Enforce C,packed, not just packed, on ULE types (unicode-org#5049)

Fixes unicode-org#5039



Caused by rust-lang/rust#125360. We were
assuming that `packed` meant `C, packed` already. This is an assumption
I've seen throughout the Rust ecosystem so there may be reasons to
revert.

Author: Manishearth

components/calendar/src/provider/chinese_based.rs

@@ -143,7 +143,7 @@ impl<'data> ChineseBasedCacheV1<'data> {
     derive(databake::Bake),
     databake(path = icu_calendar::provider),
 )]
-#[repr(packed)]
+#[repr(C, packed)]
 pub struct PackedChineseBasedYearInfo(pub u8, pub u8, pub u8);
 
 impl PackedChineseBasedYearInfo {

components/calendar/src/provider/islamic.rs

@@ -156,7 +156,7 @@ impl<'data> IslamicCacheV1<'data> {
     databake(path = icu_calendar::provider),
 )]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize))]
-#[repr(packed)]
+#[repr(C, packed)]
 pub struct PackedIslamicYearInfo(pub u8, pub u8);
 
 impl fmt::Debug for PackedIslamicYearInfo {

components/casemap/src/provider/exceptions_builder.rs

@@ -71,7 +71,7 @@ impl ExceptionHeader {
 /// In this struct the RESERVED bit is still allowed to be set, and it will produce a different
 /// exception header, but it will not have any other effects.
 #[derive(Copy, Clone, PartialEq, Eq, ULE)]
-#[repr(packed)]
+#[repr(C, packed)]
 pub struct ExceptionHeaderULE {
     slot_presence: SlotPresence,
     bits: ExceptionBitsULE,

components/properties/src/provider/bidi_data.rs

@@ -156,7 +156,7 @@ pub enum CheckedBidiPairedBracketType {
 #[doc(hidden)]
 /// needed for datagen but not intended for users
 #[derive(Copy, Clone, Hash, PartialEq, Eq, Debug)]
-#[repr(packed)]
+#[repr(C, packed)]
 pub struct MirroredPairedBracketDataULE([u8; 3]);
 
 // Safety (based on the safety checklist on the ULE trait):

utils/zerovec/derive/examples/derives.rs

@@ -6,7 +6,7 @@ use zerovec::ule::AsULE;
 use zerovec::ule::EncodeAsVarULE;
 use zerovec::*;
 
-#[repr(packed)]
+#[repr(C, packed)]
 #[derive(ule::ULE, Copy, Clone)]
 pub struct FooULE {
     a: u8,
@@ -40,7 +40,7 @@ impl AsULE for Foo {
     }
 }
 
-#[repr(packed)]
+#[repr(C, packed)]
 #[derive(ule::VarULE)]
 pub struct RelationULE {
     /// This maps to (AndOr, Polarity, Operand),

utils/zerovec/derive/src/make_ule.rs

@@ -83,7 +83,7 @@ fn make_ule_enum_impl(
     attrs: ZeroVecAttrs,
 ) -> TokenStream2 {
     // We could support more int reprs in the future if needed
-    if !utils::has_valid_repr(&input.attrs, |r| r == "u8") {
+    if !utils::ReprInfo::compute(&input.attrs).u8 {
         return Error::new(
             input.span(),
             "#[make_ule] can only be applied to #[repr(u8)] enums",

utils/zerovec/derive/src/ule.rs

@@ -10,10 +10,10 @@ use syn::spanned::Spanned;
 use syn::{Data, DeriveInput, Error};
 
 pub fn derive_impl(input: &DeriveInput) -> TokenStream2 {
-    if !utils::has_valid_repr(&input.attrs, |r| r == "packed" || r == "transparent") {
+    if !utils::ReprInfo::compute(&input.attrs).cpacked_or_transparent() {
         return Error::new(
             input.span(),
-            "derive(ULE) must be applied to a #[repr(packed)] or #[repr(transparent)] type",
+            "derive(ULE) must be applied to a #[repr(C, packed)] or #[repr(transparent)] type",
         )
         .to_compile_error();
     }

utils/zerovec/derive/src/utils.rs

@@ -11,14 +11,38 @@ use syn::punctuated::Punctuated;
 use syn::spanned::Spanned;
 use syn::{Attribute, Error, Field, Fields, Ident, Index, Result, Token};
 
-// Check that there are repr attributes satisfying the given predicate
-pub fn has_valid_repr(attrs: &[Attribute], predicate: impl Fn(&Ident) -> bool + Copy) -> bool {
-    attrs.iter().filter(|a| a.path().is_ident("repr")).any(|a| {
-        a.parse_args::<IdentListAttribute>()
-            .ok()
-            .and_then(|s| s.idents.iter().find(|s| predicate(s)).map(|_| ()))
-            .is_some()
-    })
+#[derive(Default)]
+pub struct ReprInfo {
+    pub c: bool,
+    pub transparent: bool,
+    pub u8: bool,
+    pub packed: bool,
+}
+
+impl ReprInfo {
+    pub fn compute(attrs: &[Attribute]) -> Self {
+        let mut info = ReprInfo::default();
+        for attr in attrs.iter().filter(|a| a.path().is_ident("repr")) {
+            if let Ok(pieces) = attr.parse_args::<IdentListAttribute>() {
+                for piece in pieces.idents.iter() {
+                    if piece == "C" || piece == "c" {
+                        info.c = true;
+                    } else if piece == "transparent" {
+                        info.transparent = true;
+                    } else if piece == "packed" {
+                        info.packed = true;
+                    } else if piece == "u8" {
+                        info.u8 = true;
+                    }
+                }
+            }
+        }
+        info
+    }
+
+    pub fn cpacked_or_transparent(self) -> bool {
+        (self.c && self.packed) || self.transparent
+    }
 }
 
