Don't fake borrow inside a deref pattern

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -5,7 +5,7 @@
 //! This also includes code for pattern bindings in `let` statements and
 //! function parameters.
 
-use crate::build::expr::as_place::PlaceBuilder;
+use crate::build::expr::as_place::{PlaceBase, PlaceBuilder};
 use crate::build::scope::DropKind;
 use crate::build::ForGuard::{self, OutsideGuard, RefWithinGuard};
 use crate::build::{BlockAnd, BlockAndExtension, Builder};
@@ -438,7 +438,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
  }
 
  if let Some(ref borrows) = fake_borrows {
- self.calculate_fake_borrows(borrows, scrutinee_span)
+ self.calculate_fake_borrows(borrows, scrutinee_place_builder.base(), scrutinee_span)
  } else {
  Vec::new()
  }
@@ -1936,6 +1936,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
  fn calculate_fake_borrows<'b>(
  &mut self,
  fake_borrows: &'b FxIndexSet<Place<'tcx>>,
+ scrutinee_base: PlaceBase,
  temp_span: Span,
  ) -> Vec<(Place<'tcx>, Local)> {
  let tcx = self.tcx;
@@ -1946,6 +1947,15 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
  // Insert a Shallow borrow of the prefixes of any fake borrows.
  for place in fake_borrows {
+ if let PlaceBase::Local(l) = scrutinee_base
+ && l != place.local
+ {
+ // The base of this place is a temporary created for deref patterns. We don't emit
+ // fake borrows for these as they are not initialized in all branches.
+ // FIXME(deref_patterns): is this sound?
+ continue;
+ }
+
  let mut cursor = place.projection.as_ref();
  while let [proj_base @ .., elem] = cursor {
  cursor = proj_base;

tests/ui/pattern/deref-patterns/bindings.rs

@@ -5,8 +5,7 @@
 fn simple_vec(vec: Vec<u32>) -> u32 {
  match vec {
  deref!([]) => 100,
- // FIXME(deref_patterns): fake borrows break guards
- // deref!([x]) if x == 4 => x + 4,
+ deref!([x]) if x == 4 => x + 4,
  deref!([x]) => x,
  deref!([1, x]) => x + 200,
  deref!(ref slice) => slice.iter().sum(),
@@ -29,6 +28,7 @@ fn main() {
  assert_eq!(simple_vec(vec![1]), 1);
  assert_eq!(simple_vec(vec![1, 2]), 202);
  assert_eq!(simple_vec(vec![1, 2, 3]), 6);
+ assert_eq!(simple_vec(vec![4]), 8);
 
  assert_eq!(nested_vec(vec![vec![0, 42]]), 42);
  assert_eq!(nested_vec(vec![vec![1, 42]]), 42);