add some comments explaining why MIR opts are marked as unsound

rust-lang · Oct 10, 2023 · d805b26 · d805b26
1 parent 2de4546
commit d805b26
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diff --git 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
@@ -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
@@ -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
     }