built-in derive: remove BYTE_SLICE_IN_PACKED_STRUCT_WITH_DERIVE hack and lint

Author: RalfJung

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -181,11 +181,10 @@ use crate::{deriving, errors};
 use rustc_ast::ptr::P;
 use rustc_ast::{
     self as ast, BindingMode, ByRef, EnumDef, Expr, GenericArg, GenericParamKind, Generics,
-    Mutability, PatKind, TyKind, VariantData,
+    Mutability, PatKind, VariantData,
 };
 use rustc_attr as attr;
 use rustc_expand::base::{Annotatable, ExtCtxt};
-use rustc_session::lint::builtin::BYTE_SLICE_IN_PACKED_STRUCT_WITH_DERIVE;
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{Span, DUMMY_SP};
 use std::cell::RefCell;
@@ -1599,52 +1598,11 @@ impl<'a> TraitDef<'a> {
                         ),
                     );
                     if is_packed {
-                        // In general, fields in packed structs are copied via a
-                        // block, e.g. `&{self.0}`. The two exceptions are `[u8]`
-                        // and `str` fields, which cannot be copied and also never
-                        // cause unaligned references. These exceptions are allowed
-                        // to handle the `FlexZeroSlice` type in the `zerovec`
-                        // crate within `icu4x-0.9.0`.
-                        //
-                        // Once use of `icu4x-0.9.0` has dropped sufficiently, this
-                        // exception should be removed.
-                        let is_simple_path = |ty: &P<ast::Ty>, sym| {
-                            if let TyKind::Path(None, ast::Path { segments, .. }) = &ty.kind
-                                && let [seg] = segments.as_slice()
-                                && seg.ident.name == sym
-                                && seg.args.is_none()
-                            {
-                                true
-                            } else {
-                                false
-                            }
-                        };
-
-                        let exception = if let TyKind::Slice(ty) = &struct_field.ty.kind
-                            && is_simple_path(ty, sym::u8)
-                        {
-                            Some("byte")
-                        } else if is_simple_path(&struct_field.ty, sym::str) {
-                            Some("string")
-                        } else {
-                            None
-                        };
-
-                        if let Some(ty) = exception {
-                            cx.sess.psess.buffer_lint(
-                                BYTE_SLICE_IN_PACKED_STRUCT_WITH_DERIVE,
-                                sp,
-                                ast::CRATE_NODE_ID,
-                                rustc_lint_defs::BuiltinLintDiag::ByteSliceInPackedStructWithDerive {
-                                    ty: ty.to_string(),
-                                },
-                            );
-                        } else {
-                            // Wrap the expression in `{...}`, causing a copy.
-                            field_expr = cx.expr_block(
-                                cx.block(struct_field.span, thin_vec![cx.stmt_expr(field_expr)]),
-                            );
-                        }
+                        // Fields in packed structs are wrapped in a block, e.g. `&{self.0}`,
+                        // causing a copy instead of a (potentially misaligned) reference.
+                        field_expr = cx.expr_block(
+                            cx.block(struct_field.span, thin_vec![cx.stmt_expr(field_expr)]),
+                        );
                     }
                     cx.expr_addr_of(sp, field_expr)
                 })