Auto merge of rust-lang#77755 - bugadani:perf-calc-dtor, r=ecstatic-morse

Monomorphize `calculate_dtor` instead of using function pointers

Change `calculate_dtor` to avoid dynamic dispatching. This change allows the empty functions to be optimized away.

Based on the discussion in rust-lang#77754 (comment), the performance impact of this change was measured.

Perf run results: https://perf.rust-lang.org/compare.html?start=7bc5839e99411aad9061a632b62075d1346cbb3b&end=ffec759ae9bbc4d6d2235ff40ade6723a85bc7cc

Author: bors

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -94,7 +94,7 @@ provide! { <'tcx> tcx, def_id, other, cdata,
     adt_def => { cdata.get_adt_def(def_id.index, tcx) }
     adt_destructor => {
         let _ = cdata;
-        tcx.calculate_dtor(def_id, &mut |_,_| Ok(()))
+        tcx.calculate_dtor(def_id, |_,_| Ok(()))
     }
     variances_of => { tcx.arena.alloc_from_iter(cdata.get_item_variances(def_id.index)) }
     associated_item_def_ids => {

compiler/rustc_middle/src/ty/util.rs

@@ -341,7 +341,7 @@ impl<'tcx> TyCtxt<'tcx> {
     pub fn calculate_dtor(
         self,
         adt_did: DefId,
-        validate: &mut dyn FnMut(Self, DefId) -> Result<(), ErrorReported>,
+        validate: impl Fn(Self, DefId) -> Result<(), ErrorReported>,
     ) -> Option<ty::Destructor> {
         let drop_trait = self.lang_items().drop_trait()?;
         self.ensure().coherent_trait(drop_trait);

compiler/rustc_mir/src/transform/check_const_item_mutation.rs

@@ -53,7 +53,7 @@ impl<'a, 'tcx> ConstMutationChecker<'a, 'tcx> {
         //
         //     #[const_mutation_allowed]
         //     pub const LOG: Log = Log { msg: "" };
-        match self.tcx.calculate_dtor(def_id, &mut |_, _| Ok(())) {
+        match self.tcx.calculate_dtor(def_id, |_, _| Ok(())) {
             Some(_) => None,
             None => Some(def_id),
         }

compiler/rustc_typeck/src/check/mod.rs

@@ -264,7 +264,7 @@ pub fn provide(providers: &mut Providers) {
 }
 
 fn adt_destructor(tcx: TyCtxt<'_>, def_id: DefId) -> Option<ty::Destructor> {
-    tcx.calculate_dtor(def_id, &mut dropck::check_drop_impl)
+    tcx.calculate_dtor(def_id, dropck::check_drop_impl)
 }
 
 /// If this `DefId` is a "primary tables entry", returns