Auto merge of #126505 - compiler-errors:no-vtable, r=lcnr

Only compute vtable information during codegen

This PR removes vtable information from the `Object` and `TraitUpcasting` candidate sources in the trait solvers, and defers the computation of relevant information to `Instance::resolve`. This is because vtables really aren't a thing in the trait world -- they're an implementation detail in codegen.

Previously it was just easiest to tangle this information together since we were already doing the work of looking at all the supertraits in the trait solver, and specifically because we use traits to represent when it's possible to call a method via a vtable (`Object` candidate) and do upcasting (`Unsize` candidate). but I am somewhat suspicious we're doing a *lot* of extra work, especially in polymorphic contexts, so let's see what perf says.

1 files changed, 0 insertions, 17 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/util.rs b/compiler/rustc_trait_selection/src/traits/util.rs
index ce7245d93a4..c3fe816028e 100644
--- a/compiler/rustc_trait_selection/src/traits/util.rs
+++ b/compiler/rustc_trait_selection/src/traits/util.rs
@@ -208,23 +208,6 @@ pub fn upcast_choices<'tcx>(
     supertraits(tcx, source_trait_ref).filter(|r| r.def_id() == target_trait_def_id).collect()
 }
 
-/// Given an upcast trait object described by `object`, returns the
-/// index of the method `method_def_id` (which should be part of
-/// `object.upcast_trait_ref`) within the vtable for `object`.
-pub fn get_vtable_index_of_object_method<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    vtable_base: usize,
-    method_def_id: DefId,
-) -> Option<usize> {
-    // Count number of methods preceding the one we are selecting and
-    // add them to the total offset.
-    tcx.own_existential_vtable_entries(tcx.parent(method_def_id))
-        .iter()
-        .copied()
-        .position(|def_id| def_id == method_def_id)
-        .map(|index| vtable_base + index)
-}
-
 pub fn closure_trait_ref_and_return_type<'tcx>(
     tcx: TyCtxt<'tcx>,
     fn_trait_def_id: DefId,