"note: method exists but trait bounds not satisfied" could be better

[durka]

I saw this broken code on IRC:

pub trait JoinWithString { 
  fn join(self, separator: &'static str) -> String; 
} 

impl<X> JoinWithString for X 
  where X: Iterator<Item = String> 
{ 
  fn join(self, separator: &'static str) -> String { 
    unimplemented!()
  } 
}


fn main() {
    let a: Vec<std::path::PathBuf> = Vec::new();
    a.iter().map(|x| x.to_str().unwrap()).join(":");

}

I know why it's broken, but I don't like the error message:

error: no method named `join` found for type `std::iter::Map<std::slice::Iter<'_, std::path::PathBuf>, [closure@<anon>:16:18: 16:41]>` in the current scope
  --> <anon>:16:43
   |>
16 |>     a.iter().map(|x| x.to_str().unwrap()).join(":");
   |>                                           ^^^^
note: the method `join` exists but the following trait bounds were not satisfied: `&std::iter::Map<std::slice::Iter<'_, std::path::PathBuf>, [closure@<anon>:16:18: 16:41]> : std::iter::Iterator`
help: items from traits can only be used if the trait is implemented and in scope; the following trait defines an item `join`, perhaps you need to implement it:
help: candidate #1: `JoinWithString`

• The note just told me that the problem is the trait bounds, so it is superfluous to suggest importing a trait (even without the note, it doesn't need to be imported so the help is useless).
• The note says Iterator is not satisfied, which is not helpful. The problem is that Iterator<Item=String> is not satisfied.

[therustmonk]

I want to add the case to don't duplicate an issue. Message is unsophisticated if I use different versions of crates. I stumbled it many times (

Example:

error[E0277]: the trait bound `mould_auth::AuthService<checker::Checker<r2d2::Pool<r2d2_postgres::PostgresConnectionManager>>, session::UserRole, dba::Error>: mould::service::Service<session::UserSession>` is not satisfied
  --> src\main.rs:63:11
   |
63 |     suite.register("auth-service", auth_service);
   |           ^^^^^^^^ trait `mould_auth::AuthService<checker::Checker<r2d2::Pool<r2d2_postgres::PostgresConnectionManager>>, session::UserRole, dba::Error>: mould::service::Service<session::UserSession>` not satisfied

error[E0277]: the trait bound `mould_auth::TokenService<checker::Checker<r2d2::Pool<r2d2_postgres::PostgresConnectionManager>>, session::UserRole, dba::Error>: mould::service::Service<session::UserSession>` is not satisfied
  --> src\main.rs:68:11
   |
68 |     suite.register("token-service", token_service);
   |           ^^^^^^^^ trait `mould_auth::TokenService<checker::Checker<r2d2::Pool<r2d2_postgres::PostgresConnectionManager>>, session::UserRole, dba::Error>: mould::service::Service<session::UserSession>` not satisfied

error: aborting due to 2 previous errors

There is an error, because main crate use one dependency from git, but child uses same dependency from crates.io. It was hard to found, because I thought it was as error with an implementation.

[estebank]

Current output:

error[E0599]: no method named `join` found for type `std::iter::Map<std::slice::Iter<'_, std::path::PathBuf>, [closure@src/main.rs:16:18: 16:41]>` in the current scope
  --> src/main.rs:16:43
   |
16 |     a.iter().map(|x| x.to_str().unwrap()).join(":");
   |                                           ^^^^
   |
   = note: the method `join` exists but the following trait bounds were not satisfied:
           `std::iter::Map<std::slice::Iter<'_, std::path::PathBuf>, [closure@src/main.rs:16:18: 16:41]> : JoinWithString`
           `&std::iter::Map<std::slice::Iter<'_, std::path::PathBuf>, [closure@src/main.rs:16:18: 16:41]> : JoinWithString`
           `&mut std::iter::Map<std::slice::Iter<'_, std::path::PathBuf>, [closure@src/main.rs:16:18: 16:41]> : JoinWithString`
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `join`, perhaps you need to implement it:
           candidate #1: `JoinWithString`

[IRobL]

I'm getting a similar problem here too. I'm trying to convert a Json object to a string. Rocket and serde is involved in this issue:

#[macro_use] extern crate rocket;
#[macro_use] extern crate serde_derive;

extern crate rand;
extern crate serde_json;


use rand::{thread_rng, Rng};
use rocket_contrib::json::Json;

#[derive(Deserialize)]
struct MTT2Game {
    mode: String,
    user: String,
    difficulty: String,
    reported_at: String
}

#[post("/games/mtt2/modes/progression/games", format = "application/json", data = "<game>")]
fn mtt2_progression_game(game: Json<MTT2Game>) -> String {
    println!( "{}", game.to_string() );

    return "{ \"id\": \"1\" }".to_owned();
}

The error is:

error[E0599]: no method named `to_string` found for type `rocket_contrib::json::Json<MTT2Game>` in the current scope
  --> src/main.rs:43:26
   |
43 |     println!( "{}", game.to_string() );
   |                          ^^^^^^^^^
   |
   = note: the method `to_string` exists but the following trait bounds were not satisfied:
           `MTT2Game : std::string::ToString`
           `rocket_contrib::json::Json<MTT2Game> : std::string::ToString`
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `to_string`, perhaps you need to implement it:
           candidate #1: `std::string::ToString`

It mentions that something isn't satisfied, which is helpful. Maybe this is an issue with the underlying Json object and not Rust?

