Add a debug-info cost to MIR inlining

Author: scottmcm

compiler/rustc_mir_transform/src/cost_checker.rs

@@ -7,6 +7,13 @@ const CALL_PENALTY: usize = 25;
 const LANDINGPAD_PENALTY: usize = 50;
 const RESUME_PENALTY: usize = 45;
 
+// While debug info has no runtime cost, when inlining we do need to copy it
+// into the caller MIR, which means we should give it a small cost to represent
+// the compile-time impact of doing that, since just because something has few
+// instructions doesn't necessarily mean it's cheap for us to inline.
+// The backend can, of course, still inline such things later should it so wish.
+const DEBUG_INFO_COST: usize = 4;
+
 /// Verify that the callee body is compatible with the caller.
 #[derive(Clone)]
 pub(crate) struct CostChecker<'b, 'tcx> {
@@ -41,6 +48,11 @@ impl<'b, 'tcx> CostChecker<'b, 'tcx> {
 }
 
 impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> {
+    fn visit_body(&mut self, body: &Body<'tcx>) {
+        self.cost += body.var_debug_info.len() * DEBUG_INFO_COST;
+        self.super_body(body)
+    }
+
     fn visit_statement(&mut self, statement: &Statement<'tcx>, _: Location) {
         // Don't count StorageLive/StorageDead in the inlining cost.
         match statement.kind {