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add helper to project to a field of a place

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This commit is contained in:
Ralf Jung 2026-02-07 19:11:38 +01:00
parent e5ed8643b6
commit 0df1764e60
2 changed files with 22 additions and 16 deletions
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16
compiler/rustc_middle/src/mir/statement.rs
View file

@ -432,6 +432,22 @@ impl<'tcx> Place<'tcx> {
self.as_ref().project_deeper(more_projections, tcx)
}
/// Return a place that projects to a field of the current place.
///
/// The type of the current place must be an ADT.
pub fn project_to_field(
self,
idx: FieldIdx,
local_decls: &impl HasLocalDecls<'tcx>,
tcx: TyCtxt<'tcx>,
) -> Self {
let ty = self.ty(local_decls, tcx).ty;
let ty::Adt(adt, args) = ty.kind() else { panic!("projecting to field of non-ADT {ty}") };
let field = &adt.non_enum_variant().fields[idx];
let field_ty = field.ty(tcx, args);
self.project_deeper(&[ProjectionElem::Field(idx, field_ty)], tcx)
}
pub fn ty_from<D>(
local: Local,
projection: &[PlaceElem<'tcx>],

22
compiler/rustc_mir_build/src/builder/expr/into.rs
View file

@ -409,7 +409,6 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
// cause the borrow checker to enforce that `val` lives sufficiently
// long to be stored in `b`. The above lowering does this; anything that
// involves a `*const T` or a `NonNull<T>` does not as those are covariant.
let tcx = this.tcx;
// Extract the operands, compile `b`.
let [b, val] = **args else {
@ -419,28 +418,19 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
else {
span_bug!(expr_span, "invalid init_box_via_move call")
};
// Helper to project to a field of an ADT.
let place_project_field = |place: Place<'tcx>, idx: FieldIdx| {
let ty = place.ty(&this.local_decls, tcx).ty;
let ty::Adt(adt, args) = ty.kind() else {
panic!("projecting to field of non-ADT {ty}")
};
let field = &adt.non_enum_variant().fields[idx];
let field_ty = field.ty(tcx, args);
place.project_deeper(&[ProjectionElem::Field(idx, field_ty)], tcx)
};
let tcx = this.tcx;
let decls = &this.local_decls;
// `b` is a `Box<MaybeUninit<T>>`.
let place = b.project_deeper(&[ProjectionElem::Deref], tcx);
// Current type: `MaybeUninit<T>`. Field #1 is `ManuallyDrop<T>`.
let place = place_project_field(place, FieldIdx::from_u32(1));
let place = place.project_to_field(FieldIdx::from_u32(1), decls, tcx);
// Current type: `ManuallyDrop<T>`. Field #0 is `MaybeDangling<T>`.
let place = place_project_field(place, FieldIdx::ZERO);
let place = place.project_to_field(FieldIdx::ZERO, decls, tcx);
// Current type: `MaybeDangling<T>`. Field #0 is `T`.
let place = place_project_field(place, FieldIdx::ZERO);
let place = place.project_to_field(FieldIdx::ZERO, decls, tcx);
// Sanity check.
assert_eq!(place.ty(&this.local_decls, tcx).ty, generic_args.type_at(0));
assert_eq!(place.ty(decls, tcx).ty, generic_args.type_at(0));
// Store `val` into place.
unpack!(block = this.expr_into_dest(place, block, val));