7 @@ fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
                 // LHS and RHS of the assignment must have the same type.
                 let left_ty = dest.ty(&self.body.local_decls, self.tcx).ty;
                 let right_ty = rvalue.ty(&self.body.local_decls, self.tcx);
-                if !mir_assign_valid_types(self.tcx, right_ty, left_ty) {
+                if !self.mir_assign_valid_types(right_ty, left_ty) {
                     self.fail(
                         location,
                         format!(

From 9576e307a7b8ac0c812fac927d247761662e7d1a Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Mon, 1 Jun 2020 21:04:11 +0200
Subject: [PATCH 04/12] also normalize_erasing_regions

---
 src/librustc_mir/transform/validate.rs | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/librustc_mir/transform/validate.rs b/src/librustc_mir/transform/validate.rs
index 14c67c2372c..40189ca5c12 100644
--- a/src/librustc_mir/transform/validate.rs
+++ b/src/librustc_mir/transform/validate.rs
@@ -89,6 +89,13 @@ fn mir_assign_valid_types(&self, src: Ty<'tcx>, dest: Ty<'tcx>) -> bool {
             // Equal types, all is good.
             return true;
         }
+        // Normalize projections and things like that.
+        let src = self.tcx.normalize_erasing_regions(self.param_env, src);
+        let dest = self.tcx.normalize_erasing_regions(self.param_env, dest);
+        // It's worth checking equality again.
+        if src == dest {
+            return true;
+        }
 
         // Type-changing assignments can happen for (at least) two reasons:
         // 1. `&mut T` -> `&T` gets optimized from a reborrow to a mere assignment.

From 8200771aa2cbd393a5beca819ac2462cf35e8d15 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sat, 6 Jun 2020 11:45:19 +0200
Subject: [PATCH 05/12] reveal_all when sanity-checking MIR assignment types

---
 src/librustc_mir/transform/validate.rs | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/src/librustc_mir/transform/validate.rs b/src/librustc_mir/transform/validate.rs
index 40189ca5c12..b2fbb48eefe 100644
--- a/src/librustc_mir/transform/validate.rs
+++ b/src/librustc_mir/transform/validate.rs
@@ -90,8 +90,11 @@ fn mir_assign_valid_types(&self, src: Ty<'tcx>, dest: Ty<'tcx>) -> bool {
             return true;
         }
         // Normalize projections and things like that.
-        let src = self.tcx.normalize_erasing_regions(self.param_env, src);
-        let dest = self.tcx.normalize_erasing_regions(self.param_env, dest);
+        // FIXME: We need to reveal_all, as some optimizations change types in ways
+        // that requires unfolding opaque types.
+        let param_env = self.param_env.with_reveal_all();
+        let src = self.tcx.normalize_erasing_regions(param_env, src);
+        let dest = self.tcx.normalize_erasing_regions(param_env, dest);
         // It's worth checking equality again.
         if src == dest {
             return true;
@@ -119,7 +122,7 @@ fn mir_assign_valid_types(&self, src: Ty<'tcx>, dest: Ty<'tcx>) -> bool {
                 // lead to wrong errors.
                 lt_op: |_| self.tcx.lifetimes.re_erased,
                 // Evaluate consts.
-                ct_op: |ct| ct.eval(self.tcx, self.param_env),
+                ct_op: |ct| ct.eval(self.tcx, param_env),
             })
         };
         normalize(src) == normalize(dest)

From 978470f711b3be3350a46d386a424e1dfb1ea148 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sun, 21 Jun 2020 10:04:12 +0200
Subject: [PATCH 06/12] no need to normalize mutability any more

---
 src/librustc_mir/transform/validate.rs | 20 +++++++-------------
 1 file changed, 7 insertions(+), 13 deletions(-)

diff --git a/src/librustc_mir/transform/validate.rs b/src/librustc_mir/transform/validate.rs
index b2fbb48eefe..81bdcc849e4 100644
--- a/src/librustc_mir/transform/validate.rs
+++ b/src/librustc_mir/transform/validate.rs
@@ -100,22 +100,16 @@ fn mir_assign_valid_types(&self, src: Ty<'tcx>, dest: Ty<'tcx>) -> bool {
             return true;
         }
 
