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Lifetimes bug on beta 1.31: the associated type may not live long enough #55756
Description
A reduced test case (play):
#![crate_type="lib"]
pub trait Database<'a> {
type Guard: 'a;
}
pub struct Stateful<'a, D: 'a>(&'a D);
impl<'b, D: for <'a> Database<'a>> Stateful<'b, D> {
pub fn callee<'a>(&'a self) -> <D as Database<'a>>::Guard {
unimplemented!()
}
pub fn caller<'a>(&'a self) -> <D as Database<'a>>::Guard {
let state = self.callee();
unimplemented!()
}
}Error diagnostic output in details block below
Details
error[E0311]: the associated type `<D as Database<'_>>::Guard` may not live long enough
--> src/lib.rs:14:13
|
14 | let state = self.callee();
| ^^^^^
|
= help: consider adding an explicit lifetime bound for `<D as Database<'_>>::Guard`
note: the associated type `<D as Database<'_>>::Guard` must be valid for the block suffix following statement 0 at 14:9...
--> src/lib.rs:14:9
|
14 | / let state = self.callee();
15 | | unimplemented!()
16 | | }
| |_____^
note: ...so that variable is valid at time of its declaration
--> src/lib.rs:14:13
|
14 | let state = self.callee();
| ^^^^^
error[E0311]: the associated type `<D as Database<'_>>::Guard` may not live long enough
--> src/lib.rs:14:13
|
14 | let state = self.callee();
| ^^^^^
|
= help: consider adding an explicit lifetime bound for `<D as Database<'_>>::Guard`
note: the associated type `<D as Database<'_>>::Guard` must be valid for the block at 13:63...
--> src/lib.rs:13:63
|
13 | pub fn caller<'a>(&'a self) -> <D as Database<'a>>::Guard {
| _______________________________________________________________^
14 | | let state = self.callee();
15 | | unimplemented!()
16 | | }
| |_____^
note: ...so that references are valid when the destructor runs
--> src/lib.rs:14:13
|
14 | let state = self.callee();
| ^^^^^
error[E0311]: the associated type `<D as Database<'_>>::Guard` may not live long enough
--> src/lib.rs:14:21
|
14 | let state = self.callee();
| ^^^^^^^^^^^^^
|
= help: consider adding an explicit lifetime bound for `<D as Database<'_>>::Guard`
note: the associated type `<D as Database<'_>>::Guard` must be valid for the method call at 14:21...
--> src/lib.rs:14:21
|
14 | let state = self.callee();
| ^^^^^^^^^^^^^
note: ...so type `<D as Database<'_>>::Guard` of expression is valid during the expression
--> src/lib.rs:14:21
|
14 | let state = self.callee();
| ^^^^^^^^^^^^^
error[E0311]: the associated type `<D as Database<'_>>::Guard` may not live long enough
--> src/lib.rs:14:21
|
14 | let state = self.callee();
| ^^^^^^^^^^^^^
|
= help: consider adding an explicit lifetime bound for `<D as Database<'_>>::Guard`
note: the associated type `<D as Database<'_>>::Guard` must be valid for the destruction scope surrounding statement at 14:9...
--> src/lib.rs:14:9
|
14 | let state = self.callee();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...so that references are valid when the destructor runs
--> src/lib.rs:14:21
|
14 | let state = self.callee();
| ^^^^^^^^^^^^^
error: aborting due to 4 previous errors
Original bug report
Versions
rustc 1.31.0-beta.4 (04da282 2018-11-01)
cargo 1.31.0-beta (efb7972a0 2018-10-24)
Expected behavior
Everything compiling correctly like it does on stable 1.30
Actual behavior
A lot of lifetime-related errors:
error[E0311]: the associated type `<D as trie::Database<'_>>::Guard` may not live long enough
--> /home/mike/dev/etcdev/sputnikvm/stateful/src/lib.rs:536:23
|
536 | let account = state.get(&address);
| ^^^^^^^^^^^^^^^^^^^
|
= help: consider adding an explicit lifetime bound for `<D as trie::Database<'_>>::Guard`
note: the associated type `<D as trie::Database<'_>>::Guard` must be valid for the method call at 536:23...
--> /home/mike/dev/etcdev/sputnikvm/stateful/src/lib.rs:536:23
|
536 | let account = state.get(&address);
| ^^^^^^^^^^^^^^^^^^^
note: ...so that a type/lifetime parameter is in scope here
--> /home/mike/dev/etcdev/sputnikvm/stateful/src/lib.rs:536:23
|
536 | let account = state.get(&address);
| ^^^^^^^^^^^^^^^^^^^
It has been an issue in nightly 1.31 for a long time now, but I haven't reported the issue while it was just nightly. Now the regression is in beta channel and may land in stable, breaking existing code.
My own findings on the subject: regression is related to an old lifetime system, it's not reproducible with nll feature on, so on nightly the code can be compiled with nll, but not without it. Same holds for beta now except nll isn't quite there yet.
The subjected codebase: https://github.com/ETCDEVTeam/sputnikvm/tree/master/stateful
Activity
[-]Liferimes bug on beta 1.31: the associated type may not live long enough [/-][+]Lifetimes bug on beta 1.31: the associated type may not live long enough [/+]pnkfelix commentedon Nov 8, 2018
visiting for T-compiler triage. P-high. Assigning to self for initial investigation.
Mark-Simulacrum commentedon Nov 9, 2018
Based on crater run, looks like this also affects:
pnkfelix commentedon Nov 12, 2018
Bisected against when https://source.that.world/source/evm-rs.git stopped building.
