Field accesses in debuginfo are treated as real uses of the base local

[jonas-schievink]

MIR may contain debuginfo like this after inlining:

    scope 1 {
        debug x => _3;                   // in scope 1 at $DIR/inline-closure-captures.rs:11:9: 11:10
        scope 2 {
            debug _q => _10;             // in scope 2 at $DIR/inline-closure-captures.rs:11:14: 11:16
            debug q => (*((*_6).0: &i32)); // in scope 2 at $DIR/inline-closure-captures.rs:10:23: 10:24
            debug t => (*((*_6).1: &T)); // in scope 2 at $DIR/inline-closure-captures.rs:10:17: 10:18
            let mut _9: T;               // in scope 2 at $DIR/inline-closure-captures.rs:12:5: 12:9
        }
    }

Here, the field access (*_6).0 is treated like a use of the _6 local in MIR visitors (using PlaceContext::NonMutatingUse), even though it is not an actual use of _6.

Instead, it should be using PlaceContext::NonUse(NonUseContext::VarDebugInfo).

[camelid]

@jonas-schievink Can you describe where the change needs to be made? The visit method for debuginfo seems to be correct:

rust/compiler/rustc_middle/src/mir/visit.rs

Lines 827 to 842 in 603ab5b

	fn super_var_debug_info(&mut self,
	var_debug_info: & $($mutability)? VarDebugInfo<'tcx>) {
	let VarDebugInfo {
	name: _,
	source_info,
	place,
	} = var_debug_info;
	self.visit_source_info(source_info);
	let location = START_BLOCK.start_location();
	self.visit_place(
	place,
	PlaceContext::NonUse(NonUseContext::VarDebugInfo),
	location,
	);
	}

It shells out to visit_place with PlaceContext::NonUse(NonUseContext::VarDebugInfo), and visit_place then shells out through some other methods to visit_projection_elem. visit_projection_elem isn't getting the PlaceContext that visit_place was called with (I guess because the context of a projection is potentially different than that of the local it's projecting), but it seems to do nothing for field accesses:

rust/compiler/rustc_middle/src/mir/visit.rs

Lines 974 to 990 in 603ab5b

	match elem {
	PlaceElem::Index(local) => {
	let mut new_local = local;
	self.visit_local(
	&mut new_local,
	PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy),
	location,
	);
	if new_local == local { None } else { Some(PlaceElem::Index(new_local)) }
	}
	PlaceElem::Deref
	| PlaceElem::Field(..)
	| PlaceElem::ConstantIndex { .. }
	| PlaceElem::Subslice { .. }
	| PlaceElem::Downcast(..) => None,
	}

So what part should I change? Thanks!

[jonas-schievink]

Looks like the problem is here:

rust/compiler/rustc_middle/src/mir/visit.rs

Lines 1020 to 1024 in 603ab5b

	context = if context.is_mutating_use() {
	PlaceContext::MutatingUse(MutatingUseContext::Projection)
	} else {
	PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection)
	};

[camelid]

Okay, thank you!