-        // Type-changing assignments can happen for (at least) two reasons:
-        // 1. `&mut T` -> `&T` gets optimized from a reborrow to a mere assignment.
-        // 2. Subtyping is used. While all normal lifetimes are erased, higher-ranked types
-        //    with their late-bound lifetimes are still around and can lead to type differences.
-        // Normalize both of them away.
-        // Also see the related but slightly different post-monomorphization method in `interpret/eval_context.rs`.
+        // Type-changing assignments can happen when subtyping is used. While
+        // all normal lifetimes are erased, higher-ranked types with their
+        // late-bound lifetimes are still around and can lead to type
+        // differences. Normalize both of them away.
+        // Also see the related but slightly different post-monomorphization
+        // method in `interpret/eval_context.rs`.
         let normalize = |ty: Ty<'tcx>| {
             ty.fold_with(&mut BottomUpFolder {
                 tcx: self.tcx,
-                // Normalize all references to immutable.
-                ty_op: |ty| match ty.kind {
-                    ty::Ref(_, pointee, _) => {
-                        self.tcx.mk_imm_ref(self.tcx.lifetimes.re_erased, pointee)
-                    }
-                    _ => ty,
-                },
+                ty_op: |ty| ty,
                 // We just erase all late-bound lifetimes, but this is not fully correct (FIXME):
                 // lifetimes in invariant positions could matter (e.g. through associated types).
                 // But that just means we miss some potential incompatible types, it will not

From 91f73fbca488973169b4f4b927323f712ad3d776 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sun, 21 Jun 2020 13:49:53 +0200
Subject: [PATCH 07/12] use a TypeRelation to compare the types

---
 src/librustc_mir/transform/validate.rs | 98 +++++++++++++++++++++-----
 1 file changed, 80 insertions(+), 18 deletions(-)

diff --git a/src/librustc_mir/transform/validate.rs b/src/librustc_mir/transform/validate.rs
index 81bdcc849e4..d0293131b26 100644
--- a/src/librustc_mir/transform/validate.rs
+++ b/src/librustc_mir/transform/validate.rs
@@ -7,7 +7,11 @@
         BasicBlock, Body, Location, Operand, Rvalue, Statement, StatementKind, Terminator,
         TerminatorKind,
     },
-    ty::{self, fold::BottomUpFolder, ParamEnv, Ty, TyCtxt, TypeFoldable},
+    ty::{
+        self,
+        relate::{Relate, RelateResult, TypeRelation},
+        ParamEnv, Ty, TyCtxt,
+    },
 };
 
