2229: Handle capturing a reference into a repr packed struct

RFC 1240 states that it is unsafe to capture references into a
packed-struct. This PR ensures that when a closure captures a precise
path, we aren't violating this safety constraint.

To acheive so we restrict the capture precision to the struct itself.

An interesting edge case:
```rust
struct Foo(String);

let foo: Foo;
let c = || {
    println!("{}", foo.0);
    let x = foo.0;
}
```

Given how closures get desugared today, foo.0 will be moved into the
closure, making the `println!`, safe. However this can be very subtle
and also will be unsafe if the closure gets inline.

Closes: rust-lang/project-rfc-2229#33

Author: arora-aman

compiler/rustc_typeck/src/check/upvar.rs

@@ -1053,6 +1053,52 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     }
 }
 
+/// Truncate the capture so that the place being borrowed is in accordance with RFC 1240,
+/// which states that it's unsafe to take a reference into a struct marked `repr(packed)`.
+fn restrict_repr_packed_field_ref_capture<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    place: &Place<'tcx>,
+) -> Place<'tcx> {
+    let pos = place.projections.iter().enumerate().position(|(i, p)| {
+        let ty = place.ty_before_projection(i);
+
+        // Return true for fields of packed structs, unless those fields have alignment 1.
+        match p.kind {
+            ProjectionKind::Field(..) => match ty.kind() {
+                ty::Adt(def, _) if def.repr.packed() => {
+                    match tcx.layout_raw(param_env.and(p.ty)) {
+                        Ok(layout) if layout.align.abi.bytes() == 1 => {
+                            // if the alignment is 1, the type can't be further
+                            // disaligned.
+                            debug!(
+                                "restrict_repr_packed_field_ref_capture: ({:?}) - align = 1",
+                                place
+                            );
+                            false
+                        }
+                        _ => {
+                            debug!("restrict_repr_packed_field_ref_capture: ({:?}) - true", place);
+                            true
+                        }
+                    }
+                }
+
+                _ => false,
+            },
+            _ => false,
+        }
+    });
+
+    let mut place = place.clone();
+
+    if let Some(pos) = pos {
+        place.projections.truncate(pos);
+    }
+
+    place
+}
+
 struct InferBorrowKind<'a, 'tcx> {
     fcx: &'a FnCtxt<'a, 'tcx>,
 
@@ -1391,8 +1437,15 @@ impl<'a, 'tcx> euv::Delegate<'tcx> for InferBorrowKind<'a, 'tcx> {
             place_with_id, diag_expr_id, bk
         );
 
+        let place = restrict_repr_packed_field_ref_capture(
+            self.fcx.tcx,
+            self.fcx.param_env,
+            &place_with_id.place,
+        );
+        let place_with_id = PlaceWithHirId { 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);
         }
 
         match bk {
@@ -1409,11 +1462,7 @@ impl<'a, 'tcx> euv::Delegate<'tcx> for InferBorrowKind<'a, 'tcx> {
     fn mutate(&mut self, assignee_place: &PlaceWithHirId<'tcx>, diag_expr_id: hir::HirId) {
         debug!("mutate(assignee_place={:?}, diag_expr_id={:?})", assignee_place, diag_expr_id);
 
-        if !self.capture_information.contains_key(&assignee_place.place) {
-            self.init_capture_info_for_place(assignee_place, diag_expr_id);
-        }
-
-        self.adjust_upvar_borrow_kind_for_mut(assignee_place, diag_expr_id);
+        self.borrow(assignee_place, diag_expr_id, ty::BorrowKind::MutBorrow);
     }
 }
 

src/test/ui/closures/2229_closure_analysis/diagnostics/repr_packed.rs

@@ -0,0 +1,36 @@
+// check-pass
+
+#![feature(capture_disjoint_fields)]
+//~^ WARNING: the feature `capture_disjoint_fields` is incomplete
+
+// Given how the closure desugaring is implemented (at least at the time of writing this test),
+// we don't need to truncate the captured path to a reference into a packed-struct if the field
+// being referenced will be moved into the closure, since it's safe to move out a field from a
+// packed-struct.
+//
+// However to avoid surprises for the user, or issues when the closure is
+// inlined we will truncate the capture to access just the struct regardless of if the field
+// might get moved into the closure.
+//
+// It is possible for someone to try writing the code that relies on the desugaring to access a ref
+// into a packed-struct without explicity using unsafe. Here we test that the compiler warns the
+// user that such an access is still unsafe.
+fn test_missing_unsafe_warning_on_repr_packed() {
+    #[repr(packed)]
+    struct Foo { x: String }
+
+    let foo = Foo { x: String::new() };
+
+    let c = || {
+        println!("{}", foo.x);
+        //~^ WARNING: borrow of packed field is unsafe and requires unsafe function or block
+        //~| WARNING: this was previously accepted by the compiler but is being phased out
+        let _z = foo.x;
+    };
+
+    c();
+}
+
+fn main() {
+    test_missing_unsafe_warning_on_repr_packed();
+}

