"trait not implemented" errors can be very confusing when the trait has an associated type with bounds

[jplatte]

Simple example using rayon (playground):

use rayon::prelude::*;

fn main() {
    let v: Vec<*const usize> = Vec::new();

    v.par_iter();
    (&v).into_par_iter();
    <Vec<*const usize> as IntoParallelRefMutIterator>::par_iter(&v);
    <&Vec<*const usize> as IntoParallelIterator>::into_par_iter(&v);
}

This code rightly doesn't compile because rayons IntoParallelIterator has a Send bound on its Item type. However, none of the error messages for the methods called in the code above ever mention that bound or the associated type:

error[E0576]: cannot find method or associated constant `par_iter` in trait `IntoParallelRefMutIterator`
 --> src/lib.rs:8:56
  |
8 |     <Vec<*const usize> as IntoParallelRefMutIterator>::par_iter(&v);
  |                                                        ^^^^^^^^ not found in `IntoParallelRefMutIterator`

error[E0599]: no method named `par_iter` found for type `std::vec::Vec<*const usize>` in the current scope
 --> src/lib.rs:6:7
  |
6 |     v.par_iter();
  |       ^^^^^^^^
  |
  = note: the method `par_iter` exists but the following trait bounds were not satisfied:
          `[*const usize] : rayon::iter::IntoParallelRefIterator`
          `std::vec::Vec<*const usize> : rayon::iter::IntoParallelRefIterator`

error[E0599]: no method named `into_par_iter` found for type `&std::vec::Vec<*const usize>` in the current scope
 --> src/lib.rs:7:10
  |
7 |     (&v).into_par_iter();
  |          ^^^^^^^^^^^^^
  |
  = note: the method `into_par_iter` exists but the following trait bounds were not satisfied:
          `&&std::vec::Vec<*const usize> : rayon::iter::IntoParallelIterator`
          `&mut &std::vec::Vec<*const usize> : rayon::iter::IntoParallelIterator`
          `[*const usize] : rayon::iter::IntoParallelIterator`

error[E0277]: the trait bound `&std::vec::Vec<*const usize>: rayon::iter::IntoParallelIterator` is not satisfied
 --> src/lib.rs:9:5
  |
9 |     <&Vec<*const usize> as IntoParallelIterator>::into_par_iter(&v);
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `rayon::iter::IntoParallelIterator` is not implemented for `&std::vec::Vec<*const usize>`
  |
  = help: the following implementations were found:
            <&'data mut std::vec::Vec<T> as rayon::iter::IntoParallelIterator>
            <&'data std::vec::Vec<T> as rayon::iter::IntoParallelIterator>
            <std::vec::Vec<T> as rayon::iter::IntoParallelIterator>