can't find crate when using extern command

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.

Carl Gronvald · Accepted Answer · 2021-08-24 08:13:36Z

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)

edited Aug 24, 2021 at 8:13

answered Aug 24, 2021 at 7:44

Carl Gronvald

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2 Comments

Dolphin Over a year ago

why the diesel official docs could do it like this?diesel.rs/guides/getting-started @Carl Gronvald his project is diesel_demo and create extern diesel_demo.

Carl Gronvald Over a year ago

His library crate is called diesel_demo, but everything inside the bin folder is in fact not directly a part of the library crate. It is instead a separate binary crate, that can thus refer to the library crate. If you, instead of making a file called main.rs in the root, create a file inside a bin folder, and run cargo build --bin, I think it should work.

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