Don't fake borrow below a deref pattern

Author: Nadrieril

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};
@@ -361,7 +361,7 @@ 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()
         }
@@ -1862,6 +1862,7 @@ 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;
@@ -1872,6 +1873,15 @@ 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): does this allow for subtle bugs?
+                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 {
         box [] => 100,
-        // FIXME(deref_patterns): fake borrows break guards
-        // box [x] if x == 4 => x + 4,
+        box [x] if x == 4 => x + 4,
         box [x] => x,
         box [1, x] => x + 200,
         box 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);