track caller

Add `#[track_caller]` to some functions which can panic b/c of the
caller's wrong index (this helped me debug some mistakes I did while
refactoring `ty::Alias`)

Author: WaffleLapkin

compiler/rustc_middle/src/ty/generic_args.rs

@@ -50,14 +50,17 @@ impl<'tcx> rustc_type_ir::inherent::GenericArgs<TyCtxt<'tcx>> for ty::GenericArg
         self.rebase_onto(tcx, source_ancestor, target_args)
     }
 
+    #[track_caller]
     fn type_at(self, i: usize) -> Ty<'tcx> {
         self.type_at(i)
     }
 
+    #[track_caller]
     fn region_at(self, i: usize) -> ty::Region<'tcx> {
         self.region_at(i)
     }
 
+    #[track_caller]
     fn const_at(self, i: usize) -> ty::Const<'tcx> {
         self.const_at(i)
     }

compiler/rustc_type_ir/src/ty_kind.rs

@@ -489,6 +489,7 @@ impl<I: Interner> AliasTy<I> {
 
 /// The following methods work only with (trait) associated type projections.
 impl<I: Interner> AliasTy<I> {
+    #[track_caller]
     pub fn self_ty(self) -> I::Ty {
         self.args.type_at(0)
     }