Initial impl of `repr_packed_without_abi` (#13398)

Fixes #13375

I've added the lint next to the other attribute-related ones. Not sure
if this is the correct place, since while we are looking after the
`packed`-attribute (there is nothing we can do about types defined
elsewhere), we are more concerned about the type's representation set by
the attribute (instead of "duplicate attributes" and such).

The lint simply looks at the attributes themselves without concern for
the item-kind, since items where `repr` is not allowed end up in a
compile-error anyway.

I'm somewhat concerned about the level of noise this lint would cause
if/when it goes into stable, although it does _not_ come up in
`lintcheck`.

```
changelog: [`repr_packed_without_abi`]: Initial implementation
```

13 files changed, 187 insertions, 25 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index cc966972939..529ab6c20d6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5969,6 +5969,7 @@ Released 2018-09-13
 [`repeat_once`]: https://rust-lang.github.io/rust-clippy/master/index.html#repeat_once
 [`repeat_vec_with_capacity`]: https://rust-lang.github.io/rust-clippy/master/index.html#repeat_vec_with_capacity
 [`replace_consts`]: https://rust-lang.github.io/rust-clippy/master/index.html#replace_consts
+[`repr_packed_without_abi`]: https://rust-lang.github.io/rust-clippy/master/index.html#repr_packed_without_abi
 [`reserve_after_initialization`]: https://rust-lang.github.io/rust-clippy/master/index.html#reserve_after_initialization
 [`rest_pat_in_fully_bound_structs`]: https://rust-lang.github.io/rust-clippy/master/index.html#rest_pat_in_fully_bound_structs
 [`result_expect_used`]: https://rust-lang.github.io/rust-clippy/master/index.html#result_expect_used
diff --git a/clippy_lints/src/attrs/mod.rs b/clippy_lints/src/attrs/mod.rs
index a9766597d50..92efd1a4ddc 100644
--- a/clippy_lints/src/attrs/mod.rs
+++ b/clippy_lints/src/attrs/mod.rs
@@ -7,6 +7,7 @@ mod duplicated_attributes;
 mod inline_always;
 mod mixed_attributes_style;
 mod non_minimal_cfg;
+mod repr_attributes;
 mod should_panic_without_expect;
 mod unnecessary_clippy_cfg;
 mod useless_attribute;
@@ -274,6 +275,44 @@ declare_clippy_lint! {
 
 declare_clippy_lint! {
     /// ### What it does
+    /// Checks for items with `#[repr(packed)]`-attribute without ABI qualification
+    ///
+    /// ### Why is this bad?
+    /// Without qualification, `repr(packed)` implies `repr(Rust)`. The Rust-ABI is inherently unstable.
+    /// While this is fine as long as the type is accessed correctly within Rust-code, most uses
+    /// of `#[repr(packed)]` involve FFI and/or data structures specified by network-protocols or
+    /// other external specifications. In such situations, the unstable Rust-ABI implied in
+    /// `#[repr(packed)]` may lead to future bugs should the Rust-ABI change.
+    ///
+    /// In case you are relying on a well defined and stable memory layout, qualify the type's
+    /// representation using the `C`-ABI. Otherwise, if the type in question is only ever
+    /// accessed from Rust-code according to Rust's rules, use the `Rust`-ABI explicitly.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// #[repr(packed)]
+    /// struct NetworkPacketHeader {
+    ///     header_length: u8,
+    ///     header_version: u16
+    /// }
+    /// ```
+    ///
+    /// Use instead:
+    /// ```no_run
+    /// #[repr(C, packed)]
+    /// struct NetworkPacketHeader {
+    ///     header_length: u8,
+    ///     header_version: u16
+    /// }
+    /// ```
+    #[clippy::version = "1.84.0"]
+    pub REPR_PACKED_WITHOUT_ABI,
+    suspicious,
+    "ensures that `repr(packed)` always comes with a qualified ABI"
+}
+
+declare_clippy_lint! {
+    /// ### What it does
     /// Checks for `any` and `all` combinators in `cfg` with only one condition.
     ///
     /// ### Why is this bad?
@@ -415,6 +454,7 @@ pub struct Attributes {
 
 impl_lint_pass!(Attributes => [
     INLINE_ALWAYS,
+    REPR_PACKED_WITHOUT_ABI,
 ]);
 
 impl Attributes {
@@ -431,6 +471,7 @@ impl<'tcx> LateLintPass<'tcx> for Attributes {
         if is_relevant_item(cx, item) {
             inline_always::check(cx, item.span, item.ident.name, attrs);
         }
+        repr_attributes::check(cx, item.span, attrs, &self.msrv);
     }
 
