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refactor unsafety checking of places
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@RalfJung
RalfJung committed Nov 22, 2020
1 parent df1c55a commit 571da2c
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Showing 2 changed files with 87 additions and 76 deletions.
4 changes: 3 additions & 1 deletion compiler/rustc_middle/src/mir/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1744,7 +1744,9 @@ impl<'tcx> Place<'tcx> {

/// Iterate over the projections in evaluation order, i.e., the first element is the base with
/// its projection and then subsequently more projections are added.
pub fn iter_projections(self) -> impl Iterator<Item=(PlaceRef<'tcx>, PlaceElem<'tcx>)> + DoubleEndedIterator {
pub fn iter_projections(
self,
) -> impl Iterator<Item = (PlaceRef<'tcx>, PlaceElem<'tcx>)> + DoubleEndedIterator {
self.projection.iter().enumerate().map(move |(i, proj)| {
let base = PlaceRef { local: self.local, projection: &self.projection[..i] };
(base, proj)
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159 changes: 84 additions & 75 deletions compiler/rustc_mir/src/transform/check_unsafety.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -181,6 +181,9 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
self.check_mut_borrowing_layout_constrained_field(*place, context.is_mutating_use());
}

// Check for borrows to packed fields.
// `is_disaligned` already traverses the place to consider all projections after the last
// `Deref`, so this only needs to be called once at the top level.
if context.is_borrow() {
if util::is_disaligned(self.tcx, self.body, self.param_env, *place) {
self.require_unsafe(
Expand All @@ -190,99 +193,105 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
}
}

for (i, _elem) in place.projection.iter().enumerate() {
let proj_base = &place.projection[..i];
if context.is_borrow() {
if util::is_disaligned(self.tcx, self.body, self.param_env, *place) {
// Some checks below need the extra metainfo of the local declaration.
let decl = &self.body.local_decls[place.local];

// Check the base local: it might be an unsafe-to-access static. We only check derefs of the
// temporary holding the static pointer to avoid duplicate errors
// <https://github.com/rust-lang/rust/pull/78068#issuecomment-731753506>.
if decl.internal && place.projection.first() == Some(&ProjectionElem::Deref) {
// If the projection root is an artifical local that we introduced when
// desugaring `static`, give a more specific error message
// (avoid the general "raw pointer" clause below, that would only be confusing).
if let Some(box LocalInfo::StaticRef { def_id, .. }) = decl.local_info {
if self.tcx.is_mutable_static(def_id) {
self.require_unsafe(
UnsafetyViolationKind::BorrowPacked,
UnsafetyViolationDetails::BorrowOfPackedField,
UnsafetyViolationKind::General,
UnsafetyViolationDetails::UseOfMutableStatic,
);
return;
} else if self.tcx.is_foreign_item(def_id) {
self.require_unsafe(
UnsafetyViolationKind::General,
UnsafetyViolationDetails::UseOfExternStatic,
);
return;
}
}
let source_info = self.source_info;
if let [] = proj_base {
let decl = &self.body.local_decls[place.local];
if decl.internal {
// If the projection root is an artifical local that we introduced when
// desugaring `static`, give a more specific error message
// (avoid the general "raw pointer" clause below, that would only be confusing).
if let Some(box LocalInfo::StaticRef { def_id, .. }) = decl.local_info {
if self.tcx.is_mutable_static(def_id) {
self.require_unsafe(
UnsafetyViolationKind::General,
UnsafetyViolationDetails::UseOfMutableStatic,
);
return;
} else if self.tcx.is_foreign_item(def_id) {
self.require_unsafe(
UnsafetyViolationKind::General,
UnsafetyViolationDetails::UseOfExternStatic,
);
return;
}
} else {
// Internal locals are used in the `move_val_init` desugaring.
// We want to check unsafety against the source info of the
// desugaring, rather than the source info of the RHS.
self.source_info = self.body.local_decls[place.local].source_info;
}
}