 #[derive(Copy, Clone, Debug)]
@@ -103,23 +107,81 @@ fn mir_assign_valid_types(&self, src: Ty<'tcx>, dest: Ty<'tcx>) -> bool {
         // Type-changing assignments can happen when subtyping is used. While
         // all normal lifetimes are erased, higher-ranked types with their
         // late-bound lifetimes are still around and can lead to type
-        // differences. Normalize both of them away.
-        // Also see the related but slightly different post-monomorphization
-        // method in `interpret/eval_context.rs`.
-        let normalize = |ty: Ty<'tcx>| {
-            ty.fold_with(&mut BottomUpFolder {
-                tcx: self.tcx,
-                ty_op: |ty| ty,
-                // We just erase all late-bound lifetimes, but this is not fully correct (FIXME):
-                // lifetimes in invariant positions could matter (e.g. through associated types).
-                // But that just means we miss some potential incompatible types, it will not
-                // lead to wrong errors.
-                lt_op: |_| self.tcx.lifetimes.re_erased,
-                // Evaluate consts.
-                ct_op: |ct| ct.eval(self.tcx, param_env),
-            })
-        };
-        normalize(src) == normalize(dest)
+        // differences. So we compare ignoring lifetimes.
+        struct LifetimeIgnoreRelation<'tcx> {
+            tcx: TyCtxt<'tcx>,
+            param_env: ty::ParamEnv<'tcx>,
+        }
+
+        impl TypeRelation<'tcx> for LifetimeIgnoreRelation<'tcx> {
+            fn tcx(&self) -> TyCtxt<'tcx> {
+                self.tcx
+            }
+
+            fn param_env(&self) -> ty::ParamEnv<'tcx> {
+                self.param_env
+            }
+
+            fn tag(&self) -> &'static str {
+                "librustc_mir::transform::validate"
+            }
+
+            fn a_is_expected(&self) -> bool {
+                true
+            }
+
+            fn relate_with_variance<T: Relate<'tcx>>(
+                &mut self,
+                _: ty::Variance,
+                a: &T,
+                b: &T,
+            ) -> RelateResult<'tcx, T> {
+                // Ignore variance, require types to be exactly the same.
+                self.relate(a, b)
+            }
+
+            fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+                if a == b {
+                    // Short-circuit.
+                    return Ok(a);
+                }
+                ty::relate::super_relate_tys(self, a, b)
+            }
+
+            fn regions(
+                &mut self,
+                a: ty::Region<'tcx>,
+                _b: ty::Region<'tcx>,
+            ) -> RelateResult<'tcx, ty::Region<'tcx>> {
+                // Ignore regions.
+                Ok(a)
+            }
+
+            fn consts(
+                &mut self,
+                a: &'tcx ty::Const<'tcx>,
+                b: &'tcx ty::Const<'tcx>,
+            ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
+                ty::relate::super_relate_consts(self, a, b)
+            }
+
+            fn binders<T>(
+                &mut self,
+                a: &ty::Binder<T>,
+                b: &ty::Binder<T>,
+            ) -> RelateResult<'tcx, ty::Binder<T>>
+            where
+                T: Relate<'tcx>,
+            {
+                self.relate(a.skip_binder(), b.skip_binder())?;
+                Ok(a.clone())
+            }
+        }
+
+        // Instantiate and run relation.
+        let mut relator: LifetimeIgnoreRelation<'tcx> =
+            LifetimeIgnoreRelation { tcx: self.tcx, param_env };
+        relator.relate(&src, &dest).is_ok()
     }
 }
 

From 7754322bcc2852814592c47c3b76c53fefe95a4f Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Mon, 22 Jun 2020 09:00:40 +0200
Subject: [PATCH 08/12] fix typo

Co-authored-by: Bastian Kauschke <bastian_kauschke@hotmail.de>
---
 src/librustc_mir/transform/validate.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/librustc_mir/transform/validate.rs b/src/librustc_mir/transform/validate.rs
index d0293131b26..803954d258f 100644
--- a/src/librustc_mir/transform/validate.rs
+++ b/src/librustc_mir/transform/validate.rs
@@ -95,7 +95,7 @@ fn mir_assign_valid_types(&self, src: Ty<'tcx>, dest: Ty<'tcx>) -> bool {
         }
         // Normalize projections and things like that.
         // FIXME: We need to reveal_all, as some optimizations change types in ways
-        // that requires unfolding opaque types.
+        // that require unfolding opaque types.
         let param_env = self.param_env.with_reveal_all();
         let src = self.tcx.normalize_erasing_regions(param_env, src);
         let dest = self.tcx.normalize_erasing_regions(param_env, dest);

From 7f8fe6a9851aaf493c8657fe7e98145539d466dd Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Mon, 22 Jun 2020 09:17:33 +0200
Subject: [PATCH 09/12] also use relator in interpreter assignment sanity check

---
 src/librustc_mir/interpret/eval_context.rs |  41 ++----
 src/librustc_mir/interpret/operand.rs      |   4 +-
 src/librustc_mir/interpret/place.rs        |   5 +-
 src/librustc_mir/transform/validate.rs     | 162 +++++++++++----------
 4 files changed, 109 insertions(+), 103 deletions(-)

