ICE bootstrapping rustc with NLL enabled

When I try to bootstrap rustc with NLL enabled, I get an odd ICE compiling libsyntax_ext:

error: internal compiler error: broken MIR in NodeId(16117) (_368 = move _369): bad assignment (for<'cx, 'r, 's> fn(&'cx mut syntax::ext::base::ExtCtxt<'r>, syntax_pos::Span, &'s [syntax::tokenstream::To\
kenTree]) -> std::boxed::Box<syntax::ext::base::MacResult + 'cx> = for<'r, 's, 't0> fn(&'r mut syntax::ext::base::ExtCtxt<'s>, syntax_pos::Span, &'t0 [syntax::tokenstream::TokenTree]) -> std::boxed::Box<\
syntax::ext::base::MacResult>): RegionsOverlyPolymorphic(BrNamed(crate11:DefIndex(1:1012), 'cx), '_#729r)
   --> src/libsyntax_ext/lib.rs:65:44
    |
65  |                           expander: Box::new($f as MacroExpanderFn),
    |                                              ^^^^^^^^^^^^^^^^^^^^^
...
92  | /     register! {
93  | |         line: expand_line,
94  | |         __rust_unstable_column: expand_column_gated,
95  | |         column: expand_column,
...   |
112 | |         compile_error: compile_error::expand_compile_error,
113 | |     }
    | |_____- in this macro invocation

I'd like to work on this.

@Aaron1011

@Aaron1011 any progress? want me to take a look at all?

@nikomatsakis

@nikomatsakis: I haven't yet been able to reproduce this, due to #48129 stopping compilation. I'll try to work around that issue locally, and see if I can reproduce + investigate the original issue.

@nikomatsakis

@nikomatsakis: I ran across another issue while trying to reach the ICE: #48224

@nikomatsakis

@nikomatsakis: Until the intermediate issues are resolved, I don't think I'll be able to reproduce this.

@Aaron1011

@Aaron1011 this branch #48372 clears the way

Hmm, but on that branch I am now encountering the AddAssign problems I guess. So we're still blocked on #48129

Actually, the issue-48071 branch on my repo reproduces the problem. You have to build like so:

RUSTFLAGS_STAGE_NOT_0='-Znll -Zborrowck=mir -Ztwo-phase-borrows' ./x.py build --stage 2

Error that I get is (lightly edited):

error: internal compiler error: broken MIR in NodeId(16117) (_368 = move _369): bad assignment (

for<'cx, 'r, 's> fn(&'cx mut syntax::ext::base::ExtCtxt<'r>, syntax_pos::Span, &'s [syntax::tokenstream::TokenTree]) -> std::boxed::Box<syntax::ext::base::MacResult + 'cx>
for<'r, 's, 't0> fn(&'r mut syntax::ext::base::ExtCtxt<'s>, syntax_pos::Span, &'t0 [syntax::tokenstream::TokenTree]) -> std::boxed::Box<syntax::ext::base::MacResult>): 

RegionsOverlyPolymorphic(BrNamed(crate11:DefIndex(1:1012), 'cx), '_#729r)

Removing explicit paths:

for<'cx, 'r, 's> fn(&'cx mut ExtCtxt<'r>, Span, &'s [TokenTree]) -> Box<MacResult + 'cx>
for<'r, 's, 't0> fn(&'r mut ExtCtxt<'s>, Span, &'t0 [TokenTree]) -> Box<MacResult>)

The problem seems to be the return type.

Minimized:

#![allow(warnings)]
#![feature(dyn_trait)]
#![feature(nll)]

trait Foo {
    fn foo(&self) { }
}

impl Foo for () {
}

type MakeFooFn = for<'a> fn(&'a u8) -> Box<dyn Foo + 'a>;

fn make_foo(x: &u8) -> Box<dyn Foo + 'static> {
    Box::new(())
}

fn main() {
    let x: MakeFooFn = make_foo as MakeFooFn;
}

I think the problem is that there is a missing coercion of sorts, or else that this code in the NLL type checker is too strict:

rust/src/librustc_mir/borrow_check/nll/type_check/mod.rs

Lines 1308 to 1330 in 27a046e

    
           CastKind::ReifyFnPointer => { 
        
               let fn_sig = op.ty(mir, tcx).fn_sig(tcx); 
        
               // The type that we see in the fcx is like 
        
               // `foo::<'a, 'b>`, where `foo` is the path to a 
        
               // function definition. When we extract the 
        
               // signature, it comes from the `fn_sig` query, 
        
               // and hence may contain unnormalized results. 
        
               let fn_sig = self.normalize(&fn_sig, location); 
        
               let ty_fn_ptr_from = tcx.mk_fn_ptr(fn_sig); 
        
               if let Err(terr) = self.eq_types(ty_fn_ptr_from, ty, location.at_self()) { 
        
                   span_mirbug!( 
        
                       self, 
        
                       rvalue, 
        
                       "equating {:?} with {:?} yields {:?}", 
        
                       ty_fn_ptr_from, 
        
                       ty, 
        
                       terr 
        
                   ); 
        
               } 
        
           }

In particular, it constructs the "reified" form of the fn pointer in ty_fn_ptr_from and then requires that the target type be equal to that. But in this case, the target type is a subtype of that.

Hmm. I'm going to mark this as high priority because it blocks bootstrap. I think it's a bit tricky to fix. While bike riding recently though I had some ideas here -- I realized that we could probably write a custom bit of subtyping code used only (initially) by the MIR type checker. The hope would be that this is faster than canonicalizing etc. This code could use a universe-based check. This would sidestep the limitations of our existing subtyping algorithm which are biting us here. It would also help eliminate a blocker for chalk integration later. I may take a stab at this.

OK, my branch is now working and passing tests. I want to clean it up some and perhaps try to improve its performance though.

visited for triage. this should be fixed via PR #52488 (which is WIP but hopefully will be ready soon).

Auto merge of #52488 - nikomatsakis:nll-issue-48071-universe-and-sub,…

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ICE bootstrapping rustc with NLL enabled #48071

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	CastKind::ReifyFnPointer => {
	let fn_sig = op.ty(mir, tcx).fn_sig(tcx);

	// The type that we see in the fcx is like
	// `foo::<'a, 'b>`, where `foo` is the path to a
	// function definition. When we extract the
	// signature, it comes from the `fn_sig` query,
	// and hence may contain unnormalized results.
	let fn_sig = self.normalize(&fn_sig, location);

	let ty_fn_ptr_from = tcx.mk_fn_ptr(fn_sig);

	if let Err(terr) = self.eq_types(ty_fn_ptr_from, ty, location.at_self()) {
	span_mirbug!(
	self,
	rvalue,
	"equating {:?} with {:?} yields {:?}",
	ty_fn_ptr_from,
	ty,
	terr
	);
	}
	}

ICE bootstrapping rustc with NLL enabled #48071

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Aaron1011 commented on Feb 10, 2018

nikomatsakis commented on Feb 14, 2018

Aaron1011 commented on Feb 14, 2018

Aaron1011 commented on Feb 15, 2018

Aaron1011 commented on Feb 15, 2018

nikomatsakis commented on Feb 21, 2018

nikomatsakis commented on Feb 21, 2018

nikomatsakis commented on Feb 21, 2018

nikomatsakis commented on Feb 21, 2018

nikomatsakis commented on Feb 21, 2018

nikomatsakis commented on Feb 21, 2018

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nikomatsakis commented on Jun 29, 2018

nikomatsakis commented on Jul 17, 2018

pnkfelix commented on Jul 24, 2018

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