[Nemo157]

Another variant of the “same trait from different versions” is when you have the exact same version but from different sources, e.g. crates.io and a git dependency.

[ryankurte]

another example is where trait bounds are nested, in which case the error suggests that the bounds are not met, but not which bounds or why.

a simple example (playground link):

pub struct Example;

trait A {
    fn do_a(&self);
}

trait B {
    fn do_b(&self);
}

trait Empty {}

impl <T> B for T where T: A + Empty
{
    fn do_b(&self) {
        self.do_a()
    }
}

impl A for Example {
    fn do_a(&self) {
        println!("A!");
    }
}

fn main() {
    let e = Example;
    e.do_b();
}

Error:

   |
2  | pub struct Example;
   | ------------------- method `do_b` not found for this
...
29 |     e.do_b();
   |       ^^^^ method not found in `Example`
   |
   = note: the method `do_b` exists but the following trait bounds were not satisfied:
           `Example : B`
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `do_b`, perhaps you need to implement it:
           candidate #1: `B`

it would be extremely useful in this instance to have a note to indicate why the bound is not met, in this instance as A does not implement Empty, but it's usually a lot more complicated, and with multiple bounds becomes rather a dice rolling exercise.

[estebank]

For the original case, would the following output be acceptable?

error[E0599]: no method named `join` found for struct `std::iter::Map<std::slice::Iter<'_, std::path::PathBuf>, [closure@file5.rs:18:18: 18:41]>` in the current scope
  --> file5.rs:18:43
   |
1  | pub trait JoinWithString {
   | ------------------------ this trait defines an item `join`
...
18 |     a.iter().map(|x| x.to_str().unwrap()).join(":");
   |                                           ^^^^ method not found in `std::iter::Map<std::slice::Iter<'_, std::path::PathBuf>, [closure@file5.rs:18:18: 18:41]>`
   |
   = note: the method `join` exists but the following trait bounds were not satisfied:
           `&std::iter::Map<std::slice::Iter<'_, std::path::PathBuf>, [closure@file5.rs:18:18: 18:41]>: std::iter::Iterator`
           `<&mut std::iter::Map<std::slice::Iter<'_, std::path::PathBuf>, [closure@file5.rs:18:18: 18:41]> as std::iter::Iterator>::Item = std::string::String`
           `<&std::iter::Map<std::slice::Iter<'_, std::path::PathBuf>, [closure@file5.rs:18:18: 18:41]> as std::iter::Iterator>::Item = std::string::String`
           `<std::iter::Map<std::slice::Iter<'_, std::path::PathBuf>, [closure@file5.rs:18:18: 18:41]> as std::iter::Iterator>::Item = std::string::String`
   = help: items from traits can only be used if the trait is implemented and in scope

That's the current output in #69255. I think I can change it slightly to be less verbose, but worry about it being more confusing in the general case:

error[E0599]: no method named `join` found for struct `std::iter::Map<std::slice::Iter<'_, std::path::PathBuf>, [closure@file5.rs:18:18: 18:41]>` in the current scope
  --> file5.rs:18:43
   |
1  | pub trait JoinWithString {
   | ------------------------ this trait defines an item `join`
...
18 |     a.iter().map(|x| x.to_str().unwrap()).join(":");
   |                                           ^^^^ method not found in `std::iter::Map<std::slice::Iter<'_, std::path::PathBuf>, [closure@file5.rs:18:18: 18:41]>`
   |
   = note: the method `join` exists but the following trait bounds were not satisfied:
           `&std::iter::Map<std::slice::Iter<'_, std::path::PathBuf>, [closure@file5.rs:18:18: 18:41]>: std::iter::Iterator`
           `<_ as std::iter::Iterator>::Item = std::string::String`
   = help: items from traits can only be used if the trait is implemented and in scope

The case in the last comment would now be:

error[E0599]: no method named `do_b` found for struct `Example` in the current scope
  --> file5.rs:28:7
   |
1  | pub struct Example;
   | -------------------
   | |
   | method `do_b` not found for this
   | doesn't satisfy `Example: Empty`
...
7  | trait B {
   | ------- this trait defines an item `do_b`
...
28 |     e.do_b();
   |       ^^^^ method not found in `Example`
   |
   = note: the method `do_b` exists but the following trait bounds were not satisfied:
           `Example: Empty`
   = help: items from traits can only be used if the trait is implemented and in scope

[ryankurte]

   = note: the method `do_b` exists but the following trait bounds were not satisfied:
           `Example: Empty`

looks great to me! the question then is how it generalises to work with more complex nested bounds, but, i don't have a good example handy at the moment (the snippet i posted was a minimum reproduction of something, but, i can't remember what).