offset_of: allow (unstably) taking the offset of slice tail fields

compiler/rustc_codegen_cranelift/src/base.rs

@@ -832,9 +832,10 @@ fn codegen_stmt<'tcx>(
                     let val = match null_op {
                         NullOp::SizeOf => layout.size.bytes(),
                         NullOp::AlignOf => layout.align.abi.bytes(),
-                        NullOp::OffsetOf(fields) => {
-                            layout.offset_of_subfield(fx, fields.iter()).bytes()
-                        }
+                        NullOp::OffsetOf(fields) => fx
+                            .tcx
+                            .offset_of_subfield(ParamEnv::reveal_all(), layout, fields.iter())
+                            .bytes(),
                         NullOp::UbChecks => {
                             let val = fx.tcx.sess.ub_checks();
                             let val = CValue::by_val(

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -680,7 +680,10 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                         bx.cx().const_usize(val)
                     }
                     mir::NullOp::OffsetOf(fields) => {
-                        let val = layout.offset_of_subfield(bx.cx(), fields.iter()).bytes();
+                        let val = bx
+                            .tcx()
+                            .offset_of_subfield(bx.param_env(), layout, fields.iter())
+                            .bytes();
                         bx.cx().const_usize(val)
                     }
                     mir::NullOp::UbChecks => {

compiler/rustc_const_eval/src/interpret/step.rs

@@ -253,7 +253,10 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                         Scalar::from_target_usize(val, self)
                     }
                     mir::NullOp::OffsetOf(fields) => {
-                        let val = layout.offset_of_subfield(self, fields.iter()).bytes();
+                        let val = self
+                            .tcx
+                            .offset_of_subfield(self.param_env, layout, fields.iter())
+                            .bytes();
                         Scalar::from_target_usize(val, self)
                     }
                     mir::NullOp::UbChecks => Scalar::from_bool(self.tcx.sess.ub_checks()),

compiler/rustc_feature/src/unstable.rs

@@ -559,6 +559,8 @@ declare_features! (
     (unstable, offset_of_enum, "1.75.0", Some(120141)),
     /// Allows using multiple nested field accesses in offset_of!
     (unstable, offset_of_nested, "1.77.0", Some(120140)),
+    /// Allows using fields with slice type in offset_of!
+    (unstable, offset_of_slice, "CURRENT_RUSTC_VERSION", Some(999999)),
     /// Allows using `#[optimize(X)]`.
     (unstable, optimize_attribute, "1.34.0", Some(54882)),
     /// Allows postfix match `expr.match { ... }`

compiler/rustc_hir_typeck/src/expr.rs

@@ -3391,8 +3391,19 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     {
                         let field_ty = self.field_ty(expr.span, field, args);
 
-                        // FIXME: DSTs with static alignment should be allowed
-                        self.require_type_is_sized(field_ty, expr.span, ObligationCauseCode::Misc);
+                        if self.tcx.features().offset_of_slice {
+                            self.require_type_has_static_alignment(
+                                field_ty,
+                                expr.span,
+                                ObligationCauseCode::Misc,
+                            );
+                        } else {
+                            self.require_type_is_sized(
+                                field_ty,
+                                expr.span,
+                                ObligationCauseCode::Misc,
+                            );
+                        }
 
                         if field.vis.is_accessible_from(def_scope, self.tcx) {
                             self.tcx.check_stability(field.did, Some(expr.hir_id), expr.span, None);

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -386,6 +386,26 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
+    pub fn require_type_has_static_alignment(
+        &self,
+        ty: Ty<'tcx>,
+        span: Span,
+        code: traits::ObligationCauseCode<'tcx>,
+    ) {
+        if !ty.references_error() {
+            let tail =
+                self.tcx.struct_tail_with_normalize(ty, |ty| self.normalize(span, ty), || {});
+            // Sized types have static alignment, and so do slices.
+            if tail.is_trivially_sized(self.tcx) || matches!(tail.kind(), ty::Slice(..)) {
+                // Nothing else is required here.
+            } else {
+                // We can't be sure, let's required full `Sized`.
+                let lang_item = self.tcx.require_lang_item(LangItem::Sized, None);
+                self.require_type_meets(ty, span, code, lang_item);
+            }
+        }
+    }
+
     pub fn register_bound(
         &self,
         ty: Ty<'tcx>,