diff --git a/src/librustc_mir/interpret/eval_context.rs b/src/librustc_mir/interpret/eval_context.rs
index 1a6ed41ba47..b673738cec5 100644
--- a/src/librustc_mir/interpret/eval_context.rs
+++ b/src/librustc_mir/interpret/eval_context.rs
@@ -15,7 +15,7 @@
 };
 use rustc_middle::ty::layout::{self, TyAndLayout};
 use rustc_middle::ty::{
-    self, fold::BottomUpFolder, query::TyCtxtAt, subst::SubstsRef, Ty, TyCtxt, TypeFoldable,
+    self, query::TyCtxtAt, subst::SubstsRef, ParamEnv, Ty, TyCtxt, TypeFoldable,
 };
 use rustc_span::{source_map::DUMMY_SP, Span};
 use rustc_target::abi::{Align, HasDataLayout, LayoutOf, Size, TargetDataLayout};
@@ -24,6 +24,7 @@
     Immediate, MPlaceTy, Machine, MemPlace, MemPlaceMeta, Memory, OpTy, Operand, Place, PlaceTy,
     ScalarMaybeUninit, StackPopJump,
 };
+use crate::transform::validate::equal_up_to_regions;
 use crate::util::storage::AlwaysLiveLocals;
 
 pub struct InterpCx<'mir, 'tcx, M: Machine<'mir, 'tcx>> {
@@ -220,6 +221,7 @@ fn layout_of(&self, ty: Ty<'tcx>) -> Self::TyAndLayout {
 /// This test should be symmetric, as it is primarily about layout compatibility.
 pub(super) fn mir_assign_valid_types<'tcx>(
     tcx: TyCtxt<'tcx>,
+    param_env: ParamEnv<'tcx>,
     src: TyAndLayout<'tcx>,
     dest: TyAndLayout<'tcx>,
 ) -> bool {
@@ -234,29 +236,15 @@ pub(super) fn mir_assign_valid_types<'tcx>(
         return false;
     }
 
-    // Type-changing assignments can happen for (at least) two reasons:
-    // 1. `&mut T` -> `&T` gets optimized from a reborrow to a mere assignment.
-    // 2. Subtyping is used. While all normal lifetimes are erased, higher-ranked types
-    //    with their late-bound lifetimes are still around and can lead to type differences.
-    // Normalize both of them away.
-    // Also see the related but slightly different pre-monomorphization method in `transform/validate.rs`.
-    let normalize = |ty: Ty<'tcx>| {
-        ty.fold_with(&mut BottomUpFolder {
-            tcx,
-            // Normalize all references to immutable.
-            ty_op: |ty| match ty.kind {
-                ty::Ref(_, pointee, _) => tcx.mk_imm_ref(tcx.lifetimes.re_erased, pointee),
-                _ => ty,
-            },
-            // We just erase all late-bound lifetimes, but this is not fully correct (FIXME):
-            // lifetimes in invariant positions could matter (e.g. through associated types).
-            // We rely on the fact that layout was confirmed to be equal above.
-            lt_op: |_| tcx.lifetimes.re_erased,
-            // Leave consts unchanged.
-            ct_op: |ct| ct,
-        })
-    };
-    normalize(src.ty) == normalize(dest.ty)
+    // Type-changing assignments can happen when subtyping is used. While
+    // all normal lifetimes are erased, higher-ranked types with their
+    // late-bound lifetimes are still around and can lead to type
+    // differences. So we compare ignoring lifetimes.
+    //
+    // Note that this is not fully correct (FIXME):
+    // lifetimes in invariant positions could matter (e.g. through associated types).
+    // We rely on the fact that layout was confirmed to be equal above.
+    equal_up_to_regions(tcx, param_env, src.ty, dest.ty)
 }
 
