[1.31 beta] Trait bound is not satisfied

@Marwes

The following crates fail to build on 1.30 due to trait bound errors.

combine-language 3.0.1 regressed from stable to beta (build log) cc @Marwes
postgis 0.6.0 regressed from stable to beta (build log) cc @andelf
- vectortile 0.2.1 (depends on postgis) regressed from stable to beta (build log) cc @kestred

For combine it seems like rustc no longer sees that I::Item == char despite that being specified in the where bound :/

error[E0277]: the trait bound `<I as combine::StreamOnce>::Error: combine::ParseError<char, <I as combine::StreamOnce>::Range, <I as combine::StreamOnce>::Position>` is not satisfied
Sep 21 16:12:45.476 INFO kablam!    --> src/lib.rs:163:1
Sep 21 16:12:45.476 INFO kablam!     |
Sep 21 16:12:45.476 INFO kablam! 163 | / pub struct Reserved<'a: 'b, 'b, I>
Sep 21 16:12:45.476 INFO kablam! 164 | | where
Sep 21 16:12:45.476 INFO kablam! 165 | |     I: Stream<Item = char> + 'b,
Sep 21 16:12:45.476 INFO kablam! 166 | |     I::Error: ParseError<I::Item, I::Range, I::Position>,
Sep 21 16:12:45.477 INFO kablam! 167 | | {
Sep 21 16:12:45.477 INFO kablam! 168 | |     parser: Lex<'a, 'b, Try<Skip<Str<I>, NotFollowedBy<LanguageParser<'a, 'b, I, char>>>>>,
Sep 21 16:12:45.477 INFO kablam! 169 | | }
Sep 21 16:12:45.477 INFO kablam!     | |_^ the trait `combine::ParseError<char, <I as combine::StreamOnce>::Range, <I as combine::StreamOnce>::Position>` is not implemented for `<I as combine::StreamOnce>::Error`

This looks like a "soft" deadlock on normalize_param_env_or_error caused by implied bounds.

Minified:

pub trait Stream {
    type Item;
    type Error;
}

pub trait ParseError<I> {
    type Output;
}

pub struct Lex<'a, I>
    where I: Stream,
          I::Error: ParseError<char>,
          <<I as Stream>::Error as ParseError<char>>::Output: 'a
{
    x: &'a <I::Error as ParseError<char>>::Output
}

pub struct Reserved<'a, I> where
    I: Stream<Item=char> + 'a,
    I::Error: ParseError<I::Item>,
// uncomment this to make it fail on stable too
//    <<I as Stream>::Error as ParseError<char>>::Output: 'a
    
{
    x: Lex<'a, I>
}

fn main() {}

to detail: normalize_param_env_or_error does not try to normalize the param env in some sort of fixed-point computation - that would be hard without lazy normalization or some other major refactor of the trait system. Therefore, it normalizes the param env insided the unnormalized param env.

This means that bounds such as <<I as Stream>::Error as ParseError<char>>::Output: 'a are normalized in the unnormalized param env, which contains I::Error: ParseError<I::Item>, instead of I::Error: ParseError<char>,.

I think a possible hacky fix would be to normalize the implied outlives bounds (or maybe, all outlives bounds) in the normalized param env instead of the unnormalized param env.

Note that this is broken even on older nightlies as long as #![feature(infer_outlives_requirements)] is enabled.

I'll try to get a fix done later this week.

visited at T-compiler. P-high.

Interestingly, this is not fixed on current beta (or perhaps re-regressed); I'm reopening and retagging.

Specifically at least postgis is failing to build: https://crater-reports.s3.amazonaws.com/beta-1.31-1/beta-2018-10-30/reg/vectortile-0.2.2/log.txt

@arielb1

cc @arielb1 @pnkfelix @rust-lang/compiler

Maybe we need to reconsider backporting PR #54701 ... will investigate...

Test results

postgis 0.6.0

❌ nightly (2018-11-13)
❌ beta (1.31.0-beta.9)
✅ stable (1.30.1)

combine-language 3.0.1:

✅ nightly (2018-11-13)
✅ beta (1.31.0-beta.9)
✅ stable (1.30.1)

Seems like postgis-0.6.0 simply wasn't fixed.

@Mark-Simulacrum

Trying to investigate more deeply. (Also, I realize now that @Mark-Simulacrum this is likely the issue you were talking about when you were talking about re-regressing..?)

Found the problem. Fix coming shortly.

Auto merge of #56003 - nikomatsakis:issue-54467-infer-outlives-bounds…

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[1.31 beta] Trait bound is not satisfied #54467

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[1.31 beta] Trait bound is not satisfied #54467

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Marwes commented on Sep 23, 2018

arielb1 commented on Sep 24, 2018

arielb1 commented on Sep 24, 2018

arielb1 commented on Sep 24, 2018

arielb1 commented on Sep 24, 2018

pnkfelix commented on Sep 27, 2018

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Mark-Simulacrum commented on Nov 9, 2018

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pnkfelix commented on Nov 9, 2018

nikomatsakis commented on Nov 15, 2018

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nikomatsakis commented on Nov 15, 2018

nikomatsakis commented on Nov 15, 2018

nikomatsakis commented on Nov 16, 2018

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