     fn check_impl_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx ImplItem<'_>) {
diff --git a/clippy_lints/src/attrs/repr_attributes.rs b/clippy_lints/src/attrs/repr_attributes.rs
new file mode 100644
index 00000000000..e55a85b6267
--- /dev/null
+++ b/clippy_lints/src/attrs/repr_attributes.rs
@@ -0,0 +1,43 @@
+use rustc_ast::Attribute;
+use rustc_lint::LateContext;
+use rustc_span::{Span, sym};
+
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::msrvs;
+
+use super::REPR_PACKED_WITHOUT_ABI;
+
+pub(super) fn check(cx: &LateContext<'_>, item_span: Span, attrs: &[Attribute], msrv: &msrvs::Msrv) {
+    if msrv.meets(msrvs::REPR_RUST) {
+        check_packed(cx, item_span, attrs);
+    }
+}
+
+fn check_packed(cx: &LateContext<'_>, item_span: Span, attrs: &[Attribute]) {
+    if let Some(items) = attrs.iter().find_map(|attr| {
+        if attr.ident().is_some_and(|ident| matches!(ident.name, sym::repr)) {
+            attr.meta_item_list()
+        } else {
+            None
+        }
+    }) && let Some(packed) = items
+        .iter()
+        .find(|item| item.ident().is_some_and(|ident| matches!(ident.name, sym::packed)))
+        && !items.iter().any(|item| {
+            item.ident()
+                .is_some_and(|ident| matches!(ident.name, sym::C | sym::Rust))
+        })
+    {
+        span_lint_and_then(
+            cx,
+            REPR_PACKED_WITHOUT_ABI,
+            item_span,
+            "item uses `packed` representation without ABI-qualification",
+            |diag| {
+                diag.warn("unqualified `#[repr(packed)]` defaults to `#[repr(Rust, packed)]`, which has no stable ABI")
+                    .help("qualify the desired ABI explicity via `#[repr(C, packed)]` or `#[repr(Rust, packed)]`")
+                    .span_label(packed.span(), "`packed` representation set here");
+            },
+        );
+    }
+}
diff --git a/clippy_lints/src/declared_lints.rs b/clippy_lints/src/declared_lints.rs
index c3501f6c5d7..7451fb909ef 100644
--- a/clippy_lints/src/declared_lints.rs
+++ b/clippy_lints/src/declared_lints.rs
@@ -55,6 +55,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
     crate::attrs::INLINE_ALWAYS_INFO,
     crate::attrs::MIXED_ATTRIBUTES_STYLE_INFO,
     crate::attrs::NON_MINIMAL_CFG_INFO,
+    crate::attrs::REPR_PACKED_WITHOUT_ABI_INFO,
     crate::attrs::SHOULD_PANIC_WITHOUT_EXPECT_INFO,
     crate::attrs::UNNECESSARY_CLIPPY_CFG_INFO,
     crate::attrs::USELESS_ATTRIBUTE_INFO,
diff --git a/clippy_utils/src/msrvs.rs b/clippy_utils/src/msrvs.rs
index f0381760709..35f552c15a3 100644
--- a/clippy_utils/src/msrvs.rs
+++ b/clippy_utils/src/msrvs.rs
@@ -24,6 +24,7 @@ msrv_aliases! {
     1,80,0 { BOX_INTO_ITER }
     1,77,0 { C_STR_LITERALS }
     1,76,0 { PTR_FROM_REF, OPTION_RESULT_INSPECT }
+    1,74,0 { REPR_RUST }
     1,73,0 { MANUAL_DIV_CEIL }
     1,71,0 { TUPLE_ARRAY_CONVERSIONS, BUILD_HASHER_HASH_ONE }
     1,70,0 { OPTION_RESULT_IS_VARIANT_AND, BINARY_HEAP_RETAIN }
diff --git a/tests/ui/default_union_representation.rs b/tests/ui/default_union_representation.rs
index 41308b077ba..ba63cde2fa9 100644
--- a/tests/ui/default_union_representation.rs
+++ b/tests/ui/default_union_representation.rs
@@ -1,5 +1,6 @@
 #![feature(transparent_unions)]
 #![warn(clippy::default_union_representation)]
+#![allow(clippy::repr_packed_without_abi)]
 
