Merge pull request #17692 from kcbanner/struct_field_init_pass

sema: analyze struct field bodies in a second pass, to allow them to use the layout of the struct itself

src/AstGen.zig

@@ -4951,7 +4951,10 @@ fn structDeclInner(
 
         if (have_value) {
             any_default_inits = true;
-            const ri: ResultInfo = .{ .rl = if (field_type == .none) .none else .{ .coerced_ty = field_type } };
+
+            // The decl_inst is used as here so that we can easily reconstruct a mapping
+            // between it and the field type when the fields inits are analzyed.
+            const ri: ResultInfo = .{ .rl = if (field_type == .none) .none else .{ .coerced_ty = decl_inst.toRef() } };
 
             const default_inst = try expr(&block_scope, &namespace.base, ri, member.ast.value_expr);
             if (!block_scope.endsWithNoReturn()) {

src/Autodoc.zig

@@ -3808,6 +3808,11 @@ fn walkInstruction(
                         call_ctx,
                     );
 
+                    // Inside field init bodies, the struct decl instruction is used to refer to the
+                    // field type during the second pass of analysis.
+                    try self.repurposed_insts.put(self.arena, inst, {});
+                    defer _ = self.repurposed_insts.remove(inst);
+
                     var field_type_refs: std.ArrayListUnmanaged(DocData.Expr) = .{};
                     var field_default_refs: std.ArrayListUnmanaged(?DocData.Expr) = .{};
                     var field_name_indexes: std.ArrayListUnmanaged(usize) = .{};

src/InternPool.zig

@@ -463,6 +463,7 @@ pub const Key = union(enum) {
 
         pub fn fieldInit(s: @This(), ip: *const InternPool, i: usize) Index {
             if (s.field_inits.len == 0) return .none;
+            assert(s.haveFieldInits(ip));
             return s.field_inits.get(ip)[i];
         }
 
@@ -497,6 +498,14 @@ pub const Key = union(enum) {
             return @ptrCast(&ip.extra.items[self.extra_index + flags_field_index]);
         }
 
+        /// The returned pointer expires with any addition to the `InternPool`.
+        /// Asserts that the struct is packed.
+        pub fn packedFlagsPtr(self: @This(), ip: *const InternPool) *Tag.TypeStructPacked.Flags {
+            assert(self.layout == .Packed);
+            const flags_field_index = std.meta.fieldIndex(Tag.TypeStructPacked, "flags").?;
+            return @ptrCast(&ip.extra.items[self.extra_index + flags_field_index]);
+        }
+
         pub fn assumeRuntimeBitsIfFieldTypesWip(s: @This(), ip: *InternPool) bool {
             if (s.layout == .Packed) return false;
             const flags_ptr = s.flagsPtr(ip);
@@ -546,6 +555,30 @@ pub const Key = union(enum) {
             s.flagsPtr(ip).alignment_wip = false;
         }
 
+        pub fn setInitsWip(s: @This(), ip: *InternPool) bool {
+            switch (s.layout) {
+                .Packed => {
+                    const flag = &s.packedFlagsPtr(ip).field_inits_wip;
+                    if (flag.*) return true;
+                    flag.* = true;
+                    return false;
+                },
+                .Auto, .Extern => {
+                    const flag = &s.flagsPtr(ip).field_inits_wip;
+                    if (flag.*) return true;
+                    flag.* = true;
+                    return false;
+                },
+            }
+        }
+
+        pub fn clearInitsWip(s: @This(), ip: *InternPool) void {
+            switch (s.layout) {
+                .Packed => s.packedFlagsPtr(ip).field_inits_wip = false,
+                .Auto, .Extern => s.flagsPtr(ip).field_inits_wip = false,
+            }
+        }
+
         pub fn setFullyResolved(s: @This(), ip: *InternPool) bool {
             if (s.layout == .Packed) return true;
             const flags_ptr = s.flagsPtr(ip);
@@ -588,6 +621,20 @@ pub const Key = union(enum) {
             return types.len == 0 or types[0] != .none;
         }
 
+        pub fn haveFieldInits(s: @This(), ip: *const InternPool) bool {
+            return switch (s.layout) {
+                .Packed => s.packedFlagsPtr(ip).inits_resolved,
+                .Auto, .Extern => s.flagsPtr(ip).inits_resolved,
+            };
+        }
+
+        pub fn setHaveFieldInits(s: @This(), ip: *InternPool) void {
+            switch (s.layout) {
+                .Packed => s.packedFlagsPtr(ip).inits_resolved = true,
+                .Auto, .Extern => s.flagsPtr(ip).inits_resolved = true,
+            }
+        }
+
         pub fn haveLayout(s: @This(), ip: *InternPool) bool {
             return switch (s.layout) {
                 .Packed => s.backingIntType(ip).* != .none,
@@ -3000,6 +3047,14 @@ pub const Tag = enum(u8) {
         namespace: Module.Namespace.OptionalIndex,
         backing_int_ty: Index,
         names_map: MapIndex,
+        flags: Flags,
+
+        pub const Flags = packed struct(u32) {
+            /// Dependency loop detection when resolving field inits.
+            field_inits_wip: bool,
+            inits_resolved: bool,
+            _: u30 = 0,
+        };
     };
 
