zig fmt: implement container decls

lib/std/zig/ast.zig

@@ -365,6 +365,26 @@ pub const Tree = struct {
                 }
             },
 
+            .ContainerDecl,
+            .ContainerDeclComma,
+            .ContainerDeclTwo,
+            .ContainerDeclTwoComma,
+            .ContainerDeclArg,
+            .ContainerDeclArgComma,
+            .TaggedUnion,
+            .TaggedUnionComma,
+            .TaggedUnionTwo,
+            .TaggedUnionTwoComma,
+            .TaggedUnionEnumTag,
+            .TaggedUnionEnumTagComma,
+            => {
+                const main_token = main_tokens[n];
+                switch (token_tags[main_token - 1]) {
+                    .Keyword_packed, .Keyword_extern => return main_token - 1,
+                    else => return main_token,
+                }
+            },
+
             .PtrTypeAligned => unreachable, // TODO
             .PtrTypeSentinel => unreachable, // TODO
             .PtrType => unreachable, // TODO
@@ -375,10 +395,6 @@ pub const Tree = struct {
             .While => unreachable, // TODO
             .ForSimple => unreachable, // TODO
             .For => unreachable, // TODO
-            .ContainerDecl => unreachable, // TODO
-            .ContainerDeclArg => unreachable, // TODO
-            .TaggedUnion => unreachable, // TODO
-            .TaggedUnionEnumTag => unreachable, // TODO
             .AsmOutput => unreachable, // TODO
             .AsmInput => unreachable, // TODO
             .ErrorValue => unreachable, // TODO
@@ -408,6 +424,7 @@ pub const Tree = struct {
             .Break,
             .Return,
             .Nosuspend,
+            .Comptime,
             => n = datas[n].lhs,
 
             .TestDecl,
@@ -455,7 +472,6 @@ pub const Tree = struct {
             .BoolOr,
             .AnyFrameType,
             .ErrorUnion,
-            .Comptime,
             .IfSimple,
             .WhileSimple,
             => n = datas[n].rhs,
@@ -490,13 +506,37 @@ pub const Tree = struct {
                 }
                 n = tree.extra_data[params.end - 1]; // last parameter
             },
-            .Block => {
+            .ContainerDeclArg => {
+                const members = tree.extraData(datas[n].rhs, Node.SubRange);
+                if (members.end - members.start == 0) {
+                    end_offset += 1; // for the rparen
+                    n = datas[n].lhs;
+                } else {
+                    end_offset += 1; // for the rbrace
+                    n = tree.extra_data[members.end - 1]; // last parameter
+                }
+            },
+            .ContainerDeclArgComma => {
+                const members = tree.extraData(datas[n].rhs, Node.SubRange);
+                assert(members.end - members.start > 0);
+                end_offset += 2; // for the comma + rbrace
+                n = tree.extra_data[members.end - 1]; // last parameter
+            },
+            .Block,
+            .ContainerDecl,
+            .TaggedUnion,
+            => {
                 end_offset += 1; // for the rbrace
                 if (datas[n].rhs - datas[n].lhs == 0) {
                     return main_tokens[n] + end_offset;
                 }
                 n = tree.extra_data[datas[n].rhs - 1]; // last statement
             },
+            .ContainerDeclComma, .TaggedUnionComma => {
+                assert(datas[n].rhs - datas[n].lhs > 0);
+                end_offset += 2; // for the comma + rbrace
+                n = tree.extra_data[datas[n].rhs - 1]; // last member
+            },
             .CallOne,
             .ArrayAccess,
             => {
@@ -511,6 +551,8 @@ pub const Tree = struct {
             .BuiltinCallTwo,
             .BlockTwo,
             .StructInitDotTwo,
+            .ContainerDeclTwo,
+            .TaggedUnionTwo,
             => {
                 end_offset += 1; // for the rparen/rbrace
                 if (datas[n].rhs != 0) {
@@ -523,6 +565,8 @@ pub const Tree = struct {
             },
             .ArrayInitDotTwoComma,
             .StructInitDotTwoComma,
+            .ContainerDeclTwoComma,
+            .TaggedUnionTwoComma,
             => {
                 end_offset += 2; // for the comma + rbrace
                 if (datas[n].rhs != 0) {
@@ -589,6 +633,38 @@ pub const Tree = struct {
                     }
                 }
             },
+            .ContainerFieldInit => {
+                if (datas[n].rhs != 0) {
+                    n = datas[n].rhs;
+                } else if (datas[n].lhs != 0) {
+                    n = datas[n].lhs;
+                } else {
+                    return main_tokens[n] + end_offset;
+                }
+            },
+            .ContainerFieldAlign => {
+                if (datas[n].rhs != 0) {
+                    end_offset += 1; // for the rparen
+                    n = datas[n].rhs;
+                } else if (datas[n].lhs != 0) {
+                    n = datas[n].lhs;
+                } else {
+                    return main_tokens[n] + end_offset;
+                }
+            },
+            .ContainerField => {
+                const extra = tree.extraData(datas[n].rhs, Node.ContainerField);
+                if (extra.value_expr != 0) {
+                    n = extra.value_expr;
+                } else if (extra.align_expr != 0) {
+                    end_offset += 1; // for the rparen
+                    n = extra.align_expr;
+                } else if (datas[n].lhs != 0) {
+                    n = datas[n].lhs;
+                } else {
+                    return main_tokens[n] + end_offset;
+                }
+            },
 
             // These are not supported by lastToken() because implementation would
             // require recursion due to the optional comma followed by rbrace.
@@ -600,10 +676,9 @@ pub const Tree = struct {
             .StructInit => unreachable,
             .StructInitOne => unreachable,
             .StructInitDot => unreachable,
-            .ContainerFieldInit => unreachable,
-            .ContainerFieldAlign => unreachable,
-            .ContainerField => unreachable,
 
+            .TaggedUnionEnumTag => unreachable, // TODO
+            .TaggedUnionEnumTagComma => unreachable, // TODO
             .Switch => unreachable, // TODO
             .If => unreachable, // TODO
             .Continue => unreachable, // TODO
@@ -631,10 +706,6 @@ pub const Tree = struct {
             .FnProtoMulti => unreachable, // TODO
             .FnProtoOne => unreachable, // TODO
             .FnProto => unreachable, // TODO
-            .ContainerDecl => unreachable, // TODO
-            .ContainerDeclArg => unreachable, // TODO
-            .TaggedUnion => unreachable, // TODO
-            .TaggedUnionEnumTag => unreachable, // TODO
             .AsmOutput => unreachable, // TODO
             .AsmInput => unreachable, // TODO
             .ErrorValue => unreachable, // TODO
@@ -952,6 +1023,93 @@ pub const Tree = struct {
         };
     }
 
