stage2 parsing: rework block statements AST memory layout

block statements are now directly following the Block AST node rather
than a singly linked list. This had negligible impact on performance:

throughput: 72.3 MiB/s => 72.7 MiB/s

however it greatly improves the API since the statements are laid out in
a flat array in memory.

lib/std/zig/ast.zig

@@ -804,6 +804,7 @@ pub const Node = struct {
         }
     };
 
+    /// The fields and decls Node pointers directly follow this struct in memory.
     pub const ContainerDecl = struct {
         base: Node = Node{ .id = .ContainerDecl },
         kind_token: TokenIndex,
@@ -1188,23 +1189,37 @@ pub const Node = struct {
         }
     };
 
+    /// The statements of the block follow Block directly in memory.
     pub const Block = struct {
         base: Node = Node{ .id = .Block },
-        label: ?TokenIndex,
+        statements_len: NodeIndex,
         lbrace: TokenIndex,
-        statements: StatementList,
         rbrace: TokenIndex,
+        label: ?TokenIndex,
 
-        pub const StatementList = LinkedList(*Node);
+        /// After this the caller must initialize the statements list.
+        pub fn alloc(allocator: *mem.Allocator, statements_len: NodeIndex) !*Block {
+            const bytes = try allocator.alignedAlloc(u8, @alignOf(Block), sizeInBytes(statements_len));
+            return @ptrCast(*Block, bytes.ptr);
+        }
+
+        pub fn free(self: *Block, allocator: *mem.Allocator) void {
+            const bytes = @ptrCast([*]u8, self)[0..sizeInBytes(self.statements_len)];
+            allocator.free(bytes);
+        }
 
         pub fn iterate(self: *const Block) Node.Iterator {
-            return .{ .parent_node = &self.base, .index = 0, .node = self.statements.first };
+            return .{ .parent_node = &self.base, .index = 0, .node = null };
         }
 
         pub fn iterateNext(self: *const Block, it: *Node.Iterator) ?*Node {
-            const child = it.node orelse return null;
-            it.node = child.next;
-            return child.data;
+            var i = it.index;
+            it.index += 1;
+
+            if (i < self.statements_len) return self.statementsConst()[i];
+            i -= self.statements_len;
+
+            return null;
         }
 
         pub fn firstToken(self: *const Block) TokenIndex {
@@ -1218,6 +1233,20 @@ pub const Node = struct {
         pub fn lastToken(self: *const Block) TokenIndex {
             return self.rbrace;
         }
+
+        pub fn statements(self: *Block) []*Node {
+            const decls_start = @ptrCast([*]u8, self) + @sizeOf(Block);
+            return @ptrCast([*]*Node, decls_start)[0..self.statements_len];
+        }
+
+        pub fn statementsConst(self: *const Block) []const *Node {
+            const decls_start = @ptrCast([*]const u8, self) + @sizeOf(Block);
+            return @ptrCast([*]const *Node, decls_start)[0..self.statements_len];
+        }
+
+        fn sizeInBytes(statements_len: NodeIndex) usize {
+            return @sizeOf(Block) + @sizeOf(*Node) * @as(usize, statements_len);
+        }
     };
 
     pub const Defer = struct {

lib/std/zig/parse.zig

@@ -1178,8 +1178,9 @@ const Parser = struct {
     fn parseBlock(p: *Parser) !?*Node {
         const lbrace = p.eatToken(.LBrace) orelse return null;
 
-        var statements = Node.Block.StatementList{};
-        var statements_it = &statements.first;
+        var statements = std.ArrayList(*Node).init(p.gpa);
+        defer statements.deinit();
+
         while (true) {
             const statement = (p.parseStatement() catch |err| switch (err) {
                 error.OutOfMemory => return error.OutOfMemory,
@@ -1189,18 +1190,19 @@ const Parser = struct {
                     continue;
                 },
             }) orelse break;
-            statements_it = try p.llpush(*Node, statements_it, statement);
+            try statements.append(statement);
         }
 
         const rbrace = try p.expectToken(.RBrace);
 
-        const block_node = try p.arena.allocator.create(Node.Block);
+        const block_node = try Node.Block.alloc(&p.arena.allocator, statements.items.len);
         block_node.* = .{
             .label = null,
             .lbrace = lbrace,
-            .statements = statements,
+            .statements_len = statements.items.len,
             .rbrace = rbrace,
         };
+        std.mem.copy(*Node, block_node.statements(), statements.items);
 
         return &block_node.base;
     }

lib/std/zig/render.zig

@@ -358,21 +358,20 @@ fn renderExpression(
                 try renderToken(tree, stream, tree.nextToken(label), indent, start_col, Space.Space);
             }
 
-            if (block.statements.first == null) {
+            if (block.statements_len == 0) {
                 try renderToken(tree, stream, block.lbrace, indent + indent_delta, start_col, Space.None);
                 return renderToken(tree, stream, block.rbrace, indent, start_col, space);
             } else {
                 const block_indent = indent + indent_delta;
                 try renderToken(tree, stream, block.lbrace, block_indent, start_col, Space.Newline);
 
-                var it = block.statements.first;
-                while (it) |statement_node| : (it = statement_node.next) {
-                    const statement = statement_node.data;
+                const block_statements = block.statements();
+                for (block_statements) |statement, i| {
                     try stream.writeByteNTimes(' ', block_indent);
                     try renderStatement(allocator, stream, tree, block_indent, start_col, statement);
 
-                    if (statement_node.next) |next_statement| {
-                        try renderExtraNewline(tree, stream, start_col, next_statement.data);
+                    if (i + 1 < block_statements.len) {
+                        try renderExtraNewline(tree, stream, start_col, block_statements[i + 1]);
                     }
                 }