Avoid encoding optimized MIR for constructors

We only use mir_for_ctfe for them anyway in instance_mir. This does
prevent MIR inlining of constructor calls, but constructor calls that
are inlinable during MIR optimizations are rare anyway given that MIR
building already inlines all direct calls to constructors.

Author: bjorn3

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1102,12 +1102,8 @@ fn should_encode_mir(
     def_id: LocalDefId,
 ) -> (bool, bool) {
     match tcx.def_kind(def_id) {
-        // Constructors
-        DefKind::Ctor(_, _) => {
-            let mir_opt_base = tcx.sess.opts.output_types.should_codegen()
-                || tcx.sess.opts.unstable_opts.always_encode_mir;
-            (true, mir_opt_base)
-        }
+        // instance_mir uses mir_for_ctfe rather than optimized_mir for constructors
+        DefKind::Ctor(_, _) => (true, false),
         // Constants
         DefKind::AnonConst | DefKind::InlineConst | DefKind::AssocConst | DefKind::Const => {
             (true, false)

compiler/rustc_monomorphize/src/collector.rs

@@ -1084,7 +1084,8 @@ fn should_codegen_locally<'tcx>(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) ->
         return false;
     }
 
-    if !tcx.is_mir_available(def_id) {
+    // See comment in should_encode_mir for why we don't report an error for constructors.
+    if !tcx.is_mir_available(def_id) && !matches!(tcx.def_kind(def_id), DefKind::Ctor(..)) {
         tcx.dcx().emit_fatal(NoOptimizedMir {
             span: tcx.def_span(def_id),
             crate_name: tcx.crate_name(def_id.krate),