diff options
Diffstat (limited to 'compiler/rustc_mir_transform/src')
| -rw-r--r-- | compiler/rustc_mir_transform/src/early_otherwise_branch.rs | 33 |
1 files changed, 32 insertions, 1 deletions
diff --git a/compiler/rustc_mir_transform/src/early_otherwise_branch.rs b/compiler/rustc_mir_transform/src/early_otherwise_branch.rs index 2bf97e5d43c..d72e8d16105 100644 --- a/compiler/rustc_mir_transform/src/early_otherwise_branch.rs +++ b/compiler/rustc_mir_transform/src/early_otherwise_branch.rs @@ -95,7 +95,7 @@ pub struct EarlyOtherwiseBranch; impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch { fn is_enabled(&self, sess: &rustc_session::Session) -> bool { - sess.mir_opt_level() >= 2 + sess.mir_opt_level() >= 3 && sess.opts.debugging_opts.unsound_mir_opts } fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { @@ -226,6 +226,37 @@ impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch { /// Returns true if computing the discriminant of `place` may be hoisted out of the branch fn may_hoist<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, place: Place<'tcx>) -> bool { + // FIXME(JakobDegen): This is unsound. Someone could write code like this: + // ```rust + // let Q = val; + // if discriminant(P) == otherwise { + // let ptr = &mut Q as *mut _ as *mut u8; + // unsafe { *ptr = 10; } // Any invalid value for the type + // } + // + // match P { + // A => match Q { + // A => { + // // code + // } + // _ => { + // // don't use Q + // } + // } + // _ => { + // // don't use Q + // } + // }; + // ``` + // + // Hoisting the `discriminant(Q)` out of the `A` arm causes us to compute the discriminant of an + // invalid value, which is UB. + // + // In order to fix this, we would either need to show that the discriminant computation of + // `place` is computed in all branches, including the `otherwise` branch, or we would need + // another analysis pass to determine that the place is fully initialized. It might even be best + // to have the hoisting be performed in a different pass and just do the CFG changing in this + // pass. for (place, proj) in place.iter_projections() { match proj { // Dereferencing in the computation of `place` might cause issues from one of two |