compiler/rustc_middle/src/ty/layout.rs

@@ -1351,3 +1351,37 @@ pub trait FnAbiOf<'tcx>: FnAbiOfHelpers<'tcx> {
 }
 
 impl<'tcx, C: FnAbiOfHelpers<'tcx>> FnAbiOf<'tcx> for C {}
+
+impl<'tcx> TyCtxt<'tcx> {
+    pub fn offset_of_subfield<I>(
+        self,
+        param_env: ty::ParamEnv<'tcx>,
+        mut layout: TyAndLayout<'tcx>,
+        indices: I,
+    ) -> Size
+    where
+        I: Iterator<Item = (VariantIdx, FieldIdx)>,
+    {
+        let cx = LayoutCx { tcx: self, param_env };
+        let mut offset = Size::ZERO;
+
+        for (variant, field) in indices {
+            layout = layout.for_variant(&cx, variant);
+            let index = field.index();
+            offset += layout.fields.offset(index);
+            layout = layout.field(&cx, index);
+            if !layout.is_sized() {
+                // If it is not sized, then the tail must still have at least a known static alignment.
+                let tail = self.struct_tail_erasing_lifetimes(layout.ty, param_env);
+                if !matches!(tail.kind(), ty::Slice(..)) {
+                    bug!(
+                        "offset of not-statically-aligned field (type {:?}) cannot be computed statically",
+                        layout.ty
+                    );
+                }
+            }
+        }
+
+        offset
+    }
+}

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -10,7 +10,7 @@ use rustc_middle::bug;
 use rustc_middle::mir::interpret::{InterpResult, Scalar};
 use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor};
 use rustc_middle::mir::*;
-use rustc_middle::ty::layout::LayoutOf;
+use rustc_middle::ty::layout::{HasParamEnv, LayoutOf};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_mir_dataflow::value_analysis::{
     Map, PlaceIndex, State, TrackElem, ValueAnalysis, ValueAnalysisWrapper, ValueOrPlace,
@@ -285,9 +285,11 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'_, 'tcx> {
                 let val = match null_op {
                     NullOp::SizeOf if layout.is_sized() => layout.size.bytes(),
                     NullOp::AlignOf if layout.is_sized() => layout.align.abi.bytes(),
-                    NullOp::OffsetOf(fields) => {
-                        layout.offset_of_subfield(&self.ecx, fields.iter()).bytes()
-                    }
+                    NullOp::OffsetOf(fields) => self
+                        .ecx
+                        .tcx
+                        .offset_of_subfield(self.ecx.param_env(), layout, fields.iter())
+                        .bytes(),
                     _ => return ValueOrPlace::Value(FlatSet::Top),
                 };
                 FlatSet::Elem(Scalar::from_target_usize(val, &self.tcx))

compiler/rustc_mir_transform/src/gvn.rs

@@ -95,7 +95,7 @@ use rustc_middle::bug;
 use rustc_middle::mir::interpret::GlobalAlloc;
 use rustc_middle::mir::visit::*;
 use rustc_middle::mir::*;
-use rustc_middle::ty::layout::LayoutOf;
+use rustc_middle::ty::layout::{HasParamEnv, LayoutOf};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::def_id::DefId;
 use rustc_span::DUMMY_SP;
@@ -484,9 +484,11 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                 let val = match null_op {
                     NullOp::SizeOf => layout.size.bytes(),
                     NullOp::AlignOf => layout.align.abi.bytes(),
-                    NullOp::OffsetOf(fields) => {
-                        layout.offset_of_subfield(&self.ecx, fields.iter()).bytes()
-                    }
+                    NullOp::OffsetOf(fields) => self
+                        .ecx
+                        .tcx
+                        .offset_of_subfield(self.ecx.param_env(), layout, fields.iter())
+                        .bytes(),
                     NullOp::UbChecks => return None,
                 };
                 let usize_layout = self.ecx.layout_of(self.tcx.types.usize).unwrap();

