use hir::{def_id::DefId, Body, HirId, HirIdMap};
use rustc_data_structures::stable_set::FxHashSet;
use rustc_hir as hir;
-use rustc_middle::hir::place::PlaceBase;
+use rustc_middle::hir::place::{PlaceBase, Projection, ProjectionKind};
use rustc_middle::ty::{ParamEnv, TyCtxt};
pub(super) fn find_consumed_and_borrowed<'a, 'tcx>(
.borrowed
.insert(TrackedValue::from_place_with_projections_allowed(place_with_id));
- // Keep track of whether this is a borrowed temporary (i.e. a borrow of an RValue)
- // so that later in generator_interior we can use the correct scope.
+ // Ordinarily a value is consumed by it's parent, but in the special case of a
+ // borrowed RValue, we create a reference that lives as long as the temporary scope
+ // for that expression (typically, the innermost statement, but sometimes the enclosing
+ // block). We record this fact here so that later in generator_interior
+ // we can use the correct scope.
//
- // We ignore borrows that are the result of an autoref because these will be
- // immediately consumed and should not extend the temporary's lifetime.
- if let (false, PlaceBase::Rvalue) = (is_autoref, place_with_id.place.base) {
+ // We special case borrows through a dereference (`&*x`, `&mut *x` where `x` is
+ // some rvalue expression), since these are essentially a copy of a pointer.
+ // In other words, this borrow does not refer to the
+ // temporary (`*x`), but to the referent (whatever `x` is a borrow of).
+ //
+ // We were considering that we might encounter problems down the line if somehow,
+ // some part of the compiler were to look at this result and try to use it to
+ // drive a borrowck-like analysis (this does not currently happen, as of this writing).
+ // But even this should be fine, because the lifetime of the dereferenced reference
+ // found in the rvalue is only significant as an intermediate 'link' to the value we
+ // are producing, and we separately track whether that value is live over a yield.
+ // Example:
+ //
+ // ```notrust
+ // fn identity<T>(x: &mut T) -> &mut T { x }
+ // let a: A = ...;
+ // let y: &'y mut A = &mut *identity(&'a mut a);
+ // ^^^^^^^^^^^^^^^^^^^^^^^^^ the borrow we are talking about
+ // ```
+ //
+ // The expression `*identity(...)` is a deref of an rvalue,
+ // where the `identity(...)` (the rvalue) produces a return type
+ // of `&'rv mut A`, where `'a: 'rv`. We then assign this result to
+ // `'y`, resulting in (transitively) `'a: 'y` (i.e., while `y` is in use,
+ // `a` will be considered borrowed). Other parts of the code will ensure
+ // that if `y` is live over a yield, `&'y mut A` appears in the generator
+ // state. If `'y` is live, then any sound region analysis must conclude
+ // that `'a` is also live. So if this causes a bug, blame some other
+ // part of the code!
+ let is_deref = place_with_id
+ .place
+ .projections
+ .iter()
+ .any(|Projection { kind, .. }| *kind == ProjectionKind::Deref);
+
+ if let (false, PlaceBase::Rvalue) = (is_deref, place_with_id.place.base) {
self.places.borrowed_temporaries.insert(place_with_id.hir_id);
}
}