+    pub fn containerDeclTwo(tree: Tree, buffer: *[2]Node.Index, node: Node.Index) Full.ContainerDecl {
+        assert(tree.nodes.items(.tag)[node] == .ContainerDeclTwo or
+            tree.nodes.items(.tag)[node] == .ContainerDeclTwoComma);
+        const data = tree.nodes.items(.data)[node];
+        buffer.* = .{ data.lhs, data.rhs };
+        const members = if (data.rhs != 0)
+            buffer[0..2]
+        else if (data.lhs != 0)
+            buffer[0..1]
+        else
+            buffer[0..0];
+        return tree.fullContainerDecl(.{
+            .main_token = tree.nodes.items(.main_token)[node],
+            .enum_token = null,
+            .members = members,
+            .arg = 0,
+        });
+    }
+
+    pub fn containerDecl(tree: Tree, node: Node.Index) Full.ContainerDecl {
+        assert(tree.nodes.items(.tag)[node] == .ContainerDecl or
+            tree.nodes.items(.tag)[node] == .ContainerDeclComma);
+        const data = tree.nodes.items(.data)[node];
+        return tree.fullContainerDecl(.{
+            .main_token = tree.nodes.items(.main_token)[node],
+            .enum_token = null,
+            .members = tree.extra_data[data.lhs..data.rhs],
+            .arg = 0,
+        });
+    }
+
+    pub fn containerDeclArg(tree: Tree, node: Node.Index) Full.ContainerDecl {
+        assert(tree.nodes.items(.tag)[node] == .ContainerDeclArg);
+        const data = tree.nodes.items(.data)[node];
+        const members_range = tree.extraData(data.rhs, Node.SubRange);
+        return tree.fullContainerDecl(.{
+            .main_token = tree.nodes.items(.main_token)[node],
+            .enum_token = null,
+            .members = tree.extra_data[members_range.start..members_range.end],
+            .arg = data.lhs,
+        });
+    }
+
+    pub fn taggedUnionTwo(tree: Tree, buffer: *[2]Node.Index, node: Node.Index) Full.ContainerDecl {
+        assert(tree.nodes.items(.tag)[node] == .TaggedUnionTwo);
+        const data = tree.nodes.items(.data)[node];
+        buffer.* = .{ data.lhs, data.rhs };
+        const members = if (data.rhs != 0)
+            buffer[0..2]
+        else if (data.lhs != 0)
+            buffer[0..1]
+        else
+            buffer[0..0];
+        const main_token = tree.nodes.items(.main_token)[node];
+        return tree.fullContainerDecl(.{
+            .main_token = main_token,
+            .enum_token = main_token + 2, // union lparen enum
+            .members = members,
+            .arg = 0,
+        });
+    }
+
+    pub fn taggedUnion(tree: Tree, node: Node.Index) Full.ContainerDecl {
+        assert(tree.nodes.items(.tag)[node] == .TaggedUnion);
+        const data = tree.nodes.items(.data)[node];
+        const main_token = tree.nodes.items(.main_token)[node];
+        return tree.fullContainerDecl(.{
+            .main_token = main_token,
+            .enum_token = main_token + 2, // union lparen enum
+            .members = tree.extra_data[data.lhs..data.rhs],
+            .arg = 0,
+        });
+    }
+
+    pub fn taggedUnionEnumTag(tree: Tree, node: Node.Index) Full.ContainerDecl {
+        assert(tree.nodes.items(.tag)[node] == .TaggedUnionEnumTag);
+        const data = tree.nodes.items(.data)[node];
+        const members_range = tree.extraData(data.rhs, Node.SubRange);
+        const main_token = tree.nodes.items(.main_token)[node];
+        return tree.fullContainerDecl(.{
+            .main_token = main_token,
+            .enum_token = main_token + 2, // union lparen enum
+            .members = tree.extra_data[data.lhs..data.rhs],
+            .arg = data.lhs,
+        });
+    }
+
     fn fullVarDecl(tree: Tree, info: Full.VarDecl.Ast) Full.VarDecl {
         const token_tags = tree.tokens.items(.tag);
         var result: Full.VarDecl = .{
@@ -1031,6 +1189,19 @@ pub const Tree = struct {
         };
         return result;
     }
+
+    fn fullContainerDecl(tree: Tree, info: Full.ContainerDecl.Ast) Full.ContainerDecl {
+        const token_tags = tree.tokens.items(.tag);
+        var result: Full.ContainerDecl = .{
+            .ast = info,
+            .layout_token = null,
+        };
+        switch (token_tags[info.main_token - 1]) {
+            .Keyword_extern, .Keyword_packed => result.layout_token = info.main_token - 1,
+            else => {},
+        }
+        return result;
+    }
 };
 
 /// Fully assembled AST node information.
@@ -1125,6 +1296,19 @@ pub const Full = struct {
             elem_type: Node.Index,
         };
     };
+
+    pub const ContainerDecl = struct {
+        layout_token: ?TokenIndex,
+        ast: Ast,
+
+        pub const Ast = struct {
+            main_token: TokenIndex,
+            /// Populated when main_token is Keyword_union.
+            enum_token: ?TokenIndex,
+            members: []const Node.Index,
+            arg: Node.Index,
+        };
+    };
 };
 
