WIP PoC for impl Trait in impl Fn return in fn parameters

[alsuren]

My amateurish attempts at getting rust-lang#45994 to work (I've never done rustc development before. Only contributed test cases so far).

I managed to make the ast lowering step accept the syntax, but I am definitely doing something wrong, because I'm not sure what to do with the impl Trait bound, so I'm dropping it on the floor. When the HIR lowering code tries to use it, it's not found in the HIR map, so it explodes (I added some debugging, and it's definitely the impl Trait bit that's going missing.)

vsonline:~/workspace/rust$ RUST_BACKTRACE=1 RUSTC_LOG=rustc::traits ./build/*/stage1/bin/rustc src/test/ui/impl-trait/where-allowed.rs 
FIXME: Generic params from impl Trait: [GenericParam { hir_id: HirId { owner: DefId(0:4), local_id: 18 }, name: Plain(impl Debug#0), attrs: [], bounds: [Trait(PolyTraitRef { bound_generic_params: [], trait_ref: TraitRef { path: Path { span: src/test/ui/impl-trait/where-allowed.rs:2:58: 2:63 (#0), res: Def(Trait, DefId(2:6178)), segments: [PathSegment { ident: Debug#0, hir_id: Some(HirId { owner: DefId(0:4), local_id: 16 }), res: Some(Def(Trait, DefId(2:6178))), args: None, infer_args: false }] }, hir_ref_id: HirId { owner: DefId(0:4), local_id: 17 } }, span: src/test/ui/impl-trait/where-allowed.rs:2:58: 2:63 (#0) }, None)], span: src/test/ui/impl-trait/where-allowed.rs:2:53: 2:63 (#0), pure_wrt_drop: false, kind: Type { default: None, synthetic: Some(ImplTrait) } }]

...

thread 'rustc' panicked at 'Can't find Entry for HirId HirId { owner: DefId(0:4 ~ where_allowed[317d]::in_impl_Fn_return_in_parameters[0]), local_id: 18 }', src/librustc_middle/hir/map/mod.rs:294:47
stack backtrace:
...
  16: core::option::Option<T>::unwrap_or_else
             at ./src/libcore/option.rs:430
  17: rustc_middle::hir::map::Map::get_entry
             at src/librustc_middle/hir/map/mod.rs:294
  18: rustc_middle::hir::map::Map::get_parent_node
             at src/librustc_middle/hir/map/mod.rs:525
  19: <dyn rustc_typeck::astconv::AstConv>::res_to_ty
             at src/librustc_typeck/astconv.rs:2751
  20: <dyn rustc_typeck::astconv::AstConv>::ast_ty_to_ty
             at src/librustc_typeck/astconv.rs:2839
  21: <dyn rustc_typeck::astconv::AstConv>::create_substs_for_ast_path::{{closure}}
             at src/librustc_typeck/astconv.rs:908
  22: core::iter::adapters::map_fold::{{closure}}
             at ./src/libcore/iter/adapters/mod.rs:830
  23: core::iter::traits::iterator::Iterator::fold
             at ./src/libcore/iter/traits/iterator.rs:2022
...

[alsuren]

Suggested change

// Explodes here

No longer explodes here. Now explodes in self.find_entry(id).unwrap_or_else(|| panic!("Can't find Entry for HirId {:?}", id)), below.

[alsuren]

Nothing to see here. Just my rust-analyser not being set up correctly.

[alsuren]

Just closing some old PRs. Nothing to see here.