Try to debug another miscompile

Author: saethlin

compiler/rustc_mir_transform/src/check_niches.rs

@@ -92,7 +92,8 @@ impl<'a, 'tcx> Visitor<'tcx> for NicheFinder<'a, 'tcx> {
         if let Rvalue::Cast(CastKind::Transmute, op, ty) = rvalue {
             if let Some(niche) = self.get_niche(*ty) {
                 with_no_trimmed_paths!(debug!(
-                    "Found place {op:?}: {ty:?} with niche due to Transmute"
+                    "Found place {op:?}: {ty:?} with niche {niche:?} due to Transmute: {:?}",
+                    self.body.stmt_at(location)
                 ));
                 self.places.push((op.clone(), niche));
             }
@@ -114,9 +115,8 @@ impl<'a, 'tcx> Visitor<'tcx> for NicheFinder<'a, 'tcx> {
         }
 
         let ty = place.ty(self.body, self.tcx).ty;
-        if ty == self.tcx.types.bool {
-            // LLVM defeats our attempts to insert niche checks for bools by truncating all our
-            // loads to an i1.
+        // bool is actually an i1 to LLVM which means a bool local can never be invalid.
+        if ty == self.tcx.types.bool && place.projection.is_empty() {
             return;
         }
         let Some(niche) = self.get_niche(ty) else {
@@ -136,7 +136,8 @@ impl<'a, 'tcx> Visitor<'tcx> for NicheFinder<'a, 'tcx> {
         }
 
         with_no_trimmed_paths!(debug!(
-            "Found place {place:?}: {ty:?} with niche due to {context:?}"
+            "Found place {place:?}: {ty:?} with niche {niche:?} due to {:?}",
+            self.body.stmt_at(location)
         ));
         self.places.push((Operand::Copy(*place), niche));
 
@@ -153,6 +154,10 @@ impl<'a, 'tcx> NicheFinder<'a, 'tcx> {
 
         let niche = layout.largest_niche?;
 
+        if niche.offset == Size::ZERO {
+            return None;
+        }
+
         if niche.size(self.tcx) == layout.size {
             Some(NicheKind::Full(niche))
         } else {
@@ -295,7 +300,7 @@ impl<'a, 'tcx> NicheChecker<'a, 'tcx> {
     }
 }
 
-#[derive(Clone, Copy)]
+#[derive(Clone, Copy, Debug)]
 enum NicheKind {
     Full(Niche),
     // We need the full Size of the type in order to do the transmute-to-MU approach