Hello,
I am new to Rust and trying to understand how to use external libraries. This is a noob question!
For example I want to use this library for an elliptic curve https://github.com/RustCrypto/elliptic-curves/tree/master/p256
So I cloned the repo I want to use in my disk and then I created a new folder somewhere else named "myproject".
I run then cargo init and edit the Cargo.toml by adding p256 = { path = "/Users/user/elliptic-curves/p256" } under [dependencies].
The main.rs in myproject/src is as follows:

use crate::p256;
fn main() {
println!("Hello, world!");
}

But I get an error

error: failed to select a version for the requirement `elliptic-curve = "=0.5.0-pre"`
candidate versions found which didn't match: 0.4.0, 0.3.0, 0.2.0, ...
location searched: crates.io index
required by package `p256 v0.3.0 (/Users/user/elliptic-curves/p256)`
... which is depended on by `myproject v0.1.0 (/Users/user/myproject)`

Yes I read it but I can't find this particular library version in crates.io...
The latest version is indeed 0.4.0 but it looks for 0.5.0-pre..
Am I missing something?

p256 v0.5 is not yet published on crates.io. The latest version on both crates.io and in the repository is v0.3, so it looks like you've confused versions from a different crate, e.g. elliptic-curve which defines traits implemented by p256. You could use the version directly from git like this:

But if you are new to Rust, just use v0.3. Also p256 is not an easy crate to start with, you will have to understand a broader context, which includes crates like elliptic-curve, ecdsa and signature, so I would recommend to chose a simpler crate for practice. If you are interested in cryptography crates, it would be better to start with hash functions or stream ciphers.

Got it, I just added elliptic-curve = "0.4.0" and works.. Thanks!

My (one more newb) question now: How do I access internal methods?
E.g. here is my sample code:

use elliptic_curve;
fn main() {
const UNCOMPRESSED_BASEPOINT: &str =
"046B17D1F2E12C4247F8BCE6E563A440F277037D812DEB33A0F4A13945D898C2964FE342E2FE1A7F9B8EE7EB4A7C0F9E162BCE33576B315ECECBB6406837BF51F5";
let pubkey = PublicKey::from_bytes(&hex::decode(UNCOMPRESSED_BASEPOINT).unwrap()).unwrap();
let point = AffinePoint::from_pubkey(&pubkey).unwrap();
}

Note that p256 re-exports elliptic-curve, so you don't need specify it in Cargo toml and you can import it like this:

use p256::elliptic_curve;

Your have to import types and crates first. Your example can be written like this.

Cargo.toml:

[dependencies]
p256 = "0.3"
hex = "0.4"

src/main.rs:

use p256::PublicKey;
use p256::arithmetic::AffinePoint;
use hex;
fn main() {
const UNCOMPRESSED_BASEPOINT: &str =
"046B17D1F2E12C4247F8BCE6E563A440F277037D812DEB33A0F4A13945D898C2964FE342E2FE1A7F9B8EE7EB4A7C0F9E162BCE33576B315ECECBB6406837BF51F5";
let pubkey = PublicKey::from_bytes(&hex::decode(UNCOMPRESSED_BASEPOINT).unwrap()).unwrap();
let point = AffinePoint::from_pubkey(&pubkey).unwrap();
}

Note that the arithmetic module is feature-gated behind an enabled by default feature with the same name.