Auto merge of #80865 - oliviacrain:proj_based, r=RalfJung

Use PlaceRef projection abstractions more consistently in rustc_mir

PlaceRef contains abstractions for dealing with the `projections` array. This PR uses these abstractions more consistently within the `rustc_mir` crate.

See associated issue: rust-lang/rust#80647.

r? `@RalfJung`

10 files changed, 103 insertions, 124 deletions

diff --git a/compiler/rustc_mir/src/borrow_check/diagnostics/conflict_errors.rs b/compiler/rustc_mir/src/borrow_check/diagnostics/conflict_errors.rs
index a5fb8a1cbe8..8f41bfae2fd 100644
--- a/compiler/rustc_mir/src/borrow_check/diagnostics/conflict_errors.rs
+++ b/compiler/rustc_mir/src/borrow_check/diagnostics/conflict_errors.rs
@@ -1616,20 +1616,17 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
     fn classify_drop_access_kind(&self, place: PlaceRef<'tcx>) -> StorageDeadOrDrop<'tcx> {
         let tcx = self.infcx.tcx;
-        match place.projection {
-            [] => StorageDeadOrDrop::LocalStorageDead,
-            [proj_base @ .., elem] => {
+        match place.last_projection() {
+            None => StorageDeadOrDrop::LocalStorageDead,
+            Some((place_base, elem)) => {
                 // FIXME(spastorino) make this iterate
-                let base_access = self.classify_drop_access_kind(PlaceRef {
-                    local: place.local,
-                    projection: proj_base,
-                });
+                let base_access = self.classify_drop_access_kind(place_base);
                 match elem {
                     ProjectionElem::Deref => match base_access {
                         StorageDeadOrDrop::LocalStorageDead
                         | StorageDeadOrDrop::BoxedStorageDead => {
                             assert!(
-                                Place::ty_from(place.local, proj_base, self.body, tcx).ty.is_box(),
+                                place_base.ty(self.body, tcx).ty.is_box(),
                                 "Drop of value behind a reference or raw pointer"
                             );
                             StorageDeadOrDrop::BoxedStorageDead
@@ -1637,7 +1634,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                         StorageDeadOrDrop::Destructor(_) => base_access,
                     },
                     ProjectionElem::Field(..) | ProjectionElem::Downcast(..) => {
-                        let base_ty = Place::ty_from(place.local, proj_base, self.body, tcx).ty;
+                        let base_ty = place_base.ty(self.body, tcx).ty;
                         match base_ty.kind() {
                             ty::Adt(def, _) if def.has_dtor(tcx) => {
                                 // Report the outermost adt with a destructor
@@ -1652,7 +1649,6 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                             _ => base_access,
                         }
                     }
-
                     ProjectionElem::ConstantIndex { .. }
                     | ProjectionElem::Subslice { .. }
                     | ProjectionElem::Index(_) => base_access,
diff --git a/compiler/rustc_mir/src/borrow_check/diagnostics/mod.rs b/compiler/rustc_mir/src/borrow_check/diagnostics/mod.rs
index 9f97d5c9af7..6d98bf554f1 100644
--- a/compiler/rustc_mir/src/borrow_check/diagnostics/mod.rs
+++ b/compiler/rustc_mir/src/borrow_check/diagnostics/mod.rs
@@ -338,8 +338,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     self.describe_field(PlaceRef { local, projection: proj_base }, field)
                 }
                 ProjectionElem::Downcast(_, variant_index) => {
-                    let base_ty =
-                        Place::ty_from(place.local, place.projection, self.body, self.infcx.tcx).ty;
+                    let base_ty = place.ty(self.body, self.infcx.tcx).ty;
                     self.describe_field_from_ty(&base_ty, field, Some(*variant_index))
                 }
                 ProjectionElem::Field(_, field_type) => {
@@ -473,7 +472,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
         // If we didn't find an overloaded deref or index, then assume it's a
         // built in deref and check the type of the base.
-        let base_ty = Place::ty_from(deref_base.local, deref_base.projection, self.body, tcx).ty;
+        let base_ty = deref_base.ty(self.body, tcx).ty;
         if base_ty.is_unsafe_ptr() {
             BorrowedContentSource::DerefRawPointer
         } else if base_ty.is_mutable_ptr() {
diff --git a/compiler/rustc_mir/src/borrow_check/path_utils.rs b/compiler/rustc_mir/src/borrow_check/path_utils.rs
index 934729553a7..fa3ae2367e0 100644
--- a/compiler/rustc_mir/src/borrow_check/path_utils.rs
+++ b/compiler/rustc_mir/src/borrow_check/path_utils.rs
@@ -147,27 +147,25 @@ pub(crate) fn is_upvar_field_projection(
     place_ref: PlaceRef<'tcx>,
     body: &Body<'tcx>,
 ) -> Option<Field> {
-    let mut place_projection = place_ref.projection;
+    let mut place_ref = place_ref;
     let mut by_ref = false;
 
