NLL ICE: "unresolved inference variable in outlives"

The crater run turned up a number of crates encountering an ICE with a message like "unresolved inference variable in outlives" (see list in #52054). I'm not sure what is the problem yet but I'm filing this bug to track the issue.

A good representative is combine-language-2.0.0-beta4, which failed with this log.

Self contained example. Related to #51649?

// Delayed ICE:
// error: internal compiler error: unresolved inference variable in outlives: _#0t
#![feature(nll)]

pub trait Parser {
    type Input;

    #[inline(always)]
    fn parse_first(input: &mut Self::Input);
}

impl<'a, I, P: ?Sized> Parser for &'a mut P
where
    P: Parser<Input = I>,
{
    type Input = I;

    fn parse_first(_: &mut Self::Input) {}
}

fn main() {}

triage re-assigning to self

@matthewjasper

I think this may have been fixed by #51959. At least @matthewjasper's example no longer reproduces. I'll have to try the original crate.

@matthewjasper

OK wait. @matthewjasper's example now fails to build, whereas it builds without nll. It no longer ICEs though.

Ah, I see the problem. It arises because we don't normalize types before computing the implied bounds in the universal regions code:

rust/src/librustc_mir/borrow_check/nll/universal_regions.rs

Lines 502 to 506 in fefe816

    
           // Add the implied bounds from inputs and outputs. 
        
           for ty in inputs_and_output { 
        
               debug!("build: input_or_output={:?}", ty); 
        
               self.add_implied_bounds(&indices, ty); 
        
           }

even though we just "liberated" late-bound regions:

rust/src/librustc_mir/borrow_check/nll/universal_regions.rs

Lines 487 to 492 in fefe816

    
           let inputs_and_output = self.infcx.replace_bound_regions_with_nll_infer_vars( 
        
               FR, 
        
               self.mir_def_id, 
        
               &bound_inputs_and_output, 
        
               &mut indices, 
        
           );

It's a bit tricky -- we don't just want to insert a random call to normalize until we've finished generating all the "free regions". That call could produce outlives obligations we have to propagate to the right place. It's not that big a deal, in fact at this point we have generated all the free regions:

rust/src/librustc_mir/borrow_check/nll/universal_regions.rs

Line 494 in fefe816

let num_universals = self.infcx.num_region_vars();

but we still need to route the resulting outlives obligations to the right place. This will take a touch of reorganization in the code.

I'm working on this. It's non-trivial.

But I'm getting there =)

add regression test for rust-lang#52057

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NLL ICE: "unresolved inference variable in outlives" #52057

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	// Add the implied bounds from inputs and outputs.
	for ty in inputs_and_output {
	debug!("build: input_or_output={:?}", ty);
	self.add_implied_bounds(&indices, ty);
	}

	let inputs_and_output = self.infcx.replace_bound_regions_with_nll_infer_vars(
	FR,
	self.mir_def_id,
	&bound_inputs_and_output,
	&mut indices,
	);

NLL ICE: "unresolved inference variable in outlives" #52057

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matthewjasper commented on Jul 11, 2018

pnkfelix commented on Jul 24, 2018

nikomatsakis commented on Jul 25, 2018

nikomatsakis commented on Jul 25, 2018

nikomatsakis commented on Jul 25, 2018

nikomatsakis commented on Jul 25, 2018

nikomatsakis commented on Jul 26, 2018

nikomatsakis commented on Jul 26, 2018

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