 /// Use the already known layout if given (but sanity check in debug mode),
@@ -264,6 +252,7 @@ pub(super) fn mir_assign_valid_types<'tcx>(
 #[cfg_attr(not(debug_assertions), inline(always))]
 pub(super) fn from_known_layout<'tcx>(
     tcx: TyCtxtAt<'tcx>,
+    param_env: ParamEnv<'tcx>,
     known_layout: Option<TyAndLayout<'tcx>>,
     compute: impl FnOnce() -> InterpResult<'tcx, TyAndLayout<'tcx>>,
 ) -> InterpResult<'tcx, TyAndLayout<'tcx>> {
@@ -272,7 +261,7 @@ pub(super) fn from_known_layout<'tcx>(
         Some(known_layout) => {
             if cfg!(debug_assertions) {
                 let check_layout = compute()?;
-                if !mir_assign_valid_types(tcx.tcx, check_layout, known_layout) {
+                if !mir_assign_valid_types(tcx.tcx, param_env, check_layout, known_layout) {
                     span_bug!(
                         tcx.span,
                         "expected type differs from actual type.\nexpected: {:?}\nactual: {:?}",
@@ -476,7 +465,7 @@ pub fn layout_of_local(
         // have to support that case (mostly by skipping all caching).
         match frame.locals.get(local).and_then(|state| state.layout.get()) {
             None => {
-                let layout = from_known_layout(self.tcx, layout, || {
+                let layout = from_known_layout(self.tcx, self.param_env, layout, || {
                     let local_ty = frame.body.local_decls[local].ty;
                     let local_ty =
                         self.subst_from_frame_and_normalize_erasing_regions(frame, local_ty);
diff --git a/src/librustc_mir/interpret/operand.rs b/src/librustc_mir/interpret/operand.rs
index 35e433c4bd5..27fa9b2c17c 100644
--- a/src/librustc_mir/interpret/operand.rs
+++ b/src/librustc_mir/interpret/operand.rs
@@ -472,6 +472,7 @@ pub fn eval_place_to_op(
         // Sanity-check the type we ended up with.
         debug_assert!(mir_assign_valid_types(
             *self.tcx,
+            self.param_env,
             self.layout_of(self.subst_from_current_frame_and_normalize_erasing_regions(
                 place.ty(&self.frame().body.local_decls, *self.tcx).ty
             ))?,
@@ -554,7 +555,8 @@ pub(super) fn eval_operands(
         // documentation).
         let val_val = M::adjust_global_const(self, val_val)?;
         // Other cases need layout.
-        let layout = from_known_layout(self.tcx, layout, || self.layout_of(val.ty))?;
+        let layout =
+            from_known_layout(self.tcx, self.param_env, layout, || self.layout_of(val.ty))?;
         let op = match val_val {
             ConstValue::ByRef { alloc, offset } => {
                 let id = self.tcx.create_memory_alloc(alloc);
diff --git a/src/librustc_mir/interpret/place.rs b/src/librustc_mir/interpret/place.rs
index 396aec0a8f8..98a1cea97e2 100644
--- a/src/librustc_mir/interpret/place.rs
+++ b/src/librustc_mir/interpret/place.rs
@@ -652,6 +652,7 @@ pub fn eval_place(
         // Sanity-check the type we ended up with.
         debug_assert!(mir_assign_valid_types(
             *self.tcx,
+            self.param_env,
             self.layout_of(self.subst_from_current_frame_and_normalize_erasing_regions(
                 place.ty(&self.frame().body.local_decls, *self.tcx).ty
             ))?,
@@ -855,7 +856,7 @@ fn copy_op_no_validate(
     ) -> InterpResult<'tcx> {
         // We do NOT compare the types for equality, because well-typed code can
         // actually "transmute" `&mut T` to `&T` in an assignment without a cast.
-        if !mir_assign_valid_types(*self.tcx, src.layout, dest.layout) {
+        if !mir_assign_valid_types(*self.tcx, self.param_env, src.layout, dest.layout) {
             span_bug!(
                 self.cur_span(),
                 "type mismatch when copying!\nsrc: {:?},\ndest: {:?}",
@@ -912,7 +913,7 @@ pub fn copy_op_transmute(
         src: OpTy<'tcx, M::PointerTag>,
         dest: PlaceTy<'tcx, M::PointerTag>,
     ) -> InterpResult<'tcx> {
-        if mir_assign_valid_types(*self.tcx, src.layout, dest.layout) {
+        if mir_assign_valid_types(*self.tcx, self.param_env, src.layout, dest.layout) {
             // Fast path: Just use normal `copy_op`
             return self.copy_op(src, dest);
         }
diff --git a/src/librustc_mir/transform/validate.rs b/src/librustc_mir/transform/validate.rs
index 803954d258f..5f0edd64c47 100644
--- a/src/librustc_mir/transform/validate.rs
+++ b/src/librustc_mir/transform/validate.rs
@@ -32,6 +32,93 @@ fn run_pass(&self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut Body<'
     }
 }
 