-    if let [proj_base @ .., ProjectionElem::Deref] = place_projection {
-        place_projection = proj_base;
+    if let Some((place_base, ProjectionElem::Deref)) = place_ref.last_projection() {
+        place_ref = place_base;
         by_ref = true;
     }
 
-    match place_projection {
-        [base @ .., ProjectionElem::Field(field, _ty)] => {
-            let base_ty = Place::ty_from(place_ref.local, base, body, tcx).ty;
-
+    match place_ref.last_projection() {
+        Some((place_base, ProjectionElem::Field(field, _ty))) => {
+            let base_ty = place_base.ty(body, tcx).ty;
             if (base_ty.is_closure() || base_ty.is_generator())
                 && (!by_ref || upvars[field.index()].by_ref)
             {
-                Some(*field)
+                Some(field)
             } else {
                 None
             }
         }
-
         _ => None,
     }
 }
diff --git a/compiler/rustc_mir/src/dataflow/move_paths/builder.rs b/compiler/rustc_mir/src/dataflow/move_paths/builder.rs
index ab7fadac91e..ee78ff00c9b 100644
--- a/compiler/rustc_mir/src/dataflow/move_paths/builder.rs
+++ b/compiler/rustc_mir/src/dataflow/move_paths/builder.rs
@@ -518,14 +518,10 @@ impl<'b, 'a, 'tcx> Gatherer<'b, 'a, 'tcx> {
 
         // Check if we are assigning into a field of a union, if so, lookup the place
         // of the union so it is marked as initialized again.
-        if let [proj_base @ .., ProjectionElem::Field(_, _)] = place.projection {
-            if let ty::Adt(def, _) =
-                Place::ty_from(place.local, proj_base, self.builder.body, self.builder.tcx)
-                    .ty
-                    .kind()
-            {
+        if let Some((place_base, ProjectionElem::Field(_, _))) = place.last_projection() {
+            if let ty::Adt(def, _) = place_base.ty(self.builder.body, self.builder.tcx).ty.kind() {
                 if def.is_union() {
-                    place = PlaceRef { local: place.local, projection: proj_base }
+                    place = place_base;
                 }
             }
         }
diff --git a/compiler/rustc_mir/src/transform/check_consts/qualifs.rs b/compiler/rustc_mir/src/transform/check_consts/qualifs.rs
index c66d3ed76df..0ce1980f10a 100644
--- a/compiler/rustc_mir/src/transform/check_consts/qualifs.rs
+++ b/compiler/rustc_mir/src/transform/check_consts/qualifs.rs
@@ -174,14 +174,10 @@ where
 
         Rvalue::Ref(_, _, place) | Rvalue::AddressOf(_, place) => {
             // Special-case reborrows to be more like a copy of the reference.
-            if let &[ref proj_base @ .., ProjectionElem::Deref] = place.projection.as_ref() {
-                let base_ty = Place::ty_from(place.local, proj_base, cx.body, cx.tcx).ty;
+            if let Some((place_base, ProjectionElem::Deref)) = place.as_ref().last_projection() {
+                let base_ty = place_base.ty(cx.body, cx.tcx).ty;
                 if let ty::Ref(..) = base_ty.kind() {
-                    return in_place::<Q, _>(
-                        cx,
-                        in_local,
-                        PlaceRef { local: place.local, projection: proj_base },
-                    );
+                    return in_place::<Q, _>(cx, in_local, place_base);
                 }
             }
 
@@ -209,9 +205,9 @@ where
     Q: Qualif,
     F: FnMut(Local) -> bool,
 {
-    let mut projection = place.projection;
-    while let &[ref proj_base @ .., proj_elem] = projection {
-        match proj_elem {
+    let mut place = place;
+    while let Some((place_base, elem)) = place.last_projection() {
+        match elem {
             ProjectionElem::Index(index) if in_local(index) => return true,
 