     /// At first I thought of storing the denormalized data externally, such as...
@@ -3045,6 +3100,7 @@ pub const Tag = enum(u8) {
             requires_comptime: RequiresComptime,
             is_tuple: bool,
             assumed_runtime_bits: bool,
+            assumed_pointer_aligned: bool,
             has_namespace: bool,
             any_comptime_fields: bool,
             any_default_inits: bool,
@@ -3057,14 +3113,18 @@ pub const Tag = enum(u8) {
             field_types_wip: bool,
             /// Dependency loop detection when resolving struct layout.
             layout_wip: bool,
-            /// Determines whether `size`, `alignment`, runtime field order, and
+            /// Indicates whether `size`, `alignment`, runtime field order, and
             /// field offets are populated.
             layout_resolved: bool,
+            /// Dependency loop detection when resolving field inits.
+            field_inits_wip: bool,
+            /// Indicates whether `field_inits` has been resolved.
+            inits_resolved: bool,
             // The types and all its fields have had their layout resolved. Even through pointer,
             // which `layout_resolved` does not ensure.
             fully_resolved: bool,
 
-            _: u11 = 0,
+            _: u8 = 0,
         };
     };
 };
@@ -5347,6 +5407,7 @@ pub const StructTypeInit = struct {
     is_tuple: bool,
     any_comptime_fields: bool,
     any_default_inits: bool,
+    inits_resolved: bool,
     any_aligned_fields: bool,
 };
 
@@ -5399,6 +5460,10 @@ pub fn getStructType(
                     .namespace = ini.namespace,
                     .backing_int_ty = .none,
                     .names_map = names_map,
+                    .flags = .{
+                        .field_inits_wip = false,
+                        .inits_resolved = ini.inits_resolved,
+                    },
                 }),
             });
             ip.extra.appendNTimesAssumeCapacity(@intFromEnum(Index.none), ini.fields_len);
@@ -5431,6 +5496,7 @@ pub fn getStructType(
                 .requires_comptime = ini.requires_comptime,
                 .is_tuple = ini.is_tuple,
                 .assumed_runtime_bits = false,
+                .assumed_pointer_aligned = false,
                 .has_namespace = ini.namespace != .none,
                 .any_comptime_fields = ini.any_comptime_fields,
                 .any_default_inits = ini.any_default_inits,
@@ -5440,6 +5506,8 @@ pub fn getStructType(
                 .field_types_wip = false,
                 .layout_wip = false,
                 .layout_resolved = false,
+                .field_inits_wip = false,
+                .inits_resolved = ini.inits_resolved,
                 .fully_resolved = false,
             },
         }),
@@ -6451,6 +6519,7 @@ fn addExtraAssumeCapacity(ip: *InternPool, extra: anytype) u32 {
             Tag.TypePointer.PackedOffset,
             Tag.TypeUnion.Flags,
             Tag.TypeStruct.Flags,
+            Tag.TypeStructPacked.Flags,
             Tag.Variable.Flags,
             => @bitCast(@field(extra, field.name)),
 
@@ -6525,6 +6594,7 @@ fn extraDataTrail(ip: *const InternPool, comptime T: type, index: usize) struct
             Tag.TypePointer.PackedOffset,
             Tag.TypeUnion.Flags,
             Tag.TypeStruct.Flags,
+            Tag.TypeStructPacked.Flags,
             Tag.Variable.Flags,
             FuncAnalysis,
             => @bitCast(int32),

src/Sema.zig

@@ -2699,6 +2699,7 @@ pub fn getStructType(
         .requires_comptime = if (small.known_comptime_only) .yes else .unknown,
         .any_default_inits = small.any_default_inits,
         .any_comptime_fields = small.any_comptime_fields,
+        .inits_resolved = false,
         .any_aligned_fields = small.any_aligned_fields,
     });
 
@@ -4718,6 +4719,7 @@ fn validateStructInit(
             const i: u32 = @intCast(i_usize);
             if (field_ptr != .none) continue;
 
+            try sema.resolveStructFieldInits(struct_ty);
             const default_val = struct_ty.structFieldDefaultValue(i, mod);
             if (default_val.toIntern() == .unreachable_value) {
                 const field_name = struct_ty.structFieldName(i, mod).unwrap() orelse {
@@ -4773,6 +4775,8 @@ fn validateStructInit(
     const air_tags = sema.air_instructions.items(.tag);
     const air_datas = sema.air_instructions.items(.data);
 
+    try sema.resolveStructFieldInits(struct_ty);
+
     // We collect the comptime field values in case the struct initialization
     // ends up being comptime-known.
     const field_values = try sema.arena.alloc(InternPool.Index, struct_ty.structFieldCount(mod));
@@ -17638,6 +17642,8 @@ fn zirTypeInfo(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Ai
                 };
                 struct_field_vals = try gpa.alloc(InternPool.Index, struct_type.field_types.len);
 
+                try sema.resolveStructFieldInits(ty);
+
                 for (struct_field_vals, 0..) |*field_val, i| {
                     // TODO: write something like getCoercedInts to avoid needing to dupe
                     const name = if (struct_type.fieldName(ip, i).unwrap()) |name_nts|
@@ -19213,17 +19219,20 @@ fn zirStructInit(
             const uncoerced_init = try sema.resolveInst(item.data.init);
             const field_ty = resolved_ty.structFieldType(field_index, mod);
             field_inits[field_index] = try sema.coerce(block, field_ty, uncoerced_init, field_src);
-            if (!is_packed) if (try resolved_ty.structFieldValueComptime(mod, field_index)) |default_value| {
-                const init_val = (try sema.resolveValue(field_inits[field_index])) orelse {
-                    return sema.failWithNeededComptime(block, field_src, .{
-                        .needed_comptime_reason = "value stored in comptime field must be comptime-known",
-                    });
-                };
+            if (!is_packed) {
+                try sema.resolveStructFieldInits(resolved_ty);
+                if (try resolved_ty.structFieldValueComptime(mod, field_index)) |default_value| {
+                    const init_val = (try sema.resolveValue(field_inits[field_index])) orelse {
+                        return sema.failWithNeededComptime(block, field_src, .{
+                            .needed_comptime_reason = "value stored in comptime field must be comptime-known",
+                        });
+                    };
 
