rustc should provide a suggestion when using a type/trait from a sibling module that isn't imported

[fitzgen]

Here is two examples in one, a trait and a type:

/Users/fitzgen/src/oxischeme/src/eval.rs:267:6: 267:11 error: attempt to implement a nonexistent trait `Trace`
/Users/fitzgen/src/oxischeme/src/eval.rs:267 impl Trace for Meaning {
                                                  ^~~~~
/Users/fitzgen/src/oxischeme/src/eval.rs:268:24: 268:35 error: use of undeclared type name `IterGcThing`
/Users/fitzgen/src/oxischeme/src/eval.rs:268     fn trace(&self) -> IterGcThing {
                                                                    ^~~~~~~~~~~

It would be nice UX to say something like "did you forget to import module::Type?" or something.

[fitzgen]

Sorry, forgot to say: Trace and IterGcThing are both defined in a sibling module.

[kmcallister]

Perhaps it should print a list of modules in the local crate that define something with that name.

[fitzgen]

Perhaps it should print a list of modules in the local crate that define something with that name.

That would be wonderful.

[fitzgen]

@kmcallister You marked this easy, can you give me an idea of what might be involved plus some pointers to the relevant code? If it is simple enough, I might be able to knock this out.

[jdm]

Sounds similar to #21008.

[yati-sagade]

If this is still open (I don't see suggestions in the beta channel), I'd like to take a shot.

[VladUreche]

Since it doesn't seem like there's much activity around this bug, let me try to revive it. Please correct me if this is not the desired behavior:

mod mul1 {
    pub trait Mul {}
}

mod mul2 {
    pub trait Mul {}
}

#[derive(Debug)]
struct Foo;

// When we comment the next line:
use mul1::Mul;

// We get the following error for the `impl` below:
//   error: use of undeclared trait name `Mul` [E0405]
// Ideally, when this bug is fixed, we would like to see:
//   error: use of undeclared trait name `Mul` [E0405]
//   note: Possible alternatives are:
//         use mul1::Mul;        // from module mul1 in this crate <or>
//         use mul2::Mul;        // from module mul1 in this crate <or>
//         use std::ops::Mul;    // from the standard library <or>
//         use other_crate::Mul; // from external crate other_crate ...
impl Mul for Foo { }

fn main() {
    let foo = Foo;
    println!("Hello, {:?}!", foo);
}

So, I imagine, in the ideal case, rustc would look up all the traits within the following contexts:

• modules defined by the current crate
• the standard library
• external crates

Right?

As for the implementation, I imagine I can start from the suggest_traits_to_import method in suggest.rs. WDYT?

[nikomatsakis]

I feel like changing the message to "trait name Mul is not a scope" would already be an improvement. To me, "undeclared" suggests something stronger (that is, not declared anywhere).

[nikomatsakis]

But yes if we wanted to all out and give suggestions, I think the code you pointed out would be a logical starting point, though of course it is harvesting all traits that have a method with the given name, which is somewhat more complicated.

[VladUreche]

Thank you for the feedback @nikomatsakis -- if all else fails, I will PR an update to the error message.

Outdated:

I've been following the logical thread from the trait suggestion, but I need some help:

• the trait/method search in suggest.rs is done during typechecking, in the librustc_typeck crate, and ultimately relies on the typechecker context, through the CrateCtxt struct.
• the error in this issue is generated during name resolution, in the librustc_resolve crate, which, as far as I can tell runs before the type checker and doesn't have a typer context.

The resolver also has a trait_map value, but printing its contents when the error occurs produces empty vectors:

{39: [], 36: []}

The output comes from the test case above. As expected, there are two node ids, but there are no trait definitions attached.

In this context, I have three questions:

• Is this description accurate, or have I missed something?
• Is it possible to access all traits in the crate during name resolution?
• Any pointers as to how I can achieve this?

Thanks!

PS: Is there any faster way to prototype changes faster than make rustc-stage1? I explored stripping the commands out of the makefile here, but ran into a dead end.

[VladUreche]

Some progress on this: VladUreche@f4abc9d

[VladUreche]

Okay, sloppy implementation, but I have a solution:

bug-21221.rs:22:6: 22:9 error: use of undeclared trait name `Mul`. 
Are you looking for `mul1::Mul` or `mul2::Mul` from this crate? 
Or maybe `Mul` from another crate? [E0405]
bug-21221.rs:22 impl Mul for Foo { 
                     ^~~
bug-21221.rs:22:6: 22:9 help: run `rustc --explain E0405` to see a
detailed explanation

Commit is VladUreche@9a4c9e5.

[VladUreche]

The last commit adds recommendations for both resolution errors in the bug description and the new message is more comprehensible, IMO: master...VladUreche:issue/21221

@fitzgen @nikomatsakis are you okay with the ouput for this example?