How Do You Troubleshoot the Cargo/Rust Error “use of undeclared type or module”?

Problem Scenario
You run a "cargo run" command. You get an error like one or more of the following:

use of undeclared type or module api

use of undeclared type or module serde_json

use of undeclared type or module async-std

What should you do?

Solution
Update the Cargo.toml file. The dependencies section should have an entry like one of the following (but the version number may or may not differ):

[package]
name = "in3-tutorial"
version = "0.0.1"
authors = ["reader@medium.com"]
edition = "2018"

[dependencies]
in3 = "0.1.7"
async-std = "1.5.0"
futures-executor = "0.3.5"
serde_json = "1.0"
ethabi = "13.0.0"
ethereum_abi = "0.2.0"
ethbloom = "0.10.0"

[[bin]]
name = "in3"
path = "/home/jdoe/rust/contint.rs"

If you are not sure what version is valid, go here: https://crates.io/