miri tree borrows: skip retag_reference early if there is no NewPermission

Author: RalfJung

src/tools/miri/src/borrow_tracker/tree_borrows/mod.rs

@@ -178,7 +178,7 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
         &mut self,
         place: &MPlaceTy<'tcx, Provenance>, // parent tag extracted from here
         ptr_size: Size,
-        new_perm: Option<NewPermission>,
+        new_perm: NewPermission,
         new_tag: BorTag,
     ) -> InterpResult<'tcx, Option<(AllocId, BorTag)>> {
         let this = self.eval_context_mut();
@@ -256,10 +256,6 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
             ptr_size.bytes()
         );
 
-        let Some(new_perm) = new_perm else {
-            return Ok(Some((alloc_id, orig_tag)));
-        };
-
         if let Some(protect) = new_perm.protector {
             // We register the protection in two different places.
             // This makes creating a protector slower, but checking whether a tag
@@ -305,7 +301,7 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
     fn tb_retag_reference(
         &mut self,
         val: &ImmTy<'tcx, Provenance>,
-        new_perm: Option<NewPermission>,
+        new_perm: NewPermission,
     ) -> InterpResult<'tcx, ImmTy<'tcx, Provenance>> {
         let this = self.eval_context_mut();
         // We want a place for where the ptr *points to*, so we get one.
@@ -317,7 +313,7 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
         // - if the pointer is not reborrowed (raw pointer) or if `zero_size` is set
         // then we override the size to do a zero-length reborrow.
         let reborrow_size = match new_perm {
-            Some(NewPermission { zero_size: false, .. }) =>
+            NewPermission { zero_size: false, .. } =>
                 this.size_and_align_of_mplace(&place)?
                     .map(|(size, _)| size)
                     .unwrap_or(place.layout.size),
@@ -374,7 +370,11 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 NewPermission::from_ref_ty(pointee, mutability, kind, this),
             _ => None,
         };
-        this.tb_retag_reference(val, new_perm)
+        if let Some(new_perm) = new_perm {
+            this.tb_retag_reference(val, new_perm)
+        } else {
+            Ok(val.clone())
+        }
     }
 
     /// Retag all pointers that are stored in this place.
@@ -405,9 +405,11 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 place: &PlaceTy<'tcx, Provenance>,
                 new_perm: Option<NewPermission>,
             ) -> InterpResult<'tcx> {
-                let val = self.ecx.read_immediate(&self.ecx.place_to_op(place)?)?;
-                let val = self.ecx.tb_retag_reference(&val, new_perm)?;
-                self.ecx.write_immediate(*val, place)?;
+                if let Some(new_perm) = new_perm {
+                    let val = self.ecx.read_immediate(&self.ecx.place_to_op(place)?)?;
+                    let val = self.ecx.tb_retag_reference(&val, new_perm)?;
+                    self.ecx.write_immediate(*val, place)?;
+                }
                 Ok(())
             }
         }
@@ -505,11 +507,11 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         let ptr_layout = this.layout_of(Ty::new_mut_ptr(this.tcx.tcx, place.layout.ty))?;
         let ptr = ImmTy::from_immediate(place.to_ref(this), ptr_layout);
         // Reborrow it. With protection! That is the entire point.
-        let new_perm = Some(NewPermission {
+        let new_perm = NewPermission {
             initial_state: Permission::new_active(),
             zero_size: false,
             protector: Some(ProtectorKind::StrongProtector),
-        });
+        };
         let _new_ptr = this.tb_retag_reference(&ptr, new_perm)?;
         // We just throw away `new_ptr`, so nobody can access this memory while it is protected.