Actually use the #[do_not_recommend] attribute if present

This change tweaks the error message generation to actually use the
`#[do_not_recommend]` attribute if present by just skipping the marked
trait impl in favour of the parent impl. It also adds a compile test for
this behaviour. Without this change the test would output the following
error:

```
error[E0277]: the trait bound `&str: Expression` is not satisfied
  --> /home/weiznich/Documents/rust/rust/tests/ui/diagnostic_namespace/do_not_recommend.rs:53:15
   |
LL |     SelectInt.check("bar");
   |               ^^^^^ the trait `Expression` is not implemented for `&str`, which is required by `&str: AsExpression<Integer>`
   |
   = help: the following other types implement trait `Expression`:
             Bound<T>
             SelectInt
note: required for `&str` to implement `AsExpression<Integer>`
  --> /home/weiznich/Documents/rust/rust/tests/ui/diagnostic_namespace/do_not_recommend.rs:26:13
   |
LL | impl<T, ST> AsExpression<ST> for T
   |             ^^^^^^^^^^^^^^^^     ^
LL | where
LL |     T: Expression<SqlType = ST>,
   |        ------------------------ unsatisfied trait bound introduced here
```

Note how that mentions `&str: Expression` before and now mentions `&str:
AsExpression<Integer>` instead which is much more helpful for users.

Open points for further changes before stabilization:

* We likely want to move the attribute to the `#[diagnostic]` namespace
to relax the guarantees given?
* How does it interact with the new trait solver?

compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs

@@ -418,7 +418,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 match bound_predicate.skip_binder() {
                     ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_predicate)) => {
                         let trait_predicate = bound_predicate.rebind(trait_predicate);
-                        let trait_predicate = self.resolve_vars_if_possible(trait_predicate);
+                        let mut trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 
                         // Let's use the root obligation as the main message, when we care about the
                         // most general case ("X doesn't implement Pattern<'_>") over the case that
@@ -464,6 +464,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                                 root_obligation,
                             )
                         } else {
+                            let trait_predicate = self.apply_do_not_recommend(&mut trait_predicate, &mut obligation);
                             (trait_predicate, &obligation)
                         };
                         let trait_ref = main_trait_predicate.to_poly_trait_ref();
@@ -1000,6 +1001,51 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         err.emit()
     }
 
+    fn apply_do_not_recommend<'b>(
+        &self,
+        trait_predicate: &'_ mut ty::Binder<'b, ty::TraitPredicate<'tcx>>,
+        obligation: &'_ mut PredicateObligation<'tcx>,
+    ) -> ty::Binder<'b, ty::TraitPredicate<'tcx>>
+    where
+        'tcx: 'b,
+    {
+        let mut temp_trait_predicate = trait_predicate.clone();
+        let mut base_cause = obligation.cause.code().clone();
+        // we first need to check whether the current impl
+        // is marked as `#[do_not_recommend]`
+        if let ObligationCauseCode::ImplDerivedObligation(ref c) = base_cause {
+            if self.tcx.has_attr(c.impl_or_alias_def_id, sym::do_not_recommend) {
+                use std::ops::Deref;
+
+                let code = c.derived.parent_code.deref().clone();
+                obligation.cause.map_code(|_| code);
+                obligation.predicate = c.derived.parent_trait_pred.to_predicate(self.tcx);
+                temp_trait_predicate = c.derived.parent_trait_pred.clone();
+                *trait_predicate = c.derived.parent_trait_pred;
+            }
+        }
+
+        // later we need to walk the parent tree
+        // and check any impl there as well
+        while let Some((parent_cause, parent_pred)) = base_cause.parent() {
+            if let Some(parent_pred) = parent_pred {
+                if matches!(
+                    parent_cause,
+                    ObligationCauseCode::ImplDerivedObligation(ref c)
+                        if self.tcx.has_attr(c.impl_or_alias_def_id, sym::do_not_recommend)
+                ) {
+                    let code = parent_cause.clone();
+                    obligation.cause.map_code(|_| code);
+                    obligation.predicate = parent_pred.to_predicate(self.tcx);
+                    temp_trait_predicate = parent_pred.clone();
+                    *trait_predicate = parent_pred;
+                }
+            }
+            base_cause = parent_cause.clone();
+        }
+        temp_trait_predicate
+    }
+
     fn emit_specialized_closure_kind_error(
         &self,
         obligation: &PredicateObligation<'tcx>,

tests/ui/diagnostic_namespace/do_not_recommend/as_expression.rs

@@ -0,0 +1,55 @@
+#![feature(do_not_recommend)]
+
+pub trait Expression {
+    type SqlType;
+}
+
+pub trait AsExpression<ST> {
+    type Expression: Expression<SqlType = ST>;
+}
+
+pub struct Text;
+pub struct Integer;
+
+pub struct Bound<T>(T);
+pub struct SelectInt;
+
+impl Expression for SelectInt {
+    type SqlType = Integer;
+}
+
+impl<T> Expression for Bound<T> {
+    type SqlType = T;
+}
+
+#[do_not_recommend]
+impl<T, ST> AsExpression<ST> for T
+where
+    T: Expression<SqlType = ST>,
+{
+    type Expression = T;
+}
+
+impl AsExpression<Integer> for i32 {
+    type Expression = Bound<Integer>;
+}
+
+impl AsExpression<Text> for &'_ str {
+    type Expression = Bound<Text>;
+}
+
+trait Foo: Expression + Sized {
+    fn check<T>(&self, _: T) -> <T as AsExpression<<Self as Expression>::SqlType>>::Expression
+    where
+        T: AsExpression<Self::SqlType>,
+    {
+        todo!()
+    }
+}
+
+impl<T> Foo for T where T: Expression {}
+
+fn main() {
+    SelectInt.check("bar");
+    //~^ERROR the trait bound `&str: AsExpression<Integer>` is not satisfied
+}

tests/ui/diagnostic_namespace/do_not_recommend/as_expression.stderr

@@ -0,0 +1,12 @@
+error[E0277]: the trait bound `&str: AsExpression<Integer>` is not satisfied
+  --> $DIR/as_expression.rs:53:15
+   |
+LL |     SelectInt.check("bar");
+   |               ^^^^^ the trait `AsExpression<Integer>` is not implemented for `&str`
+   |
+   = help: the trait `AsExpression<Text>` is implemented for `&str`
+   = help: for that trait implementation, expected `Text`, found `Integer`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.