-                if (!init_val.eql(default_value, resolved_ty.structFieldType(field_index, mod), mod)) {
-                    return sema.failWithInvalidComptimeFieldStore(block, field_src, resolved_ty, field_index);
+                    if (!init_val.eql(default_value, resolved_ty.structFieldType(field_index, mod), mod)) {
+                        return sema.failWithInvalidComptimeFieldStore(block, field_src, resolved_ty, field_index);
+                    }
                 }
-            };
+            }
         }
 
         return sema.finishStructInit(block, src, src, field_inits, resolved_ty, result_ty, is_ref);
@@ -19376,6 +19385,8 @@ fn finishStructInit(
                     continue;
                 }
 
+                try sema.resolveStructFieldInits(struct_ty);
+
                 const field_init = struct_type.fieldInit(ip, i);
                 if (field_init == .none) {
                     const field_name = struct_type.field_names.get(ip)[i];
@@ -21140,6 +21151,7 @@ fn reifyStruct(
         // struct types.
         .any_comptime_fields = true,
         .any_default_inits = true,
+        .inits_resolved = true,
         .any_aligned_fields = true,
     });
     // TODO: figure out InternPool removals for incremental compilation
@@ -26640,6 +26652,7 @@ fn finishFieldCallBind(
 
     const container_ty = ptr_ty.childType(mod);
     if (container_ty.zigTypeTag(mod) == .Struct) {
+        try sema.resolveStructFieldInits(container_ty);
         if (try container_ty.structFieldValueComptime(mod, field_index)) |default_val| {
             return .{ .direct = Air.internedToRef(default_val.toIntern()) };
         }
@@ -26855,6 +26868,7 @@ fn structFieldPtrByIndex(
     const ptr_field_ty = try sema.ptrType(ptr_ty_data);
 
     if (struct_type.fieldIsComptime(ip, field_index)) {
+        try sema.resolveStructFieldInits(struct_ty);
         const val = try mod.intern(.{ .ptr = .{
             .ty = ptr_field_ty.toIntern(),
             .addr = .{ .comptime_field = struct_type.field_inits.get(ip)[field_index] },
@@ -26891,6 +26905,7 @@ fn structFieldVal(
     assert(struct_ty.zigTypeTag(mod) == .Struct);
 
     try sema.resolveTypeFields(struct_ty);
+
     switch (ip.indexToKey(struct_ty.toIntern())) {
         .struct_type => |struct_type| {
             if (struct_type.isTuple(ip))
@@ -26899,6 +26914,7 @@ fn structFieldVal(
             const field_index = struct_type.nameIndex(ip, field_name) orelse
                 return sema.failWithBadStructFieldAccess(block, struct_type, field_name_src, field_name);
             if (struct_type.fieldIsComptime(ip, field_index)) {
+                try sema.resolveStructFieldInits(struct_ty);
                 return Air.internedToRef(struct_type.field_inits.get(ip)[field_index]);
             }
 
@@ -31290,6 +31306,7 @@ fn coerceTupleToStruct(
     const mod = sema.mod;
     const ip = &mod.intern_pool;
     try sema.resolveTypeFields(struct_ty);
+    try sema.resolveStructFieldInits(struct_ty);
 
     if (struct_ty.isTupleOrAnonStruct(mod)) {
         return sema.coerceTupleToTuple(block, struct_ty, inst, inst_src);
@@ -34272,6 +34289,8 @@ fn resolvePeerTypesInner(
                 var comptime_val: ?Value = null;
                 for (peer_tys) |opt_ty| {
                     const struct_ty = opt_ty orelse continue;
+                    try sema.resolveStructFieldInits(struct_ty);
+
                     const uncoerced_field_val = try struct_ty.structFieldValueComptime(mod, field_idx) orelse {
                         comptime_val = null;
                         break;
@@ -34613,8 +34632,7 @@ pub fn resolveStructAlignment(
         // We'll guess "pointer-aligned", if the struct has an
         // underaligned pointer field then some allocations
         // might require explicit alignment.
-        //TODO write this bit and emit an error later if incorrect
-        //struct_type.flagsPtr(ip).assumed_pointer_aligned = true;
+        struct_type.flagsPtr(ip).assumed_pointer_aligned = true;
         const result = Alignment.fromByteUnits(@divExact(target.ptrBitWidth(), 8));
         struct_type.flagsPtr(ip).alignment = result;
         return result;
@@ -34626,8 +34644,7 @@ pub fn resolveStructAlignment(
         // We'll guess "pointer-aligned", if the struct has an
         // underaligned pointer field then some allocations
         // might require explicit alignment.
-        //TODO write this bit and emit an error later if incorrect
-        //struct_type.flagsPtr(ip).assumed_pointer_aligned = true;
+        struct_type.flagsPtr(ip).assumed_pointer_aligned = true;
         const result = Alignment.fromByteUnits(@divExact(target.ptrBitWidth(), 8));
         struct_type.flagsPtr(ip).alignment = result;
         return result;
@@ -34718,6 +34735,18 @@ fn resolveStructLayout(sema: *Sema, ty: Type) CompileError!void {
         return sema.failWithOwnedErrorMsg(null, msg);
     }
 
