2229: Reduce the size of closures with capture_disjoint_fields

compiler/rustc_typeck/src/check/upvar.rs

@@ -1609,11 +1609,17 @@ impl<'a, 'tcx> euv::Delegate<'tcx> for InferBorrowKind<'a, 'tcx> {
             "consume(place_with_id={:?}, diag_expr_id={:?}, mode={:?})",
             place_with_id, diag_expr_id, mode
         );
+
+        let place_with_id = PlaceWithHirId {
+            place: truncate_capture_for_optimization(&place_with_id.place),
+            ..*place_with_id
+        };
+
         if !self.capture_information.contains_key(&place_with_id.place) {
-            self.init_capture_info_for_place(place_with_id, diag_expr_id);
+            self.init_capture_info_for_place(&place_with_id, diag_expr_id);
         }
 
-        self.adjust_upvar_borrow_kind_for_consume(place_with_id, diag_expr_id, mode);
+        self.adjust_upvar_borrow_kind_for_consume(&place_with_id, diag_expr_id, mode);
     }
 
     fn borrow(
@@ -1636,6 +1642,8 @@ impl<'a, 'tcx> euv::Delegate<'tcx> for InferBorrowKind<'a, 'tcx> {
             &place_with_id.place,
         );
 
+        let place = truncate_capture_for_optimization(&place);
+
         let place_with_id = PlaceWithHirId { place, ..*place_with_id };
 
         if !self.capture_information.contains_key(&place_with_id.place) {
@@ -1967,6 +1975,48 @@ fn determine_place_ancestry_relation(
     }
 }
 
+/// Reduces the precision of the captured place when the precision doesn't yeild any benefit from
+/// borrow checking prespective, allowing us to save us on the size of the capture.
+///
+///
+/// Fields that are read through a shared reference will always be read via a shared ref or a copy,
+/// and therefore capturing precise paths yields no benefit. This optimization truncates the
+/// rightmost deref of the capture if the deref is applied to a shared ref.
+///
+/// Reason we only drop the last deref is because of the following edge case:
+///
+/// ```rust
+/// struct MyStruct<'a> {
+///    a: &'static A,
+///    b: B,
+///    c: C<'a>,
+/// }
+///
+/// fn foo<'a, 'b>(m: &'a MyStruct<'b>) -> impl FnMut() + 'static {
+///     let c = || drop(&*m.a.field_of_a);
+///     // Here we really do want to capture `*m.a` because that outlives `'static`
+///
+///     // If we capture `m`, then the closure no longer outlives `'static'
+///     // it is constrained to `'a`
+/// }
+/// ```
+fn truncate_capture_for_optimization<'tcx>(place: &Place<'tcx>) -> Place<'tcx> {
+    let is_shared_ref = |ty: Ty<'_>| matches!(ty.kind(), ty::Ref(.., hir::Mutability::Not));
+
+    // Find the right-most deref (if any). All the projections that come after this
+    // are fields or other "in-place pointer adjustments"; these refer therefore to
+    // data owned by whatever pointer is being dereferenced here.
+    let idx = place.projections.iter().rposition(|proj| ProjectionKind::Deref == proj.kind);
+
+    match idx {
+        // If that pointer is a shared reference, then we don't need those fields.
+        Some(idx) if is_shared_ref(place.ty_before_projection(idx)) => {
+            Place { projections: place.projections[0..=idx].to_vec(), ..place.clone() }
+        }
+        None | Some(_) => place.clone(),
+    }
+}
+
 /// Precise capture is enabled if the feature gate `capture_disjoint_fields` is enabled or if
 /// user is using Rust Edition 2021 or higher.
 ///

src/test/ui/closures/2229_closure_analysis/diagnostics/cant-mutate-imm-borrow.rs

@@ -8,10 +8,10 @@ fn main() {
     let mut y = (&x, "Y");
     let z = (&mut y, "Z");
 
-    // `x.0` is mutable but we access `x` via `z.0.0`, which is an immutable reference and
+    // `x.0` is mutable but we access `x` via `*z.0.0`, which is an immutable reference and
     // therefore can't be mutated.
     let mut c = || {
-    //~^ ERROR: cannot borrow `z.0.0.0` as mutable, as it is behind a `&` reference
+    //~^ ERROR: cannot borrow `*z.0.0` as mutable, as it is behind a `&` reference
         z.0.0.0 = format!("X1");
     };
 

src/test/ui/closures/2229_closure_analysis/diagnostics/cant-mutate-imm-borrow.stderr

@@ -1,11 +1,11 @@
-error[E0596]: cannot borrow `z.0.0.0` as mutable, as it is behind a `&` reference
+error[E0596]: cannot borrow `*z.0.0` as mutable, as it is behind a `&` reference
   --> $DIR/cant-mutate-imm-borrow.rs:13:17
    |
 LL |     let mut c = || {
    |                 ^^ cannot borrow as mutable
 LL |
 LL |         z.0.0.0 = format!("X1");
-   |         ------- mutable borrow occurs due to use of `z.0.0.0` in closure
+   |         ------- mutable borrow occurs due to use of `*z.0.0` in closure
 
 error: aborting due to previous error
 

src/test/ui/closures/2229_closure_analysis/move_closure.rs

@@ -78,8 +78,8 @@ fn struct_contains_ref_to_another_struct_2() {
     //~^ ERROR: First Pass analysis includes:
     //~| ERROR: Min Capture analysis includes:
         let _t = t.0.0;
-        //~^ NOTE: Capturing t[(0, 0),Deref,(0, 0)] -> ImmBorrow
-        //~| NOTE: Min Capture t[(0, 0),Deref,(0, 0)] -> ImmBorrow
+        //~^ NOTE: Capturing t[(0, 0),Deref] -> ImmBorrow
+        //~| NOTE: Min Capture t[(0, 0),Deref] -> ImmBorrow
     };
 