 union NoAttribute {
     //~^ ERROR: this union has the default representation
diff --git a/tests/ui/default_union_representation.stderr b/tests/ui/default_union_representation.stderr
index c7ef70a0b8e..d558a3e8de1 100644
--- a/tests/ui/default_union_representation.stderr
+++ b/tests/ui/default_union_representation.stderr
@@ -1,5 +1,5 @@
 error: this union has the default representation
-  --> tests/ui/default_union_representation.rs:4:1
+  --> tests/ui/default_union_representation.rs:5:1
    |
 LL | / union NoAttribute {
 LL | |
@@ -13,7 +13,7 @@ LL | | }
    = help: to override `-D warnings` add `#[allow(clippy::default_union_representation)]`
 
 error: this union has the default representation
-  --> tests/ui/default_union_representation.rs:17:1
+  --> tests/ui/default_union_representation.rs:18:1
    |
 LL | / union ReprPacked {
 LL | |
@@ -25,7 +25,7 @@ LL | | }
    = help: consider annotating `ReprPacked` with `#[repr(C)]` to explicitly specify memory layout
 
 error: this union has the default representation
-  --> tests/ui/default_union_representation.rs:36:1
+  --> tests/ui/default_union_representation.rs:37:1
    |
 LL | / union ReprAlign {
 LL | |
@@ -37,7 +37,7 @@ LL | | }
    = help: consider annotating `ReprAlign` with `#[repr(C)]` to explicitly specify memory layout
 
 error: this union has the default representation
-  --> tests/ui/default_union_representation.rs:57:1
+  --> tests/ui/default_union_representation.rs:58:1
    |
 LL | / union ZSTAndTwoFields {
 LL | |
diff --git a/tests/ui/derive.rs b/tests/ui/derive.rs
index 3647b242505..dabc06bd7d1 100644
--- a/tests/ui/derive.rs
+++ b/tests/ui/derive.rs
@@ -2,6 +2,7 @@
     clippy::non_canonical_clone_impl,
     clippy::non_canonical_partial_ord_impl,
     clippy::needless_lifetimes,
+    clippy::repr_packed_without_abi,
     dead_code
 )]
 #![warn(clippy::expl_impl_clone_on_copy)]
diff --git a/tests/ui/derive.stderr b/tests/ui/derive.stderr
index c072a9a6277..0eb4b3c1ada 100644
--- a/tests/ui/derive.stderr
+++ b/tests/ui/derive.stderr
@@ -1,5 +1,5 @@
 error: you are implementing `Clone` explicitly on a `Copy` type
-  --> tests/ui/derive.rs:12:1
+  --> tests/ui/derive.rs:13:1
    |
 LL | / impl Clone for Qux {
 LL | |
@@ -10,7 +10,7 @@ LL | | }
    | |_^
    |
 note: consider deriving `Clone` or removing `Copy`
-  --> tests/ui/derive.rs:12:1
+  --> tests/ui/derive.rs:13:1
    |
 LL | / impl Clone for Qux {
 LL | |
@@ -23,7 +23,7 @@ LL | | }
    = help: to override `-D warnings` add `#[allow(clippy::expl_impl_clone_on_copy)]`
 
 error: you are implementing `Clone` explicitly on a `Copy` type
-  --> tests/ui/derive.rs:37:1
+  --> tests/ui/derive.rs:38:1
    |
 LL | / impl<'a> Clone for Lt<'a> {
 LL | |
@@ -34,7 +34,7 @@ LL | | }
    | |_^
    |
 note: consider deriving `Clone` or removing `Copy`
-  --> tests/ui/derive.rs:37:1
+  --> tests/ui/derive.rs:38:1
    |
 LL | / impl<'a> Clone for Lt<'a> {
 LL | |
@@ -45,7 +45,7 @@ LL | | }
    | |_^
 
 error: you are implementing `Clone` explicitly on a `Copy` type
-  --> tests/ui/derive.rs:49:1
+  --> tests/ui/derive.rs:50:1
    |
 LL | / impl Clone for BigArray {
 LL | |
@@ -56,7 +56,7 @@ LL | | }
    | |_^
    |
 note: consider deriving `Clone` or removing `Copy`
-  --> tests/ui/derive.rs:49:1
+  --> tests/ui/derive.rs:50:1
    |
 LL | / impl Clone for BigArray {
 LL | |
@@ -67,7 +67,7 @@ LL | | }
    | |_^
 
 error: you are implementing `Clone` explicitly on a `Copy` type
-  --> tests/ui/derive.rs:61:1
+  --> tests/ui/derive.rs:62:1
    |
 LL | / impl Clone for FnPtr {
 LL | |
@@ -78,7 +78,7 @@ LL | | }
    | |_^
    |
 note: consider deriving `Clone` or removing `Copy`
-  --> tests/ui/derive.rs:61:1
+  --> tests/ui/derive.rs:62:1
    |
 LL | / impl Clone for FnPtr {
 LL | |
@@ -89,7 +89,7 @@ LL | | }
    | |_^
 