 pub const Error = union(enum) {
@@ -1543,9 +1727,11 @@ pub const Node = struct {
         StructInitOne,
         /// `.{.a = lhs, .b = rhs}`. lhs and rhs can be omitted.
         /// main_token is the lbrace.
+        /// No trailing comma before the rbrace.
         StructInitDotTwo,
         /// Same as `StructInitDotTwo` except there is known to be a trailing comma
-        /// before the final rbrace.
+        /// before the final rbrace. This tag exists to facilitate lastToken() implemented
+        /// without recursion.
         StructInitDotTwoComma,
         /// `.{.a = b, .c = d}`. `sub_list[lhs..rhs]`.
         /// main_token is the lbrace.
@@ -1655,21 +1841,50 @@ pub const Node = struct {
         /// `error{a, b}`.
         /// lhs and rhs both unused.
         ErrorSetDecl,
-        /// `struct {}`, `union {}`, etc. `sub_list[lhs..rhs]`.
+        /// `struct {}`, `union {}`, `opaque {}`, `enum {}`. `extra_data[lhs..rhs]`.
+        /// main_token is `struct`, `union`, `opaque`, `enum` keyword.
         ContainerDecl,
-        /// `union(lhs)` / `enum(lhs)`. `sub_range_list[rhs]`.
+        /// Same as ContainerDecl but there is known to be a trailing comma before the rbrace.
+        ContainerDeclComma,
+        /// `struct {lhs, rhs}`, `union {lhs, rhs}`, `opaque {lhs, rhs}`, `enum {lhs, rhs}`.
+        /// lhs or rhs can be omitted.
+        /// main_token is `struct`, `union`, `opaque`, `enum` keyword.
+        ContainerDeclTwo,
+        /// Same as ContainerDeclTwo except there is known to be a trailing comma
+        /// before the rbrace.
+        ContainerDeclTwoComma,
+        /// `union(lhs)` / `enum(lhs)`. `SubRange[rhs]`.
         ContainerDeclArg,
+        /// Same as ContainerDeclArg but there is known to be a trailing comma before the rbrace.
+        ContainerDeclArgComma,
         /// `union(enum) {}`. `sub_list[lhs..rhs]`.
         /// Note that tagged unions with explicitly provided enums are represented
         /// by `ContainerDeclArg`.
         TaggedUnion,
-        /// `union(enum(lhs)) {}`. `sub_list_range[rhs]`.
+        /// Same as TaggedUnion but there is known to be a trailing comma before the rbrace.
+        TaggedUnionComma,
+        /// `union(enum) {lhs, rhs}`. lhs or rhs may be omitted.
+        /// Note that tagged unions with explicitly provided enums are represented
+        /// by `ContainerDeclArg`.
+        TaggedUnionTwo,
+        /// Same as TaggedUnionTwo but there is known to be a trailing comma before the rbrace.
+        TaggedUnionTwoComma,
+        /// `union(enum(lhs)) {}`. `SubRange[rhs]`.
         TaggedUnionEnumTag,
+        /// Same as TaggedUnionEnumTag but there is known to be a trailing comma
+        /// before the rbrace.
+        TaggedUnionEnumTagComma,
         /// `a: lhs = rhs,`. lhs and rhs can be omitted.
+        /// main_token is the field name identifier.
+        /// lastToken() does not include the possible trailing comma.
         ContainerFieldInit,
         /// `a: lhs align(rhs),`. rhs can be omitted.
+        /// main_token is the field name identifier.
+        /// lastToken() does not include the possible trailing comma.
         ContainerFieldAlign,
         /// `a: lhs align(c) = d,`. `container_field_list[rhs]`.
+        /// main_token is the field name identifier.
+        /// lastToken() does not include the possible trailing comma.
         ContainerField,
         /// `anytype`. both lhs and rhs unused.
         /// Used by `ContainerField`.
@@ -1699,6 +1914,17 @@ pub const Node = struct {
         ErrorValue,
         /// `lhs!rhs`. main_token is the `!`.
         ErrorUnion,
+
+        pub fn isContainerField(tag: Tag) bool {
+            return switch (tag) {
+                .ContainerFieldInit,
+                .ContainerFieldAlign,
+                .ContainerField,
+                => true,
+
+                else => false,
+            };
+        }
     };
 
     pub const Data = struct {

lib/std/zig/parse.zig

@@ -64,9 +64,10 @@ pub fn parse(gpa: *Allocator, source: []const u8) Allocator.Error!Tree {
             .rhs = undefined,
         },
     });
-    const root_decls = try parser.parseContainerMembers(true);
+    const root_members = try parser.parseContainerMembers();
-    // parseContainerMembers will try to skip as much
+    const root_decls = try root_members.toSpan(&parser);
-    // invalid tokens as it can, so we are now at EOF.
+    // parseContainerMembers will try to skip as much invalid tokens as
+    // it can, so we are now at EOF.
     assert(parser.token_tags[parser.tok_i] == .Eof);
     parser.nodes.items(.data)[0] = .{
         .lhs = root_decls.start,
@@ -108,6 +109,22 @@ const Parser = struct {
         }
     };
 
+    const Members = struct {
+        len: usize,
+        lhs: Node.Index,
+        rhs: Node.Index,
+        trailing_comma: bool,
+
+        fn toSpan(self: Members, p: *Parser) !Node.SubRange {
+            if (self.len <= 2) {
+                const nodes = [2]Node.Index{ self.lhs, self.rhs };
+                return p.listToSpan(nodes[0..self.len]);
+            } else {
+                return Node.SubRange{ .start = self.lhs, .end = self.rhs };
+            }
+        }
+    };
+
     fn listToSpan(p: *Parser, list: []const Node.Index) !Node.SubRange {
         try p.extra_data.appendSlice(p.gpa, list);
         return Node.SubRange{
@@ -151,171 +168,227 @@ const Parser = struct {
     ///      / ContainerField COMMA ContainerMembers
     ///      / ContainerField
     ///      /
-    fn parseContainerMembers(p: *Parser, top_level: bool) !Node.SubRange {
+    /// TopLevelComptime <- KEYWORD_comptime BlockExpr
+    fn parseContainerMembers(p: *Parser) !Members {
         var list = std.ArrayList(Node.Index).init(p.gpa);
         defer list.deinit();
 
         var field_state: union(enum) {
-            /// no fields have been seen
+            /// No fields have been seen.
             none,
-            /// currently parsing fields
+            /// Currently parsing fields.
             seen,
-            /// saw fields and then a declaration after them.
+            /// Saw fields and then a declaration after them.
-            /// payload is first token of previous declaration.
+            /// Payload is first token of previous declaration.
-            end: TokenIndex,
+            end: Node.Index,
-            /// ther was a declaration between fields, don't report more errors
+            /// There was a declaration between fields, don't report more errors.
             err,
         } = .none;
 
         // Skip container doc comments.
         while (p.eatToken(.ContainerDocComment)) |_| {}
 
+        var trailing_comma = false;
         while (true) {
             const doc_comment = p.eatDocComments();
 