Successfully built: rustc 1.30.0-nightly (4141a40 2018-09-25)
Error during compilation: rustc 1.30.0-nightly (ae7fe84 2018-09-26)
pnkfelix commentedon Nov 12, 2018
Heres the bors merge record from that time frame:
% git log 4141a40..ae7fe84 --format=oneline --author=bors
ae7fe84 Auto merge of #54561 - matthiaskrgr:clippy_up, r=kennytm
c7df1f5 Auto merge of #54453 - nikomatsakis:nll-issue-53121-shred-outlives, r=pnkfelix
6846f22 Auto merge of #51946 - japaric:emit-stack-sizes, r=nikomatsakis
e783d2b Auto merge of #54199 - nikomatsakis:predicate_may_hold-failure, r=eddyb
c3a1a0d Auto merge of #53824 - ljedrz:begone_onevector, r=michaelwoerister
a2b27c1 Auto merge of #54497 - ralexstokes:stabilize_pattern_parentheses, r=nikomatsakis
beff387 Auto merge of #54548 - nrc:update, r=kennytm
0b2eae7 Auto merge of #54164 - mikhail-m1:54131, r=pnkfelix
92aff72 Auto merge of #54575 - pietroalbini:rollup, r=pietroalbini
%
pnkfelix commentedon Nov 12, 2018
The most obvious culprit to my eye is PR #54453. That PR might have been intended to be restricted to NLL, I'm not sure (it touches code outside of rustc_mir, certainly). It does seem like the crate in question is not opting into NLL nor the 2018 editon.
Update: of course they aren't opting into NLL. This issue is tagged NLL-fixed-by-NLL
pnkfelix commentedon Nov 12, 2018
Here is a reduced test case (play):
pnkfelix commentedon Nov 12, 2018
I am not sure if I'm going to have time to look into this further before I go on leave (for ~2 weeks), and yet it seems like a potentially important thing to address. Unassigning self.
pnkfelix commentedon Nov 12, 2018
I have now confirmed that PR #54453 is what injected this regression.
cc @nikomatsakis
1 remaining item
nikomatsakis commentedon Nov 13, 2018
Also, it's interesting that NLL makes this test case pass. Off hand, I'm not sure entirely sure why that is.
nikomatsakis commentedon Nov 13, 2018
OK, so I investigated NLL a bit. The reason is actually somewhat interesting.
NLL introduces an additional liveness check -- it says:
This rule is much "cruder" than the outlives check here. In particular, if you have a variable
x: <D as Database<'a>>::Guard, then for it to pass this liveness check,'amust include all points wherexoccurs.We could adopt a similar distinction in lexical region checking with relative ease -- basically a "live at" relation that is more conservative than outlives. I'm not sure how well justified it is though.
(This is a bit weird because of eager normalization: if we were able to normalize
<D as Database<'a>>::Guardto another type (e.g.,u32), then we would not impose this requirement on'a, because we would be representing its type as merelyu32. This will change with lazy normalization. It is conceivable that this could cause programs to stop type-checking. I guess we'll worry about it then.)nagisa commentedon Nov 15, 2018
@nikomatsakis says they know what the problem is and hope to have a fix soon (today).
nikomatsakis commentedon Nov 15, 2018
Hmm, so, I have a fix. Though looking more closely at the test, I think I see a nicer fix.
The problem is that we now observe two potentially applicable rules when trying to find bounds for
<T as Database<'0>>::Guard:Because of this, we decide to take no action to influence inference, which means that
'0winds up unconstrained, leading to the ultimate error.My current fix basically overrides this with a coarser rule (one which works fine, however, in this example).
A smarter fix would be to observe that the first rule (the one talking about
'a) is in fact "specialization" of the latter, in which case we could conclude that we should relate'0to the bound we need.I am trying to decide which approach is lowest risk -- the current fix, as I wrote, actually makes us compatible with how NLL is handling things. I am planning to overhaul this whole setup in the not-that-distinct future, since the current solver is essentially "unnecessarily stupid".
Some options:
Given the deadline, I'm leaning towards the first path.
Rollup merge of rust-lang#56043 - nikomatsakis:issue-55756-via-outliv…
nikomatsakis commentedon Nov 19, 2018
Testing the more conservative fix from #56043:
nikomatsakis commentedon Nov 19, 2018
@mersinvald
btw, in the future, we would definitely appreciate bug reports against nightly! That is indeed part of the point of nightly, to help us find regressions before they make it into beta =)
nikomatsakis commentedon Nov 19, 2018
I don't know how to test the final two cases that @Mark-Simulacrum cited -- I guess I need to checkout some github repositories? -- but based on the logs they seem like the same root problem and hence I expect it is fixed.
Mark-Simulacrum commentedon Nov 19, 2018
Yes, you'd need to build https://github.com/ETCDEVTeam/sputnikvm-dev and https://github.com/TheWaWaR/cita-create-genesis to check them locally.
Rollup merge of rust-lang#56043 - nikomatsakis:issue-55756-via-outliv…
TheWaWaR commentedon Nov 20, 2018
@Mark-Simulacrum
https://github.com/TheWaWaR/cita-create-genesis
Confirm success build both on:
rustc 1.30.1 (1433507eb 2018-11-07)rustc 1.32.0-nightly (5aff30734 2018-11-19)mersinvald commentedon Nov 20, 2018
Sure, I understand I should've report it once encountered. Would be less stressful for both parties. Thanks for fixing the issue!