Try an inliner tweak

Author: saethlin

compiler/rustc_mir_transform/src/cost_checker.rs

@@ -2,6 +2,7 @@ use rustc_middle::bug;
 use rustc_middle::mir::visit::*;
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, ParamEnv, Ty, TyCtxt};
+use rustc_span::sym;
 
 const INSTR_COST: usize = 5;
 const CALL_PENALTY: usize = 25;
@@ -127,14 +128,24 @@ impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> {
                 }
             }
             TerminatorKind::Call { func, unwind, .. } => {
-                self.penalty += if let Some((def_id, ..)) = func.const_fn_def()
-                    && self.tcx.intrinsic(def_id).is_some()
-                {
-                    // Don't give intrinsics the extra penalty for calls
-                    INSTR_COST
-                } else {
-                    CALL_PENALTY
-                };
+                let mut is_intrinsic = false;
+                let mut is_ubcheck = false;
+                if let Some((def_id, ..)) = func.const_fn_def() {
+                    if self.tcx.intrinsic(def_id).is_some() {
+                        is_intrinsic = true;
+                    }
+                    if self.tcx.has_attr(def_id, sym::rustc_no_mir_inline) {
+                        is_ubcheck = true;
+                    }
+                }
+                if !is_ubcheck {
+                    self.penalty += if is_intrinsic {
+                        // Don't give intrinsics the extra penalty for calls
+                        INSTR_COST
+                    } else {
+                        CALL_PENALTY
+                    };
+                }
                 if let UnwindAction::Cleanup(_) = unwind {
                     self.penalty += LANDINGPAD_PENALTY;
                 }