-            const test_decl_node = p.parseTestDecl() catch |err| switch (err) {
+            switch (p.token_tags[p.tok_i]) {
-                error.OutOfMemory => return error.OutOfMemory,
+                .Keyword_test => {
-                error.ParseError => {
+                    const test_decl_node = try p.expectTestDeclRecoverable();
-                    p.findNextContainerMember();
-                    continue;
-                },
-            };
                     if (test_decl_node != 0) {
                         if (field_state == .seen) {
-                    field_state = .{ .end = p.nodes.items(.main_token)[test_decl_node] };
+                            field_state = .{ .end = test_decl_node };
                         }
                         try list.append(test_decl_node);
-                continue;
                     }
-
+                    trailing_comma = false;
-            const comptime_node = p.parseTopLevelComptime() catch |err| switch (err) {
-                error.OutOfMemory => return error.OutOfMemory,
-                error.ParseError => {
-                    p.findNextContainerMember();
-                    continue;
                 },
-            };
+                .Keyword_comptime => switch (p.token_tags[p.tok_i + 1]) {
-            if (comptime_node != 0) {
+                    .Identifier => {
-                if (field_state == .seen) {
+                        p.tok_i += 1;
-                    field_state = .{ .end = p.nodes.items(.main_token)[comptime_node] };
+                        const container_field = try p.expectContainerFieldRecoverable();
-                }
-                try list.append(comptime_node);
-                continue;
-            }
-
-            const visib_token = p.eatToken(.Keyword_pub);
-
-            const top_level_decl = p.parseTopLevelDecl() catch |err| switch (err) {
-                error.OutOfMemory => return error.OutOfMemory,
-                error.ParseError => {
-                    p.findNextContainerMember();
-                    continue;
-                },
-            };
-            if (top_level_decl != 0) {
-                if (field_state == .seen) {
-                    field_state = .{
-                        .end = visib_token orelse p.nodes.items(.main_token)[top_level_decl],
-                    };
-                }
-                try list.append(top_level_decl);
-                continue;
-            }
-
-            if (visib_token != null) {
-                try p.warn(.{ .ExpectedPubItem = .{ .token = p.tok_i } });
-                // ignore this pub
-                continue;
-            }
-
-            const container_field = p.parseContainerField() catch |err| switch (err) {
-                error.OutOfMemory => return error.OutOfMemory,
-                error.ParseError => {
-                    // attempt to recover
-                    p.findNextContainerMember();
-                    continue;
-                },
-            };
                         if (container_field != 0) {
                             switch (field_state) {
                                 .none => field_state = .seen,
                                 .err, .seen => {},
-                    .end => |tok| {
+                                .end => |node| {
-                        try p.warn(.{ .DeclBetweenFields = .{ .token = tok } });
+                                    try p.warn(.{
-                        // continue parsing, error will be reported later
+                                        .DeclBetweenFields = .{ .token = p.nodes.items(.main_token)[node] },
+                                    });
+                                    // Continue parsing; error will be reported later.
                                     field_state = .err;
                                 },
                             }
                             try list.append(container_field);
-                const comma = p.eatToken(.Comma) orelse {
+                            switch (p.token_tags[p.tok_i]) {
-                    // try to continue parsing
+                                .Comma => {
-                    const index = p.tok_i;
+                                    p.tok_i += 1;
-                    p.findNextContainerMember();
+                                    trailing_comma = true;
-                    const next = p.token_tags[p.tok_i];
+                                    continue;
-                    switch (next) {
+                                },
-                        .Eof => {
+                                .RBrace, .Eof => {
-                            // no invalid tokens were found
+                                    trailing_comma = false;
-                            if (index == p.tok_i) break;
+                                    break;
-
+                                },
-                            // Invalid tokens, add error and exit
+                                else => {},
+                            }
+                            // There is not allowed to be a decl after a field with no comma.
+                            // Report error but recover parser.
                             try p.warn(.{
-                                .ExpectedToken = .{ .token = index, .expected_id = .Comma },
+                                .ExpectedToken = .{ .token = p.tok_i, .expected_id = .Comma },
                             });
+                            p.findNextContainerMember();
+                        }
+                    },
+                    .LBrace => {
+                        const comptime_token = p.nextToken();
+                        const block = p.parseBlock() catch |err| switch (err) {
+                            error.OutOfMemory => return error.OutOfMemory,
+                            error.ParseError => blk: {
+                                p.findNextContainerMember();
+                                break :blk null_node;
+                            },
+                        };
+                        if (block != 0) {
+                            const comptime_node = try p.addNode(.{
+                                .tag = .Comptime,
+                                .main_token = comptime_token,
+                                .data = .{
+                                    .lhs = block,
+                                    .rhs = undefined,
+                                },
+                            });
+                            if (field_state == .seen) {
+                                field_state = .{ .end = comptime_node };
+                            }
+                            try list.append(comptime_node);
+                        }
+                        trailing_comma = false;
+                    },
+                    else => {
+                        p.tok_i += 1;
+                        try p.warn(.{ .ExpectedBlockOrField = .{ .token = p.tok_i } });
+                    },
+                },
+                .Keyword_pub => {
+                    p.tok_i += 1;
+                    const top_level_decl = try p.expectTopLevelDeclRecoverable();
+                    if (top_level_decl != 0) {
+                        if (field_state == .seen) {
+                            field_state = .{ .end = top_level_decl };
+                        }
+                        try list.append(top_level_decl);
+                    }
+                    trailing_comma = false;
+                },
+                .Keyword_usingnamespace => {
+                    const node = try p.expectUsingNamespaceRecoverable();
+                    if (node != 0) {
+                        if (field_state == .seen) {
+                            field_state = .{ .end = node };
+                        }
+                        try list.append(node);
+                    }
+                    trailing_comma = false;
+                },
+                .Keyword_const,
+                .Keyword_var,
+                .Keyword_threadlocal,
+                .Keyword_export,
+                .Keyword_extern,
+                .Keyword_inline,
+                .Keyword_noinline,
+                .Keyword_fn,
+                => {
+                    const top_level_decl = try p.expectTopLevelDeclRecoverable();
+                    if (top_level_decl != 0) {
+                        if (field_state == .seen) {
+                            field_state = .{ .end = top_level_decl };
+                        }
+                        try list.append(top_level_decl);
+                    }
+                    trailing_comma = false;
+                },
+                .Identifier => {
+                    const container_field = try p.expectContainerFieldRecoverable();
+                    if (container_field != 0) {
+                        switch (field_state) {
+                            .none => field_state = .seen,
+                            .err, .seen => {},
+                            .end => |node| {
+                                try p.warn(.{
+                                    .DeclBetweenFields = .{ .token = p.nodes.items(.main_token)[node] },
+                                });
+                                // Continue parsing; error will be reported later.
+                                field_state = .err;
+                            },
+                        }
+                        try list.append(container_field);
+                        switch (p.token_tags[p.tok_i]) {
+                            .Comma => {
+                                p.tok_i += 1;
+                                trailing_comma = true;
+                                continue;
+                            },
+                            .RBrace, .Eof => {
+                                trailing_comma = false;
+                                break;
+                            },
+                            else => {},
+                        }
+                        // There is not allowed to be a decl after a field with no comma.
+                        // Report error but recover parser.
+                        try p.warn(.{
+                            .ExpectedToken = .{ .token = p.tok_i, .expected_id = .Comma },
+                        });
+                        p.findNextContainerMember();
+                    }
+                },
+                .Eof, .RBrace => {
+                    if (doc_comment) |tok| {
+                        try p.warn(.{ .UnattachedDocComment = .{ .token = tok } });
+                    }
                     break;
                 },
                 else => {
-                            if (next == .RBrace) {
+                    try p.warn(.{ .ExpectedContainerMembers = .{ .token = p.tok_i } });
-                                if (!top_level) break;
+                    // This was likely not supposed to end yet; try to find the next declaration.
-                                p.tok_i += 1;
+                    p.findNextContainerMember();
-                            }
-
-                            // add error and continue
-                            try p.warn(.{
-                                .ExpectedToken = .{ .token = index, .expected_id = .Comma },
-                            });
-                            continue;
                 },
             }
-                };
-                continue;
         }
 
