can't find crate when using extern command - rust

Question

I am learning rust, now I tried to use Diesel to do some operation to database. I write the main.rs like this:
#[macro_use]
extern crate reddwarf_music;
fn main(){
}
then compile the project using command cargo build, shows error like this:
~/Documents/GitHub/reddwarf_music on  develop! ⌚ 14:50:50
$ cargo build ‹ruby-2.7.2›
Compiling reddwarf_music v0.1.0 (/Users/dolphin/Documents/GitHub/reddwarf_music)
error[E0463]: can't find crate for `reddwarf_music`
--> src/main.rs:3:1
|
3 | extern crate reddwarf_music;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't find crate
error: aborting due to previous error
For more information about this error, try `rustc --explain E0463`.
error: could not compile `reddwarf_music`
To learn more, run the command again with --verbose.
(base)
and this is my Cargo.toml:
[package]
name = "reddwarf_music"
version = "0.1.0"
edition = "2018"
[dependencies]
rocket = { version = "0.5.0-rc.1", features = ["json"] }
rand = "0.8.4"
serde = { version = "1.0.64", features = ["derive"] }
serde_json = "1.0.64"
reqwest = "0.11.4"
# database
diesel = { version = "1.4.4", features = ["postgres"] }
dotenv = "0.15.0"
am I doing the right way? the next step I want to use the mod like this:
use reddwarf_music::schema::posts::dsl::*;
I am follow the docs step by step from here to using diesel. what should I do to make it work as expect? the diesel official docs use it like this way. In my project, it did not work.

Answer 1

As far as I can see, the crate you are trying to compile is called reddwarf_music. extern crate tries to include a crate into the crate you're currently compiling. You're in effect trying to include the crate reddwarf_music into itself in order to use proc_macros from reddwarf_music in reddwarf_music. This is unfortunately not possible.
What the official Diesel documentation does is slightly different. Their library crate is called diesel_demo, but the code run when running cargo run --bin is actually the code inside the folder bin, which is not directly a part of the library crate, and instead part of a separate binary crate. If you instead of the file main.rs in the root, create a file inside a bin folder and run cargo build --bin, I think it should work.
(This has been edited to more directly answer the question)