stage2: lower each struct field type, align, init separately

Previously, struct types, alignment values, and initialization
expressions were all lowered into the same ZIR body, which caused false
positive "depends on itself" errors when the initialization expression
depended on the size of the struct.

This also uses ResultLoc.coerced_ty for struct field alignment and
initialization values. The resulting ZIR encoding ends up being roughly
the same, neither smaller nor larger than previously.

Closes ziglang#12029

Author: andrewrk

src/AstGen.zig

@@ -4148,7 +4148,6 @@ fn structDeclInner(
             .src_node = node,
             .layout = layout,
             .fields_len = 0,
-            .body_len = 0,
             .decls_len = 0,
             .known_non_opv = false,
             .known_comptime_only = false,
@@ -4192,6 +4191,19 @@ fn structDeclInner(
     var wip_members = try WipMembers.init(gpa, &astgen.scratch, decl_count, field_count, bits_per_field, max_field_size);
     defer wip_members.deinit();
 
+    // We will use the scratch buffer, starting here, for the bodies:
+    //    bodies: { // for every fields_len
+    //        field_type_body_inst: Inst, // for each field_type_body_len
+    //        align_body_inst: Inst, // for each align_body_len
+    //        init_body_inst: Inst, // for each init_body_len
+    //    }
+    // Note that the scratch buffer is simultaneously being used by WipMembers, however
+    // it will not access any elements beyond this point in the ArrayList. It also
+    // accesses via the ArrayList items field so it can handle the scratch buffer being
+    // reallocated.
+    // No defer needed here because it is handled by `wip_members.deinit()` above.
+    const bodies_start = astgen.scratch.items.len;
+
     var known_non_opv = false;
     var known_comptime_only = false;
     for (container_decl.ast.members) |member_node| {
@@ -4203,57 +4215,78 @@ fn structDeclInner(
         const field_name = try astgen.identAsString(member.ast.name_token);
         wip_members.appendToField(field_name);
 
+        const doc_comment_index = try astgen.docCommentAsString(member.firstToken());
+        wip_members.appendToField(doc_comment_index);
+
         if (member.ast.type_expr == 0) {
             return astgen.failTok(member.ast.name_token, "struct field missing type", .{});
         }
 
         const field_type = try typeExpr(&block_scope, &namespace.base, member.ast.type_expr);
-        wip_members.appendToField(@enumToInt(field_type));
-
-        const doc_comment_index = try astgen.docCommentAsString(member.firstToken());
-        wip_members.appendToField(doc_comment_index);
-
+        const have_type_body = !block_scope.isEmpty();
         const have_align = member.ast.align_expr != 0;
         const have_value = member.ast.value_expr != 0;
         const is_comptime = member.comptime_token != null;
-        const unused = false;
 
         if (!is_comptime) {
             known_non_opv = known_non_opv or
                 nodeImpliesMoreThanOnePossibleValue(tree, member.ast.type_expr);
             known_comptime_only = known_comptime_only or
                 nodeImpliesComptimeOnly(tree, member.ast.type_expr);
         }
-        wip_members.nextField(bits_per_field, .{ have_align, have_value, is_comptime, unused });
+        wip_members.nextField(bits_per_field, .{ have_align, have_value, is_comptime, have_type_body });
+
+        if (have_type_body) {
+            if (!block_scope.endsWithNoReturn()) {
+                _ = try block_scope.addBreak(.break_inline, decl_inst, field_type);
+            }
+            const body = block_scope.instructionsSlice();
+            const old_scratch_len = astgen.scratch.items.len;
+            try astgen.scratch.ensureUnusedCapacity(gpa, countBodyLenAfterFixups(astgen, body));
+            appendBodyWithFixupsArrayList(astgen, &astgen.scratch, body);
+            wip_members.appendToField(@intCast(u32, astgen.scratch.items.len - old_scratch_len));
+            block_scope.instructions.items.len = block_scope.instructions_top;
+        } else {
+            wip_members.appendToField(@enumToInt(field_type));
+        }
 
         if (have_align) {
             if (layout == .Packed) {
                 try astgen.appendErrorNode(member.ast.align_expr, "unable to override alignment of packed struct fields", .{});
             }
-            const align_inst = try expr(&block_scope, &namespace.base, align_rl, member.ast.align_expr);
-            wip_members.appendToField(@enumToInt(align_inst));
+            const align_ref = try expr(&block_scope, &namespace.base, coerced_align_rl, member.ast.align_expr);
+            if (!block_scope.endsWithNoReturn()) {
+                _ = try block_scope.addBreak(.break_inline, decl_inst, align_ref);
+            }
+            const body = block_scope.instructionsSlice();
+            const old_scratch_len = astgen.scratch.items.len;
+            try astgen.scratch.ensureUnusedCapacity(gpa, countBodyLenAfterFixups(astgen, body));
+            appendBodyWithFixupsArrayList(astgen, &astgen.scratch, body);
+            wip_members.appendToField(@intCast(u32, astgen.scratch.items.len - old_scratch_len));
+            block_scope.instructions.items.len = block_scope.instructions_top;
         }
+
         if (have_value) {
-            const rl: ResultLoc = if (field_type == .none) .none else .{ .ty = field_type };
+            const rl: ResultLoc = if (field_type == .none) .none else .{ .coerced_ty = field_type };
 
