nll: respect user type annotations with constants in expressions #54571

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1. nikomatsakis

   added

   T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.

   A-NLLArea: Non-lexical lifetimes (NLL)Area: Non-lexical lifetimes (NLL)

   on Sep 25, 2018
2. nikomatsakis

   added this to the Edition 2018 RC 2 milestone on Sep 25, 2018
3. nikomatsakis

   self-assigned this

   on Sep 25, 2018
4. nikomatsakis commented on Sep 25, 2018

   

   nikomatsakis

   on Sep 25, 2018

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   Assigning to myself to investigate, write up some instructions, etc
5. matthewjasper

   added

   NLL-soundWorking towards the "invalid code does not compile" goalWorking towards the "invalid code does not compile" goal

   on Sep 26, 2018
6. nikomatsakis commented on Oct 2, 2018

   

   nikomatsakis

   on Oct 2, 2018

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   Test case #1 (should not compile, does):

   #![feature(nll)]
   
   trait Foo<'a> {
       const C: &'a u32;
   }
   
   impl<'a, T> Foo<'a> for T {
       const C: &'a u32 = &22;
   }
   
   fn foo<'a>(_: &'a u32) -> &'static u32 {
       <() as Foo<'a>>::C
   }
   
   fn main() {
   }
7. nikomatsakis commented on Oct 2, 2018

   

   nikomatsakis

   on Oct 2, 2018

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   Test case #2 (same story):

   #![feature(nll)]
   
   struct Foo<'a> { x: &'a u32 }
   
   impl<'a> Foo<'a> {
       const C: &'a u32 = &22;
   }
   
   fn foo<'a>(_: &'a u32) -> &'static u32 {
       <Foo<'a>>::C
   }
   
   fn main() {
   }
8. nikomatsakis commented on Oct 2, 2018

   

   nikomatsakis

   on Oct 2, 2018

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   Test case #3:

   #![feature(nll)]
   
   trait Foo<'a> {
       const C: &'a u32;
   }
   
   impl<'a, T> Foo<'a> for T {
       const C: &'a u32 = &22;
   }
   
   fn foo<'a, T: Foo<'a>>() -> &'static u32 {
       <T as Foo<'a>>::C
   }
   
   fn main() {
   }
9. nikomatsakis commented on Oct 15, 2018

   

   nikomatsakis

   on Oct 15, 2018

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   The fix for this will have to build on #55093
10. nikomatsakis commented on Oct 15, 2018

    

    nikomatsakis

    on Oct 15, 2018

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    That PR adds the ability for user type annotations to be more complex than they presently are. I think we need to extend the UserTypeAnnotation type to account for constants. In the branch, it is:

    pub enum UserTypeAnnotation<'tcx> {
        Ty(CanonicalTy<'tcx>),
        FnDef(DefId, CanonicalUserSubsts<'tcx>),
        AdtDef(&'tcx AdtDef, CanonicalUserSubsts<'tcx>),
    }

    So something like this:

    pub enum UserTypeAnnotation<'tcx> {
        Ty(CanonicalTy<'tcx>),
        FnDef(DefId, CanonicalUserSubsts<'tcx>),
        AdtDef(&'tcx AdtDef, CanonicalUserSubsts<'tcx>),
        ConstantDef(DefId, CanonicalUserSubsts<'tcx>),
    }

    Next, there is code that instantiates these "user type annotations" in various ways in:

    src/librustc_mir/borrow_check/nll/type_check/relate_tys.rs
    

    It looks sort of like this:

        match user_ty {
            UserTypeAnnotation::Ty(canonical_ty) => ...,
            UserTypeAnnotation::FnDef(def_id, canonical_substs) => ...,
            ...
        }

    I think we need to extend this with a case for ConstantDef -- or maybe we can actually adapt the FnDef code so it works for both cases (maybe even merge the two variants).

    It would work like so:

    • Instantiate the canonical_substs with fresh variables (like FnDef)
    • Fetch the type_of for def_id and substitute the canonical_substs (different from FnDef, but that same code would probably work for fns too)
    • Relate the types and apply the user_self_ty exactly as with FnDef

    If we do that, then the only thing we have left to do is to find the FIXME(#47184) spots that pertain to constants and propagate the user-substs onto those spots.
11. davidtwco

    assigned

    davidtwco

    and unassigned

    davidtwco

    on Oct 15, 2018
12. nikomatsakis commented on Oct 17, 2018

    

    nikomatsakis

    on Oct 17, 2018

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    Fix in #55152
13. bors

    closed this as completedin #55152on Oct 19, 2018