-            // Dangling doc comment
+        switch (list.items.len) {
-            if (doc_comment) |tok| {
+            0 => return Members{
-                try p.warn(.{
+                .len = 0,
-                    .UnattachedDocComment = .{ .token = tok },
+                .lhs = 0,
-                });
+                .rhs = 0,
-            }
+                .trailing_comma = trailing_comma,
-
+            },
-            const next = p.token_tags[p.tok_i];
+            1 => return Members{
-            switch (next) {
+                .len = 1,
-                .Eof => break,
+                .lhs = list.items[0],
-                .Keyword_comptime => {
+                .rhs = 0,
-                    p.tok_i += 1;
+                .trailing_comma = trailing_comma,
-                    try p.warn(.{
+            },
-                        .ExpectedBlockOrField = .{ .token = p.tok_i },
+            2 => return Members{
-                    });
+                .len = 2,
+                .lhs = list.items[0],
+                .rhs = list.items[1],
+                .trailing_comma = trailing_comma,
             },
             else => {
-                    const index = p.tok_i;
+                const span = try p.listToSpan(list.items);
-                    if (next == .RBrace) {
+                return Members{
-                        if (!top_level) break;
+                    .len = list.items.len,
-                        p.tok_i += 1;
+                    .lhs = span.start,
-                    }
+                    .rhs = span.end,
-
+                    .trailing_comma = trailing_comma,
-                    // this was likely not supposed to end yet,
+                };
-                    // try to find the next declaration
-                    p.findNextContainerMember();
-                    try p.warn(.{
-                        .ExpectedContainerMembers = .{ .token = index },
-                    });
             },
         }
     }
 
-        return p.listToSpan(list.items);
-    }
-
     /// Attempts to find next container member by searching for certain tokens
     fn findNextContainerMember(p: *Parser) void {
         var level: u32 = 0;
@@ -398,44 +471,36 @@ const Parser = struct {
     }
 
     /// TestDecl <- KEYWORD_test STRINGLITERALSINGLE? Block
-    fn parseTestDecl(p: *Parser) !Node.Index {
+    fn expectTestDecl(p: *Parser) !Node.Index {
-        const test_token = p.eatToken(.Keyword_test) orelse return null_node;
+        const test_token = try p.expectToken(.Keyword_test);
-        const name_token = try p.expectToken(.StringLiteral);
+        const name_token = p.eatToken(.StringLiteral);
         const block_node = try p.parseBlock();
         if (block_node == 0) return p.fail(.{ .ExpectedLBrace = .{ .token = p.tok_i } });
         return p.addNode(.{
             .tag = .TestDecl,
             .main_token = test_token,
             .data = .{
-                .lhs = name_token,
+                .lhs = name_token orelse 0,
                 .rhs = block_node,
             },
         });
     }
 
-    /// TopLevelComptime <- KEYWORD_comptime BlockExpr
+    fn expectTestDeclRecoverable(p: *Parser) error{OutOfMemory}!Node.Index {
-    fn parseTopLevelComptime(p: *Parser) !Node.Index {
+        return p.expectTestDecl() catch |err| switch (err) {
-        if (p.token_tags[p.tok_i] == .Keyword_comptime and
+            error.OutOfMemory => return error.OutOfMemory,
-            p.token_tags[p.tok_i + 1] == .LBrace)
+            error.ParseError => {
-        {
+                p.findNextContainerMember();
-            return p.addNode(.{
-                .tag = .Comptime,
-                .main_token = p.nextToken(),
-                .data = .{
-                    .lhs = try p.parseBlock(),
-                    .rhs = undefined,
-                },
-            });
-        } else {
                 return null_node;
-        }
+            },
+        };
     }
 
     /// TopLevelDecl
     ///     <- (KEYWORD_export / KEYWORD_extern STRINGLITERALSINGLE? / (KEYWORD_inline / KEYWORD_noinline))? FnProto (SEMICOLON / Block)
     ///      / (KEYWORD_export / KEYWORD_extern STRINGLITERALSINGLE?)? KEYWORD_threadlocal? VarDecl
     ///      / KEYWORD_usingnamespace Expr SEMICOLON
-    fn parseTopLevelDecl(p: *Parser) !Node.Index {
+    fn expectTopLevelDecl(p: *Parser) !Node.Index {
         const extern_export_inline_token = p.nextToken();
         var expect_fn: bool = false;
         var exported: bool = false;
@@ -496,7 +561,21 @@ const Parser = struct {
             return p.fail(.{ .ExpectedVarDeclOrFn = .{ .token = p.tok_i } });
         }
 
-        const usingnamespace_token = p.eatToken(.Keyword_usingnamespace) orelse return null_node;
+        return p.expectUsingNamespace();
+    }
+
+    fn expectTopLevelDeclRecoverable(p: *Parser) error{OutOfMemory}!Node.Index {
+        return p.expectTopLevelDecl() catch |err| switch (err) {
+            error.OutOfMemory => return error.OutOfMemory,
+            error.ParseError => {
+                p.findNextContainerMember();
+                return null_node;
+            },
+        };
+    }
+
+    fn expectUsingNamespace(p: *Parser) !Node.Index {
+        const usingnamespace_token = try p.expectToken(.Keyword_usingnamespace);
         const expr = try p.expectExpr();
         const semicolon_token = try p.expectToken(.Semicolon);
         try p.parseAppendedDocComment(semicolon_token);
@@ -510,6 +589,16 @@ const Parser = struct {
         });
     }
 
+    fn expectUsingNamespaceRecoverable(p: *Parser) error{OutOfMemory}!Node.Index {
+        return p.expectUsingNamespace() catch |err| switch (err) {
+            error.OutOfMemory => return error.OutOfMemory,
+            error.ParseError => {
+                p.findNextContainerMember();
+                return null_node;
+            },
+        };
+    }
+
     /// FnProto <- KEYWORD_fn IDENTIFIER? LPAREN ParamDeclList RPAREN ByteAlign? LinkSection? CallConv? EXCLAMATIONMARK? (Keyword_anytype / TypeExpr)
     fn parseFnProto(p: *Parser) !Node.Index {
         const fn_token = p.eatToken(.Keyword_fn) orelse return null_node;
@@ -648,12 +737,9 @@ const Parser = struct {
     }
 
     /// ContainerField <- KEYWORD_comptime? IDENTIFIER (COLON TypeExpr ByteAlign?)? (EQUAL Expr)?
-    fn parseContainerField(p: *Parser) !Node.Index {
+    fn expectContainerField(p: *Parser) !Node.Index {
         const comptime_token = p.eatToken(.Keyword_comptime);
-        const name_token = p.eatToken(.Identifier) orelse {
+        const name_token = try p.expectToken(.Identifier);
-            if (comptime_token) |_| p.tok_i -= 1;
-            return null_node;
-        };
 
         var align_expr: Node.Index = 0;
         var type_expr: Node.Index = 0;
@@ -708,6 +794,16 @@ const Parser = struct {
         }
     }
 
