Misleading "error: source trait is private" when a trait is not imported

[gkoz]

This example uses rust-crypto 0.2.14. Failing to import the Encryptor trait that implements encrypt causes an unexpected error message. The trait isn't private and importing it compiles as expected.

extern crate crypto;

use crypto::{ buffer, aes, blockmodes };
use crypto::buffer::{ ReadBuffer, WriteBuffer };

fn enc(key: &[u8], iv: &[u8], data: &[u8]) {
    // Don't import the trait that implements encrypt()
    //use crypto::symmetriccipher::Encryptor;

    let mut encryptor = aes::cbc_encryptor(aes::KeySize::KeySize256,
                key, iv, blockmodes::PkcsPadding);

    let mut read_buffer = buffer::RefReadBuffer::new(data);
    let mut buffer = [0; 4096];
    let mut write_buffer = buffer::RefWriteBuffer::new(&mut buffer);

    // Expected error: type `_` does not implement any method in scope named `encrypt`
    // Get error: source trait is private
    encryptor.encrypt(&mut read_buffer, &mut write_buffer, true);
}

fn main() {
}

src/main.rs:19:5: 19:65 error: source trait is private
src/main.rs:19     encryptor.encrypt(&mut read_buffer, &mut write_buffer, true);
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

rustc 1.0.0-nightly (706be5ba1 2015-02-05 23:14:28 +0000)

[GuillaumeGomez]

The new error message would need to be something like "error: source trait not found" I guess ?

[shepmaster]

Another person got bit

[Wilfred]

For what it's worth, I was confused by this issue today. I was trying to use methods on quickcheck's Gen trait, which inherits from rand::Rng.

I tried adding use rand::Rng but the same error occurred. I had to add to my Cargo.toml:

[dependencies]
rand = "*"

This wasn't obvious to me at all.

[steveklabnik]

me too

[GuillaumeGomez]

So should I replace it by "error: source trait not found" ?

[steveklabnik]

I would argue it should be

error: use of undeclared trait name `Foo`

Because that's what happens when you do something like

fn foo<T: Foo>() {}

fn main() {

}

[Wilfred]

@GuillaumeGomez That would be clearer, yep. It could also suggest adding an extern crate... or an entry in Cargo.toml, although perhaps this occurs in other cases that haven't occurred to me.

[steveklabnik]

well, i guess that's different code than the OP's. Basically, the error should mirror what happens when it doesn't exist

[gkoz]

Am I wrong to expect

error: no method named `encrypt` found for type `_` in the current scope

Possibly with a hint about importing the correct trait. Y'know like it usually happens.

[jethrogb]

Also expecting the error message @gkoz mentions. What is different about this than, for example:

let mut buf=[0u8;32];
(&mut buf[..]).write_all(&b"test");

which yields

<anon>:3:20: 3:39 error: no method named `write_all` found for type `&mut [u8]` in the current scope
<anon>:3     (&mut buf[..]).write_all(&b"test");
                            ^~~~~~~~~~~~~~~~~~~
<anon>:3:20: 3:39 help: items from traits can only be used if the trait is in scope; the following trait is implemented but not in scope, perhaps add a `use` for it:
<anon>:3:20: 3:39 help: candidate #1: use `std::io::Write`

[naufraghi]

Got the same problem (1.8.0):

//extern crate num;
extern crate nalgebra;

pub fn fps_look_at<T: nalgebra::BaseFloat>(eye: Vector3<T>, pitch: T, yaw: T) -> Matrix4<T> {
    let (sin_pitch, cos_pitch) = pitch.sin_cos();
}

Gives the user no hints where to search for the sin_cos impl.

If the compiler knows which is the private trait that is missing, an useful message could show that name as @steveklabnik suggested. Even better with the suggested extern crate.

Update:
I see from the master that the first half of the request has been already implemented, but without the extern crate suggestion.

Even better, on the latest nightly there is no need to include the num crate at all.

This issue is perhaps obsolete and can be closed.