 // An attribute that is a list of idents
@@ -60,7 +84,7 @@ pub fn repr_for(f: &Fields) -> TokenStream2 {
     if f.len() == 1 {
         quote!(transparent)
     } else {
-        quote!(packed)
+        quote!(C, packed)
     }
 }
 

utils/zerovec/derive/src/varule.rs

@@ -15,10 +15,10 @@ pub fn derive_impl(
     input: &DeriveInput,
     custom_varule_validator: Option<TokenStream2>,
 ) -> TokenStream2 {
-    if !utils::has_valid_repr(&input.attrs, |r| r == "packed" || r == "transparent") {
+    if !utils::ReprInfo::compute(&input.attrs).cpacked_or_transparent() {
         return Error::new(
             input.span(),
-            "derive(VarULE) must be applied to a #[repr(packed)] or #[repr(transparent)] type",
+            "derive(VarULE) must be applied to a #[repr(C, packed)] or #[repr(transparent)] type",
         )
         .to_compile_error();
     }

utils/zerovec/src/flexzerovec/slice.rs

@@ -13,7 +13,7 @@ use core::ops::Range;
 const USIZE_WIDTH: usize = mem::size_of::<usize>();
 
 /// A zero-copy "slice" that efficiently represents `[usize]`.
-#[repr(packed)]
+#[repr(C, packed)]
 pub struct FlexZeroSlice {
     // Hard Invariant: 1 <= width <= USIZE_WIDTH (which is target_pointer_width)
     // Soft Invariant: width == the width of the largest element

utils/zerovec/src/ule/mod.rs

@@ -61,7 +61,7 @@ use core::{mem, slice};
 /// 6. Acknowledge the following note about the equality invariant.
 ///
 /// If the ULE type is a struct only containing other ULE types (or other types which satisfy invariants 1 and 2,
-/// like `[u8; N]`), invariants 1 and 2 can be achieved via `#[repr(packed)]` or `#[repr(transparent)]`.
+/// like `[u8; N]`), invariants 1 and 2 can be achieved via `#[repr(C, packed)]` or `#[repr(transparent)]`.
 ///
 /// # Equality invariant
 ///
@@ -271,7 +271,7 @@ where
 /// 7. Acknowledge the following note about the equality invariant.
 ///
 /// If the ULE type is a struct only containing other ULE/VarULE types (or other types which satisfy invariants 1 and 2,
-/// like `[u8; N]`), invariants 1 and 2 can be achieved via `#[repr(packed)]` or `#[repr(transparent)]`.
+/// like `[u8; N]`), invariants 1 and 2 can be achieved via `#[repr(C, packed)]` or `#[repr(transparent)]`.
 ///
 /// # Equality invariant
 ///

utils/zerovec/src/ule/option.rs

@@ -28,7 +28,7 @@ use core::mem::{self, MaybeUninit};
 // Invariants:
 // The MaybeUninit is zeroed when None (bool = false),
 // and is valid when Some (bool = true)
-#[repr(packed)]
+#[repr(C, packed)]
 pub struct OptionULE<U>(bool, MaybeUninit<U>);
 
 impl<U: Copy> OptionULE<U> {
@@ -62,11 +62,11 @@ impl<U: Copy + core::fmt::Debug> core::fmt::Debug for OptionULE<U> {
 
 // Safety (based on the safety checklist on the ULE trait):
 //  1. OptionULE does not include any uninitialized or padding bytes.
-//     (achieved by `#[repr(packed)]` on a struct containing only ULE fields,
+//     (achieved by `#[repr(C, packed)]` on a struct containing only ULE fields,
 //     in the context of this impl. The MaybeUninit is valid for all byte sequences, and we only generate
 ///    zeroed or valid-T byte sequences to fill it)
 //  2. OptionULE is aligned to 1 byte.
-//     (achieved by `#[repr(packed)]` on a struct containing only ULE fields, in the context of this impl)
+//     (achieved by `#[repr(C, packed)]` on a struct containing only ULE fields, in the context of this impl)
 //  3. The impl of validate_byte_slice() returns an error if any byte is not valid.
 //  4. The impl of validate_byte_slice() returns an error if there are extra bytes.
 //  5. The other ULE methods use the default impl.

utils/zerovec/src/ule/tuple.rs

@@ -30,15 +30,15 @@ use core::mem;
 macro_rules! tuple_ule {
     ($name:ident, $len:literal, [ $($t:ident $i:tt),+ ]) => {
         #[doc = concat!("ULE type for tuples with ", $len, " elements.")]
-        #[repr(packed)]
+        #[repr(C, packed)]
         #[allow(clippy::exhaustive_structs)] // stable
         pub struct $name<$($t),+>($(pub $t),+);
 
         // Safety (based on the safety checklist on the ULE trait):
         //  1. TupleULE does not include any uninitialized or padding bytes.
-        //     (achieved by `#[repr(packed)]` on a struct containing only ULE fields)
+        //     (achieved by `#[repr(C, packed)]` on a struct containing only ULE fields)
         //  2. TupleULE is aligned to 1 byte.
-        //     (achieved by `#[repr(packed)]` on a struct containing only ULE fields)
+        //     (achieved by `#[repr(C, packed)]` on a struct containing only ULE fields)
         //  3. The impl of validate_byte_slice() returns an error if any byte is not valid.
         //  4. The impl of validate_byte_slice() returns an error if there are extra bytes.
         //  5. The other ULE methods use the default impl.