Wrong Clone bound suggestion

[Banyc]

Code

Playground: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=cda41942df9b5feddbbf2417959e717a

use std::collections::HashMap;

#[derive(Clone)]
struct Ctx<A> {
    a_map: HashMap<String, B<A>>,
}

#[derive(Clone)]
// help: consider annotating `B<A>` with `#[derive(Clone)]`
struct B<A> {
    // ----------- doesn't satisfy `B<A>: Clone`
    a: A,
}

fn foo<A>(ctx: &mut Ctx<A>) {
    let a_map = ctx.a_map.clone();
    //                    ^^^^^ method cannot be called on `HashMap<String, B<A>>` due to unsatisfied trait bounds
}

Current output

error[E0599]: the method `clone` exists for struct `HashMap<String, B<A>>`, but its trait bounds were not satisfied
   --> src/lib.rs:15:27
    |
10  | struct B<A> {
    | ----------- doesn't satisfy `B<A>: Clone`
...
15  |     let a_map = ctx.a_map.clone();
    |                           ^^^^^ method cannot be called on `HashMap<String, B<A>>` due to unsatisfied trait bounds
    |
   ::: /Users/USERNAME/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/collections/hash/map.rs:214:1
    |
214 | pub struct HashMap<K, V, S = RandomState> {
    | ----------------------------------------- doesn't satisfy `HashMap<String, B<A>>: Clone`
    |
    = note: the following trait bounds were not satisfied:
            `B<A>: Clone`
            which is required by `HashMap<String, B<A>>: Clone`
help: consider annotating `B<A>` with `#[derive(Clone)]`
    |
10  | #[derive(Clone)]
    |

For more information about this error, try `rustc --explain E0599`.
error: could not compile `test_clone_err` (lib) due to previous error
warning: build failed, waiting for other jobs to finish...
error: could not compile `test_clone_err` (lib test) due to previous error

Desired output

error[E0599]: the method `clone` exists for mutable reference `&mut Ctx<A>`, but its trait bounds were not satisfied
  --> src/lib.rs:16:9
   |
4  | struct Ctx<A> {
   | ------------- doesn't satisfy `Ctx<A>: Clone`
...
16 |     ctx.clone();
   |         ^^^^^ method cannot be called on `&mut Ctx<A>` due to unsatisfied trait bounds
   |
note: trait bound `A: Clone` was not satisfied
  --> src/lib.rs:3:10
   |
3  | #[derive(Clone)]
   |          ^^^^^ unsatisfied trait bound introduced in this `derive` macro
help: consider restricting the type parameter to satisfy the trait bound
   |
15 | fn foo<A>(ctx: &mut Ctx<A>) where A: Clone {
   |                             ++++++++++++++

For more information about this error, try `rustc --explain E0599`.
error: could not compile `test_clone_err` (lib test) due to previous error
warning: build failed, waiting for other jobs to finish...
error: could not compile `test_clone_err` (lib) due to previous error

Rationale and extra context

The root cause is that P in foo<P> has not been restricted by the Clone bound, but the compiler suggests users to add #[derive(Clone)] on struct B<A>, which has already been done.

Other cases

No response

Anything else?

rustc versions:

rustc 1.70.0-nightly (88fb1b922 2023-04-10)
rustc 1.71.0-nightly (d0f204e4d 2023-04-16)

I have tested the same code under both of the versions and the outputs are the same.

[Ezrashaw]

I think the problem here is that, for some reason, rustc isn't "peeling back" the trait bounds through the auto-derived impl (seems the same with a manual impl too). It says:

note: the following trait bounds were not satisfied:
            `B<A>: Clone`
            which is required by `HashMap<String, B<A>>: Clone`

Well, B<A>: Clone iff A: Clone which it doesn't seem to be picking up on.

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