Allow turbofishing late bound vars

[nxsaken]

Metadata
Contact	@oli-obk
Team(s)	compiler, lang, types (@BoxyUwU)
Goal document	2026/turbofishing-late-bound
Funding contact	RustNL

Summary

Opaque types and function item types are handling late bound lifetimes very hackily.

We think something along the lines of the ideas below can work, but mainly we want to try out lots of things here and land whatever sticks to the wall ("sticky" meaning "improves things")

• make FnDef's args higher ranked (i.e. put them behind a binder)
• update all the HIR ty lowering logic for args to functions to properly map stuff to late bound parameters and check them
• track late bound function parameters in generics_of
• try to make opaque types nicer by stopping to duplicae args/params all over the place

Sparse on purpose, we're figuring things out as we go

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[nxsaken]

This issue is intended for status updates only.

For general questions or comments, please contact the owner(s) directly.

[oli-obk]

@addiesh worked on this during her internship here at RustNL.

First she added binders to ty::FnDef's generic arg list, then simplified most FnDef creations by going through type_of instead of manually creating it via Ty::new_fn_def.

At this time all those binders are dummy binders, but as a next step they will actually have bound vars with rust-lang/rust#159403, after which we can also start adding the late bound vars to the args list and subsequently allow turbofishing them.