 error: you are implementing `Clone` explicitly on a `Copy` type
-  --> tests/ui/derive.rs:82:1
+  --> tests/ui/derive.rs:83:1
    |
 LL | / impl<T: Clone> Clone for Generic2<T> {
 LL | |
@@ -100,7 +100,7 @@ LL | | }
    | |_^
    |
 note: consider deriving `Clone` or removing `Copy`
-  --> tests/ui/derive.rs:82:1
+  --> tests/ui/derive.rs:83:1
    |
 LL | / impl<T: Clone> Clone for Generic2<T> {
 LL | |
diff --git a/tests/ui/repr_packed_without_abi.rs b/tests/ui/repr_packed_without_abi.rs
new file mode 100644
index 00000000000..16b5ededee9
--- /dev/null
+++ b/tests/ui/repr_packed_without_abi.rs
@@ -0,0 +1,37 @@
+#![deny(clippy::repr_packed_without_abi)]
+
+#[repr(packed)]
+struct NetworkPacketHeader {
+    header_length: u8,
+    header_version: u16,
+}
+
+#[repr(packed)]
+union Foo {
+    a: u8,
+    b: u16,
+}
+
+#[repr(C, packed)]
+struct NoLintCNetworkPacketHeader {
+    header_length: u8,
+    header_version: u16,
+}
+
+#[repr(Rust, packed)]
+struct NoLintRustNetworkPacketHeader {
+    header_length: u8,
+    header_version: u16,
+}
+
+#[repr(packed, C)]
+union NotLintCFoo {
+    a: u8,
+    b: u16,
+}
+
+#[repr(packed, Rust)]
+union NotLintRustFoo {
+    a: u8,
+    b: u16,
+}
diff --git a/tests/ui/repr_packed_without_abi.stderr b/tests/ui/repr_packed_without_abi.stderr
new file mode 100644
index 00000000000..4f7acd00db3
--- /dev/null
+++ b/tests/ui/repr_packed_without_abi.stderr
@@ -0,0 +1,35 @@
+error: item uses `packed` representation without ABI-qualification
+  --> tests/ui/repr_packed_without_abi.rs:4:1
+   |
+LL |   #[repr(packed)]
+   |          ------ `packed` representation set here
+LL | / struct NetworkPacketHeader {
+LL | |     header_length: u8,
+LL | |     header_version: u16,
+LL | | }
+   | |_^
+   |
+   = warning: unqualified `#[repr(packed)]` defaults to `#[repr(Rust, packed)]`, which has no stable ABI
+   = help: qualify the desired ABI explicity via `#[repr(C, packed)]` or `#[repr(Rust, packed)]`
+note: the lint level is defined here
+  --> tests/ui/repr_packed_without_abi.rs:1:9
+   |
+LL | #![deny(clippy::repr_packed_without_abi)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: item uses `packed` representation without ABI-qualification
+  --> tests/ui/repr_packed_without_abi.rs:10:1
+   |
+LL |   #[repr(packed)]
+   |          ------ `packed` representation set here
+LL | / union Foo {
+LL | |     a: u8,
+LL | |     b: u16,
+LL | | }
+   | |_^
+   |
+   = warning: unqualified `#[repr(packed)]` defaults to `#[repr(Rust, packed)]`, which has no stable ABI
+   = help: qualify the desired ABI explicity via `#[repr(C, packed)]` or `#[repr(Rust, packed)]`
+
+error: aborting due to 2 previous errors
+
diff --git a/tests/ui/trailing_empty_array.rs b/tests/ui/trailing_empty_array.rs
index 3d06c262168..309a5920dfd 100644
--- a/tests/ui/trailing_empty_array.rs
+++ b/tests/ui/trailing_empty_array.rs
@@ -1,4 +1,5 @@
 #![warn(clippy::trailing_empty_array)]
+#![allow(clippy::repr_packed_without_abi)]
 
