Auto merge of #86701 - sexxi-goose:optimization, r=nikomatsakis

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`
}
```

r? `@nikomatsakis`

compiler/rustc_typeck/src/check/upvar.rs

@@ -1618,11 +1618,17 @@ 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(
@@ -1645,6 +1651,8 @@ 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) {
@@ -1980,6 +1988,48 @@ 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.
 ///