Auto merge of #104429 - nnethercote:more-deriving-on-packed-structs, r=RalfJung

More deriving on packed structs

See [here](https://github.com/rust-lang/rust/pull/104429#issuecomment-1320909245) for the t-lang nomination summary, and [here](https://github.com/rust-lang/rust/pull/104429#issuecomment-1360077895) for the approval.

r? `@RalfJung`

2 files changed, 34 insertions, 0 deletions

diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
index 4a38ab2159f..d731c10f46e 100644
--- a/compiler/rustc_lint_defs/src/builtin.rs
+++ b/compiler/rustc_lint_defs/src/builtin.rs
@@ -3381,6 +3381,7 @@ declare_lint_pass! {
         REPR_TRANSPARENT_EXTERNAL_PRIVATE_FIELDS,
         NAMED_ARGUMENTS_USED_POSITIONALLY,
         IMPLIED_BOUNDS_ENTAILMENT,
+        BYTE_SLICE_IN_PACKED_STRUCT_WITH_DERIVE,
     ]
 }
 
@@ -4115,3 +4116,35 @@ declare_lint! {
         reason: FutureIncompatibilityReason::FutureReleaseErrorReportNow,
     };
 }
+
+declare_lint! {
+    /// The `byte_slice_in_packed_struct_with_derive` lint detects cases where a byte slice field
+    /// (`[u8]`) is used in a `packed` struct that derives one or more built-in traits.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// #[repr(packed)]
+    /// #[derive(Hash)]
+    /// struct FlexZeroSlice {
+    ///     width: u8,
+    ///     data: [u8],
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// This was previously accepted but is being phased out, because fields in packed structs are
+    /// now required to implement `Copy` for `derive` to work. Byte slices are a temporary
+    /// exception because certain crates depended on them.
+    pub BYTE_SLICE_IN_PACKED_STRUCT_WITH_DERIVE,
+    Warn,
+    "`[u8]` slice used in a packed struct with `derive`",
+    @future_incompatible = FutureIncompatibleInfo {
+        reference: "issue #107457 <https://github.com/rust-lang/rust/issues/107457>",
+        reason: FutureIncompatibilityReason::FutureReleaseErrorReportNow,
+    };
+    report_in_external_macro
+}
diff --git a/compiler/rustc_lint_defs/src/lib.rs b/compiler/rustc_lint_defs/src/lib.rs
index 7054d1e9f10..6efbf5ce9ee 100644
--- a/compiler/rustc_lint_defs/src/lib.rs
+++ b/compiler/rustc_lint_defs/src/lib.rs
@@ -521,6 +521,7 @@ pub enum BuiltinLintDiagnostics {
         /// Indicates if the named argument is used as a width/precision for formatting
         is_formatting_arg: bool,
     },
+    ByteSliceInPackedStructWithDerive,
 }
 
 /// Lints that are buffered up early on in the `Session` before the