 // Do lint:
 
diff --git a/tests/ui/trailing_empty_array.stderr b/tests/ui/trailing_empty_array.stderr
index 756381478f2..7ebff372cf7 100644
--- a/tests/ui/trailing_empty_array.stderr
+++ b/tests/ui/trailing_empty_array.stderr
@@ -1,5 +1,5 @@
 error: trailing zero-sized array in a struct which is not marked with a `repr` attribute
-  --> tests/ui/trailing_empty_array.rs:5:1
+  --> tests/ui/trailing_empty_array.rs:6:1
    |
 LL | / struct RarelyUseful {
 LL | |
@@ -13,7 +13,7 @@ LL | | }
    = help: to override `-D warnings` add `#[allow(clippy::trailing_empty_array)]`
 
 error: trailing zero-sized array in a struct which is not marked with a `repr` attribute
-  --> tests/ui/trailing_empty_array.rs:11:1
+  --> tests/ui/trailing_empty_array.rs:12:1
    |
 LL | / struct OnlyField {
 LL | |
@@ -24,7 +24,7 @@ LL | | }
    = help: consider annotating `OnlyField` with `#[repr(C)]` or another `repr` attribute
 
 error: trailing zero-sized array in a struct which is not marked with a `repr` attribute
-  --> tests/ui/trailing_empty_array.rs:16:1
+  --> tests/ui/trailing_empty_array.rs:17:1
    |
 LL | / struct GenericArrayType<T> {
 LL | |
@@ -36,7 +36,7 @@ LL | | }
    = help: consider annotating `GenericArrayType` with `#[repr(C)]` or another `repr` attribute
 
 error: trailing zero-sized array in a struct which is not marked with a `repr` attribute
-  --> tests/ui/trailing_empty_array.rs:23:1
+  --> tests/ui/trailing_empty_array.rs:24:1
    |
 LL | / struct OnlyAnotherAttribute {
 LL | |
@@ -48,7 +48,7 @@ LL | | }
    = help: consider annotating `OnlyAnotherAttribute` with `#[repr(C)]` or another `repr` attribute
 
 error: trailing zero-sized array in a struct which is not marked with a `repr` attribute
-  --> tests/ui/trailing_empty_array.rs:30:1
+  --> tests/ui/trailing_empty_array.rs:31:1
    |
 LL | / struct OnlyADeriveAttribute {
 LL | |
@@ -60,7 +60,7 @@ LL | | }
    = help: consider annotating `OnlyADeriveAttribute` with `#[repr(C)]` or another `repr` attribute
 
 error: trailing zero-sized array in a struct which is not marked with a `repr` attribute
-  --> tests/ui/trailing_empty_array.rs:37:1
+  --> tests/ui/trailing_empty_array.rs:38:1
    |
 LL | / struct ZeroSizedWithConst {
 LL | |
@@ -72,7 +72,7 @@ LL | | }
    = help: consider annotating `ZeroSizedWithConst` with `#[repr(C)]` or another `repr` attribute
 
 error: trailing zero-sized array in a struct which is not marked with a `repr` attribute
-  --> tests/ui/trailing_empty_array.rs:47:1
+  --> tests/ui/trailing_empty_array.rs:48:1
    |
 LL | / struct ZeroSizedWithConstFunction {
 LL | |
@@ -84,7 +84,7 @@ LL | | }
    = help: consider annotating `ZeroSizedWithConstFunction` with `#[repr(C)]` or another `repr` attribute
 
 error: trailing zero-sized array in a struct which is not marked with a `repr` attribute
-  --> tests/ui/trailing_empty_array.rs:56:1
+  --> tests/ui/trailing_empty_array.rs:57:1
    |
 LL | / struct ZeroSizedWithConstFunction2 {
 LL | |
@@ -96,7 +96,7 @@ LL | | }
    = help: consider annotating `ZeroSizedWithConstFunction2` with `#[repr(C)]` or another `repr` attribute
 
 error: trailing zero-sized array in a struct which is not marked with a `repr` attribute
-  --> tests/ui/trailing_empty_array.rs:62:1
+  --> tests/ui/trailing_empty_array.rs:63:1
    |
 LL | struct ZeroSizedArrayWrapper([usize; 0]);
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -104,7 +104,7 @@ LL | struct ZeroSizedArrayWrapper([usize; 0]);
    = help: consider annotating `ZeroSizedArrayWrapper` with `#[repr(C)]` or another `repr` attribute
 
 error: trailing zero-sized array in a struct which is not marked with a `repr` attribute
-  --> tests/ui/trailing_empty_array.rs:65:1
+  --> tests/ui/trailing_empty_array.rs:66:1
    |
 LL | struct TupleStruct(i32, [usize; 0]);
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -112,7 +112,7 @@ LL | struct TupleStruct(i32, [usize; 0]);
    = help: consider annotating `TupleStruct` with `#[repr(C)]` or another `repr` attribute
 
 error: trailing zero-sized array in a struct which is not marked with a `repr` attribute
-  --> tests/ui/trailing_empty_array.rs:68:1
+  --> tests/ui/trailing_empty_array.rs:69:1
    |
 LL | / struct LotsOfFields {
 LL | |