Polymorphization should look at the runtime MIR of const fn

Author: oli-obk

compiler/rustc_mir/src/monomorphize/polymorphize.rs

@@ -56,7 +56,7 @@ fn unused_generic_params(tcx: TyCtxt<'_>, def_id: DefId) -> FiniteBitSet<u32> {
     // Exit early when there is no MIR available.
     let context = tcx.hir().body_const_context(def_id.expect_local());
     match context {
-        None if !tcx.is_mir_available(def_id) => {
+        Some(ConstContext::Fn) | None if !tcx.is_mir_available(def_id) => {
             debug!("unused_generic_params: (no mir available) def_id={:?}", def_id);
             return FiniteBitSet::new_empty();
         }
@@ -78,10 +78,9 @@ fn unused_generic_params(tcx: TyCtxt<'_>, def_id: DefId) -> FiniteBitSet<u32> {
 
     // Visit MIR and accumululate used generic parameters.
     let body = match context {
-        None => tcx.optimized_mir(def_id),
-        // FIXME(oli-obk): since this is solely used for codegen (I think?), should we keep using
-        // the optimized MIR for `const fn`? Need to adjust the above `is_mir_available` check
-        // in that case.
+        // Const functions are actually called and should thus be considered for polymorphization
+        // via their runtime MIR
+        Some(ConstContext::Fn) | None => tcx.optimized_mir(def_id),
         Some(_) => tcx.mir_for_ctfe(def_id),
     };
     let mut vis = MarkUsedGenericParams { tcx, def_id, unused_parameters: &mut unused_parameters };