+/// Returns whether the two types are equal up to lifetimes.
+/// All lifetimes, including higher-ranked ones, get ignored for this comparison.
+/// (This is unlike the `erasing_regions` methods, which keep higher-ranked lifetimes for soundness reasons.)
+///
+/// The point of this function is to approximate "equal up to subtyping".  However,
+/// the approximation is incorrect as variance is ignored.
+pub fn equal_up_to_regions(
+    tcx: TyCtxt<'tcx>,
+    param_env: ParamEnv<'tcx>,
+    src: Ty<'tcx>,
+    dest: Ty<'tcx>,
+) -> bool {
+    struct LifetimeIgnoreRelation<'tcx> {
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    }
+
+    impl TypeRelation<'tcx> for LifetimeIgnoreRelation<'tcx> {
+        fn tcx(&self) -> TyCtxt<'tcx> {
+            self.tcx
+        }
+
+        fn param_env(&self) -> ty::ParamEnv<'tcx> {
+            self.param_env
+        }
+
+        fn tag(&self) -> &'static str {
+            "librustc_mir::transform::validate"
+        }
+
+        fn a_is_expected(&self) -> bool {
+            true
+        }
+
+        fn relate_with_variance<T: Relate<'tcx>>(
+            &mut self,
+            _: ty::Variance,
+            a: &T,
+            b: &T,
+        ) -> RelateResult<'tcx, T> {
+            // Ignore variance, require types to be exactly the same.
+            self.relate(a, b)
+        }
+
+        fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+            if a == b {
+                // Short-circuit.
+                return Ok(a);
+            }
+            ty::relate::super_relate_tys(self, a, b)
+        }
+
+        fn regions(
+            &mut self,
+            a: ty::Region<'tcx>,
+            _b: ty::Region<'tcx>,
+        ) -> RelateResult<'tcx, ty::Region<'tcx>> {
+            // Ignore regions.
+            Ok(a)
+        }
+
+        fn consts(
+            &mut self,
+            a: &'tcx ty::Const<'tcx>,
+            b: &'tcx ty::Const<'tcx>,
+        ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
+            ty::relate::super_relate_consts(self, a, b)
+        }
+
+        fn binders<T>(
+            &mut self,
+            a: &ty::Binder<T>,
+            b: &ty::Binder<T>,
+        ) -> RelateResult<'tcx, ty::Binder<T>>
+        where
+            T: Relate<'tcx>,
+        {
+            self.relate(a.skip_binder(), b.skip_binder())?;
+            Ok(a.clone())
+        }
+    }
+
+    // Instantiate and run relation.
+    let mut relator: LifetimeIgnoreRelation<'tcx> = LifetimeIgnoreRelation { tcx: tcx, param_env };
+    relator.relate(&src, &dest).is_ok()
+}
+
 struct TypeChecker<'a, 'tcx> {
     when: &'a str,
     source: MirSource<'tcx>,
@@ -108,80 +195,7 @@ fn mir_assign_valid_types(&self, src: Ty<'tcx>, dest: Ty<'tcx>) -> bool {
         // all normal lifetimes are erased, higher-ranked types with their
         // late-bound lifetimes are still around and can lead to type
         // differences. So we compare ignoring lifetimes.
-        struct LifetimeIgnoreRelation<'tcx> {
-            tcx: TyCtxt<'tcx>,
-            param_env: ty::ParamEnv<'tcx>,
-        }
-
-        impl TypeRelation<'tcx> for LifetimeIgnoreRelation<'tcx> {
-            fn tcx(&self) -> TyCtxt<'tcx> {
-                self.tcx
-            }
-
-            fn param_env(&self) -> ty::ParamEnv<'tcx> {
-                self.param_env
-            }
-
-            fn tag(&self) -> &'static str {
-                "librustc_mir::transform::validate"
-            }
-
-            fn a_is_expected(&self) -> bool {
-                true
-            }
-
-            fn relate_with_variance<T: Relate<'tcx>>(
-                &mut self,
-                _: ty::Variance,
-                a: &T,
-                b: &T,
-            ) -> RelateResult<'tcx, T> {
-                // Ignore variance, require types to be exactly the same.
-                self.relate(a, b)
-            }
-
-            fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
-                if a == b {
-                    // Short-circuit.
-                    return Ok(a);
-                }
-                ty::relate::super_relate_tys(self, a, b)
-            }
-
-            fn regions(
-                &mut self,
-                a: ty::Region<'tcx>,
-                _b: ty::Region<'tcx>,
-            ) -> RelateResult<'tcx, ty::Region<'tcx>> {
-                // Ignore regions.
-                Ok(a)
-            }
-
-            fn consts(
-                &mut self,
-                a: &'tcx ty::Const<'tcx>,
-                b: &'tcx ty::Const<'tcx>,
-            ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
-                ty::relate::super_relate_consts(self, a, b)
-            }
-
-            fn binders<T>(
-                &mut self,
-                a: &ty::Binder<T>,
-                b: &ty::Binder<T>,
-            ) -> RelateResult<'tcx, ty::Binder<T>>
-            where
-                T: Relate<'tcx>,
-            {
-                self.relate(a.skip_binder(), b.skip_binder())?;
-                Ok(a.clone())
-            }
-        }
-
-        // Instantiate and run relation.
-        let mut relator: LifetimeIgnoreRelation<'tcx> =
-            LifetimeIgnoreRelation { tcx: self.tcx, param_env };
-        relator.relate(&src, &dest).is_ok()
+        equal_up_to_regions(self.tcx, param_env, src, dest)
     }
 }
 

