From 9e5c162d729b2d892bab6b43166be03242b69ebd Mon Sep 17 00:00:00 2001
From: Ben Kimock <kimockb@gmail.com>
Date: Thu, 8 Jun 2023 21:40:22 -0400
Subject: [PATCH] Move the implementation to codegen, to use monomorphized
 types

---
 compiler/rustc_codegen_ssa/src/mir/mod.rs     | 27 ++++++++++++-
 compiler/rustc_mir_transform/src/lib.rs       |  2 -
 compiler/rustc_mir_transform/src/ralf_refs.rs | 38 -------------------
 3 files changed, 26 insertions(+), 41 deletions(-)
 delete mode 100644 compiler/rustc_mir_transform/src/ralf_refs.rs

diff --git a/compiler/rustc_codegen_ssa/src/mir/mod.rs b/compiler/rustc_codegen_ssa/src/mir/mod.rs
index 2809ec2deb550..089d99488d56e 100644
--- a/compiler/rustc_codegen_ssa/src/mir/mod.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/mod.rs
@@ -153,7 +153,7 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
 
     let llfn = cx.get_fn(instance);
 
-    let mir = cx.tcx().instance_mir(instance.def);
+    let mut mir = cx.tcx().instance_mir(instance.def);
 
     let fn_abi = cx.fn_abi_of_instance(instance, ty::List::empty());
     debug!("fn_abi: {:?}", fn_abi);
@@ -260,6 +260,31 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     // Apply debuginfo to the newly allocated locals.
     fx.debug_introduce_locals(&mut start_bx);
 
+    if mir.args_iter().any(|arg| {
+        // Find the monomorphized type of our argument
+        let arg_decl = &mir.local_decls[arg];
+        let arg_ty = fx.monomorphize(arg_decl.ty);
+
+        // And check if it is a reference to an uninhabited type
+        let ty = arg_ty.peel_refs();
+        start_bx.layout_of(ty).abi.is_uninhabited()
+    }) {
+        // We're probably not supposed to mutate MIR here.
+        // But since we have an arena, we can just create a clone and replace all references to
+        // the old MIR with the new.
+        let mut body = mir.clone();
+        let blocks = body.basic_blocks.as_mut();
+        blocks.truncate(1);
+        let block = &mut blocks[rustc_middle::mir::START_BLOCK];
+        block.statements.clear();
+        block.terminator.as_mut().unwrap().kind = rustc_middle::mir::TerminatorKind::Unreachable;
+        block.is_cleanup = false;
+
+        // Now we have new MIR, stash it in the TyCtxt arena and swap it in
+        mir = fx.cx.tcx().arena.alloc(body);
+        fx.mir = mir;
+    }
+
     // The builders will be created separately for each basic block at `codegen_block`.
     // So drop the builder of `start_llbb` to avoid having two at the same time.
     drop(start_bx);
diff --git a/compiler/rustc_mir_transform/src/lib.rs b/compiler/rustc_mir_transform/src/lib.rs
index 9acf876d3d480..7d9f6c38e36a4 100644
--- a/compiler/rustc_mir_transform/src/lib.rs
+++ b/compiler/rustc_mir_transform/src/lib.rs
@@ -86,7 +86,6 @@ 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;
@@ -546,7 +545,6 @@ 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),
diff --git a/compiler/rustc_mir_transform/src/ralf_refs.rs b/compiler/rustc_mir_transform/src/ralf_refs.rs
deleted file mode 100644
index fea0c71c98ad7..0000000000000
--- a/compiler/rustc_mir_transform/src/ralf_refs.rs
+++ /dev/null
@@ -1,38 +0,0 @@
-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;
-            }
-        }
-    }
-}