Fix borrow checker unsoundness with unions #47689

davidtwco · 2018-01-23T22:53:10Z

Fixes #45157. After discussion with @nikomatsakis on Gitter, this PR only adds a test since the original issue was resolved elsewhere.

r? @nikomatsakis

davidtwco · 2018-01-23T23:19:34Z

~~Renamed this to be "WIP" since I've seen it fail some compile-fail errors that I'll need to look into.~~ See PR description, only adding a test now.

matthewjasper · 2018-01-24T09:40:19Z

src/test/ui/issue-45157.rs

+        let mut u = U { s: Default::default() };
+
+        let mref = &mut u.s.a; //~ ERROR cannot assign twice to immutable variable `mref` [E0384]
+        let err = &u.z.c; //~ ERROR cannot assign twice to immutable variable `err` [E0384]


The code as is should compile with nll. There needs to be a second use of mref to keep the borrow active for the second assignment.

nikomatsakis · 2018-01-24T09:46:42Z

src/librustc_mir/borrow_check/mod.rs

+                            debug!("place_element_conflict: DISJOINT-LOCAL");
+                            Overlap::Disjoint
+                        },
+                    }


I think this is not quite right. Local variables should always be disjoint from one another. For example, borrowing x while y is borrowed should never be an error, no matter the type of x.

davidtwco · 2018-01-24T10:05:11Z

Going to modify this PR to add a test to handle the case @nikomatsakis mentioned in the issue since whichever problem this issue originally referred to is now being handled.

nikomatsakis · 2018-01-24T19:22:36Z

src/test/ui/issue-45157.stderr

+   |                    ^^^^^^ immutable borrow occurs here
+
+error[E0502]: cannot borrow `u.s.a` as mutable because it is also borrowed as immutable
+  --> $DIR/issue-45157.rs:39:27


This second error is a bit surprising. I don't quite understand what it is saying, it looks a bit fishy.

cc @pnkfelix -- agreed?

Are you saying that there should be no error here at all? Or just that the error should be focused on prefixes of the field projections it is currently highlighting?

I do not think there should be an error at all, and certainly not with these spans. Usually something like this:

let mut a = 2; let mref = &mut a; let nref = &a; println("{}{}", mref, nref);

gives only one error, right?

i.e., one borrow comes first, and it "wins"

Example: https://play.rust-lang.org/?gist=695ae34722e06a4060c38df82a45c92d&version=nightly

So, looking into this a little bit, here is what I've figured out:

rust/src/librustc_mir/util/borrowck_errors.rs

Lines 225 to 247 in fd0f292

fn cannot_reborrow_already_borrowed(&self,

span: Span,

desc_new: &str,

msg_new: &str,

kind_new: &str,

old_span: Span,

noun_old: &str,

kind_old: &str,

msg_old: &str,

old_load_end_span: Option<Span>,

o: Origin)

-> DiagnosticBuilder

{

let mut err = struct_span_err!(self, span, E0502,

"cannot borrow `{}`{} as {} because {} is also borrowed as {}{}{OGN}",

desc_new, msg_new, kind_new, noun_old, kind_old, msg_old, OGN=o);

err.span_label(span, format!("{} borrow occurs here{}", kind_new, msg_new));

err.span_label(old_span, format!("{} borrow occurs here{}", kind_old, msg_old));

if let Some(old_load_end_span) = old_load_end_span {

err.span_label(old_load_end_span, format!("{} borrow ends here", kind_old));

}

self.cancel_if_wrong_origin(err, o)

}

The error is reported in the above function. That is called by the following function:

rust/src/librustc_mir/borrow_check/error_reporting.rs

Lines 259 to 272 in fdecb05

(BorrowKind::Shared, lft, _, BorrowKind::Mut, _, rgt) |

(BorrowKind::Mut, _, lft, BorrowKind::Shared, rgt, _) => self.tcx

.cannot_reborrow_already_borrowed(

span,

&desc_place,

"",

lft,

issued_span,

"it",

rgt,

"",

end_issued_loan_span,

Origin::Mir,

),

This is quite similar to the work done for #47607, in that PR, I added a set that contains the place/span of any errors reported so that they aren't reported again. In this case, the span on line 37 on the below error (that we want) would be in this set:

error[E0502]: cannot borrow `u.z.c` as immutable because it is also borrowed as mutable --> src/main.rs:37:20 | 34 | let mref = &mut u.s.a; //~ ERROR cannot assign twice to immutable variable `mref` [E0384] | ---------- mutable borrow occurs here ... 37 | let nref = &u.z.c; //~ ERROR cannot assign twice to immutable variable `err` [E0384] | ^^^^^^ immutable borrow occurs here

I'm not entirely sure what the unintended side effects of the following might be, but we could check if the issued_span is in the set (in the above example, that is the error on line 34, but in the second unintended error, that would refer to the same location as above on line 37) and if it is, skip this error. It would essentially silence errors that overlap where the second borrow location of the first error is the first borrow location in subsequent errors. Thoughts?

@davidtwco do you think you could gist the output from -Zdump-mir=nll for this function?