src/test/ui/closures/2229_closure_analysis/diagnostics/repr_packed.stderr

@@ -0,0 +1,22 @@
+warning: the feature `capture_disjoint_fields` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/repr_packed.rs:3:12
+   |
+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: borrow of packed field is unsafe and requires unsafe function or block (error E0133)
+  --> $DIR/repr_packed.rs:25:24
+   |
+LL |         println!("{}", foo.x);
+   |                        ^^^^^
+   |
+   = note: `#[warn(safe_packed_borrows)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #46043 <https://github.com/rust-lang/rust/issues/46043>
+   = note: fields of packed structs might be misaligned: dereferencing a misaligned pointer or even just creating a misaligned reference is undefined behavior
+
+warning: 2 warnings emitted
+

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

@@ -0,0 +1,103 @@
+#![feature(capture_disjoint_fields)]
+//~^ WARNING: the feature `capture_disjoint_fields` is incomplete
+//~| `#[warn(incomplete_features)]` on by default
+//~| see issue #53488 <https://github.com/rust-lang/rust/issues/53488>
+
+#![feature(rustc_attrs)]
+
+// `u8` aligned at a byte and are unaffected by repr(packed).
+// Therefore we can precisely (and safely) capture references to both the fields.
+fn test_alignment_not_affected() {
+    #[repr(packed)]
+    struct Foo { x: u8, y: u8 }
+
+    let mut foo = Foo { x: 0, y: 0 };
+
+    let mut c = #[rustc_capture_analysis]
+    //~^ 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:
+        let z1: &u8 = &foo.x;
+        //~^ NOTE: Capturing foo[(0, 0)] -> ImmBorrow
+        //~| NOTE: Min Capture foo[(0, 0)] -> ImmBorrow
+        let z2: &mut u8 = &mut foo.y;
+        //~^ NOTE: Capturing foo[(1, 0)] -> MutBorrow
+        //~| NOTE: Min Capture foo[(1, 0)] -> MutBorrow
+
+        *z2 = 42;
+
+        println!("({}, {})", z1, z2);
+    };
+
+    c();
+}
+
+// `String`, `u16` are not aligned at a one byte boundry and are thus affected by repr(packed).
+//
+// Here we test that the closure doesn't capture a reference point to `foo.x` but
+// rather capture `foo` entirely.
+fn test_alignment_affected() {
+    #[repr(packed)]
+    struct Foo { x: String, y: u16 }
+
+    let mut foo = Foo { x: String::new(), y: 0 };
+
+    let mut c = #[rustc_capture_analysis]
+    //~^ 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:
+        let z1: &String = &foo.x;
+        let z2: &mut u16 = &mut foo.y;
+        //~^ NOTE: Capturing foo[] -> MutBorrow
+        //~| NOTE: Min Capture foo[] -> MutBorrow
+
+
+        *z2 = 42;
+
+        println!("({}, {})", z1, z2);
+    };
+
+    c();
+}
+
+// Given how the closure desugaring is implemented (at least at the time of writing this test),
+// we don't need to truncate the captured path to a reference into a packed-struct if the field
+// being referenced will be moved into the closure, since it's safe to move out a field from a
+// packed-struct.
+//
+// However to avoid surprises for the user, or issues when the closure is
+// inlined we will truncate the capture to access just the struct regardless of if the field
+// might get moved into the closure.
+fn test_truncation_when_ref_and_move() {
+    #[repr(packed)]
+    struct Foo { x: String }
+
+    let mut foo = Foo { x: String::new() };
+
+    let c = #[rustc_capture_analysis]
+    //~^ 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:
+        println!("{}", foo.x);
+        //~^ NOTE: Capturing foo[] -> ImmBorrow
+        //~| NOTE: Min Capture foo[] -> ByValue
+        //~| NOTE: foo[] used here
+        let _z = foo.x;
+        //~^ NOTE: Capturing foo[(0, 0)] -> ByValue
+        //~| NOTE: foo[] captured as ByValue here
+    };
+
+    c();
+}
+
+fn main() {
+    test_truncation_when_ref_and_move();
+    test_alignment_affected();
+    test_alignment_not_affected();
+}