experiment: do not mess about with required_consts and mentioned_items in inliner

Author: RalfJung

compiler/rustc_mir_transform/src/inline.rs

@@ -565,8 +565,7 @@ impl<'tcx> Inliner<'tcx> {
         mut callee_body: Body<'tcx>,
     ) {
         let terminator = caller_body[callsite.block].terminator.take().unwrap();
-        let TerminatorKind::Call { func, args, destination, unwind, target, .. } = terminator.kind
-        else {
+        let TerminatorKind::Call { args, destination, unwind, target, .. } = terminator.kind else {
             bug!("unexpected terminator kind {:?}", terminator.kind);
         };
 
@@ -705,42 +704,6 @@ impl<'tcx> Inliner<'tcx> {
             source_info: callsite.source_info,
             kind: TerminatorKind::Goto { target: integrator.map_block(START_BLOCK) },
         });
-
-        // Copy only unevaluated constants from the callee_body into the caller_body.
-        // Although we are only pushing `ConstKind::Unevaluated` consts to
-        // `required_consts`, here we may not only have `ConstKind::Unevaluated`
-        // because we are calling `instantiate_and_normalize_erasing_regions`.
-        caller_body.required_consts.extend(callee_body.required_consts.iter().copied().filter(
-            |&ct| match ct.const_ {
-                Const::Ty(_) => {
-                    bug!("should never encounter ty::UnevaluatedConst in `required_consts`")
-                }
-                Const::Val(..) | Const::Unevaluated(..) => true,
-            },
-        ));
-        // Now that we incorporated the callee's `required_consts`, we can remove the callee from
-        // `mentioned_items` -- but we have to take their `mentioned_items` in return.
-        // This does some extra work here to save the monomorphization collector work later.
-        // FIXME: benchmark which option is better.
-        let callee_item = {
-            // We need to reconstruct the `required_item` for the callee so that we can find and
-            // remove it.
-            let func_ty = func.ty(caller_body, self.tcx);
-            match func_ty.kind() {
-                ty::FnDef(def_id, args) => MentionedItem::Fn(*def_id, args),
-                _ => bug!(),
-            }
-        };
-        if let Some(idx) =
-            caller_body.mentioned_items.iter().position(|item| item.node == callee_item)
-        {
-            // We found the callee, so remove it and add its items instead.
-            caller_body.mentioned_items.remove(idx);
-            caller_body.mentioned_items.extend(callee_body.mentioned_items);
-        } else {
-            // If we can't find the callee, there's no point in adding its items.
-            // Probably it already got removed by being inlined elsewhere in the same function.
-        }
     }
 
     fn make_call_args(

tests/ui/inline-const/required-const.stderr

@@ -12,6 +12,12 @@ note: erroneous constant encountered
 LL |         const { panic!() }
    |         ^^^^^^^^^^^^^^^^^^
 
+note: the above error was encountered while instantiating `fn foo::<i32>`
+  --> $DIR/required-const.rs:12:5
+   |
+LL |     foo::<i32>();
+   |     ^^^^^^^^^^
+
 error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0080`.