+    if (struct_type.flagsPtr(ip).assumed_pointer_aligned and
+        big_align.compareStrict(.neq, Alignment.fromByteUnits(@divExact(mod.getTarget().ptrBitWidth(), 8))))
+    {
+        const msg = try Module.ErrorMsg.create(
+            sema.gpa,
+            mod.declPtr(struct_type.decl.unwrap().?).srcLoc(mod),
+            "struct layout depends on being pointer aligned",
+            .{},
+        );
+        return sema.failWithOwnedErrorMsg(null, msg);
+    }
+
     if (struct_type.hasReorderedFields()) {
         const runtime_order = struct_type.runtime_order.get(ip);
 
@@ -35337,6 +35366,32 @@ pub fn resolveTypeFieldsStruct(
     try semaStructFields(mod, sema.arena, struct_type);
 }
 
+pub fn resolveStructFieldInits(sema: *Sema, ty: Type) CompileError!void {
+    const mod = sema.mod;
+    const ip = &mod.intern_pool;
+    const struct_type = mod.typeToStruct(ty) orelse return;
+    const owner_decl = struct_type.decl.unwrap() orelse return;
+
+    // Inits can start as resolved
+    if (struct_type.haveFieldInits(ip)) return;
+
+    try sema.resolveStructLayout(ty);
+
+    if (struct_type.setInitsWip(ip)) {
+        const msg = try Module.ErrorMsg.create(
+            sema.gpa,
+            mod.declPtr(owner_decl).srcLoc(mod),
+            "struct '{}' depends on itself",
+            .{ty.fmt(mod)},
+        );
+        return sema.failWithOwnedErrorMsg(null, msg);
+    }
+    defer struct_type.clearInitsWip(ip);
+
+    try semaStructFieldInits(mod, sema.arena, struct_type);
+    struct_type.setHaveFieldInits(ip);
+}
+
 pub fn resolveTypeFieldsUnion(sema: *Sema, ty: Type, union_type: InternPool.Key.UnionType) CompileError!void {
     const mod = sema.mod;
     const ip = &mod.intern_pool;
@@ -35518,24 +35573,18 @@ fn resolveInferredErrorSetTy(
     }
 }
 
-fn semaStructFields(
-    mod: *Module,
-    arena: Allocator,
-    struct_type: InternPool.Key.StructType,
-) CompileError!void {
-    const gpa = mod.gpa;
-    const ip = &mod.intern_pool;
-    const decl_index = struct_type.decl.unwrap() orelse return;
-    const decl = mod.declPtr(decl_index);
-    const namespace_index = struct_type.namespace.unwrap() orelse decl.src_namespace;
-    const zir = mod.namespacePtr(namespace_index).file_scope.zir;
-    const zir_index = struct_type.zir_index;
+fn structZirInfo(zir: Zir, zir_index: Zir.Inst.Index) struct {
+    /// fields_len
+    usize,
+    Zir.Inst.StructDecl.Small,
+    /// extra_index
+    usize,
+} {
     const extended = zir.instructions.items(.data)[@intFromEnum(zir_index)].extended;
     assert(extended.opcode == .struct_decl);
     const small: Zir.Inst.StructDecl.Small = @bitCast(extended.small);
     var extra_index: usize = extended.operand;
 
-    const src = LazySrcLoc.nodeOffset(0);
     extra_index += @intFromBool(small.has_src_node);
 
     const fields_len = if (small.has_fields_len) blk: {
@@ -35566,6 +35615,25 @@ fn semaStructFields(
     while (decls_it.next()) |_| {}
     extra_index = decls_it.extra_index;
 
+    return .{ fields_len, small, extra_index };
+}
+
+fn semaStructFields(
+    mod: *Module,
+    arena: Allocator,
+    struct_type: InternPool.Key.StructType,
+) CompileError!void {
+    const gpa = mod.gpa;
+    const ip = &mod.intern_pool;
+    const decl_index = struct_type.decl.unwrap() orelse return;
+    const decl = mod.declPtr(decl_index);
+    const namespace_index = struct_type.namespace.unwrap() orelse decl.src_namespace;
+    const zir = mod.namespacePtr(namespace_index).file_scope.zir;
+    const zir_index = struct_type.zir_index;
+
+    const src = LazySrcLoc.nodeOffset(0);
+    const fields_len, const small, var extra_index = structZirInfo(zir, zir_index);
+
     if (fields_len == 0) switch (struct_type.layout) {
         .Packed => {
             try semaBackingIntType(mod, struct_type);
@@ -35693,7 +35761,6 @@ fn semaStructFields(
 
     // Next we do only types and alignments, saving the inits for a second pass,
     // so that init values may depend on type layout.
-    const bodies_index = extra_index;
 
     for (fields, 0..) |zir_field, field_i| {
         const field_ty: Type = ty: {
@@ -35817,44 +35884,161 @@ fn semaStructFields(
         extra_index += zir_field.init_body_len;
     }
 
