MIR-borrowck: emit "`foo` does not live long enough" instead of borrow errors #45989

davidtwco · 2017-11-14T19:47:41Z

Fixes #45360. As of writing, contains deduplication of existing errors.

nikomatsakis · 2017-11-14T22:38:55Z

I didn't get a chance to look closely, but I wanted to point you at this gist:

https://gist.github.com/nikomatsakis/a0d5dee0bd3c4d4f0442914a9abdf74c

It describes how to update the existing AST borrow check tests so that we can simultaneously test: (a) the current behavior on stable and (b) the behavior with MIR-borrowck enabled and compare the two. It'd be helpful if you update a test or two to show how the current PR behaves.

davidtwco · 2017-11-14T22:41:03Z

@nikomatsakis That's great. I tested it locally running the same example from the issue - I'll update some tests tomorrow based on that gist.

davidtwco · 2017-11-15T09:33:01Z

@nikomatsakis I updated the test given as an example in the issue to reflect the expected result once this PR is fully implemented. For now it demonstrates the deduplication. This is the updated test.

I'd be happy to update any other tests that this PR would fix.

nikomatsakis · 2017-11-15T17:20:01Z

src/test/compile-fail/issue-36082.rs

-    //~^ ERROR borrowed value does not live long enough
+    let val: &_ = x.borrow().0; //[ast]~ ERROR borrowed value does not live long enough [E0597]
+    //[mir]~^ ERROR borrowed value does not live long enough (Ast) [E0597]
+    //[mir]~^ ERROR borrowed value does not live long enough (Mir) [E0597]


This needs to be //[mir]~|, I believe.

Resolved this, my bad.

nikomatsakis · 2017-11-15T17:20:14Z

src/test/compile-fail/issue-36082.rs

    //~| temporary value dropped here while still borrowed
    //~| temporary value created here
    //~| consider using a `let` binding to increase its lifetime
    println!("{}", val);
 }
-//~^ temporary value needs to live until here
+//[ast]~^ temporary value needs to live until here
+//[mir]~^ temporary value needs to live until here


Resolved this, my bad.

nikomatsakis · 2017-11-15T17:25:21Z

src/librustc_mir/borrow_check.rs

+            _ => false,
+        };
+
+        if !has_storage_drop_or_dead_error_reported {


So @arielb1 pointed out that there is a risk here. The calls to access_lvalue are not guaranteed to report an error -- and, in fact, DROP will report errors in a broader set of conditions than StorageDead (I think strictly broader). So it would be better to populate the map (and hence skip the latter check) only if an error was actually reported.

What @arielb1 suggested was issuing precisely the same error message in both cases, and letting rustc deduplicate (which it will do). That's an option, though I don't think it's the best path, since I think that the error messages should perhaps be different between the two cases.

Alternatively, we could make access_lvalue return a boolean or something, indicating whether an error was reported, and then do:

let has_storage_drop_or_dead_error_reported = self.storage_drop_or_dead_error_reported.contains_key(...); let error_reported; if has_storage_drop_or_dead_error_reported { error_reported = false; } else { if moves_by_default { error_reported = self.access_lvalue(..); } else { ... } } if error_reported { self.storage_drop_or_dead_error_reported.insert(local); }

It would also be good to comment about this. =)

It's hard to come up with a test case that would necesarily show off this problem though -- specifically, Drop will always come before StorageDead, and hence I imagine we'll visit it first, so the bug won't show itself. But I wouldn't want to rely on that.

I've modified what was there based on this, should be fine now.

nikomatsakis

Looks great! Left a few nits.

nikomatsakis · 2017-11-16T15:50:33Z

src/librustc_mir/borrow_check.rs


        let (sd, rw) = kind;

        // Check permissions
        self.check_access_permissions(lvalue_span, rw);

+        let mut has_storage_drop_or_dead_error_reported = false;


Nit: we should probably just call this error_reported

Resolved this, thanks.

nikomatsakis · 2017-11-16T15:50:41Z

src/librustc_mir/borrow_check.rs

@@ -444,13 +455,14 @@ impl<'c, 'b, 'a: 'b+'c, 'gcx, 'tcx: 'a> MirBorrowckCtxt<'c, 'b, 'a, 'gcx, 'tcx>
                     context: Context,
                     lvalue_span: (&Lvalue<'tcx>, Span),
                     kind: (ShallowOrDeep, ReadOrWrite),
-                     flow_state: &InProgress<'b, 'gcx, 'tcx>) {
+                     flow_state: &InProgress<'b, 'gcx, 'tcx>) -> bool {


Can you add a comment to the function indicating what the return value means?

I see there is no existing comment, either! How about something like this:

Checks an access to the given lvalue to see if it is allowed. Examines the set of borrows that are in scope, as well as which paths have been initialized, to ensure that (a) the lvalue is initialized and (b) it is not borrowed in some way that would prevent this access.