From 5e5ae8b0875ce70b1a729bef442e753bb3d2502f Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Mon, 22 Jun 2020 10:26:29 +0200
Subject: [PATCH 10/12] expand a comment

---
 src/librustc_mir/interpret/eval_context.rs | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/librustc_mir/interpret/eval_context.rs b/src/librustc_mir/interpret/eval_context.rs
index b673738cec5..56a9355650e 100644
--- a/src/librustc_mir/interpret/eval_context.rs
+++ b/src/librustc_mir/interpret/eval_context.rs
@@ -226,7 +226,9 @@ pub(super) fn mir_assign_valid_types<'tcx>(
     dest: TyAndLayout<'tcx>,
 ) -> bool {
     if src.ty == dest.ty {
-        // Equal types, all is good.
+        // Equal types, all is good. Layout will also be equal.
+        // (Enum variants would be an exception here as they have the type of the enum but different layout.
+        // However, those are never the type of an assignment.)
         return true;
     }
     if src.layout != dest.layout {

From a593728fc753f85df575b54aadc64c3fd2c06d11 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Mon, 22 Jun 2020 11:07:39 +0200
Subject: [PATCH 11/12] make layout check a mere sanity check

---
 src/librustc_mir/interpret/eval_context.rs | 25 ++++++----------------
 src/librustc_mir/transform/validate.rs     | 10 +++++----
 2 files changed, 13 insertions(+), 22 deletions(-)