-    // TODO: there seems to be no mechanism to catch when an init depends on
-    // another init that hasn't been resolved.
+    struct_type.clearTypesWip(ip);
+    if (!any_inits) struct_type.setHaveFieldInits(ip);
+
+    for (comptime_mutable_decls.items) |ct_decl_index| {
+        const ct_decl = mod.declPtr(ct_decl_index);
+        _ = try ct_decl.internValue(mod);
+    }
+}
+
+// This logic must be kept in sync with `semaStructFields`
+fn semaStructFieldInits(
+    mod: *Module,
+    arena: Allocator,
+    struct_type: InternPool.Key.StructType,
+) CompileError!void {
+    const gpa = mod.gpa;
+    const ip = &mod.intern_pool;
+
+    assert(!struct_type.haveFieldInits(ip));
+
+    const decl_index = struct_type.decl.unwrap() orelse return;
+    const decl = mod.declPtr(decl_index);
+    const namespace_index = struct_type.namespace.unwrap() orelse decl.src_namespace;
+    const zir = mod.namespacePtr(namespace_index).file_scope.zir;
+    const zir_index = struct_type.zir_index;
+    const fields_len, const small, var extra_index = structZirInfo(zir, zir_index);
+
+    var comptime_mutable_decls = std.ArrayList(Decl.Index).init(gpa);
+    defer comptime_mutable_decls.deinit();
+
+    var sema: Sema = .{
+        .mod = mod,
+        .gpa = gpa,
+        .arena = arena,
+        .code = zir,
+        .owner_decl = decl,
+        .owner_decl_index = decl_index,
+        .func_index = .none,
+        .func_is_naked = false,
+        .fn_ret_ty = Type.void,
+        .fn_ret_ty_ies = null,
+        .owner_func_index = .none,
+        .comptime_mutable_decls = &comptime_mutable_decls,
+    };
+    defer sema.deinit();
+
+    var block_scope: Block = .{
+        .parent = null,
+        .sema = &sema,
+        .src_decl = decl_index,
+        .namespace = namespace_index,
+        .wip_capture_scope = try mod.createCaptureScope(decl.src_scope),
+        .instructions = .{},
+        .inlining = null,
+        .is_comptime = true,
+    };
+    defer assert(block_scope.instructions.items.len == 0);
+
+    const Field = struct {
+        type_body_len: u32 = 0,
+        align_body_len: u32 = 0,
+        init_body_len: u32 = 0,
+    };
+    const fields = try sema.arena.alloc(Field, fields_len);
+
+    var any_inits = false;
+
+    {
+        const bits_per_field = 4;
+        const fields_per_u32 = 32 / bits_per_field;
+        const bit_bags_count = std.math.divCeil(usize, fields_len, fields_per_u32) catch unreachable;
+        const flags_index = extra_index;
+        var bit_bag_index: usize = flags_index;
+        extra_index += bit_bags_count;
+        var cur_bit_bag: u32 = undefined;
+        var field_i: u32 = 0;
+        while (field_i < fields_len) : (field_i += 1) {
+            if (field_i % fields_per_u32 == 0) {
+                cur_bit_bag = zir.extra[bit_bag_index];
+                bit_bag_index += 1;
+            }
+            const has_align = @as(u1, @truncate(cur_bit_bag)) != 0;
+            cur_bit_bag >>= 1;
+            const has_init = @as(u1, @truncate(cur_bit_bag)) != 0;
+            cur_bit_bag >>= 2;
+            const has_type_body = @as(u1, @truncate(cur_bit_bag)) != 0;
+            cur_bit_bag >>= 1;
+
+            if (!small.is_tuple) {
+                extra_index += 1;
+            }
+            extra_index += 1; // doc_comment
+
+            fields[field_i] = .{};
+
+            if (has_type_body) fields[field_i].type_body_len = zir.extra[extra_index];
+            extra_index += 1;
+
+            if (has_align) {
+                fields[field_i].align_body_len = zir.extra[extra_index];
+                extra_index += 1;
+            }
+            if (has_init) {
+                fields[field_i].init_body_len = zir.extra[extra_index];
+                extra_index += 1;
+                any_inits = true;
+            }
+        }
+    }
 
     if (any_inits) {
-        extra_index = bodies_index;
         for (fields, 0..) |zir_field, field_i| {
-            const field_ty = struct_type.field_types.get(ip)[field_i].toType();
             extra_index += zir_field.type_body_len;
             extra_index += zir_field.align_body_len;
-            if (zir_field.init_body_len > 0) {
-                const body = zir.bodySlice(extra_index, zir_field.init_body_len);
-                extra_index += body.len;
-                const init = try sema.resolveBody(&block_scope, body, zir_index);
-                const coerced = sema.coerce(&block_scope, field_ty, init, .unneeded) catch |err| switch (err) {
-                    error.NeededSourceLocation => {
-                        const init_src = mod.fieldSrcLoc(decl_index, .{
-                            .index = field_i,
-                            .range = .value,
-                        }).lazy;
-                        _ = try sema.coerce(&block_scope, field_ty, init, init_src);
-                        unreachable;
-                    },
-                    else => |e| return e,
-                };
-                const default_val = (try sema.resolveValue(coerced)) orelse {
+            const body = zir.bodySlice(extra_index, zir_field.init_body_len);
+            extra_index += zir_field.init_body_len;
+
+            if (body.len == 0) continue;
+
+            // Pre-populate the type mapping the body expects to be there.
+            // In init bodies, the zir index of the struct itself is used
+            // to refer to the current field type.
+
+            const field_ty = struct_type.field_types.get(ip)[field_i].toType();
+            const type_ref = Air.internedToRef(field_ty.toIntern());
+            try sema.inst_map.ensureSpaceForInstructions(sema.gpa, &.{zir_index});
+            sema.inst_map.putAssumeCapacity(zir_index, type_ref);
+
+            const init = try sema.resolveBody(&block_scope, body, zir_index);
+            const coerced = sema.coerce(&block_scope, field_ty, init, .unneeded) catch |err| switch (err) {
+                error.NeededSourceLocation => {
                     const init_src = mod.fieldSrcLoc(decl_index, .{
                         .index = field_i,
                         .range = .value,
                     }).lazy;
-                    return sema.failWithNeededComptime(&block_scope, init_src, .{
-                        .needed_comptime_reason = "struct field default value must be comptime-known",
-                    });
-                };
-                const field_init = try default_val.intern(field_ty, mod);
-                struct_type.field_inits.get(ip)[field_i] = field_init;
-            }
+                    _ = try sema.coerce(&block_scope, field_ty, init, init_src);
+                    unreachable;
+                },
+                else => |e| return e,
+            };
+            const default_val = (try sema.resolveValue(coerced)) orelse {
+                const init_src = mod.fieldSrcLoc(decl_index, .{
+                    .index = field_i,
+                    .range = .value,
+                }).lazy;
+                return sema.failWithNeededComptime(&block_scope, init_src, .{
+                    .needed_comptime_reason = "struct field default value must be comptime-known",
+                });
+            };
+
+            const field_init = try default_val.intern(field_ty, mod);
+            struct_type.field_inits.get(ip)[field_i] = field_init;
         }
     }
+
     for (comptime_mutable_decls.items) |ct_decl_index| {
         const ct_decl = mod.declPtr(ct_decl_index);
         _ = try ct_decl.internValue(mod);
@@ -36682,6 +36866,7 @@ pub fn typeHasOnePossibleValue(sema: *Sema, ty: Type) CompileError!?Value {
                     );
                     for (field_vals, 0..) |*field_val, i| {
                         if (struct_type.fieldIsComptime(ip, i)) {
+                            try sema.resolveStructFieldInits(ty);
                             field_val.* = struct_type.field_inits.get(ip)[i];
                             continue;
                         }