             ProjectionElem::Deref
@@ -222,16 +218,16 @@ where
             | ProjectionElem::Index(_) => {}
         }
 
-        let base_ty = Place::ty_from(place.local, proj_base, cx.body, cx.tcx);
-        let proj_ty = base_ty.projection_ty(cx.tcx, proj_elem).ty;
+        let base_ty = place_base.ty(cx.body, cx.tcx);
+        let proj_ty = base_ty.projection_ty(cx.tcx, elem).ty;
         if !Q::in_any_value_of_ty(cx, proj_ty) {
             return false;
         }
 
-        projection = proj_base;
+        place = place_base;
     }
 
-    assert!(projection.is_empty());
+    assert!(place.projection.is_empty());
     in_local(place.local)
 }
 
diff --git a/compiler/rustc_mir/src/transform/check_consts/validation.rs b/compiler/rustc_mir/src/transform/check_consts/validation.rs
index 88b2378cd06..3d8192649a0 100644
--- a/compiler/rustc_mir/src/transform/check_consts/validation.rs
+++ b/compiler/rustc_mir/src/transform/check_consts/validation.rs
@@ -1007,27 +1007,26 @@ fn place_as_reborrow(
     body: &Body<'tcx>,
     place: Place<'tcx>,
 ) -> Option<&'a [PlaceElem<'tcx>]> {
-    place.projection.split_last().and_then(|(outermost, inner)| {
-        if outermost != &ProjectionElem::Deref {
-            return None;
-        }
-
-        // A borrow of a `static` also looks like `&(*_1)` in the MIR, but `_1` is a `const`
-        // that points to the allocation for the static. Don't treat these as reborrows.
-        if body.local_decls[place.local].is_ref_to_static() {
-            return None;
-        }
-
-        // Ensure the type being derefed is a reference and not a raw pointer.
-        //
-        // This is sufficient to prevent an access to a `static mut` from being marked as a
-        // reborrow, even if the check above were to disappear.
-        let inner_ty = Place::ty_from(place.local, inner, body, tcx).ty;
-        match inner_ty.kind() {
-            ty::Ref(..) => Some(inner),
-            _ => None,
+    match place.as_ref().last_projection() {
+        Some((place_base, ProjectionElem::Deref)) => {
+            // A borrow of a `static` also looks like `&(*_1)` in the MIR, but `_1` is a `const`
+            // that points to the allocation for the static. Don't treat these as reborrows.
+            if body.local_decls[place_base.local].is_ref_to_static() {
+                None
+            } else {
+                // Ensure the type being derefed is a reference and not a raw pointer.
+                //
+                // This is sufficient to prevent an access to a `static mut` from being marked as a
+                // reborrow, even if the check above were to disappear.
+                let inner_ty = place_base.ty(body, tcx).ty;
+                match inner_ty.kind() {
+                    ty::Ref(..) => Some(place_base.projection),
+                    _ => None,
+                }
+            }
         }
-    })
+        _ => None,
+    }
 }
 