+    fn expectContainerFieldRecoverable(p: *Parser) error{OutOfMemory}!Node.Index {
+        return p.expectContainerField() catch |err| switch (err) {
+            error.OutOfMemory => return error.OutOfMemory,
+            error.ParseError => {
+                p.findNextContainerMember();
+                return null_node;
+            },
+        };
+    }
+
     /// Statement
     ///     <- KEYWORD_comptime? VarDecl
     ///      / KEYWORD_comptime BlockExprStatement
@@ -3333,16 +3429,20 @@ const Parser = struct {
                             _ = try p.expectToken(.RParen);
 
                             _ = try p.expectToken(.LBrace);
-                            const members = try p.parseContainerMembers(false);
+                            const members = try p.parseContainerMembers();
+                            const members_span = try members.toSpan(p);
                             _ = try p.expectToken(.RBrace);
                             return p.addNode(.{
-                                .tag = .TaggedUnionEnumTag,
+                                .tag = switch (members.trailing_comma) {
+                                    true => .TaggedUnionEnumTagComma,
+                                    false => .TaggedUnionEnumTag,
+                                },
                                 .main_token = main_token,
                                 .data = .{
                                     .lhs = enum_tag_expr,
                                     .rhs = try p.addExtra(Node.SubRange{
-                                        .start = members.start,
+                                        .start = members_span.start,
-                                        .end = members.end,
+                                        .end = members_span.end,
                                     }),
                                 },
                             });
@@ -3350,16 +3450,34 @@ const Parser = struct {
                             _ = try p.expectToken(.RParen);
 
                             _ = try p.expectToken(.LBrace);
-                            const members = try p.parseContainerMembers(false);
+                            const members = try p.parseContainerMembers();
                             _ = try p.expectToken(.RBrace);
+                            if (members.len <= 2) {
                                 return p.addNode(.{
-                                .tag = .TaggedUnion,
+                                    .tag = switch (members.trailing_comma) {
+                                        true => .TaggedUnionTwoComma,
+                                        false => .TaggedUnionTwo,
+                                    },
                                     .main_token = main_token,
                                     .data = .{
-                                    .lhs = members.start,
+                                        .lhs = members.lhs,
-                                    .rhs = members.end,
+                                        .rhs = members.rhs,
                                     },
                                 });
+                            } else {
+                                const span = try members.toSpan(p);
+                                return p.addNode(.{
+                                    .tag = switch (members.trailing_comma) {
+                                        true => .TaggedUnionComma,
+                                        false => .TaggedUnion,
+                                    },
+                                    .main_token = main_token,
+                                    .data = .{
+                                        .lhs = span.start,
+                                        .rhs = span.end,
+                                    },
+                                });
+                            }
                         }
                     } else {
                         const expr = try p.expectExpr();
@@ -3373,26 +3491,48 @@ const Parser = struct {
             else => unreachable,
         };
         _ = try p.expectToken(.LBrace);
-        const members = try p.parseContainerMembers(false);
+        const members = try p.parseContainerMembers();
         _ = try p.expectToken(.RBrace);
         if (arg_expr == 0) {
+            if (members.len <= 2) {
                 return p.addNode(.{
-                .tag = .ContainerDecl,
+                    .tag = switch (members.trailing_comma) {
+                        true => .ContainerDeclTwoComma,
+                        false => .ContainerDeclTwo,
+                    },
                     .main_token = main_token,
                     .data = .{
-                    .lhs = members.start,
+                        .lhs = members.lhs,
-                    .rhs = members.end,
+                        .rhs = members.rhs,
                     },
                 });
             } else {
+                const span = try members.toSpan(p);
                 return p.addNode(.{
-                .tag = .ContainerDeclArg,
+                    .tag = switch (members.trailing_comma) {
+                        true => .ContainerDeclComma,
+                        false => .ContainerDecl,
+                    },
+                    .main_token = main_token,
+                    .data = .{
+                        .lhs = span.start,
+                        .rhs = span.end,
+                    },
+                });
+            }
+        } else {
+            const span = try members.toSpan(p);
+            return p.addNode(.{
+                .tag = switch (members.trailing_comma) {
+                    true => .ContainerDeclArgComma,
+                    false => .ContainerDeclArg,
+                },
                 .main_token = main_token,
                 .data = .{
                     .lhs = arg_expr,
                     .rhs = try p.addExtra(Node.SubRange{
-                        .start = members.start,
+                        .start = span.start,
-                        .end = members.end,
+                        .end = span.end,
                     }),
                 },
             });

lib/std/zig/parser_test.zig

@@ -149,45 +149,82 @@ test "zig fmt: nosuspend block" {
     );
 }
 
-//test "zig fmt: nosuspend await" {
+test "zig fmt: nosuspend await" {
-//    try testCanonical(
+    try testCanonical(
-//        \\fn foo() void {
+        \\fn foo() void {
-//        \\    x = nosuspend await y;
+        \\    x = nosuspend await y;
-//        \\}
+        \\}
-//        \\
+        \\
-//    );
+    );
-//}
+}
-//
+
-//test "zig fmt: trailing comma in container declaration" {
+test "zig fmt: container declaration, single line" {
-//    try testCanonical(
+    try testCanonical(
-//        \\const X = struct { foo: i32 };
+        \\const X = struct { foo: i32 };
-//        \\const X = struct { foo: i32, bar: i32 };
+        \\const X = struct { foo: i32, bar: i32 };
-//        \\const X = struct { foo: i32 = 1, bar: i32 = 2 };
+        \\const X = struct { foo: i32 = 1, bar: i32 = 2 };
-//        \\const X = struct { foo: i32 align(4), bar: i32 align(4) };
+        \\const X = struct { foo: i32 align(4), bar: i32 align(4) };
-//        \\const X = struct { foo: i32 align(4) = 1, bar: i32 align(4) = 2 };
+        \\const X = struct { foo: i32 align(4) = 1, bar: i32 align(4) = 2 };
-//        \\
+        \\
-//    );
+    );
-//    try testCanonical(
+}
-//        \\test "" {
+
-//        \\    comptime {
+test "zig fmt: container declaration, one item, multi line trailing comma" {
-//        \\        const X = struct {
+    try testCanonical(
-//        \\            x: i32
+        \\test "" {
-//        \\        };
+        \\    comptime {
-//        \\    }
+        \\        const X = struct {
-//        \\}
+        \\            x: i32,
-//        \\
+        \\        };
-//    );
+        \\    }
-//    try testTransform(
+        \\}
-//        \\const X = struct {
+        \\
-//        \\    foo: i32, bar: i8 };
+    );
-//    ,
+}
-//        \\const X = struct {
+
-//        \\    foo: i32, bar: i8
+test "zig fmt: container declaration, no trailing comma on separate line" {
-//        \\};
+    try testTransform(
-//        \\
+        \\test "" {
-//    );
+        \\    comptime {
-//}
+        \\        const X = struct {
-//
+        \\            x: i32
+        \\        };
+        \\    }
+        \\}
+        \\
+    ,
+        \\test "" {
+        \\    comptime {
+        \\        const X = struct { x: i32 };
+        \\    }
+        \\}
+        \\
+    );
+}
+
+test "zig fmt: container declaration, line break, no trailing comma" {
+    try testTransform(
+        \\const X = struct {
+        \\    foo: i32, bar: i8 };
+    ,
+        \\const X = struct { foo: i32, bar: i8 };
+        \\
+    );
+}
+
+test "zig fmt: container declaration, transform trailing comma" {
+    try testTransform(
+        \\const X = struct {
+        \\    foo: i32, bar: i8, };
+    ,
+        \\const X = struct {
+        \\    foo: i32,
+        \\    bar: i8,
+        \\};
+        \\
+    );
+}
+
 //test "zig fmt: trailing comma in fn parameter list" {
 //    try testCanonical(
 //        \\pub fn f(

