在机箱根目录中找不到[包]

如何让铁锈识别机箱中的模块？我认为在lib.rs中声明mod [module name]就足够了。不是吗？

错误：

error[E0432]: unresolved import `crate::a`
--> src/bin/b.rs:2:12
|
2 | use crate::a::f;
|            ^ could not find `a` in the crate root

src/a.rs:

pub fn f() {}

src/bin/b.rs:

use crate::a::f;
fn main() {}

src/lib.rs:

mod a;

Cargo.toml:

[package]
name = "m"
version = "0.1.0"
edition = "2021"
# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
[[bin]]
name = "b"
path = "src/bin/b.rs"
[dependencies]