NLL Diagnostic Review 3: missing a note describing the type of the non-Copy moved variable

[davidtwco]

The following note is missing on the output of the NLL borrow checker for ui/use/use-after-move-self.nll.stderr:

   |
   = note: move occurs because `self` has type `S`, which does not implement the `Copy` trait

[davidtwco]

Here are some mentoring instructions to get started on this issue. If you're reading this and want to give this issue a go, drop a comment here and feel free to ask for any help or clarification on Zulip.

If this is your first contribution then there are instructions on getting a local build of the compiler available in the rustc guide.

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These instructions are up to date as of master being 850fc6a4791b3b0ab668f9fb67c35dddd838b01f.

If we grep in the source, we can find that both the AST borrow checker and the NLL borrow checker have code that is supposed to emit this note, so rather than adding that note, we'll want to improve how we're detecting when to emit it.

This error is emitted from the cannot_act_on_moved_value function:

rust/src/librustc_mir/util/borrowck_errors.rs

Lines 485 to 494 in 850fc6a

	let err = struct_span_err!(
	self,
	use_span,
	E0382,
	"{} of {}moved value{}{OGN}",
	verb,
	optional_adverb_for_moved,
	moved_path,
	OGN = o
	);

If we grep in the source, we can find that it is invoked in the report_use_of_moved_or_uninitialized function:

rust/src/librustc_mir/borrow_check/error_reporting.rs

Lines 128 to 134 in 850fc6a

	let mut err = self.infcx.tcx.cannot_act_on_moved_value(
	span,
	desired_action.as_noun(),
	msg,
	self.describe_place_with_options(&moved_place, IncludingDowncast(true)),
	Origin::Mir,
	);

In that same function, we can see where the diagnostic note should be being added:

rust/src/librustc_mir/borrow_check/error_reporting.rs

Lines 210 to 224 in 850fc6a

	if let Some(ty) = self.retrieve_type_for_place(place) {
	let note_msg = match self.describe_place_with_options(
	place,
	IncludingDowncast(true),
	) {
	Some(name) => format!("`{}`", name),
	None => "value".to_owned(),
	};
	err.note(&format!(
	"move occurs because {} has type `{}`, \
	which does not implement the `Copy` trait",
	note_msg, ty
	));
	}

Given we know this note should be being added (because the AST borrow checker has it) then that means one of the conditions between the main body of the function and the diagnostic being emitted isn't robust enough and is stopping the error from being reported.

If we look at the used_place variable (the first condition just outside what is shown in the preview above), then we'll see that it is printed as (*(_1.0: std::box::Boxed<isize>)) (equivalent of the *self.x line in the actual source) - a deref of a field projection. Notably, the retrieve_type_of_place function doesn't handle the ProjectionElem::Deref case:

rust/src/librustc_mir/borrow_check/error_reporting.rs

Lines 1677 to 1690 in 850fc6a

	fn retrieve_type_for_place(&self, place: &Place<'tcx>) -> Option<ty::Ty> {
	match place {
	Place::Local(local) => {
	let local = &self.mir.local_decls[*local];
	Some(local.ty)
	}
	Place::Promoted(ref prom) => Some(prom.1),
	Place::Static(ref st) => Some(st.ty),
	Place::Projection(ref proj) => match proj.elem {
	ProjectionElem::Field(_, ty) => Some(ty),
	_ => None,
	},
	}
	}

That may be the issue, consider trying to use the Place::ty (then PlaceTy::to_ty) function instead.

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If you've got this working then you should run the tests and update any that change correctly and then open a PR with a line saying r? @davidtwco in the description so I'll be assigned to review it.