Skip to content

Commit

Permalink
Unions reinitialized after assignment into field.
Browse files Browse the repository at this point in the history
This commit makes two changes:

First, it updates the dataflow builder to add an init for the place
containing a union if there is an assignment into the field of
that union.

Second, it stops a "use of uninitialized" error occuring when there is an
assignment into the field of an uninitialized union that was previously
initialized. Making this assignment would re-initialize the union, as
tested in `src/test/ui/borrowck/borrowck-union-move-assign.nll.stderr`.
The check for previous initialization ensures that we do not start
supporting partial initialization yet (cc rust-lang#21232, rust-lang#54499, rust-lang#54986).
  • Loading branch information
davidtwco committed Nov 3, 2018
1 parent 87a3c1e commit a4e0945
Show file tree
Hide file tree
Showing 4 changed files with 50 additions and 31 deletions.
29 changes: 23 additions & 6 deletions src/librustc_mir/borrow_check/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1719,12 +1719,13 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
}
}

fn check_parent_of_field<'cx, 'gcx, 'tcx>(this: &mut MirBorrowckCtxt<'cx, 'gcx, 'tcx>,
context: Context,
base: &Place<'tcx>,
span: Span,
flow_state: &Flows<'cx, 'gcx, 'tcx>)
{
fn check_parent_of_field<'cx, 'gcx, 'tcx>(
this: &mut MirBorrowckCtxt<'cx, 'gcx, 'tcx>,
context: Context,
base: &Place<'tcx>,
span: Span,
flow_state: &Flows<'cx, 'gcx, 'tcx>,
) {
// rust-lang/rust#21232: Until Rust allows reads from the
// initialized parts of partially initialized structs, we
// will, starting with the 2018 edition, reject attempts
Expand Down Expand Up @@ -1776,6 +1777,22 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
}

if let Some((prefix, mpi)) = shortest_uninit_seen {
// Check for a reassignment into a uninitialized field of a union (for example,
// after a move out). In this case, do not report a error here. There is an
// exception, if this is the first assignment into the union (that is, there is
// no move out from an earlier location) then this is an attempt at initialization
// of the union - we should error in that case.
let tcx = this.infcx.tcx;
if let ty::TyKind::Adt(def, _) = base.ty(this.mir, tcx).to_ty(tcx).sty {
let moved_before_this = this.move_data.path_map[mpi].iter().any(|moi| {
this.move_data.moves[*moi].source < context.loc
});

if def.is_union() && moved_before_this {
return;
}
}

this.report_use_of_moved_or_uninitialized(
context,
InitializationRequiringAction::PartialAssignment,
Expand Down
14 changes: 14 additions & 0 deletions src/librustc_mir/dataflow/move_paths/builder.rs
Original file line number Diff line number Diff line change
Expand Up @@ -430,6 +430,20 @@ impl<'b, 'a, 'gcx, 'tcx> Gatherer<'b, 'a, 'gcx, 'tcx> {
fn gather_init(&mut self, place: &Place<'tcx>, kind: InitKind) {
debug!("gather_init({:?}, {:?})", self.loc, place);

let place = match place {
// 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.
Place::Projection(box Projection {
base,
elem: ProjectionElem::Field(_, _),
}) if match base.ty(self.builder.mir, self.builder.tcx).to_ty(self.builder.tcx).sty {
ty::TyKind::Adt(def, _) if def.is_union() => true,
_ => false,
} => base,
// Otherwise, lookup the place.
_ => place,
};

if let LookupResult::Exact(path) = self.builder.data.rev_lookup.find(place) {
let init = self.builder.data.inits.push(Init {
location: InitLocation::Statement(self.loc),
Expand Down
24 changes: 1 addition & 23 deletions src/test/ui/borrowck/borrowck-union-move-assign.nll.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -8,28 +8,6 @@ LL | let a = u.a; //~ ERROR use of moved value: `u.a`
|
= note: move occurs because `u` has type `U`, which does not implement the `Copy` trait

error[E0382]: use of moved value: `u`
--> $DIR/borrowck-union-move-assign.rs:33:21
|
LL | let a = u.a;
| --- value moved here
LL | u.a = A;
LL | let a = u.a; // OK
| ^^^ value used here after move
|
= note: move occurs because `u` has type `U`, which does not implement the `Copy` trait

error[E0382]: use of moved value: `u`
--> $DIR/borrowck-union-move-assign.rs:39:21
|
LL | let a = u.a;
| --- value moved here
LL | u.b = B;
LL | let a = u.a; // OK
| ^^^ value used here after move
|
= note: move occurs because `u` has type `U`, which does not implement the `Copy` trait

error: aborting due to 3 previous errors
error: aborting due to previous error

For more information about this error, try `rustc --explain E0382`.
Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,16 @@ error[E0381]: assign to part of possibly uninitialized variable: `x`
LL | x.a = 1; //~ ERROR
| ^^^^^^^ use of possibly uninitialized `x`

error: aborting due to previous error
error[E0594]: cannot assign to `x.b`, as `x` is not declared as mutable
--> $DIR/reassignment_immutable_fields_overlapping.rs:23:5
|
LL | let x: Foo;
| - help: consider changing this to be mutable: `mut x`
LL | x.a = 1; //~ ERROR
LL | x.b = 22; //~ ERROR
| ^^^^^^^^ cannot assign

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0381`.
Some errors occurred: E0381, E0594.
For more information about an error, try `rustc --explain E0381`.

0 comments on commit a4e0945

Please sign in to comment.