 fn is_int_bool_or_char(ty: Ty<'_>) -> bool {
diff --git a/compiler/rustc_mir/src/transform/check_unsafety.rs b/compiler/rustc_mir/src/transform/check_unsafety.rs
index 0732d2568ad..bac47d0b547 100644
--- a/compiler/rustc_mir/src/transform/check_unsafety.rs
+++ b/compiler/rustc_mir/src/transform/check_unsafety.rs
@@ -399,17 +399,13 @@ impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
         place: Place<'tcx>,
         is_mut_use: bool,
     ) {
-        let mut cursor = place.projection.as_ref();
-        while let &[ref proj_base @ .., elem] = cursor {
-            cursor = proj_base;
-
+        for (place_base, elem) in place.iter_projections().rev() {
             match elem {
                 // Modifications behind a dereference don't affect the value of
                 // the pointer.
                 ProjectionElem::Deref => return,
                 ProjectionElem::Field(..) => {
-                    let ty =
-                        Place::ty_from(place.local, proj_base, &self.body.local_decls, self.tcx).ty;
+                    let ty = place_base.ty(&self.body.local_decls, self.tcx).ty;
                     if let ty::Adt(def, _) = ty.kind() {
                         if self.tcx.layout_scalar_valid_range(def.did)
                             != (Bound::Unbounded, Bound::Unbounded)
diff --git a/compiler/rustc_mir/src/transform/instcombine.rs b/compiler/rustc_mir/src/transform/instcombine.rs
index 990ca313c5d..b0c70372b16 100644
--- a/compiler/rustc_mir/src/transform/instcombine.rs
+++ b/compiler/rustc_mir/src/transform/instcombine.rs
@@ -277,11 +277,9 @@ impl OptimizationFinder<'b, 'tcx> {
 impl Visitor<'tcx> for OptimizationFinder<'b, 'tcx> {
     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
         if let Rvalue::Ref(_, _, place) = rvalue {
-            if let PlaceRef { local, projection: &[ref proj_base @ .., ProjectionElem::Deref] } =
-                place.as_ref()
-            {
+            if let Some((place_base, ProjectionElem::Deref)) = place.as_ref().last_projection() {
                 // The dereferenced place must have type `&_`.
-                let ty = Place::ty_from(local, proj_base, self.body, self.tcx).ty;
+                let ty = place_base.ty(self.body, self.tcx).ty;
                 if let ty::Ref(_, _, Mutability::Not) = ty.kind() {
                     self.optimizations.and_stars.insert(location);
                 }
diff --git a/compiler/rustc_mir/src/transform/promote_consts.rs b/compiler/rustc_mir/src/transform/promote_consts.rs
index ea92e23e9bf..cac5abb1059 100644
--- a/compiler/rustc_mir/src/transform/promote_consts.rs
+++ b/compiler/rustc_mir/src/transform/promote_consts.rs
@@ -445,43 +445,50 @@ impl<'tcx> Validator<'_, 'tcx> {
     }
 
     fn validate_place(&self, place: PlaceRef<'tcx>) -> Result<(), Unpromotable> {
-        match place {
-            PlaceRef { local, projection: [] } => self.validate_local(local),
-            PlaceRef { local, projection: [proj_base @ .., elem] } => {
+        match place.last_projection() {
+            None => self.validate_local(place.local),
+            Some((place_base, elem)) => {
                 // Validate topmost projection, then recurse.
-                match *elem {
+                match elem {
                     ProjectionElem::Deref => {
                         let mut promotable = false;
-                        // This is a special treatment for cases like *&STATIC where STATIC is a
-                        // global static variable.
-                        // This pattern is generated only when global static variables are directly
-                        // accessed and is qualified for promotion safely.
-                        if let TempState::Defined { location, .. } = self.temps[local] {
-                            let def_stmt =
-                                self.body[location.block].statements.get(location.statement_index);
-                            if let Some(Statement {
-                                kind:
-                                    StatementKind::Assign(box (_, Rvalue::Use(Operand::Constant(c)))),
-                                ..
-                            }) = def_stmt
+                        // The `is_empty` predicate is introduced to exclude the case
+                        // where the projection operations are [ .field, * ].
+                        // The reason is because promotion will be illegal if field
+                        // accesses precede the dereferencing.
+                        // Discussion can be found at
+                        // https://github.com/rust-lang/rust/pull/74945#discussion_r463063247
+                        // There may be opportunity for generalization, but this needs to be
+                        // accounted for.
+                        if place_base.projection.is_empty() {
+                            // This is a special treatment for cases like *&STATIC where STATIC is a
+                            // global static variable.
+                            // This pattern is generated only when global static variables are directly
+                            // accessed and is qualified for promotion safely.
+                            if let TempState::Defined { location, .. } =
+                                self.temps[place_base.local]
                             {
-                                if let Some(did) = c.check_static_ptr(self.tcx) {
-                                    // Evaluating a promoted may not read statics except if it got
-                                    // promoted from a static (this is a CTFE check). So we
-                                    // can only promote static accesses inside statics.
-                                    if let Some(hir::ConstContext::Static(..)) = self.const_kind {
-                                        // The `is_empty` predicate is introduced to exclude the case
-                                        // where the projection operations are [ .field, * ].
-                                        // The reason is because promotion will be illegal if field
-                                        // accesses precede the dereferencing.
-                                        // Discussion can be found at
-                                        // https://github.com/rust-lang/rust/pull/74945#discussion_r463063247
-                                        // There may be opportunity for generalization, but this needs to be
-                                        // accounted for.
-                                        if proj_base.is_empty()
-                                            && !self.tcx.is_thread_local_static(did)
+                                let def_stmt = self.body[location.block]
+                                    .statements
+                                    .get(location.statement_index);
+                                if let Some(Statement {
+                                    kind:
+                                        StatementKind::Assign(box (
+                                            _,
+                                            Rvalue::Use(Operand::Constant(c)),
+                                        )),
+                                    ..
+                                }) = def_stmt
+                                {
+                                    if let Some(did) = c.check_static_ptr(self.tcx) {
+                                        // Evaluating a promoted may not read statics except if it got
+                                        // promoted from a static (this is a CTFE check). So we
+                                        // can only promote static accesses inside statics.
+                                        if let Some(hir::ConstContext::Static(..)) = self.const_kind
                                         {
-                                            promotable = true;
+                                            if !self.tcx.is_thread_local_static(did) {
+                                                promotable = true;
+                                            }
                                         }
                                     }
                                 }
@@ -502,8 +509,7 @@ impl<'tcx> Validator<'_, 'tcx> {
                     }
 