diff --git a/src/librustc_mir/interpret/eval_context.rs b/src/librustc_mir/interpret/eval_context.rs
index 56a9355650e..25860c43add 100644
--- a/src/librustc_mir/interpret/eval_context.rs
+++ b/src/librustc_mir/interpret/eval_context.rs
@@ -225,28 +225,17 @@ pub(super) fn mir_assign_valid_types<'tcx>(
     src: TyAndLayout<'tcx>,
     dest: TyAndLayout<'tcx>,
 ) -> bool {
-    if src.ty == dest.ty {
-        // Equal types, all is good. Layout will also be equal.
-        // (Enum variants would be an exception here as they have the type of the enum but different layout.
-        // However, those are never the type of an assignment.)
-        return true;
-    }
-    if src.layout != dest.layout {
-        // Layout differs, definitely not equal.
-        // We do this here because Miri would *do the wrong thing* if we allowed layout-changing
-        // assignments.
-        return false;
-    }
-
     // Type-changing assignments can happen when subtyping is used. While
     // all normal lifetimes are erased, higher-ranked types with their
     // late-bound lifetimes are still around and can lead to type
     // differences. So we compare ignoring lifetimes.
-    //
-    // Note that this is not fully correct (FIXME):
-    // lifetimes in invariant positions could matter (e.g. through associated types).
-    // We rely on the fact that layout was confirmed to be equal above.
-    equal_up_to_regions(tcx, param_env, src.ty, dest.ty)
+    if equal_up_to_regions(tcx, param_env, src.ty, dest.ty) {
+        // Make sure the layout is equal, too -- just to be safe. Miri really needs layout equality.
+        assert_eq!(src.layout, dest.layout);
+        true
+    } else {
+        false
+    }
 }
 
 /// Use the already known layout if given (but sanity check in debug mode),
diff --git a/src/librustc_mir/transform/validate.rs b/src/librustc_mir/transform/validate.rs
index 5f0edd64c47..a293751d5b2 100644
--- a/src/librustc_mir/transform/validate.rs
+++ b/src/librustc_mir/transform/validate.rs
@@ -44,6 +44,11 @@ pub fn equal_up_to_regions(
     src: Ty<'tcx>,
     dest: Ty<'tcx>,
 ) -> bool {
+    // Fast path.
+    if src == dest {
+        return true;
+    }
+
     struct LifetimeIgnoreRelation<'tcx> {
         tcx: TyCtxt<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
@@ -176,6 +181,7 @@ fn check_edge(&self, location: Location, bb: BasicBlock, edge_kind: EdgeKind) {
     /// Check if src can be assigned into dest.
     /// This is not precise, it will accept some incorrect assignments.
     fn mir_assign_valid_types(&self, src: Ty<'tcx>, dest: Ty<'tcx>) -> bool {
+        // Fast path before we normalize.
         if src == dest {
             // Equal types, all is good.
             return true;
@@ -186,10 +192,6 @@ fn mir_assign_valid_types(&self, src: Ty<'tcx>, dest: Ty<'tcx>) -> bool {
         let param_env = self.param_env.with_reveal_all();
         let src = self.tcx.normalize_erasing_regions(param_env, src);
         let dest = self.tcx.normalize_erasing_regions(param_env, dest);
-        // It's worth checking equality again.
-        if src == dest {
-            return true;
-        }
 
         // Type-changing assignments can happen when subtyping is used. While
         // all normal lifetimes are erased, higher-ranked types with their

From 35911eeb93bae9585b6881bb3d6620df3e6a38ff Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Wed, 24 Jun 2020 09:03:01 +0200
Subject: [PATCH 12/12] reduce sanity check in debug mode

---
 src/librustc_mir/interpret/eval_context.rs | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/librustc_mir/interpret/eval_context.rs b/src/librustc_mir/interpret/eval_context.rs
index 25860c43add..ec792d5b224 100644
--- a/src/librustc_mir/interpret/eval_context.rs
+++ b/src/librustc_mir/interpret/eval_context.rs
@@ -230,8 +230,14 @@ pub(super) fn mir_assign_valid_types<'tcx>(
     // late-bound lifetimes are still around and can lead to type
     // differences. So we compare ignoring lifetimes.
     if equal_up_to_regions(tcx, param_env, src.ty, dest.ty) {
-        // Make sure the layout is equal, too -- just to be safe. Miri really needs layout equality.
-        assert_eq!(src.layout, dest.layout);
+        // Make sure the layout is equal, too -- just to be safe. Miri really
+        // needs layout equality. For performance reason we skip this check when
+        // the types are equal. Equal types *can* have different layouts when
+        // enum downcast is involved (as enum variants carry the type of the
+        // enum), but those should never occur in assignments.
+        if cfg!(debug_assertions) || src.ty != dest.ty {
+            assert_eq!(src.layout, dest.layout);
+        }
         true
     } else {
         false