官方教程：https://doc.rust-lang.org/std/fmt/
注意，标准库里的格式控制字符串必须是字面量：
https://www.reddit.com/r/rust/comments/48jzw0/formatting_with_a_static_string_literal/
https://stackoverflow.com/questions/46788199/how-to-create-a-constant-string-literal-in-rust
https://stackoverflow.com/questions/32572486/how-can-i-use-a-dynamic-format-string-with-the-format-macro
如果想要动态的格式控制字符串，可以试试strfmt:
https://github.com/vitiral/strfmt
¶ 固定宽度左边补零
rust print固定宽度左边补零¶ 浮点数打印时忽略浮点误差
rust不支持%g:
Pretty print isn't pretty for floats
https://github.com/sampsyo/bril/pull/218
不过可以用crate gpoint实现：https://crates.io/crates/gpoint