             const default_inst = try expr(&block_scope, &namespace.base, rl, member.ast.value_expr);
-            wip_members.appendToField(@enumToInt(default_inst));
+            if (!block_scope.endsWithNoReturn()) {
+                _ = try block_scope.addBreak(.break_inline, decl_inst, default_inst);
+            }
+            const body = block_scope.instructionsSlice();
+            const old_scratch_len = astgen.scratch.items.len;
+            try astgen.scratch.ensureUnusedCapacity(gpa, countBodyLenAfterFixups(astgen, body));
+            appendBodyWithFixupsArrayList(astgen, &astgen.scratch, body);
+            wip_members.appendToField(@intCast(u32, astgen.scratch.items.len - old_scratch_len));
+            block_scope.instructions.items.len = block_scope.instructions_top;
         } else if (member.comptime_token) |comptime_token| {
             return astgen.failTok(comptime_token, "comptime field without default initialization value", .{});
         }
     }
 
-    if (!block_scope.isEmpty()) {
-        _ = try block_scope.addBreak(.break_inline, decl_inst, .void_value);
-    }
-
-    const body = block_scope.instructionsSlice();
-    const body_len = astgen.countBodyLenAfterFixups(body);
-
     try gz.setStruct(decl_inst, .{
         .src_node = node,
         .layout = layout,
-        .body_len = body_len,
         .fields_len = field_count,
         .decls_len = decl_count,
         .known_non_opv = known_non_opv,
@@ -4263,10 +4296,11 @@ fn structDeclInner(
     wip_members.finishBits(bits_per_field);
     const decls_slice = wip_members.declsSlice();
     const fields_slice = wip_members.fieldsSlice();
-    try astgen.extra.ensureUnusedCapacity(gpa, decls_slice.len + body_len + fields_slice.len);
+    const bodies_slice = astgen.scratch.items[bodies_start..];
+    try astgen.extra.ensureUnusedCapacity(gpa, decls_slice.len + fields_slice.len + bodies_slice.len);
     astgen.extra.appendSliceAssumeCapacity(decls_slice);
-    astgen.appendBodyWithFixups(body);
     astgen.extra.appendSliceAssumeCapacity(fields_slice);
+    astgen.extra.appendSliceAssumeCapacity(bodies_slice);
 
     block_scope.unstack();
     try gz.addNamespaceCaptures(&namespace);
@@ -10981,7 +11015,6 @@ const GenZir = struct {
 
     fn setStruct(gz: *GenZir, inst: Zir.Inst.Index, args: struct {
         src_node: Ast.Node.Index,
-        body_len: u32,
         fields_len: u32,
         decls_len: u32,
         layout: std.builtin.Type.ContainerLayout,
@@ -10998,9 +11031,6 @@ const GenZir = struct {
             const node_offset = gz.nodeIndexToRelative(args.src_node);
             astgen.extra.appendAssumeCapacity(@bitCast(u32, node_offset));
         }
-        if (args.body_len != 0) {
-            astgen.extra.appendAssumeCapacity(args.body_len);
-        }
         if (args.fields_len != 0) {
             astgen.extra.appendAssumeCapacity(args.fields_len);
         }
@@ -11013,7 +11043,6 @@ const GenZir = struct {
                 .opcode = .struct_decl,
                 .small = @bitCast(u16, Zir.Inst.StructDecl.Small{
                     .has_src_node = args.src_node != 0,
-                    .has_body_len = args.body_len != 0,
                     .has_fields_len = args.fields_len != 0,
                     .has_decls_len = args.decls_len != 0,
                     .known_non_opv = args.known_non_opv,

src/Module.zig

@@ -916,13 +916,14 @@ pub const Struct = struct {
     /// one possible value.
     known_non_opv: bool,
     requires_comptime: PropertyBoolean = .unknown,
+    have_field_inits: bool = false,
 
     pub const Fields = std.StringArrayHashMapUnmanaged(Field);
 
     /// The `Type` and `Value` memory is owned by the arena of the Struct's owner_decl.
     pub const Field = struct {
         /// Uses `noreturn` to indicate `anytype`.
-        /// undefined until `status` is `have_field_types` or `have_layout`.
+        /// undefined until `status` is >= `have_field_types`.
         ty: Type,
         /// Uses `unreachable_value` to indicate no default.
         default_val: Value,