[1.31 beta] Trait bound is not satisfied

[emilyalbini]

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

[Marwes]

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`

[arielb1]

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() {}

[arielb1]

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.

[arielb1]

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

[arielb1]

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

[pnkfelix]

visited at T-compiler. P-high.