Applying `do_not_recommend` to tuple implementations causes them to not be decomposed in diagnostics

[Nemo157]

Code

#[diagnostic::on_unimplemented(note = "this is implemented on tuples of implementors too")]
trait SomeTrait {}

struct AnImplementor;
struct AnotherImplementor;

impl SomeTrait for AnImplementor {}
impl SomeTrait for AnotherImplementor {}

#[diagnostic::do_not_recommend]
impl<T: SomeTrait> SomeTrait for (T,) {}
#[diagnostic::do_not_recommend]
impl<T: SomeTrait, U: SomeTrait> SomeTrait for (T, U) {}
#[diagnostic::do_not_recommend]
impl<T: SomeTrait, U: SomeTrait, V: SomeTrait> SomeTrait for (T, U, V) {}

fn foo(_: impl SomeTrait) {}

pub fn bar() {
    foo((AnImplementor, 5u32));
}

Current output

error[E0277]: the trait bound `(AnImplementor, u32): SomeTrait` is not satisfied
  --> src/lib.rs:20:25
   |
20 |     foo((AnImplementor, 5u32));
   |                         ^^^^ the trait `SomeTrait` is not implemented for `(AnImplementor, u32)`
   |
   = note: this is implemented on tuples of implementors too
   = help: the following other types implement trait `SomeTrait`:
             AnImplementor
             AnotherImplementor
note: required by a bound in `foo`
  --> src/lib.rs:17:16
   |
17 | fn foo(_: impl SomeTrait) {}
   |                ^^^^^^^^^ required by this bound in `foo`

Desired output

error[E0277]: the trait bound `u32: SomeTrait` is not satisfied
  --> src/lib.rs:21:29
   |
21 |         foo((AnImplementor, 5u32));
   |                             ^^^^ the trait `SomeTrait` is not implemented for `u32`
   |
   = note: this is implemented on tuples of implementors too
   = help: the following other types implement trait `SomeTrait`:
             AnImplementor
             AnotherImplementor
note: required by a bound in `foo`
  --> src/lib.rs:18:20
   |
18 |     fn foo(_: impl SomeTrait) {}
   |                    ^^^^^^^^^ required by this bound in `foo`

Rationale and extra context

Playground containing both with/without diagnostic attributes.

For variadic tuple implementations it's not useful to see all the tuples in the "the following other types implement trait", but they're still useful to be decomposed to only inform the user of the specific element of the tuple that is violating the trait impl.

It's interesting to see in the current error message it does point directly to the u32, even though it refers to the type of the whole tuple.

Other cases

Without the diagnostic attributes

trait SomeTrait {}

struct AnImplementor;
struct AnotherImplementor;

impl SomeTrait for AnImplementor {}
impl SomeTrait for AnotherImplementor {}

impl<T: SomeTrait> SomeTrait for (T,) {}
impl<T: SomeTrait, U: SomeTrait> SomeTrait for (T, U) {}
impl<T: SomeTrait, U: SomeTrait, V: SomeTrait> SomeTrait for (T, U, V) {}

fn foo(_: impl SomeTrait) {}

pub fn bar() {
    foo((AnImplementor, 5u32));
}

The output looks like

error[E0277]: the trait bound `u32: SomeTrait` is not satisfied
  --> src/lib.rs:16:25
   |
16 |     foo((AnImplementor, 5u32));
   |     ---                 ^^^^ the trait `SomeTrait` is not implemented for `u32`
   |     |
   |     required by a bound introduced by this call
   |
   = help: the following other types implement trait `SomeTrait`:
             (T, U)
             (T, U, V)
             (T,)
             AnImplementor
             AnotherImplementor
note: required for `(AnImplementor, u32)` to implement `SomeTrait`
  --> src/lib.rs:10:34
   |
10 | impl<T: SomeTrait, U: SomeTrait> SomeTrait for (T, U) {}
   |                       ---------  ^^^^^^^^^     ^^^^^^
   |                       |
   |                       unsatisfied trait bound introduced here
note: required by a bound in `foo`
  --> src/lib.rs:13:16
   |
13 | fn foo(_: impl SomeTrait) {}
   |                ^^^^^^^^^ required by this bound in `foo`

Rust Version

1.90.0-nightly (2025-06-23 706f244db581212cabf2)

Anything else?

No response

[compiler-errors]

This is intended behavior of the do_not_recommend attribute. We don't walk into the tuple implementation even though it shallowly applies,thus the emitted trait obligation that is reported as an error is the tuple-implements-trait goal (and not one of its sub-obligations).

If we want to have some other functionality to just ignore the impl for the purposes of "the following other types implement trait" note, that needs to be implemented separately.