2229: Reduce the size of closures with capture_disjoint_fields

One key observation while going over the closure size profile of rustc
was that we are disjointly capturing one or more fields starting at an
immutable reference.

Disjoint capture over immutable reference doesn't add too much value
because the fields can either be borrowed immutably or copied.

One possible edge case of the optimization is when a fields of a struct
have a longer lifetime than the structure, therefore we can't completely
get rid of all the accesses on top of sharef refs, only the rightmost
one. Here is a possible example:

```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`
}
```

Author: arora-aman

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 +1975,44 @@ 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));
+
+    let idx = place.projections.iter().rposition(|proj| ProjectionKind::Deref == proj.kind);
+
+    match idx {
+        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 +1,37 @@
+#![feature(capture_disjoint_fields)]
+//~^ WARNING: the feature `capture_disjoint_fields` is incomplete
+//~| NOTE: `#[warn(incomplete_features)]` on by default
+//~| NOTE: see issue #53488 <https://github.com/rust-lang/rust/issues/53488>
+
+#![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 +1,45 @@
+error[E0658]: attributes on expressions are experimental
+  --> $DIR/edge_case.rs:23:13
+   |
+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
+
+warning: the feature `capture_disjoint_fields` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/edge_case.rs:1:12
+   |
+LL | #![feature(capture_disjoint_fields)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #53488 <https://github.com/rust-lang/rust/issues/53488> for more information
+
+error: First Pass analysis includes:
+  --> $DIR/edge_case.rs:23:39
+   |
+LL |     let c = #[rustc_capture_analysis] || drop(&m.a.0);
+   |                                       ^^^^^^^^^^^^^^^
+   |
+note: Capturing m[Deref,(0, 0),Deref] -> ImmBorrow
+  --> $DIR/edge_case.rs:23:48
+   |
+LL |     let c = #[rustc_capture_analysis] || drop(&m.a.0);
+   |                                                ^^^^^
+
+error: Min Capture analysis includes:
+  --> $DIR/edge_case.rs:23:39
+   |
+LL |     let c = #[rustc_capture_analysis] || drop(&m.a.0);
+   |                                       ^^^^^^^^^^^^^^^
+   |
+note: Min Capture m[Deref,(0, 0),Deref] -> ImmBorrow
+  --> $DIR/edge_case.rs:23:48
+   |
+LL |     let c = #[rustc_capture_analysis] || drop(&m.a.0);
+   |                                                ^^^^^
+
+error: aborting due to 3 previous errors; 1 warning emitted
+
+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 +1,31 @@
+#![feature(capture_disjoint_fields)]
+//~^ WARNING: the feature `capture_disjoint_fields` is incomplete
+//~| NOTE: `#[warn(incomplete_features)]` on by default
+//~| NOTE: see issue #53488 <https://github.com/rust-lang/rust/issues/53488>
+
+// 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);
+}

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

@@ -0,0 +1,11 @@
+warning: the feature `capture_disjoint_fields` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/edge_case_run_pass.rs:1:12
+   |
+LL | #![feature(capture_disjoint_fields)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #53488 <https://github.com/rust-lang/rust/issues/53488> for more information
+
+warning: 1 warning emitted
+