In presence of `impl <T: Trait1> Trait2 for T`, when `X: Trait2` fails, Rust guides you towards satisfying `X: Trait1` instead of `X: Trait2`

[GrigorenkoPV]

Code

use rayon::iter::{IntoParallelIterator, ParallelIterator};

pub fn parallelize<T>(v: Vec<T>) -> impl ParallelIterator<Item = T>
where
    // T: Send,
{
    v.into_par_iter()
}

Current output

error[E0599]: the method `into_par_iter` exists for struct `Vec<T>`, but its trait bounds were not satisfied
 --> src/lib.rs:7:7
  |
7 |     v.into_par_iter()
  |       ^^^^^^^^^^^^^
  |
  = note: the following trait bounds were not satisfied:
          `[T]: Sized`
          which is required by `[T]: IntoParallelIterator`
          `[T]: ParallelIterator`
          which is required by `[T]: IntoParallelIterator`

warning: unused import: `IntoParallelIterator`
 --> src/lib.rs:1:19
  |
1 | use rayon::iter::{IntoParallelIterator, ParallelIterator};
  |                   ^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

Desired output

What's really missing is a T: Send bound.

Rationale and extra context

No response

Other cases

No response

Rust Version

rustc 1.79.0-beta.3 (f5d04caa7 2024-05-03)
binary: rustc
commit-hash: f5d04caa74a1dfa5ffc4082c2c8f621f25336bbc
commit-date: 2024-05-03
host: x86_64-unknown-linux-gnu
release: 1.79.0-beta.3
LLVM version: 18.1.4

Anything else?

I will try to provide a MCVE. In the meantime, @rustbot label +E-needs-mcve

[GrigorenkoPV]

Here's a somewhat minimized example:
https://github.com/GrigorenkoPV/rust-issue-124802

error[E0599]: the method `into_pi` exists for enum `Option<T>`, but its trait bounds were not satisfied
 --> src/lib.rs:7:7
  |
7 |     v.into_pi()
  |       ^^^^^^^
  |
  = note: the following trait bounds were not satisfied:
          `&Option<T>: PI`
          which is required by `&Option<T>: IntoPI`
          `&mut Option<T>: PI`
          which is required by `&mut Option<T>: IntoPI`

warning: unused import: `IntoPI`
 --> src/lib.rs:1:10
  |
1 | use pi::{IntoPI, PI};
  |          ^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

If you inline pi intro the main crate, you get a longer error message

error[E0599]: the method `into_pi` exists for enum `Option<T>`, but its trait bounds were not satisfied
  --> src/lib.rs:39:7
   |
39 |     v.into_pi()
   |       ^^^^^^^
   |
note: the following trait bounds were not satisfied:
      `&Option<T>: PI`
      `&mut Option<T>: PI`
  --> src/lib.rs:13:9
   |
13 | impl<T: PI> IntoPI for T {
   |         ^^  ------     -
   |         |
   |         unsatisfied trait bound introduced here
   = help: items from traits can only be used if the trait is implemented and in scope
note: `IntoPI` defines an item `into_pi`, perhaps you need to implement it
  --> src/lib.rs:8:1
   |
8  | pub trait IntoPI {
   | ^^^^^^^^^^^^^^^^

indeed the culprit behind such weird diagnostic messages is this impl

impl<T: PI> IntoPI for T {
    type I = T;
    fn into_pi(self) -> Self::I {
        self
    }
}

without it, you get the correct suggestion to add the T: Send bound.

But if instead you remove the IntoPI for Option<T> impl, you'll get

   = note: the following trait bounds were not satisfied:
            `Option<T>: PI`
            which is required by `Option<T>: IntoPI`
            `&Option<T>: PI`
            which is required by `&Option<T>: IntoPI`
            `&mut Option<T>: PI`
            which is required by `&mut Option<T>: IntoPI`

which now has 3 items instead of two.

Similarly, adding IntoPI for &{mut,} Option<T> without removing any of the present impls, will affect the message. And with all 3 impls in place, you get

error[E0599]: no method named `into_pi` found for enum `Option` in the current scope
 --> src/lib.rs:7:7
  |
7 |     v.into_pi()
  |       ^^^^^^^
  |
help: there is a method `into` with a similar name
  |
7 |     v.into()
  |       ~~~~

warning: unused import: `IntoPI`
 --> src/lib.rs:1:10
  |
1 | use pi::{IntoPI, PI};
  |          ^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

TL;DR

So, to recap. You have two traits. I and IntoI. You also have a blanket <T: I> IntoI for T impl. You also have a number of IntoI for X impl. Then if some conditions for this IntoI for X impl do not get satisfied, Rust will completely ignore it. And instead of suggesting to add some extra clauses to satisfy this impl, it will instead guide you towards satisfying X: I/&X: I/&mut X: I for that blanket impl to kick in.

So I guess @rustbot label -E-needs-mcve

[brownie-in-motion]

In this case, we get a suggestion to restrict T: S only when impl<T: S> A for T {} is commented out.

trait S {}
trait A {}

// impl<T: S> A for T {}
impl<T: S> A for Option<T> {}

fn f<T>(v: Option<T>) -> impl A { v }

Is that similar to the issue that's being brought up?

[GrigorenkoPV]

Is that similar to the issue that's being brought up?

I think so

[dianne]

@rustbot claim