// Check for raw pointer `Deref`.
for (base, proj) in place.iter_projections() {
if proj == ProjectionElem::Deref {
let source_info = self.source_info; // Backup source_info so we can restore it later.
if base.projection.is_empty() && decl.internal {
// Internal locals are used in the `move_val_init` desugaring.
// We want to check unsafety against the source info of the
// desugaring, rather than the source info of the RHS.
self.source_info = self.body.local_decls[place.local].source_info;
}
let base_ty = base.ty(self.body, self.tcx).ty;
if base_ty.is_unsafe_ptr() {
self.require_unsafe(
UnsafetyViolationKind::GeneralAndConstFn,
UnsafetyViolationDetails::DerefOfRawPointer,
)
}
self.source_info = source_info; // Restore backed-up source_info.
}
}

// Check for union fields. For this we traverse right-to-left, as the last `Deref` changes
// whether we *read* the union field or potentially *write* to it (if this place is being assigned to).
let mut saw_deref = false;
for (base, proj) in place.iter_projections().rev() {
if proj == ProjectionElem::Deref {
saw_deref = true;
continue;
}
let base_ty = Place::ty_from(place.local, proj_base, self.body, self.tcx).ty;
match base_ty.kind() {
ty::RawPtr(..) => self.require_unsafe(
UnsafetyViolationKind::GeneralAndConstFn,
UnsafetyViolationDetails::DerefOfRawPointer,
),
ty::Adt(adt, _) if adt.is_union() => {
let assign_to_field = matches!(

let base_ty = base.ty(self.body, self.tcx).ty;
if base_ty.ty_adt_def().map_or(false, |adt| adt.is_union()) {
// If we did not hit a `Deref` yet and the overall place use is an assignment, the
// rules are different.
let assign_to_field = !saw_deref
&& matches!(
context,
PlaceContext::MutatingUse(
MutatingUseContext::Store
| MutatingUseContext::Drop
| MutatingUseContext::AsmOutput
)
);
// If there is a `Deref` further along the projection chain, this is *not* an
// assignment to a union field. In that case the union field is just read to
// obtain the pointer/reference.
let assign_to_field = assign_to_field
&& !place.projection[i..]
.iter()
.any(|elem| matches!(elem, ProjectionElem::Deref));
// If this is just an assignment, determine if the assigned type needs dropping.
if assign_to_field {
// We have to check the actual type of the assignment, as that determines if the
// old value is being dropped.
let assigned_ty = place.ty(&self.body.local_decls, self.tcx).ty;
// To avoid semver hazard, we only consider `Copy` and `ManuallyDrop` non-dropping.
let manually_drop = assigned_ty
.ty_adt_def()
.map_or(false, |adt_def| adt_def.is_manually_drop());
let nodrop = manually_drop
|| assigned_ty.is_copy_modulo_regions(
self.tcx.at(self.source_info.span),
self.param_env,
);
if !nodrop {
self.require_unsafe(
UnsafetyViolationKind::GeneralAndConstFn,
UnsafetyViolationDetails::AssignToDroppingUnionField,
);
} else {
// write to non-drop union field, safe
}
} else {
// If this is just an assignment, determine if the assigned type needs dropping.
if assign_to_field {
// We have to check the actual type of the assignment, as that determines if the
// old value is being dropped.
let assigned_ty = place.ty(&self.body.local_decls, self.tcx).ty;
// To avoid semver hazard, we only consider `Copy` and `ManuallyDrop` non-dropping.
let manually_drop = assigned_ty
.ty_adt_def()
.map_or(false, |adt_def| adt_def.is_manually_drop());
let nodrop = manually_drop
|| assigned_ty.is_copy_modulo_regions(
self.tcx.at(self.source_info.span),
self.param_env,
);
if !nodrop {
self.require_unsafe(
UnsafetyViolationKind::GeneralAndConstFn,
UnsafetyViolationDetails::AccessToUnionField,
)
UnsafetyViolationDetails::AssignToDroppingUnionField,
);
} else {
// write to non-drop union field, safe
}
} else {
self.require_unsafe(
UnsafetyViolationKind::GeneralAndConstFn,
UnsafetyViolationDetails::AccessToUnionField,
)
}
_ => {}
}
self.source_info = source_info;
}
}
}
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