compiler/rustc_mir_transform/src/known_panics_lint.rs

@@ -625,9 +625,10 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
                 let val = match null_op {
                     NullOp::SizeOf => op_layout.size.bytes(),
                     NullOp::AlignOf => op_layout.align.abi.bytes(),
-                    NullOp::OffsetOf(fields) => {
-                        op_layout.offset_of_subfield(self, fields.iter()).bytes()
-                    }
+                    NullOp::OffsetOf(fields) => self
+                        .tcx
+                        .offset_of_subfield(self.param_env, op_layout, fields.iter())
+                        .bytes(),
                     NullOp::UbChecks => return None,
                 };
                 ImmTy::from_scalar(Scalar::from_target_usize(val, self), layout).into()

compiler/rustc_span/src/symbol.rs

@@ -1304,6 +1304,7 @@ symbols! {
         offset_of,
         offset_of_enum,
         offset_of_nested,
+        offset_of_slice,
         ok_or_else,
         omit_gdb_pretty_printer_section,
         on,

compiler/rustc_target/src/abi/mod.rs

@@ -256,29 +256,6 @@ impl<'a, Ty> TyAndLayout<'a, Ty> {
         Ty::is_transparent(self)
     }
 
-    pub fn offset_of_subfield<C, I>(self, cx: &C, indices: I) -> Size
-    where
-        Ty: TyAbiInterface<'a, C>,
-        I: Iterator<Item = (VariantIdx, FieldIdx)>,
-    {
-        let mut layout = self;
-        let mut offset = Size::ZERO;
-
-        for (variant, field) in indices {
-            layout = layout.for_variant(cx, variant);
-            let index = field.index();
-            offset += layout.fields.offset(index);
-            layout = layout.field(cx, index);
-            assert!(
-                layout.is_sized(),
-                "offset of unsized field (type {:?}) cannot be computed statically",
-                layout.ty
-            );
-        }
-
-        offset
-    }
-
     /// Finds the one field that is not a 1-ZST.
     /// Returns `None` if there are multiple non-1-ZST fields or only 1-ZST-fields.
     pub fn non_1zst_field<C>(&self, cx: &C) -> Option<(usize, Self)>

tests/ui/feature-gates/feature-gate-offset-of-slice.rs

@@ -0,0 +1,20 @@
+use std::mem::offset_of;
+
+struct S {
+    a: u8,
+    b: (u8, u8),
+    c: [i32],
+}
+
+struct T {
+    x: i32,
+    y: S,
+}
+
+fn main() {
+    offset_of!(S, c); //~ ERROR size for values of type `[i32]` cannot be known at compilation time
+}
+
+fn other() {
+    offset_of!(T, y); //~ ERROR size for values of type `[i32]` cannot be known at compilation time
+}

tests/ui/feature-gates/feature-gate-offset-of-slice.stderr

@@ -0,0 +1,26 @@
+error[E0277]: the size for values of type `[i32]` cannot be known at compilation time
+  --> $DIR/feature-gate-offset-of-slice.rs:15:5
+   |
+LL |     offset_of!(S, c);
+   |     ^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[i32]`
+   = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: the size for values of type `[i32]` cannot be known at compilation time
+  --> $DIR/feature-gate-offset-of-slice.rs:19:5
+   |
+LL |     offset_of!(T, y);
+   |     ^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: within `S`, the trait `Sized` is not implemented for `[i32]`, which is required by `S: Sized`
+note: required because it appears within the type `S`
+  --> $DIR/feature-gate-offset-of-slice.rs:3:8
+   |
+LL | struct S {
+   |        ^
+   = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/offset-of/offset-of-slice.rs

@@ -0,0 +1,23 @@
+//@run-pass
+#![feature(offset_of_slice, offset_of_nested)]
+
+use std::mem::offset_of;
+
+#[repr(C)]
+struct S {
+    a: u8,
+    b: (u8, u8),
+    c: [i32],
+}
+
+#[repr(C)]
+struct T {
+    x: i8,
+    y: S,
+}
+
+fn main() {
+    assert_eq!(offset_of!(S, c), 4);
+    assert_eq!(offset_of!(T, y), 4);
+    assert_eq!(offset_of!(T, y.c), 8);
+}