src/arch/wasm/CodeGen.zig

@@ -3372,10 +3372,14 @@ fn lowerConstant(func: *CodeGen, val: Value, ty: Type) InnerError!WValue {
         },
         .un => |un| {
             // in this case we have a packed union which will not be passed by reference.
-            const union_obj = mod.typeToUnion(ty).?;
-            const field_index = mod.unionTagFieldIndex(union_obj, un.tag.toValue()).?;
-            const field_ty = union_obj.field_types.get(ip)[field_index].toType();
-            return func.lowerConstant(un.val.toValue(), field_ty);
+            const constant_ty = if (un.tag == .none)
+                try ty.unionBackingType(mod)
+            else field_ty: {
+                const union_obj = mod.typeToUnion(ty).?;
+                const field_index = mod.unionTagFieldIndex(union_obj, un.tag.toValue()).?;
+                break :field_ty union_obj.field_types.get(ip)[field_index].toType();
+            };
+            return func.lowerConstant(un.val.toValue(), constant_ty);
         },
         .memoized_call => unreachable,
     }

src/codegen/c.zig

@@ -1499,56 +1499,85 @@ pub const DeclGen = struct {
                 else => unreachable,
             },
             .un => |un| {
-                if (!location.isInitializer()) {
-                    try writer.writeByte('(');
-                    try dg.renderType(writer, ty);
-                    try writer.writeByte(')');
-                }
-
                 const union_obj = mod.typeToUnion(ty).?;
-                const field_i = mod.unionTagFieldIndex(union_obj, un.tag.toValue()).?;
-                const field_ty = union_obj.field_types.get(ip)[field_i].toType();
-                const field_name = union_obj.field_names.get(ip)[field_i];
-                if (union_obj.getLayout(ip) == .Packed) {
-                    if (field_ty.hasRuntimeBits(mod)) {
-                        if (field_ty.isPtrAtRuntime(mod)) {
-                            try writer.writeByte('(');
-                            try dg.renderType(writer, ty);
-                            try writer.writeByte(')');
-                        } else if (field_ty.zigTypeTag(mod) == .Float) {
-                            try writer.writeByte('(');
-                            try dg.renderType(writer, ty);
-                            try writer.writeByte(')');
-                        }
-                        try dg.renderValue(writer, field_ty, un.val.toValue(), initializer_type);
-                    } else {
-                        try writer.writeAll("0");
-                    }
-                    return;
-                }
+                if (un.tag == .none) {
+                    const backing_ty = try ty.unionBackingType(mod);
+                    switch (union_obj.getLayout(ip)) {
+                        .Packed => {
+                            if (!location.isInitializer()) {
+                                try writer.writeByte('(');
+                                try dg.renderType(writer, backing_ty);
+                                try writer.writeByte(')');
+                            }
+                            try dg.renderValue(writer, backing_ty, un.val.toValue(), initializer_type);
+                        },
+                        .Extern => {
+                            if (location == .StaticInitializer) {
+                                return dg.fail("TODO: C backend: implement extern union backing type rendering in static initializers", .{});
+                            }
 
