Drop tracking ignores borrows with projections

[tmiasko]

For example:

fn main() {
    let _ = async {
        let mut s = (String::new(),);
        s.0.push_str("abc");
        std::mem::drop(s);
        async {}.await;
    };
}

$ rustc +nightly-2022-01-22 --edition=2021 b.rs
error: internal compiler error: compiler/rustc_mir_transform/src/generator.rs:755:13: Broken MIR: generator contains type (String,) in MIR, but typeck only knows about {ResumeTy, impl Future<Output = [async output]>, ()} and []

The TryFrom<&PlaceWithHirId<'_>> for TrackedValue fails for places with projections, so in that case borrow is never recorded:

rust/compiler/rustc_typeck/src/check/generator_interior/drop_ranges.rs

Lines 133 to 143 in 4c55c83

	impl TryFrom<&PlaceWithHirId<'_>> for TrackedValue {
	type Error = TrackedValueConversionError;
	fn try_from(place_with_id: &PlaceWithHirId<'_>) -> Result<Self, Self::Error> {
	if !place_with_id.place.projections.is_empty() {
	debug!(
	"TrackedValue from PlaceWithHirId: {:?} has projections, which are not supported.",
	place_with_id
	);
	return Err(TrackedValueConversionError::PlaceProjectionsNotSupported);
	}

rust/compiler/rustc_typeck/src/check/generator_interior/drop_ranges/record_consumed_borrow.rs

Lines 93 to 102 in 4c55c83

	fn borrow(
	&mut self,
	place_with_id: &expr_use_visitor::PlaceWithHirId<'tcx>,
	_diag_expr_id: HirId,
	_bk: rustc_middle::ty::BorrowKind,
	) {
	place_with_id
	.try_into()
	.map_or(false, |tracked_value| self.places.borrowed.insert(tracked_value));
	}

[eholk]

Thanks for the test case and the analysis!

It looks like I got the logic around this wrong. Basically, to goal was to have it so that consumes of a projection (e.g. drop(x.y)) are ignored, and reinitializations (x.y = foo) count as reinitializing all of x. It seems the code that ignores projections is too aggressive in ignoring things.

I'll try to get a fix out soon!

@rustbot claim