Auto merge of #100064 - RalfJung:disaligned, r=petrochenkov

fix is_disaligned logic for nested packed structs

#83605 broke the `is_disaligned` logic by bailing out of the loop in `is_within_packed` early. This PR fixes that problem and adds suitable tests.

Fixes #99838

Author: bors

Diff for: compiler/rustc_const_eval/src/util/alignment.rs

@@ -48,20 +48,16 @@ fn is_within_packed<'tcx, L>(
 where
     L: HasLocalDecls<'tcx>,
 {
-    for (place_base, elem) in place.iter_projections().rev() {
-        match elem {
-            // encountered a Deref, which is ABI-aligned
-            ProjectionElem::Deref => break,
-            ProjectionElem::Field(..) => {
-                let ty = place_base.ty(local_decls, tcx).ty;
-                match ty.kind() {
-                    ty::Adt(def, _) => return def.repr().pack,
-                    _ => {}
-                }
-            }
-            _ => {}
-        }
-    }
-
-    None
+    place
+        .iter_projections()
+        .rev()
+        // Stop at `Deref`; standard ABI alignment applies there.
+        .take_while(|(_base, elem)| !matches!(elem, ProjectionElem::Deref))
+        // Consider the packed alignments at play here...
+        .filter_map(|(base, _elem)| {
+            base.ty(local_decls, tcx).ty.ty_adt_def().and_then(|adt| adt.repr().pack)
+        })
+        // ... and compute their minimum.
+        // The overall smallest alignment is what matters.
+        .min()
 }

Diff for: src/test/ui/issues/issue-99838.rs

@@ -0,0 +1,40 @@
+// run-pass
+#![feature(bench_black_box)]
+use std::hint;
+
+struct U16(u16);
+
+impl Drop for U16 {
+    fn drop(&mut self) {
+        // Prevent LLVM from optimizing away our alignment check.
+        assert!(hint::black_box(self as *mut U16 as usize) % 2 == 0);
+    }
+}
+
+struct HasDrop;
+
+impl Drop for HasDrop {
+    fn drop(&mut self) {}
+}
+
+struct Wrapper {
+    _a: U16,
+    b: HasDrop,
+}
+
+#[repr(packed)]
+struct Misalign(u8, Wrapper);
+
+fn main() {
+    let m = Misalign(
+        0,
+        Wrapper {
+            _a: U16(10),
+            b: HasDrop,
+        },
+    );
+    // Put it somewhere definitely even (so the `a` field is definitely at an odd address).
+    let m: ([u16; 0], Misalign) = ([], m);
+    // Move out one field, so we run custom per-field drop logic below.
+    let _x = m.1.1.b;
+}

Diff for: src/test/ui/lint/unaligned_references.rs

@@ -47,4 +47,57 @@ fn main() {
         let _ = &packed2.y; // ok, has align 2 in packed(2) struct
         let _ = &packed2.z; // ok, has align 1
     }
+
+    unsafe {
+        struct U16(u16);
+
+        impl Drop for U16 {
+            fn drop(&mut self) {
+                println!("{:p}", self);
+            }
+        }
+
+        struct HasDrop;
+
+        impl Drop for HasDrop {
+            fn drop(&mut self) {}
+        }
+
+        #[allow(unused)]
+        struct Wrapper {
+            a: U16,
+            b: HasDrop,
+        }
+        #[allow(unused)]
+        #[repr(packed(2))]
+        struct Wrapper2 {
+            a: U16,
+            b: HasDrop,
+        }
+
+        // An outer struct with more restrictive packing than the inner struct -- make sure we
+        // notice that!
+        #[repr(packed)]
+        struct Misalign<T>(u8, T);
+
+        let m1 = Misalign(
+            0,
+            Wrapper {
+                a: U16(10),
+                b: HasDrop,
+            },
+        );
+        let _ref = &m1.1.a; //~ ERROR reference to packed field
+        //~^ previously accepted
+
+        let m2 = Misalign(
+            0,
+            Wrapper2 {
+                a: U16(10),
+                b: HasDrop,
+            },
+        );
+        let _ref = &m2.1.a; //~ ERROR reference to packed field
+        //~^ previously accepted
+    }
 }

Diff for: src/test/ui/lint/unaligned_references.stderr

@@ -80,7 +80,29 @@ LL |         let _ = &packed2.x;
    = note: fields of packed structs are not properly aligned, and creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
    = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
 
-error: aborting due to 7 previous errors
+error: reference to packed field is unaligned
+  --> $DIR/unaligned_references.rs:90:20
+   |
+LL |         let _ref = &m1.1.a;
+   |                    ^^^^^^^
+   |
+   = 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 #82523 <https://github.com/rust-lang/rust/issues/82523>
+   = note: fields of packed structs are not properly aligned, and creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
+   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
+
+error: reference to packed field is unaligned
+  --> $DIR/unaligned_references.rs:100:20
+   |
+LL |         let _ref = &m2.1.a;
+   |                    ^^^^^^^
+   |
+   = 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 #82523 <https://github.com/rust-lang/rust/issues/82523>
+   = note: fields of packed structs are not properly aligned, and creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
+   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
+
+error: aborting due to 9 previous errors
 
 Future incompatibility report: Future breakage diagnostic:
 error: reference to packed field is unaligned
@@ -201,3 +223,37 @@ LL | #![deny(unaligned_references)]
    = note: fields of packed structs are not properly aligned, and creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
    = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
 
+Future breakage diagnostic:
+error: reference to packed field is unaligned
+  --> $DIR/unaligned_references.rs:90:20
+   |
+LL |         let _ref = &m1.1.a;
+   |                    ^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/unaligned_references.rs:1:9
+   |
+LL | #![deny(unaligned_references)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+   = 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 #82523 <https://github.com/rust-lang/rust/issues/82523>
+   = note: fields of packed structs are not properly aligned, and creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
+   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
+
+Future breakage diagnostic:
+error: reference to packed field is unaligned
+  --> $DIR/unaligned_references.rs:100:20
+   |
+LL |         let _ref = &m2.1.a;
+   |                    ^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/unaligned_references.rs:1:9
+   |
+LL | #![deny(unaligned_references)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+   = 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 #82523 <https://github.com/rust-lang/rust/issues/82523>
+   = note: fields of packed structs are not properly aligned, and creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
+   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
+