Cargo.toml has two [[bin]] sections:

[[bin]]
name = "main1"
path = "src/main1.rs"

[[bin]]
name = "main2"
path = "src/main2.rs"

Both main1.rs and main2.rs have the same main function that calls
some::submodule::hello():

fn main() {
    some::submodule::hello();
}

Why does main2.rs fail to compile even though main1.rs compiles fine?

$ cargo build
   Compiling rust_second_bin v0.1.0 (/path/to/rust_second_bin)
error[E0433]: failed to resolve: use of undeclared crate or module `some`
 --> src/main2.rs:2:5
  |
2 |     some::submodule::hello();
  |     ^^^^ use of undeclared crate or module `some`

See the source code here.

$ cargo --version
cargo 1.76.0 (c84b36747 2024-01-18)
$ rustc --version
rustc 1.76.0 (07dca489a 2024-02-04)
$ uname -a
Darwin mymac.local 22.6.0 Darwin Kernel Version 22.6.0: Mon Apr 22 20:51:33 PDT 2024; root:xnu-8796.141.3.705.2~1/RELEASE_ARM64_T8112 arm64