Rollup merge of #146744 - beepster4096:derefs_in_ref_prop, r=cjgillot

Deref related cleanups in ref_prop Cherry picked from #146710 r? cjgillot

Author: Zalathar

compiler/rustc_mir_transform/src/ref_prop.rs

@@ -195,18 +195,18 @@ fn compute_replacement<'tcx>(
  // including DEF. This violates the DEF dominates USE condition, and so is impossible.
  let is_constant_place = |place: Place<'_>| {
  // We only allow `Deref` as the first projection, to avoid surprises.
- if place.projection.first() == Some(&PlaceElem::Deref) {
+ if let Some((&PlaceElem::Deref, rest)) = place.projection.split_first() {
  // `place == (*some_local).xxx`, it is constant only if `some_local` is constant.
  // We approximate constness using SSAness.
- ssa.is_ssa(place.local) && place.projection[1..].iter().all(PlaceElem::is_stable_offset)
+ ssa.is_ssa(place.local) && rest.iter().all(PlaceElem::is_stable_offset)
  } else {
  storage_live.has_single_storage(place.local)
  && place.projection[..].iter().all(PlaceElem::is_stable_offset)
  }
  };
 
  let mut can_perform_opt = |target: Place<'tcx>, loc: Location| {
- if target.projection.first() == Some(&PlaceElem::Deref) {
+ if target.is_indirect_first_projection() {
  // We are creating a reborrow. As `place.local` is a reference, removing the storage
  // statements should not make it much harder for LLVM to optimize.
  storage_to_remove.insert(target.local);
@@ -266,15 +266,15 @@ fn compute_replacement<'tcx>(
  Rvalue::Ref(_, _, place) | Rvalue::RawPtr(_, place) => {
  let mut place = *place;
  // Try to see through `place` in order to collapse reborrow chains.
- if place.projection.first() == Some(&PlaceElem::Deref)
+ if let Some((&PlaceElem::Deref, rest)) = place.projection.split_first()
  && let Value::Pointer(target, inner_needs_unique) = targets[place.local]
  // Only see through immutable reference and pointers, as we do not know yet if
  // mutable references are fully replaced.
  && !inner_needs_unique
  // Only collapse chain if the pointee is definitely live.
  && can_perform_opt(target, location)
  {
- place = target.project_deeper(&place.projection[1..], tcx);
+ place = target.project_deeper(rest, tcx);
  }
  assert_ne!(place.local, local);
  if is_constant_place(place) {
@@ -323,7 +323,7 @@ fn compute_replacement<'tcx>(
  return;
  }
 
- if place.projection.first() != Some(&PlaceElem::Deref) {
+ if !place.is_indirect_first_projection() {
  // This is not a dereference, nothing to do.
  return;
  }
@@ -392,20 +392,15 @@ impl<'tcx> MutVisitor<'tcx> for Replacer<'tcx> {
  }
 
  fn visit_var_debug_info(&mut self, debuginfo: &mut VarDebugInfo<'tcx>) {
- // If the debuginfo is a pointer to another place:
- // - if it's a reborrow, see through it;
- // - if it's a direct borrow, increase `debuginfo.references`.
+ // If the debuginfo is a pointer to another place
+ // and it's a reborrow: see through it
  while let VarDebugInfoContents::Place(ref mut place) = debuginfo.value
  && place.projection.is_empty()
  && let Value::Pointer(target, _) = self.targets[place.local]
- && target.projection.iter().all(|p| p.can_use_in_debuginfo())
+ && let &[PlaceElem::Deref] = &target.projection[..]
  {
- if let Some((&PlaceElem::Deref, rest)) = target.projection.split_last() {
- *place = Place::from(target.local).project_deeper(rest, self.tcx);
- self.any_replacement = true;
- } else {
- break;
- }
+ *place = Place::from(target.local);
+ self.any_replacement = true;
  }
 
  // Simplify eventual projections left inside `debuginfo`.
@@ -414,9 +409,7 @@ impl<'tcx> MutVisitor<'tcx> for Replacer<'tcx> {
 
  fn visit_place(&mut self, place: &mut Place<'tcx>, ctxt: PlaceContext, loc: Location) {
  loop {
- if place.projection.first() != Some(&PlaceElem::Deref) {
- return;
- }
+ let Some((&PlaceElem::Deref, rest)) = place.projection.split_first() else { return };
 
  let Value::Pointer(target, _) = self.targets[place.local] else { return };
 
@@ -432,7 +425,7 @@ impl<'tcx> MutVisitor<'tcx> for Replacer<'tcx> {
  return;
  }
 
- *place = target.project_deeper(&place.projection[1..], self.tcx);
+ *place = target.project_deeper(rest, self.tcx);
  self.any_replacement = true;
  }
  }