parseBlockExprPrefix

    pub fn parseBlockExprPrefix(p: *Parser) Error!bool {
        const def_index = @backingInt(Def.defBlockExprPrefix);
        if (p.depths[def_index] > max_depth) return error.MaxDepth;
        p.depths[def_index] += 1;
        defer p.depths[def_index] -= 1;
        return blk_0: {
            const pos_0 = p.i;
            if ((try p.parseBlockLabel() or true) and try p.parseLBRACE()) break :blk_0 true;
            p.i = pos_0;
            break :blk_0 false;
        };
    }