Make function bodies with &! arguments unreachable

Author: saethlin

compiler/rustc_mir_transform/src/lib.rs

@@ -86,6 +86,7 @@ mod multiple_return_terminators;
 mod normalize_array_len;
 mod nrvo;
 mod prettify;
+mod ralf_refs;
 mod ref_prop;
 mod remove_noop_landing_pads;
 mod remove_storage_markers;
@@ -545,6 +546,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &check_alignment::CheckAlignment,
             &reveal_all::RevealAll, // has to be done before inlining, since inlined code is in RevealAll mode.
             &lower_slice_len::LowerSliceLenCalls, // has to be done before inlining, otherwise actual call will be almost always inlined. Also simple, so can just do first
+            &ralf_refs::RalfRefs,
             &unreachable_prop::UnreachablePropagation,
             &uninhabited_enum_branching::UninhabitedEnumBranching,
             &o1(simplify::SimplifyCfg::AfterUninhabitedEnumBranching),

compiler/rustc_mir_transform/src/ralf_refs.rs

@@ -0,0 +1,38 @@
+use crate::MirPass;
+use rustc_middle::mir::*;
+use rustc_middle::ty::print::with_no_trimmed_paths;
+use rustc_middle::ty::TyCtxt;
+
+pub struct RalfRefs;
+
+impl<'tcx> MirPass<'tcx> for RalfRefs {
+    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
+        sess.mir_opt_level() >= 1
+    }
+
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        let def_id = body.source.def_id();
+        if tcx.type_of(body.source.def_id()).subst_identity().is_generator() {
+            return;
+        }
+        let param_env = tcx.param_env_reveal_all_normalized(def_id);
+        for arg in body.local_decls.iter().skip(1).take(body.arg_count) {
+            let ty = arg.ty.peel_refs();
+            let Ok(layout) = tcx.layout_of(param_env.and(ty)) else { continue; };
+            if layout.abi.is_uninhabited() {
+                debug!(
+                    "Deleting {}::{}",
+                    tcx.crate_name(def_id.krate),
+                    with_no_trimmed_paths!(tcx.def_path_str(def_id))
+                );
+                let blocks = body.basic_blocks.as_mut();
+                blocks.truncate(1);
+                let block = &mut blocks[START_BLOCK];
+                block.statements.clear();
+                block.terminator.as_mut().unwrap().kind = TerminatorKind::Unreachable;
+                block.is_cleanup = false;
+                return;
+            }
+        }
+    }
+}