Change in lifetime inference?

[ltratt]

On lrtable (at least as of commit softdevteam/lrtable@ce03d95), @snim2 found that the current Rust nightly gives this error:

$ cargo check
   Compiling lrtable v0.1.0 (file:///home/snim2/Desktop/working/softdev/lrtable)
error[E0495]: cannot infer an appropriate lifetime for lifetime parameter in function call due to conflicting requirements
  --> src/lib/stategraph.rs:61:21
   |
61 |         self.states.iter().map(|x| &x.1)
   |                     ^^^^
   |
note: first, the lifetime cannot outlive the lifetime 'a as defined on the method body at 60:5...
  --> src/lib/stategraph.rs:60:5
   |
60 | /     pub fn iter_closed_states<'a>(&'a self) -> impl Iterator<Item=&Itemset> + 'a {
61 | |         self.states.iter().map(|x| &x.1)
62 | |     }
   | |_____^
note: ...so that reference does not outlive borrowed content
  --> src/lib/stategraph.rs:61:9
   |
61 |         self.states.iter().map(|x| &x.1)
   |         ^^^^^^^^^^^
   = note: but, the lifetime must be valid for the static lifetime...
note: ...so that types are compatible (expected &itemset::Itemset, found &'static itemset::Itemset)
  --> src/lib/stategraph.rs:60:48
   |
60 |     pub fn iter_closed_states<'a>(&'a self) -> impl Iterator<Item=&Itemset> + 'a {
   |                                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: internal compiler error: /checkout/src/librustc_typeck/check/writeback.rs:485: writeback: `[Deref(Some(OverloadedDeref { region: ReStatic, mutbl: MutImmutable })) -> [(itemset::Itemset, itemset::Itemset)], Borrow(Ref(ReStatic, MutImmutable)) -> &'static [(itemset::Itemset, itemset::Itemset)]]` missing from the global type context
  --> src/lib/stategraph.rs:61:9
   |
61 |         self.states.iter().map(|x| &x.1)
   |         ^^^^^^^^^^^

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.23.0-nightly (63739ab7b 2017-11-21) running on x86_64-unknown-linux-gnu

thread 'rustc' panicked at 'Box<Any>', /checkout/src/librustc_errors/lib.rs:448:8
note: Run with `RUST_BACKTRACE=1` for a backtrace.

error: Could not compile `lrtable`.

To learn more, run the command again with --verbose.

We've hacked around this temporarily by returning a Box instead of the iterator directly, but this code used to compile, and I'm not sure if it's intentional that it doesn't compile any more or not?

[ltratt]

Ah, I wonder if I fingered the wrong suspect. The impl trait stuff that's recently been merged (#45918) seems a more plausible culprit?

[phil-opp]

This affects embed-rs/net as well:

> cargo +nightly-2017-11-21 build
   Compiling net v0.1.0 (file:///home/philipp/Documents/net)
    Finished dev [unoptimized + debuginfo] target(s) in 0.90 secs

> cargo +nightly-2017-11-22 build
   Compiling net v0.1.0 (file:///home/philipp/Documents/net)
error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements
   --> src/tcp.rs:215:27
    |
215 |         self.packet_queue.values()
    |                           ^^^^^^
    |
note: first, the lifetime cannot outlive the lifetime 'a as defined on the method body at 214:5...
   --> src/tcp.rs:214:5
    |
214 | /     pub fn packets<'a>(&'a mut self) -> impl Iterator<Item = &TcpPacket<Box<[u8]>>> {
215 | |         self.packet_queue.values()
216 | |     }
    | |_____^
note: ...so that reference does not outlive borrowed content
   --> src/tcp.rs:215:9
    |
215 |         self.packet_queue.values()
    |         ^^^^^^^^^^^^^^^^^
    = note: but, the lifetime must be valid for the static lifetime...
note: ...so that return value is valid for the call
   --> src/tcp.rs:214:41
    |
214 |     pub fn packets<'a>(&'a mut self) -> impl Iterator<Item = &TcpPacket<Box<[u8]>>> {
    |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[ltratt]

#46205 may also be relevant?

[nikomatsakis]

Can one of you isolate these examples at all? We did make changes to how impl Trait interacts with lifetimes that could be affecting these errors.

[ltratt]

With a recent nightly, I no longer get an error on that version of lrtable.

[If you want to replicate it, clone https://github.com/softdevteam/lrtable/commit/ce03d95d4a5b722b4335c19fc2cc459777ce4195 then edit this line in Cargo.toml:

cfgrammar = { git = "https://github.com/softdevteam/cfgrammar" }      

to:

cfgrammar = { git = "https://github.com/softdevteam/cfgrammar", rev = "907703afc5243f6ebf8b3aec9df5d849cfacd09d" }      

]

[estebank]

Closing as per @ltratt's prior comment.