                     ProjectionElem::Field(..) => {
-                        let base_ty =
-                            Place::ty_from(place.local, proj_base, self.body, self.tcx).ty;
+                        let base_ty = place_base.ty(self.body, self.tcx).ty;
                         if let Some(def) = base_ty.ty_adt_def() {
                             // No promotion of union field accesses.
                             if def.is_union() {
@@ -513,7 +519,7 @@ impl<'tcx> Validator<'_, 'tcx> {
                     }
                 }
 
-                self.validate_place(PlaceRef { local: place.local, projection: proj_base })
+                self.validate_place(place_base)
             }
         }
     }
@@ -660,13 +666,11 @@ impl<'tcx> Validator<'_, 'tcx> {
             Rvalue::AddressOf(_, place) => {
                 // We accept `&raw *`, i.e., raw reborrows -- creating a raw pointer is
                 // no problem, only using it is.
-                if let [proj_base @ .., ProjectionElem::Deref] = place.projection.as_ref() {
-                    let base_ty = Place::ty_from(place.local, proj_base, self.body, self.tcx).ty;
+                if let Some((place_base, ProjectionElem::Deref)) = place.as_ref().last_projection()
+                {
+                    let base_ty = place_base.ty(self.body, self.tcx).ty;
                     if let ty::Ref(..) = base_ty.kind() {
-                        return self.validate_place(PlaceRef {
-                            local: place.local,
-                            projection: proj_base,
-                        });
+                        return self.validate_place(place_base);
                     }
                 }
                 return Err(Unpromotable);
@@ -675,12 +679,12 @@ impl<'tcx> Validator<'_, 'tcx> {
             Rvalue::Ref(_, kind, place) => {
                 // Special-case reborrows to be more like a copy of the reference.
                 let mut place_simplified = place.as_ref();
-                if let [proj_base @ .., ProjectionElem::Deref] = &place_simplified.projection {
-                    let base_ty =
-                        Place::ty_from(place_simplified.local, proj_base, self.body, self.tcx).ty;
+                if let Some((place_base, ProjectionElem::Deref)) =
+                    place_simplified.last_projection()
+                {
+                    let base_ty = place_base.ty(self.body, self.tcx).ty;
                     if let ty::Ref(..) = base_ty.kind() {
-                        place_simplified =
-                            PlaceRef { local: place_simplified.local, projection: proj_base };
+                        place_simplified = place_base;
                     }
                 }
 
diff --git a/compiler/rustc_mir/src/util/alignment.rs b/compiler/rustc_mir/src/util/alignment.rs
index a0728a6a630..f567c9cfaab 100644
--- a/compiler/rustc_mir/src/util/alignment.rs
+++ b/compiler/rustc_mir/src/util/alignment.rs
@@ -38,15 +38,12 @@ fn is_within_packed<'tcx, L>(tcx: TyCtxt<'tcx>, local_decls: &L, place: Place<'t
 where
     L: HasLocalDecls<'tcx>,
 {
-    let mut cursor = place.projection.as_ref();
-    while let &[ref proj_base @ .., elem] = cursor {
-        cursor = proj_base;
-
+    for (place_base, elem) in place.iter_projections().rev() {
         match elem {
             // encountered a Deref, which is ABI-aligned
             ProjectionElem::Deref => break,
             ProjectionElem::Field(..) => {
-                let ty = Place::ty_from(place.local, proj_base, local_decls, tcx).ty;
+                let ty = place_base.ty(local_decls, tcx).ty;
                 match ty.kind() {
                     ty::Adt(def, _) if def.repr.packed() => return true,
                     _ => {}