Rollup merge of rust-lang#101141 - compiler-errors:get-trait-ref-is-a-misleading-name, r=oli-obk

Simplify `get_trait_ref` fn used for `virtual_function_elimination`

1. The name `get_trait_ref` is misleading, so I renamed it to something more like `expect_...` because it ICEs if used incorrectly.
2. No need to manually go through the existential trait refs, we already have `.principal()` for that.

Author: matthiaskrgr

compiler/rustc_codegen_ssa/src/meth.rs

@@ -1,6 +1,6 @@
 use crate::traits::*;
 
-use rustc_middle::ty::{self, subst::GenericArgKind, ExistentialPredicate, Ty, TyCtxt};
+use rustc_middle::ty::{self, subst::GenericArgKind, Ty};
 use rustc_session::config::Lto;
 use rustc_symbol_mangling::typeid_for_trait_ref;
 use rustc_target::abi::call::FnAbi;
@@ -29,7 +29,7 @@ impl<'a, 'tcx> VirtualIndex {
             && bx.cx().sess().lto() == Lto::Fat
         {
             let typeid =
-                bx.typeid_metadata(typeid_for_trait_ref(bx.tcx(), get_trait_ref(bx.tcx(), ty)));
+                bx.typeid_metadata(typeid_for_trait_ref(bx.tcx(), expect_dyn_trait_in_self(ty)));
             let vtable_byte_offset = self.0 * bx.data_layout().pointer_size.bytes();
             let type_checked_load = bx.type_checked_load(llvtable, vtable_byte_offset, typeid);
             let func = bx.extract_value(type_checked_load, 0);
@@ -64,17 +64,13 @@ impl<'a, 'tcx> VirtualIndex {
     }
 }
 
-fn get_trait_ref<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> ty::PolyExistentialTraitRef<'tcx> {
+/// This takes a valid `self` receiver type and extracts the principal trait
+/// ref of the type.
+fn expect_dyn_trait_in_self<'tcx>(ty: Ty<'tcx>) -> ty::PolyExistentialTraitRef<'tcx> {
     for arg in ty.peel_refs().walk() {
         if let GenericArgKind::Type(ty) = arg.unpack() {
-            if let ty::Dynamic(trait_refs, _) = ty.kind() {
-                return trait_refs[0].map_bound(|trait_ref| match trait_ref {
-                    ExistentialPredicate::Trait(tr) => tr,
-                    ExistentialPredicate::Projection(proj) => proj.trait_ref(tcx),
-                    ExistentialPredicate::AutoTrait(_) => {
-                        bug!("auto traits don't have functions")
-                    }
-                });
+            if let ty::Dynamic(data, _) = ty.kind() {
+                return data.principal().expect("expected principal trait object");
             }
         }
     }