@nikomatsakis is this what you're looking for?

https://gist.github.com/nikomatsakis/b0ac3440933b3ae1d4dc3db02d738111

yep but I was hoping for a gist :) kind of hard to digest inline...

updated your comment =)

Here is a gist with some logging added in access_place and check_access_for_conflict that should show the variable values. Lines 13730 to 13759 for the first error and lines 13912 to 13959 for the second error.

kennytm · 2018-01-31T13:32:04Z

Ping from triage @nikomatsakis!

pietroalbini · 2018-02-05T10:43:23Z

@nikomatsakis ping from triage!

nikomatsakis · 2018-02-06T18:45:12Z

So I did a bit of digging here, IIRC, and found that the strange errors arose from two-phase borrows. I did not however get far enough to decide if this was a distinct bug or what.

pietroalbini · 2018-02-12T11:18:45Z

@nikomatsakis and @rust-lang/compiler, can we get a review on this?

nikomatsakis · 2018-02-13T22:03:41Z

I was still hoping that @pnkfelix would weigh in. But I still hope to carve out some time to dig in to the two-phase borrow logic and try to understand just why it's triggering in the way that it is and how we can improve it.

pnkfelix · 2018-02-16T11:56:41Z

@nikomatsakis oh, sorry, I didn't know you were still waiting on further input from me

nikomatsakis · 2018-02-20T17:59:09Z

@pnkfelix so I think the problem was that whenever we saw a Gen bit for an activation, we treated that as an activation that needs checking. I think that should only be required for loans that are not already activated. I made that change in the most recent commit. Can you review it?

nikomatsakis · 2018-02-21T15:03:01Z

hmm, in gitter just now, @pnkfelix and I were saying we had some concern about the possibility of an activation being overlooked due to an activation from a previous loop

nikomatsakis · 2018-02-22T01:30:17Z

So it seems like what we need is some way to know when one activation is dominated by another (in which case it can be ignored). Because activations are "unioned" on control-flow join, we don't get that for free. Annoying. We could make a "must be activated" dataflow, but it feels like overkill to me.

nikomatsakis · 2018-02-22T01:30:58Z

That said, this seems orthogonal from the original PR. Why don't we do this -- I'll peel off my last two commits and we'll open up a separate issue to discuss the extra errors resulting from two-phase borrows. Then we can land this PR, which fixes a legit problem.

nikomatsakis · 2018-02-22T01:51:19Z

@bors r+ rollup

bors · 2018-02-22T01:51:20Z

📌 Commit e6376a1 has been approved by nikomatsakis

nikomatsakis · 2018-02-22T01:51:27Z

I opened #48418

@nikomatsakis

Fix borrow checker unsoundness with unions Fixes rust-lang#45157. After discussion with @nikomatsakis on Gitter, this PR only adds a test since the original issue was resolved elsewhere. r? @nikomatsakis

Rollup of 15 pull requests - Successful merges: #47689, #48110, #48197, #48296, #48386, #48392, #48404, #48415, #48441, #48448, #48452, #48481, #48490, #48499, #48503 - Failed merges:

rust-highfive assigned nikomatsakis Jan 23, 2018

davidtwco changed the title ~~Fix borrow checker unsoundness with unions~~ WIP: Fix borrow checker unsoundness with unions Jan 23, 2018

matthewjasper reviewed Jan 24, 2018

View reviewed changes

nikomatsakis reviewed Jan 24, 2018

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kennytm added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jan 24, 2018

davidtwco force-pushed the issue-45157 branch from 3ff0190 to 18fd3f6 Compare January 24, 2018 13:15

Added test for rust-lang#45157

e6376a1

davidtwco force-pushed the issue-45157 branch from 18fd3f6 to e6376a1 Compare January 24, 2018 18:36

davidtwco changed the title ~~WIP: Fix borrow checker unsoundness with unions~~ Fix borrow checker unsoundness with unions Jan 24, 2018

nikomatsakis reviewed Jan 24, 2018

View reviewed changes

kennytm added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jan 25, 2018

nikomatsakis mentioned this pull request Feb 21, 2018

two-phase borrows are causing #[feature(nll)] to reject a mutable borrow incorrectly #48070

Closed

nikomatsakis mentioned this pull request Feb 22, 2018

two-phase borrows creates extra error messages #48418

Closed

nikomatsakis force-pushed the issue-45157 branch from c27e29e to e6376a1 Compare February 22, 2018 01:51

bors removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Feb 22, 2018

bors added the S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. label Feb 22, 2018

Manishearth mentioned this pull request Feb 24, 2018

Rollup of 15 pull requests #48520

Merged

bors merged commit e6376a1 into rust-lang:master Feb 25, 2018

davidtwco deleted the issue-45157 branch February 25, 2018 12:16

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Fix borrow checker unsoundness with unions #47689

Fix borrow checker unsoundness with unions #47689

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nikomatsakis commented Feb 22, 2018

	fn cannot_reborrow_already_borrowed(&self,
	span: Span,
	desc_new: &str,
	msg_new: &str,
	kind_new: &str,
	old_span: Span,
	noun_old: &str,
	kind_old: &str,
	msg_old: &str,
	old_load_end_span: Option<Span>,
	o: Origin)
	-> DiagnosticBuilder
	{
	let mut err = struct_span_err!(self, span, E0502,
	"cannot borrow `{}`{} as {} because {} is also borrowed as {}{}{OGN}",
	desc_new, msg_new, kind_new, noun_old, kind_old, msg_old, OGN=o);
	err.span_label(span, format!("{} borrow occurs here{}", kind_new, msg_new));
	err.span_label(old_span, format!("{} borrow occurs here{}", kind_old, msg_old));
	if let Some(old_load_end_span) = old_load_end_span {
	err.span_label(old_load_end_span, format!("{} borrow ends here", kind_old));
	}
	self.cancel_if_wrong_origin(err, o)
	}

	(BorrowKind::Shared, lft, _, BorrowKind::Mut, _, rgt) \|
	(BorrowKind::Mut, _, lft, BorrowKind::Shared, rgt, _) => self.tcx
	.cannot_reborrow_already_borrowed(
	span,
	&desc_place,
	"",
	lft,
	issued_span,
	"it",
	rgt,
	"",
	end_issued_loan_span,
	Origin::Mir,
	),

Fix borrow checker unsoundness with unions #47689

Fix borrow checker unsoundness with unions #47689

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