lib/std/zig/render.zig

@@ -68,7 +68,7 @@ fn renderRoot(ais: *Ais, tree: ast.Tree) Error!void {
     const root_decls = tree.extra_data[nodes_data[0].lhs..nodes_data[0].rhs];
 
     for (root_decls) |decl| {
-        try renderContainerDecl(ais, tree, decl, .Newline);
+        try renderMember(ais, tree, decl, .Newline);
     }
 }
 
@@ -84,7 +84,7 @@ fn renderExtraNewlineToken(ais: *Ais, tree: ast.Tree, first_token: ast.TokenInde
     }
 }
 
-fn renderContainerDecl(ais: *Ais, tree: ast.Tree, decl: ast.Node.Index, space: Space) Error!void {
+fn renderMember(ais: *Ais, tree: ast.Tree, decl: ast.Node.Index, space: Space) Error!void {
     const token_tags = tree.tokens.items(.tag);
     const main_tokens = tree.nodes.items(.main_token);
     const datas = tree.nodes.items(.data);
@@ -158,6 +158,8 @@ fn renderContainerDecl(ais: *Ais, tree: ast.Tree, decl: ast.Node.Index, space: S
         .ContainerFieldAlign => return renderContainerField(ais, tree, tree.containerFieldAlign(decl), space),
         .ContainerField => return renderContainerField(ais, tree, tree.containerField(decl), space),
         .Comptime => return renderExpression(ais, tree, decl, space),
+
+        .Root => unreachable,
         else => unreachable,
     }
 }
@@ -195,7 +197,7 @@ fn renderExpression(ais: *Ais, tree: ast.Tree, node: ast.Node.Index, space: Spac
         //    return renderToken(ais, tree, any_type.token, space);
         //},
         .BlockTwo => {
-            var statements = [2]ast.Node.Index{ datas[node].lhs, datas[node].rhs };
+            const statements = [2]ast.Node.Index{ datas[node].lhs, datas[node].rhs };
             if (datas[node].lhs == 0) {
                 return renderBlock(ais, tree, main_tokens[node], statements[0..0], space);
             } else if (datas[node].rhs == 0) {
@@ -667,124 +669,29 @@ fn renderExpression(ais: *Ais, tree: ast.Tree, node: ast.Node.Index, space: Spac
         //    return renderToken(ais, tree, grouped_expr.rparen, space);
         //},
 
-        .ContainerDecl => unreachable, // TODO
+        .ContainerDecl,
-        .ContainerDeclArg => unreachable, // TODO
+        .ContainerDeclComma,
-        .TaggedUnion => unreachable, // TODO
+        => return renderContainerDecl(ais, tree, tree.containerDecl(node), space),
-        .TaggedUnionEnumTag => unreachable, // TODO
-        //.ContainerDecl => {
-        //    const container_decl = @fieldParentPtr(ast.Node.ContainerDecl, "base", base);
 
-        //    if (container_decl.layout_token) |layout_token| {
+        .ContainerDeclTwo, .ContainerDeclTwoComma => {
-        //        try renderToken(ais, tree, layout_token, Space.Space);
+            var buffer: [2]ast.Node.Index = undefined;
-        //    }
+            return renderContainerDecl(ais, tree, tree.containerDeclTwo(&buffer, node), space);
+        },
+        .ContainerDeclArg,
+        .ContainerDeclArgComma,
+        => return renderContainerDecl(ais, tree, tree.containerDeclArg(node), space),
 
-        //    switch (container_decl.init_arg_expr) {
+        .TaggedUnion,
-        //        .None => {
+        .TaggedUnionComma,
-        //            try renderToken(ais, tree, container_decl.kind_token, Space.Space); // union
+        => return renderContainerDecl(ais, tree, tree.taggedUnion(node), space),
-        //        },
-        //        .Enum => |enum_tag_type| {
-        //            try renderToken(ais, tree, container_decl.kind_token, Space.None); // union
 