-                try writer.writeByte('{');
-                if (ty.unionTagTypeSafety(mod)) |tag_ty| {
-                    const layout = mod.getUnionLayout(union_obj);
-                    if (layout.tag_size != 0) {
-                        try writer.writeAll(" .tag = ");
-                        try dg.renderValue(writer, tag_ty, un.tag.toValue(), initializer_type);
+                            const ptr_ty = try mod.singleConstPtrType(ty);
+                            try writer.writeAll("*((");
+                            try dg.renderType(writer, ptr_ty);
+                            try writer.writeAll(")(");
+                            try dg.renderType(writer, backing_ty);
+                            try writer.writeAll("){");
+                            try dg.renderValue(writer, backing_ty, un.val.toValue(), initializer_type);
+                            try writer.writeAll("})");
+                        },
+                        else => unreachable,
                     }
-                    if (ty.unionHasAllZeroBitFieldTypes(mod)) return try writer.writeByte('}');
-                    if (layout.tag_size != 0) try writer.writeByte(',');
-                    try writer.writeAll(" .payload = {");
+                } else {
+                    if (!location.isInitializer()) {
+                        try writer.writeByte('(');
+                        try dg.renderType(writer, ty);
+                        try writer.writeByte(')');
+                    }
+
+                    const field_i = mod.unionTagFieldIndex(union_obj, un.tag.toValue()).?;
+                    const field_ty = union_obj.field_types.get(ip)[field_i].toType();
+                    const field_name = union_obj.field_names.get(ip)[field_i];
+                    if (union_obj.getLayout(ip) == .Packed) {
+                        if (field_ty.hasRuntimeBits(mod)) {
+                            if (field_ty.isPtrAtRuntime(mod)) {
+                                try writer.writeByte('(');
+                                try dg.renderType(writer, ty);
+                                try writer.writeByte(')');
+                            } else if (field_ty.zigTypeTag(mod) == .Float) {
+                                try writer.writeByte('(');
+                                try dg.renderType(writer, ty);
+                                try writer.writeByte(')');
+                            }
+                            try dg.renderValue(writer, field_ty, un.val.toValue(), initializer_type);
+                        } else {
+                            try writer.writeAll("0");
+                        }
+                        return;
+                    }
+
+                    try writer.writeByte('{');
+                    if (ty.unionTagTypeSafety(mod)) |tag_ty| {
+                        const layout = mod.getUnionLayout(union_obj);
+                        if (layout.tag_size != 0) {
+                            try writer.writeAll(" .tag = ");
+                            try dg.renderValue(writer, tag_ty, un.tag.toValue(), initializer_type);
+                        }
+                        if (ty.unionHasAllZeroBitFieldTypes(mod)) return try writer.writeByte('}');
+                        if (layout.tag_size != 0) try writer.writeByte(',');
+                        try writer.writeAll(" .payload = {");
+                    }
+                    if (field_ty.hasRuntimeBits(mod)) {
+                        try writer.print(" .{ } = ", .{fmtIdent(ip.stringToSlice(field_name))});
+                        try dg.renderValue(writer, field_ty, un.val.toValue(), initializer_type);
+                        try writer.writeByte(' ');
+                    } else for (union_obj.field_types.get(ip)) |this_field_ty| {
+                        if (!this_field_ty.toType().hasRuntimeBits(mod)) continue;
+                        try dg.renderValue(writer, this_field_ty.toType(), Value.undef, initializer_type);
+                        break;
+                    }
+                    if (ty.unionTagTypeSafety(mod)) |_| try writer.writeByte('}');
+                    try writer.writeByte('}');
                 }
-                if (field_ty.hasRuntimeBits(mod)) {
-                    try writer.print(" .{ } = ", .{fmtIdent(ip.stringToSlice(field_name))});
-                    try dg.renderValue(writer, field_ty, un.val.toValue(), initializer_type);
-                    try writer.writeByte(' ');
-                } else for (union_obj.field_types.get(ip)) |this_field_ty| {
-                    if (!this_field_ty.toType().hasRuntimeBits(mod)) continue;
-                    try dg.renderValue(writer, this_field_ty.toType(), Value.undef, initializer_type);
-                    break;
-                }
-                if (ty.unionTagTypeSafety(mod)) |_| try writer.writeByte('}');
-                try writer.writeByte('}');
             },
         }
     }

src/type.zig

@@ -2415,6 +2415,7 @@ pub const Type = struct {
                     for (field_vals, 0..) |*field_val, i_usize| {
                         const i: u32 = @intCast(i_usize);
                         if (struct_type.fieldIsComptime(ip, i)) {
+                            assert(struct_type.haveFieldInits(ip));
                             field_val.* = struct_type.field_inits.get(ip)[i];
                             continue;
                         }
@@ -3014,6 +3015,7 @@ pub const Type = struct {
         const ip = &mod.intern_pool;
         switch (ip.indexToKey(ty.toIntern())) {
             .struct_type => |struct_type| {
+                assert(struct_type.haveFieldInits(ip));
                 if (struct_type.fieldIsComptime(ip, index)) {
                     return struct_type.field_inits.get(ip)[index].toValue();
                 } else {

test/behavior/struct.zig

@@ -1785,3 +1785,60 @@ test "comptimeness of optional and error union payload is analyzed properly" {
     const x = (try c).?.x;
     try std.testing.expectEqual(3, x);
 }
+
+test "initializer uses own alignment" {
+    const S = struct {
+        x: u32 = @alignOf(@This()) + 1,
+    };
+
+    var s: S = .{};
+    try expectEqual(4, @alignOf(S));
+    try expectEqual(@as(usize, 5), s.x);
+}
+
+test "initializer uses own size" {
+    const S = struct {
+        x: u32 = @sizeOf(@This()) + 1,
+    };
+
+    var s: S = .{};
+    try expectEqual(4, @sizeOf(S));
+    try expectEqual(@as(usize, 5), s.x);
+}
+
+test "initializer takes a pointer to a variable inside its struct" {
+    const namespace = struct {
+        const S = struct {
+            s: *S = &S.instance,
+            var instance: S = undefined;
+        };
+
+        fn doTheTest() !void {
+            var foo: S = .{};
+            try expectEqual(&S.instance, foo.s);
+        }
+    };
+
+    try namespace.doTheTest();
+    comptime try namespace.doTheTest();
+}
+
+test "circular dependency through pointer field of a struct" {
+    const S = struct {
+        const StructInner = extern struct {
+            outer: StructOuter = std.mem.zeroes(StructOuter),
+        };
+
+        const StructMiddle = extern struct {
+            outer: ?*StructInner,
+            inner: ?*StructOuter,
+        };
+
+        const StructOuter = extern struct {
+            middle: StructMiddle = std.mem.zeroes(StructMiddle),
+        };
+    };
+    var outer: S.StructOuter = .{};
+    try expect(outer.middle.outer == null);
+    try expect(outer.middle.inner == null);
+}

test/behavior/union.zig

@@ -1869,6 +1869,126 @@ test "reinterpret packed union inside packed struct" {
     try S.doTheTest();
 }
 