     c();
@@ -100,7 +100,7 @@ fn struct_contains_ref_to_another_struct_3() {
     //~^ ERROR: First Pass analysis includes:
     //~| ERROR: Min Capture analysis includes:
         let _t = t.0.0;
-        //~^ NOTE: Capturing t[(0, 0),Deref,(0, 0)] -> ImmBorrow
+        //~^ NOTE: Capturing t[(0, 0),Deref] -> ImmBorrow
         //~| NOTE: Capturing t[(0, 0)] -> ByValue
         //~| NOTE: Min Capture t[(0, 0)] -> ByValue
     };

src/test/ui/closures/2229_closure_analysis/move_closure.stderr

@@ -190,7 +190,7 @@ LL | |
 LL | |     };
    | |_____^
    |
-note: Capturing t[(0, 0),Deref,(0, 0)] -> ImmBorrow
+note: Capturing t[(0, 0),Deref] -> ImmBorrow
   --> $DIR/move_closure.rs:80:18
    |
 LL |         let _t = t.0.0;
@@ -208,7 +208,7 @@ LL | |
 LL | |     };
    | |_____^
    |
-note: Min Capture t[(0, 0),Deref,(0, 0)] -> ImmBorrow
+note: Min Capture t[(0, 0),Deref] -> ImmBorrow
   --> $DIR/move_closure.rs:80:18
    |
 LL |         let _t = t.0.0;
@@ -226,7 +226,7 @@ LL | |
 LL | |     };
    | |_____^
    |
-note: Capturing t[(0, 0),Deref,(0, 0)] -> ImmBorrow
+note: Capturing t[(0, 0),Deref] -> ImmBorrow
   --> $DIR/move_closure.rs:102:18
    |
 LL |         let _t = t.0.0;

src/test/ui/closures/2229_closure_analysis/optimization/edge_case.rs

@@ -0,0 +1,34 @@
+// edition:2021
+
+#![feature(rustc_attrs)]
+#![allow(unused)]
+#![allow(dead_code)]
+
+struct Int(i32);
+struct B<'a>(&'a i32);
+
+const I : Int = Int(0);
+const REF_I : &'static Int = &I;
+
+
+struct MyStruct<'a> {
+   a: &'static Int,
+   b: B<'a>,
+}
+
+fn foo<'a, 'b>(m: &'a MyStruct<'b>) -> impl FnMut() + 'static {
+    let c = #[rustc_capture_analysis] || drop(&m.a.0);
+    //~^ ERROR: attributes on expressions are experimental
+    //~| NOTE: see issue #15701 <https://github.com/rust-lang/rust/issues/15701>
+    //~| ERROR: First Pass analysis includes:
+    //~| ERROR: Min Capture analysis includes:
+    //~| NOTE: Capturing m[Deref,(0, 0),Deref] -> ImmBorrow
+    //~| NOTE: Min Capture m[Deref,(0, 0),Deref] -> ImmBorrow
+    c
+}
+
+fn main() {
+    let t = 0;
+    let s = MyStruct { a: REF_I, b: B(&t) };
+    let _ = foo(&s);
+}

src/test/ui/closures/2229_closure_analysis/optimization/edge_case.stderr

@@ -0,0 +1,36 @@
+error[E0658]: attributes on expressions are experimental
+  --> $DIR/edge_case.rs:20:13
+   |
+LL |     let c = #[rustc_capture_analysis] || drop(&m.a.0);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
+   = help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable
+
+error: First Pass analysis includes:
+  --> $DIR/edge_case.rs:20:39
+   |
+LL |     let c = #[rustc_capture_analysis] || drop(&m.a.0);
+   |                                       ^^^^^^^^^^^^^^^
+   |
+note: Capturing m[Deref,(0, 0),Deref] -> ImmBorrow
+  --> $DIR/edge_case.rs:20:48
+   |
+LL |     let c = #[rustc_capture_analysis] || drop(&m.a.0);
+   |                                                ^^^^^
+
+error: Min Capture analysis includes:
+  --> $DIR/edge_case.rs:20:39
+   |
+LL |     let c = #[rustc_capture_analysis] || drop(&m.a.0);
+   |                                       ^^^^^^^^^^^^^^^
+   |
+note: Min Capture m[Deref,(0, 0),Deref] -> ImmBorrow
+  --> $DIR/edge_case.rs:20:48
+   |
+LL |     let c = #[rustc_capture_analysis] || drop(&m.a.0);
+   |                                                ^^^^^
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/closures/2229_closure_analysis/optimization/edge_case_run_pass.rs

@@ -0,0 +1,27 @@
+// edition:2021
+// run-pass
+
+#![allow(unused)]
+#![allow(dead_code)]
+
+struct Int(i32);
+struct B<'a>(&'a i32);
+
+const I : Int = Int(0);
+const REF_I : &'static Int = &I;
+
+struct MyStruct<'a> {
+   a: &'static Int,
+   b: B<'a>,
+}
+
+fn foo<'a, 'b>(m: &'a MyStruct<'b>) -> impl FnMut() + 'static {
+    let c = || drop(&m.a.0);
+    c
+}
+
+fn main() {
+    let t = 0;
+    let s = MyStruct { a: REF_I, b: B(&t) };
+    let _ = foo(&s);
+}