-        //            const lparen = tree.nextToken(container_decl.kind_token);
+        .TaggedUnionTwo, .TaggedUnionTwoComma => {
-        //            const enum_token = tree.nextToken(lparen);
+            var buffer: [2]ast.Node.Index = undefined;
-
+            return renderContainerDecl(ais, tree, tree.taggedUnionTwo(&buffer, node), space);
-        //            try renderToken(ais, tree, lparen, Space.None); // (
+        },
-        //            try renderToken(ais, tree, enum_token, Space.None); // enum
+        .TaggedUnionEnumTag,
-
+        .TaggedUnionEnumTagComma,
-        //            if (enum_tag_type) |expr| {
+        => return renderContainerDecl(ais, tree, tree.taggedUnionEnumTag(node), space),
-        //                try renderToken(ais, tree, tree.nextToken(enum_token), Space.None); // (
-        //                try renderExpression(ais, tree, expr, Space.None);
-
-        //                const rparen = tree.nextToken(expr.lastToken());
-        //                try renderToken(ais, tree, rparen, Space.None); // )
-        //                try renderToken(ais, tree, tree.nextToken(rparen), Space.Space); // )
-        //            } else {
-        //                try renderToken(ais, tree, tree.nextToken(enum_token), Space.Space); // )
-        //            }
-        //        },
-        //        .Type => |type_expr| {
-        //            try renderToken(ais, tree, container_decl.kind_token, Space.None); // union
-
-        //            const lparen = tree.nextToken(container_decl.kind_token);
-        //            const rparen = tree.nextToken(type_expr.lastToken());
-
-        //            try renderToken(ais, tree, lparen, Space.None); // (
-        //            try renderExpression(ais, tree, type_expr, Space.None);
-        //            try renderToken(ais, tree, rparen, Space.Space); // )
-        //        },
-        //    }
-
-        //    if (container_decl.fields_and_decls_len == 0) {
-        //        {
-        //            ais.pushIndentNextLine();
-        //            defer ais.popIndent();
-        //            try renderToken(ais, tree, container_decl.lbrace_token, Space.None); // lbrace
-        //        }
-        //        return renderToken(ais, tree, container_decl.rbrace_token, space); // rbrace
-        //    }
-
-        //    const src_has_trailing_comma = blk: {
-        //        var maybe_comma = tree.prevToken(container_decl.lastToken());
-        //        // Doc comments for a field may also appear after the comma, eg.
-        //        // field_name: T, // comment attached to field_name
-        //        if (tree.token_tags[maybe_comma] == .DocComment)
-        //            maybe_comma = tree.prevToken(maybe_comma);
-        //        break :blk tree.token_tags[maybe_comma] == .Comma;
-        //    };
-
-        //    const fields_and_decls = container_decl.fieldsAndDecls();
-
-        //    // Check if the first declaration and the { are on the same line
-        //    const src_has_newline = !tree.tokensOnSameLine(
-        //        container_decl.lbrace_token,
-        //        fields_and_decls[0].firstToken(),
-        //    );
-
-        //    // We can only print all the elements in-line if all the
-        //    // declarations inside are fields
-        //    const src_has_only_fields = blk: {
-        //        for (fields_and_decls) |decl| {
-        //            if (decl.tag != .ContainerField) break :blk false;
-        //        }
-        //        break :blk true;
-        //    };
-
-        //    if (src_has_trailing_comma or !src_has_only_fields) {
-        //        // One declaration per line
-        //        ais.pushIndentNextLine();
-        //        defer ais.popIndent();
-        //        try renderToken(ais, tree, container_decl.lbrace_token, .Newline); // lbrace
-
-        //        for (fields_and_decls) |decl, i| {
-        //            try renderContainerDecl(allocator, ais, tree, decl, .Newline);
-
-        //            if (i + 1 < fields_and_decls.len) {
-        //                try renderExtraNewline(ais, tree, fields_and_decls[i + 1]);
-        //            }
-        //        }
-        //    } else if (src_has_newline) {
-        //        // All the declarations on the same line, but place the items on
-        //        // their own line
-        //        try renderToken(ais, tree, container_decl.lbrace_token, .Newline); // lbrace
-
-        //        ais.pushIndent();
-        //        defer ais.popIndent();
-
-        //        for (fields_and_decls) |decl, i| {
-        //            const space_after_decl: Space = if (i + 1 >= fields_and_decls.len) .Newline else .Space;
-        //            try renderContainerDecl(allocator, ais, tree, decl, space_after_decl);
-        //        }
-        //    } else {
-        //        // All the declarations on the same line
-        //        try renderToken(ais, tree, container_decl.lbrace_token, .Space); // lbrace
-
-        //        for (fields_and_decls) |decl| {
-        //            try renderContainerDecl(allocator, ais, tree, decl, .Space);
-        //        }
-        //    }
-
-        //    return renderToken(ais, tree, container_decl.rbrace_token, space); // rbrace
-        //},
 
         .ErrorSetDecl => unreachable, // TODO
         //.ErrorSetDecl => {
@@ -1949,6 +1856,94 @@ fn renderArrayInit(
     }
 }
 
+fn renderContainerDecl(
+    ais: *Ais,
+    tree: ast.Tree,
+    container_decl: ast.Full.ContainerDecl,
+    space: Space,
+) Error!void {
+    const token_tags = tree.tokens.items(.tag);
+    const node_tags = tree.nodes.items(.tag);
+
+    if (container_decl.layout_token) |layout_token| {
+        try renderToken(ais, tree, layout_token, .Space);
+    }
+
+    var lbrace: ast.TokenIndex = undefined;
+    if (container_decl.ast.enum_token) |enum_token| {
+        try renderToken(ais, tree, container_decl.ast.main_token, .None); // union
+        try renderToken(ais, tree, enum_token - 1, .None); // lparen
+        try renderToken(ais, tree, enum_token, .None); // enum
+        if (container_decl.ast.arg != 0) {
+            try renderToken(ais, tree, enum_token + 1, .None); // lparen
+            try renderExpression(ais, tree, container_decl.ast.arg, .None);
+            const rparen = tree.lastToken(container_decl.ast.arg) + 1;
+            try renderToken(ais, tree, rparen, .None); // rparen
+            try renderToken(ais, tree, rparen + 1, .Space); // rparen
+            lbrace = rparen + 2;
+        } else {
+            try renderToken(ais, tree, enum_token + 1, .Space); // rparen
+            lbrace = enum_token + 2;
+        }
+    } else if (container_decl.ast.arg != 0) {
+        try renderToken(ais, tree, container_decl.ast.main_token, .None); // union
+        try renderToken(ais, tree, container_decl.ast.main_token + 1, .None); // lparen
+        try renderExpression(ais, tree, container_decl.ast.arg, .None);
+        const rparen = tree.lastToken(container_decl.ast.arg) + 1;
+        try renderToken(ais, tree, rparen, .Space); // rparen
+        lbrace = rparen + 1;
+    } else {
+        try renderToken(ais, tree, container_decl.ast.main_token, .Space); // union
+        lbrace = container_decl.ast.main_token + 1;
+    }
+
+    if (container_decl.ast.members.len == 0) {
+        try renderToken(ais, tree, lbrace, Space.None); // lbrace
+        return renderToken(ais, tree, lbrace + 1, space); // rbrace
+    }
+
+    const last_member = container_decl.ast.members[container_decl.ast.members.len - 1];
+    const last_member_token = tree.lastToken(last_member);
+    const rbrace = switch (token_tags[last_member_token + 1]) {
+        .DocComment => last_member_token + 2,
+        .Comma => switch (token_tags[last_member_token + 2]) {
+            .DocComment => last_member_token + 3,
+            .RBrace => last_member_token + 2,
+            else => unreachable,
+        },
+        .RBrace => last_member_token + 1,
+        else => unreachable,
+    };
+    const src_has_trailing_comma = token_tags[last_member_token + 1] == .Comma;
+
+    if (!src_has_trailing_comma) one_line: {
+        // We can only print all the members in-line if all the members are fields.
+        for (container_decl.ast.members) |member| {
+            if (!node_tags[member].isContainerField()) break :one_line;
+        }
+        // All the declarations on the same line.
+        try renderToken(ais, tree, lbrace, .Space); // lbrace
+        for (container_decl.ast.members) |member| {
+            try renderMember(ais, tree, member, .Space);
+        }
+        return renderToken(ais, tree, rbrace, space); // rbrace
+    }
+
+    // One member per line.
+    ais.pushIndent();
+    try renderToken(ais, tree, lbrace, .Newline); // lbrace
+    for (container_decl.ast.members) |member, i| {
+        try renderMember(ais, tree, member, .Newline);
+
+        if (i + 1 < container_decl.ast.members.len) {
+            try renderExtraNewline(ais, tree, container_decl.ast.members[i + 1]);
+        }
+    }
+    ais.popIndent();
+
+    return renderToken(ais, tree, rbrace, space); // rbrace
+}
+
 /// Render an expression, and the comma that follows it, if it is present in the source.
 fn renderExpressionComma(ais: *Ais, tree: ast.Tree, node: ast.Node.Index, space: Space) Error!void {
     const token_tags = tree.tokens.items(.tag);