Rollup merge of rust-lang#116595 - RalfJung:mir-enable-passes, r=oli-obk

-Zmir-enable-passes: document that this may enable unsound passes

also add some comments explaining why MIR opts are marked as unsound

Author: matthiaskrgr

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
     }
 

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>) {

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
     }
 

compiler/rustc_session/src/options.rs

@@ -1599,9 +1599,10 @@ options! {
         "emit Retagging MIR statements, interpreted e.g., by miri; implies -Zmir-opt-level=0 \
         (default: no)"),
     mir_enable_passes: Vec<(String, bool)> = (Vec::new(), parse_list_with_polarity, [TRACKED],
-        "use like `-Zmir-enable-passes=+DestinationPropagation,-InstSimplify`. Forces the specified passes to be \
-        enabled, overriding all other checks. Passes that are not specified are enabled or \
-        disabled by other flags as usual."),
+        "use like `-Zmir-enable-passes=+DestinationPropagation,-InstSimplify`. Forces the \
+        specified passes to be enabled, overriding all other checks. In particular, this will \
+        enable unsound (known-buggy and hence usually disabled) passes without further warning! \
+        Passes that are not specified are enabled or disabled by other flags as usual."),
     mir_include_spans: bool = (false, parse_bool, [UNTRACKED],
         "use line numbers relative to the function in mir pretty printing"),
     mir_keep_place_mention: bool = (false, parse_bool, [TRACKED],