[NLL] check other sorts of rvalues beyond aggregates #45959

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1. nikomatsakis

   added

   E-needs-mentor

   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.

   WG-compiler-nll

   on Nov 13, 2017
2. nikomatsakis

   changed the title [-][NLL] check other sorts of rvalues beyond [/-] [+][NLL] check other sorts of rvalues beyond aggregates[/+] on Nov 13, 2017
3. nikomatsakis commented on Nov 13, 2017

   

   nikomatsakis

   on Nov 13, 2017

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   Mentoring instructions

   As noted above, the main thing we need is some kind of function to check that a given trait reference is satisfied at a particular location. Let's call it prove. I imagine it would have a signature like this:

   fn prove(&mut self, trait_ref: TraitRef<'tcx>, location: Location)

   Inernally, like normalize, it should use fully_perform_op and look a little something like this:

   self.fully_perform_op(location.at_self(), |this| {
       let mut selcx = traits::SelectionContext::new(this.infcx);
       let cause = traits::ObligationCause::misc(this.last_span, ast::CRATE_NODE_ID);
       let obligation = traits::Obligation::new(cause, this.param_env, trait_ref.to_poly_trait_ref());
       if let Some(vtable) = selcx.select(&obligation)? {
           Ok(InferOk { value: (), obligations: vtable.nested_obligations() })
       } else {
           mir_span_bug!(...) // typeck should have ensured that this will succeed
       }
   }).unwrap()

   This basically invokes the trait checker (selcx.select) and gets back a (hopefully) successful resolution (vtable) which may carry with it some more work to do, which we then propagate upwards via vtable.nested_obligations().
4. spastorino commented on Nov 13, 2017

   

   spastorino

   on Nov 13, 2017

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   I'm taking this one, I've already talk with @nikomatsakis about it :).
5. bors

   added a commit that references this issue on Nov 23, 2017

   Auto merge of #46054 - nikomatsakis:nll-master-to-rust-master-1, r=ar…

   ...

   b9b82fd
6. nikomatsakis commented on Nov 27, 2017

   

   nikomatsakis

   on Nov 27, 2017

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   OK, so, some notes.

   ReifyFnPointer. This coercion casts from the TyFnDef type (a zero-sized type given to each function) to a TyFnPtr type (the "reified" form, a function pointer whose origin we do not statically know). We basically want to check that the "target type" of the cast is equal to a TyFnPtr with the signature that we can derive from the source type ty (which is a TyFnDef).

   To compute the signature, we do ty.fn_sig(tcx) -- this gives a PolyFnSig. This can be then converted into a TyFnPtr via tcx.mk_fn_ptr(ty.fn_sig(tcx)). We can then equate that with the target type of the cast.
7. nikomatsakis commented on Dec 5, 2017

   

   nikomatsakis

   on Dec 5, 2017

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   Done (in nll-master)
8. nikomatsakis

   closed this as completedon Dec 5, 2017