From d805b265dbc254cb9b8cee8c9a1eff9cd161a87a Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Tue, 10 Oct 2023 11:17:27 +0200
Subject: [PATCH] add some comments explaining why MIR opts are marked as
 unsound

---
 compiler/rustc_mir_transform/src/early_otherwise_branch.rs | 1 +
 compiler/rustc_mir_transform/src/large_enums.rs            | 3 +++
 compiler/rustc_mir_transform/src/nrvo.rs                   | 2 +-
 3 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/compiler/rustc_mir_transform/src/early_otherwise_branch.rs b/compiler/rustc_mir_transform/src/early_otherwise_branch.rs
index 319fb4eaf3e..6eb6cb069fe 100644
--- a/compiler/rustc_mir_transform/src/early_otherwise_branch.rs
+++ b/compiler/rustc_mir_transform/src/early_otherwise_branch.rs
@@ -95,6 +95,7 @@ pub struct EarlyOtherwiseBranch;
 
 impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
+        // unsound: https://github.com/rust-lang/rust/issues/95162
         sess.mir_opt_level() >= 3 && sess.opts.unstable_opts.unsound_mir_opts
     }
 
diff --git a/compiler/rustc_mir_transform/src/large_enums.rs b/compiler/rustc_mir_transform/src/large_enums.rs
index 886ff760481..0a8b13d6677 100644
--- a/compiler/rustc_mir_transform/src/large_enums.rs
+++ b/compiler/rustc_mir_transform/src/large_enums.rs
@@ -30,6 +30,9 @@ pub struct EnumSizeOpt {
 
 impl<'tcx> MirPass<'tcx> for EnumSizeOpt {
     fn is_enabled(&self, sess: &Session) -> bool {
+        // There are some differences in behavior on wasm and ARM that are not properly
+        // understood, so we conservatively treat this optimization as unsound:
+        // https://github.com/rust-lang/rust/pull/85158#issuecomment-1101836457
         sess.opts.unstable_opts.unsound_mir_opts || sess.mir_opt_level() >= 3
     }
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
diff --git a/compiler/rustc_mir_transform/src/nrvo.rs b/compiler/rustc_mir_transform/src/nrvo.rs
index e1298b0654f..ff309bd10ec 100644
--- a/compiler/rustc_mir_transform/src/nrvo.rs
+++ b/compiler/rustc_mir_transform/src/nrvo.rs
@@ -34,7 +34,7 @@ pub struct RenameReturnPlace;
 
 impl<'tcx> MirPass<'tcx> for RenameReturnPlace {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
-        // #111005
+        // unsound: #111005
         sess.mir_opt_level() > 0 && sess.opts.unstable_opts.unsound_mir_opts
     }