fix - don't panic on ATPIT with method-local generics

[im-lunex]

when an opaque type has no generic parameters but its hidden type
references a method-local generic... instantiate would panic because
the parameter has no corresponding slot in the opaque's args

reject such uses as non-defining before it gets there and guard the
instantiate call to fall back to an error type instead of crashing.

fixes #23125

(this is my 1st PR to this repo... and please correct me if i did something wrong here for lack of experience)

[ChayimFriedman2]

That is, unfortunately, not correct. It only catches the simple case where the ATPIT has no args at all, but it can have different args.

rustc has quite complicated logic to handle that IIRC.

View changes since this review

[im-lunex]

@ChayimFriedman2 thanks for review and i see.. i need a bit more time on this PR and will try to fix it as soon as possible...

[im-lunex]

this version walks the hidden type and drops any use whose params are not covered by the
opaques own args by index and kind instead of only catching the empty-args case so it also
handles non-empty but mismatched args plus const and lifetime params without panicking

[ChayimFriedman2]

Inventing your own way to handle that is not a wise idea. You should look at how rustc handles that and copy it.

[im-lunex]

i got what you are saying.. would it fix the issue if we copy rustcs remap_generic_params_to_declaration_params / ReverseMapper? im a bit confused here.. it would be great if you could guide me a bit here... (i also tried with those 2 and that worked fine and passed all tests)

[im-lunex]

we can do it like the way rustc does with a ReverseMapper folder? we check opaque_type_has_defining_use_args, then map generic params back to declaration.. if we see any unmapped method-local generics, we map them to TyKind::Error / Const::Error instead of rejecting