Returns true if an error is reported, false otherwise.

Resolved this, thanks.

nikomatsakis · 2017-11-16T16:29:20Z

So, in terms of how to change the error message. The current error message arises from this call here:

https://github.com/davidtwco/rust/blob/d906a6e94dfaac3a1344db96237d59bb9285bf5b/src/librustc_mir/borrow_check.rs#L637-L638

Note that we pass Write(WriteKind::Move). That seems bogus. Per @arielb1's suggestion, we should rename WriteKind::StorageDead to WriteKind::StorageDeadOrDrop and pass that here, I think. (Note that we sometimes want Move, we'll have to look at the consume_via_drop argument to decide.)

That will then flow down to this case:

https://github.com/davidtwco/rust/blob/d906a6e94dfaac3a1344db96237d59bb9285bf5b/src/librustc_mir/borrow_check.rs#L492-L514

You can see that WriteKind::StorageDead (which we will have renamed) is treated the same as Mutate. We want to give it its own case. Then we want to invoke this helper function:

https://github.com/davidtwco/rust/blob/d906a6e94dfaac3a1344db96237d59bb9285bf5b/src/librustc_mir/util/borrowck_errors.rs#L375-L384

The other cases invoke similar helpers, but they use shallow wrappers like report_move_out_while_borrowed so we probably want to write one of those too.

davidtwco · 2017-11-16T22:23:03Z

Rebased and producing the correct error message now.

bors · 2017-11-16T23:30:31Z

☔ The latest upstream changes (presumably #45825) made this pull request unmergeable. Please resolve the merge conflicts.

davidtwco · 2017-11-17T00:14:08Z

Rebased on top of the conflicting changes and to squash my own commits. Error message now looks more or less what we're seeing with the Ast version, only difference is the symbols used and I can't see a way around that. Other than that, I think this is done.

@bors r=nikomatsakis

Edit: I have no idea how bors works apparently.

bors · 2017-11-17T00:14:09Z

@davidtwco: 🔑 Insufficient privileges: Not in reviewers

bors · 2017-11-17T00:14:37Z

@davidtwco: 🔑 Insufficient privileges: Not in reviewers

nikomatsakis · 2017-11-17T16:23:40Z

@bors r+

bors · 2017-11-17T16:23:41Z

📋 Looks like this PR is still in progress, ignoring approval

…s from the same line in other tests.

davidtwco · 2017-11-17T20:00:08Z

Pushed a commit that fixes the error on the Travis build in an unrelated test. Should be good now, apologies.

nikomatsakis · 2017-11-17T20:54:21Z

@bors r+

bors · 2017-11-17T20:54:22Z

📌 Commit a1d55be has been approved by nikomatsakis

bors · 2017-11-18T03:11:24Z

⌛ Testing commit a1d55be with merge 79a1385...

@nikomatsakis

MIR-borrowck: emit "`foo` does not live long enough" instead of borrow errors Fixes #45360. As of writing, contains deduplication of existing errors. r? @nikomatsakis

bors · 2017-11-18T05:36:31Z

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing 79a1385 to master...

davidtwco mentioned this pull request Nov 14, 2017

MIR-borrowck: emit "foo does not live long enough" instead of borrow errors #45360

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kennytm added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Nov 14, 2017

kennytm assigned nikomatsakis Nov 14, 2017

nikomatsakis requested changes Nov 15, 2017

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nikomatsakis requested changes Nov 16, 2017

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davidtwco added 3 commits November 16, 2017 23:37

Deduplicated borrow checking errors.

7db49b9

Updated test to reflect expected Mir output.

a98aff7

Modified to output intended error.

ce8967a

Corrected error output with many labels.

bf76ffe

nikomatsakis approved these changes Nov 17, 2017

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davidtwco changed the title ~~WIP: MIR-borrowck: emit "foo does not live long enough" instead of borrow errors~~ MIR-borrowck: emit "foo does not live long enough" instead of borrow errors Nov 17, 2017

Fixed bug with the implementation that was suppressing expected error…

a1d55be

…s from the same line in other tests.

bors merged commit a1d55be into rust-lang:master Nov 18, 2017

davidtwco deleted the issue-45360 branch December 1, 2017 16:22

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MIR-borrowck: emit "`foo` does not live long enough" instead of borrow errors #45989

MIR-borrowck: emit "`foo` does not live long enough" instead of borrow errors #45989

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MIR-borrowck: emit "foo does not live long enough" instead of borrow errors #45989

MIR-borrowck: emit "foo does not live long enough" instead of borrow errors #45989

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MIR-borrowck: emit "`foo` does not live long enough" instead of borrow errors #45989

MIR-borrowck: emit "`foo` does not live long enough" instead of borrow errors #45989

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