Remove unhelpful comments and add helpful ones

Author: WaffleLapkin

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -2323,6 +2323,10 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                                 let src_tail = tcx.struct_tail_without_normalization(src.ty);
                                 let dst_tail = tcx.struct_tail_without_normalization(dst.ty);
 
+                                // This checks (lifetime part of) vtable validity for pointer casts,
+                                // which is irrelevant when there are aren't principal traits on both sides (aka only auto traits).
+                                //
+                                // Note that other checks (such as denying `dyn Send` -> `dyn Debug`) are in `rustc_hir_typeck`.
                                 if let ty::Dynamic(src_tty, ..) = src_tail.kind()
                                     && let ty::Dynamic(dst_tty, ..) = dst_tail.kind()
                                     && src_tty.principal().is_some()

compiler/rustc_hir_typeck/src/cast.rs

@@ -838,6 +838,9 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                         // Check that the traits are actually the same
                         // (this is required as the `Unsize` check below would allow upcasting, etc)
                         // N.B.: this is only correct as long as we don't support `trait A<T>: A<()>`.
+                        //
+                        // Note that trait upcasting goes through a different mechanism (`coerce_unsized`)
+                        // and is unaffected by this check.
                         if src_principal.def_id() != dst_principal.def_id() {
                             return Err(CastError::DifferingKinds);
                         }
@@ -902,10 +905,6 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                             )
                         }
 
-                        // FIXME: ideally we'd maybe add a flag here, so that borrowck knows that
-                        //        it needs to borrowck this ptr cast. this is made annoying by the
-                        //        fact that `thir` does not have `CastKind` and mir restores it
-                        //        from types.
                         Ok(CastKind::PtrPtrCast)
                     }
 

compiler/rustc_hir_typeck/src/errors.rs

@@ -255,7 +255,6 @@ pub struct LossyProvenanceInt2Ptr<'tcx> {
 
 #[derive(LintDiagnostic)]
 #[diag(hir_typeck_ptr_cast_add_auto_to_object)]
-//#[help]
 pub struct PtrCastAddAutoToObject {
     pub traits_len: usize,
     pub traits: DiagSymbolList<String>,