Add -Zprecise-enum-drop-elaboration

Its purpose is to assist in debugging rust-lang#77382 and rust-lang#74551.

compiler/rustc_interface/src/tests.rs

@@ -568,6 +568,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(osx_rpath_install_name, true);
     tracked!(panic_abort_tests, true);
     tracked!(plt, Some(true));
+    tracked!(precise_enum_drop_elaboration, false);
     tracked!(print_fuel, Some("abc".to_string()));
     tracked!(profile, true);
     tracked!(profile_emit, Some(PathBuf::from("abc")));

compiler/rustc_mir/src/dataflow/impls/mod.rs

@@ -358,6 +358,10 @@ impl<'tcx> GenKillAnalysis<'tcx> for MaybeInitializedPlaces<'_, 'tcx> {
         discr: &mir::Operand<'tcx>,
         edge_effects: &mut impl SwitchIntEdgeEffects<G>,
     ) {
+        if !self.tcx.sess.opts.debugging_opts.precise_enum_drop_elaboration {
+            return;
+        }
+
         let enum_ = discr.place().and_then(|discr| {
             switch_on_enum_discriminant(self.tcx, &self.body, &self.body[block], discr)
         });
@@ -469,6 +473,10 @@ impl<'tcx> GenKillAnalysis<'tcx> for MaybeUninitializedPlaces<'_, 'tcx> {
         discr: &mir::Operand<'tcx>,
         edge_effects: &mut impl SwitchIntEdgeEffects<G>,
     ) {
+        if !self.tcx.sess.opts.debugging_opts.precise_enum_drop_elaboration {
+            return;
+        }
+
         if !self.mark_inactive_variants_as_uninit {
             return;
         }

compiler/rustc_session/src/options.rs

@@ -1008,6 +1008,10 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "a single extra argument to prepend the linker invocation (can be used several times)"),
     pre_link_args: Vec<String> = (Vec::new(), parse_list, [UNTRACKED],
         "extra arguments to prepend to the linker invocation (space separated)"),
+    precise_enum_drop_elaboration: bool = (true, parse_bool, [TRACKED],
+        "use a more precise version of drop elaboration for matches on enums (default: yes). \
+        This results in better codegen, but has caused miscompilations on some tier 2 platforms. \
+        See #77382 and #74551."),
     print_fuel: Option<String> = (None, parse_opt_string, [TRACKED],
         "make rustc print the total optimization fuel used by a crate"),
     print_link_args: bool = (false, parse_bool, [UNTRACKED],