+test "inner struct initializer uses union layout" {
+    const namespace = struct {
+        const U = union {
+            a: struct {
+                x: u32 = @alignOf(U) + 1,
+            },
+            b: struct {
+                y: u16 = @sizeOf(U) + 2,
+            },
+        };
+    };
+
+    {
+        const u: namespace.U = .{ .a = .{} };
+        try expectEqual(4, @alignOf(namespace.U));
+        try expectEqual(@as(usize, 5), u.a.x);
+    }
+
+    {
+        const u: namespace.U = .{ .b = .{} };
+        try expectEqual(@as(usize, @sizeOf(namespace.U) + 2), u.b.y);
+    }
+}
+
+test "inner struct initializer uses packed union layout" {
+    const namespace = struct {
+        const U = packed union {
+            a: packed struct {
+                x: u32 = @alignOf(U) + 1,
+            },
+            b: packed struct {
+                y: u16 = @sizeOf(U) + 2,
+            },
+        };
+    };
+
+    {
+        const u: namespace.U = .{ .a = .{} };
+        try expectEqual(4, @alignOf(namespace.U));
+        try expectEqual(@as(usize, 5), u.a.x);
+    }
+
+    {
+        const u: namespace.U = .{ .b = .{} };
+        try expectEqual(@as(usize, @sizeOf(namespace.U) + 2), u.b.y);
+    }
+}
+
+test "extern union initialized via reintepreted struct field initializer" {
+    const bytes = [_]u8{ 0xaa, 0xbb, 0xcc, 0xdd };
+
+    const U = extern union {
+        a: u32,
+        b: u8,
+    };
+
+    const S = extern struct {
+        u: U = std.mem.bytesAsValue(U, &bytes).*,
+    };
+
+    const s: S = .{};
+    try expect(s.u.a == littleToNativeEndian(u32, 0xddccbbaa));
+    try expect(s.u.b == 0xaa);
+}
+
+test "packed union initialized via reintepreted struct field initializer" {
+    const bytes = [_]u8{ 0xaa, 0xbb, 0xcc, 0xdd };
+
+    const U = packed union {
+        a: u32,
+        b: u8,
+    };
+
+    const S = packed struct {
+        u: U = std.mem.bytesAsValue(U, &bytes).*,
+    };
+
+    var s: S = .{};
+    try expect(s.u.a == littleToNativeEndian(u32, 0xddccbbaa));
+    try expect(s.u.b == if (endian == .little) 0xaa else 0xdd);
+}
+
+test "store of comptime reinterpreted memory to extern union" {
+    const bytes = [_]u8{ 0xaa, 0xbb, 0xcc, 0xdd };
+
+    const U = extern union {
+        a: u32,
+        b: u8,
+    };
+
+    const reinterpreted = comptime b: {
+        var u: U = undefined;
+        u = std.mem.bytesAsValue(U, &bytes).*;
+        break :b u;
+    };
+
+    var u: U = reinterpreted;
+    try expect(u.a == littleToNativeEndian(u32, 0xddccbbaa));
+    try expect(u.b == 0xaa);
+}
+
+test "store of comptime reinterpreted memory to packed union" {
+    const bytes = [_]u8{ 0xaa, 0xbb, 0xcc, 0xdd };
+
+    const U = packed union {
+        a: u32,
+        b: u8,
+    };
+
+    const reinterpreted = comptime b: {
+        var u: U = undefined;
+        u = std.mem.bytesAsValue(U, &bytes).*;
+        break :b u;
+    };
+
+    var u: U = reinterpreted;
+    try expect(u.a == littleToNativeEndian(u32, 0xddccbbaa));
+    try expect(u.b == if (endian == .little) 0xaa else 0xdd);
+}
+
 test "union field is a pointer to an aligned version of itself" {
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
@@ -1902,3 +2022,23 @@ test "pass register-sized field as non-register-sized union" {
     try S.untaggedUnion(.{ .x = x });
     try S.externUnion(.{ .x = x });
 }
+
+test "circular dependency through pointer field of a union" {
+    const S = struct {
+        const UnionInner = extern struct {
+            outer: UnionOuter = std.mem.zeroes(UnionOuter),
+        };
+
+        const UnionMiddle = extern union {
+            outer: ?*UnionOuter,
+            inner: ?*UnionInner,
+        };
+
+        const UnionOuter = extern struct {
+            u: UnionMiddle = std.mem.zeroes(UnionMiddle),
+        };
+    };
+    var outer: S.UnionOuter = .{};
+    try expect(outer.u.outer == null);
+    try expect(outer.u.inner == null);
+}

test/cases/compile_errors/struct_depends_on_pointer_alignment.zig

@@ -0,0 +1,11 @@
+const S = struct {
+    next: ?*align(1) S align(128),
+};
+
+export fn entry() usize {
+    return @alignOf(S);
+}
+
+// error
+//
+// :1:11: error: struct layout depends on being pointer aligned