Rust「failed to resolve: use of undeclared crate or module」の原因と直し方

$ cargo build
Compiling demo v0.1.0 (/tmp/tmp.sWkP2T5inv/demo)
error[E0433]: cannot find module or crate `utils` in this scope
 --> src/main.rs:2:5
  |
2 |     utils::helper();
  |     ^^^^^ use of unresolved module or unlinked crate `utils`
  |
help: to make use of source file src/utils.rs, use `mod utils` in this file to declare the module
  |
1 + mod utils;
  |
 
For more information about this error, try `rustc --explain E0433`.
error: could not compile `demo` (bin "demo") due to 1 previous error
$ echo $?
101

$ cargo build
Compiling demo v0.1.0 (/tmp/tmp.sWkP2T5inv/demo)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.14s
$ echo $?
0

$ cargo run
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.00s
     Running `target/debug/demo`
helper called

$ cargo build
Compiling demo v0.1.0 (/tmp/tmp.sWkP2T5inv/demo)
error[E0432]: unresolved import `serde_json`
 --> src/main.rs:3:5
  |
3 | use serde_json::Value;
  |     ^^^^^^^^^^ use of unresolved module or unlinked crate `serde_json`
  |
  = help: if you wanted to use a crate named `serde_json`, use `cargo add serde_json` to add it to your `Cargo.toml`
 
error[E0433]: cannot find module or crate `serde_json` in this scope
 --> src/main.rs:7:20
  |
7 |     let v: Value = serde_json::from_str(r#"{"a":1}"#).unwrap();
  |                    ^^^^^^^^^^ use of unresolved module or unlinked crate `serde_json`
  |
  = help: if you wanted to use a crate named `serde_json`, use `cargo add serde_json` to add it to your `Cargo.toml`
 
Some errors have detailed explanations: E0432, E0433.
For more information about an error, try `rustc --explain E0432`.
error: could not compile `demo` (bin "demo") due to 2 previous errors
$ echo $?
101

$ cargo add serde_json
Updating crates.io index
      Adding serde_json v1.0.151 to dependencies
             Features:
             + std
             - alloc
             - arbitrary_precision
             - float_roundtrip
             - indexmap
             - preserve_order
             - raw_value
             - unbounded_depth
    Updating crates.io index
     Locking 11 packages to latest Rust 1.98.1 compatible versions
      Adding itoa v1.0.18
      Adding memchr v2.8.3
      Adding proc-macro2 v1.0.107
      Adding quote v1.0.47
      Adding serde v1.0.229
      Adding serde_core v1.0.229
      Adding serde_derive v1.0.229
      Adding serde_json v1.0.151
      Adding syn v3.0.5
      Adding unicode-ident v1.0.24
      Adding zmij v1.0.23

$ cargo build
Downloading crates ...
  Downloaded itoa v1.0.18
  Downloaded zmij v1.0.23
  Downloaded serde_json v1.0.151
  Downloaded serde_core v1.0.229
  Downloaded memchr v2.8.3
   Compiling zmij v1.0.23
   Compiling serde_core v1.0.229
   Compiling serde_json v1.0.151
   Compiling memchr v2.8.3
   Compiling itoa v1.0.18
   Compiling demo v0.1.0 (/tmp/tmp.sWkP2T5inv/demo)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.77s
$ echo $?
0

$ cargo run
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.00s
     Running `target/debug/demo`
helper called
{"a":1}

cargo add serde_json

[dependencies]
serde_json = "1"

// src/main.rs
mod utils;              // これが無いと utils::helper は E0433 になる
 
fn main() {
    utils::helper();
}

// src/utils/mod.rs
pub mod text;
pub mod path;

[dependencies]
serde-json = "1"     # ハイフンでも受け付けられる

use serde_json::Value;   // コード側は必ずアンダースコア

[dependencies]
my-core = { path = "../my-core" }