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?

Author: weiznich

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 +1001,35 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         err.emit()
     }
 
+    fn apply_do_not_recommend(
+        &self,
+        trait_predicate: &'_ mut ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
+        obligation: &'_ mut PredicateObligation<'tcx>,
+    ) -> ty::Binder<'tcx, ty::TraitPredicate<'tcx>> {
+        let mut temp_trait_predicate = trait_predicate.clone();
+        let mut base_cause = obligation.cause.code().clone();
+        loop {
+            // 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) {
+                    let code = (*c.derived.parent_code).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;
+                }
+            }
+            if let Some((parent_cause, _parent_pred)) = base_cause.parent() {
+                base_cause = parent_cause.clone();
+            } else {
+                break;
+            }
+        }
+
+        temp_trait_predicate
+    }
+
     fn emit_specialized_closure_kind_error(
         &self,
         obligation: &PredicateObligation<'tcx>,

tests/ui/diagnostic_namespace/do_not_recommend/as_expression.current.stderr

@@ -0,0 +1,12 @@
+error[E0277]: the trait bound `&str: AsExpression<Integer>` is not satisfied
+  --> $DIR/as_expression.rs:57:15
+   |
+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`.

tests/ui/diagnostic_namespace/do_not_recommend/as_expression.next.stderr

@@ -0,0 +1,27 @@
+error[E0277]: the trait bound `&str: AsExpression<<SelectInt as Expression>::SqlType>` is not satisfied
+  --> $DIR/as_expression.rs:57:21
+   |
+LL |     SelectInt.check("bar");
+   |               ----- ^^^^^ the trait `AsExpression<<SelectInt as Expression>::SqlType>` is not implemented for `&str`
+   |               |
+   |               required by a bound introduced by this call
+   |
+   = help: the trait `AsExpression<Text>` is implemented for `&str`
+note: required by a bound in `Foo::check`
+  --> $DIR/as_expression.rs:48:12
+   |
+LL |     fn check<T>(&self, _: T) -> <T as AsExpression<<Self as Expression>::SqlType>>::Expression
+   |        ----- required by a bound in this associated function
+LL |     where
+LL |         T: AsExpression<Self::SqlType>,
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Foo::check`
+
+error: the type `fn(&SelectInt, &str) -> <&str as AsExpression<<SelectInt as Expression>::SqlType>>::Expression` is not well-formed
+  --> $DIR/as_expression.rs:57:15
+   |
+LL |     SelectInt.check("bar");
+   |               ^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/diagnostic_namespace/do_not_recommend/as_expression.rs

@@ -0,0 +1,61 @@
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
+#![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");
+    //[next]~^ ERROR the trait bound `&str: AsExpression<<SelectInt as Expression>::SqlType>` is not satisfied
+    //[next]~| ERROR the type `fn(&SelectInt, &str) -> <&str as AsExpression<<SelectInt as Expression>::SqlType>>::Expression` is not well-formed
+    //[current]~^^^ ERROR the trait bound `&str: AsExpression<Integer>` is not satisfied
+}

tests/ui/diagnostic_namespace/do_not_recommend/stacked.current.stderr

@@ -0,0 +1,25 @@
+error[E0277]: the trait bound `(): Root` is not satisfied
+  --> $DIR/stacked.rs:19:18
+   |
+LL |     needs_root::<()>();
+   |                  ^^ the trait `Other` is not implemented for `()`, which is required by `(): Root`
+   |
+note: required for `()` to implement `DontRecommend`
+  --> $DIR/stacked.rs:14:9
+   |
+LL | impl<T> DontRecommend for T where T: Other {}
+   |         ^^^^^^^^^^^^^     ^          ----- unsatisfied trait bound introduced here
+note: required for `()` to implement `Root`
+  --> $DIR/stacked.rs:12:9
+   |
+LL | impl<T> Root for T where T: DontRecommend {}
+   |         ^^^^     ^          ------------- unsatisfied trait bound introduced here
+note: required by a bound in `needs_root`
+  --> $DIR/stacked.rs:16:18
+   |
+LL | fn needs_root<T: Root>() {}
+   |                  ^^^^ required by this bound in `needs_root`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/diagnostic_namespace/do_not_recommend/stacked.next.stderr

@@ -0,0 +1,15 @@
+error[E0277]: the trait bound `(): Root` is not satisfied
+  --> $DIR/stacked.rs:19:18
+   |
+LL |     needs_root::<()>();
+   |                  ^^ the trait `Root` is not implemented for `()`
+   |
+note: required by a bound in `needs_root`
+  --> $DIR/stacked.rs:16:18
+   |
+LL | fn needs_root<T: Root>() {}
+   |                  ^^^^ required by this bound in `needs_root`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/diagnostic_namespace/do_not_recommend/stacked.rs

@@ -0,0 +1,21 @@
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
+#![feature(do_not_recommend)]
+
+trait Root {}
+trait DontRecommend {}
+trait Other {}
+
+#[do_not_recommend]
+impl<T> Root for T where T: DontRecommend {}
+
+impl<T> DontRecommend for T where T: Other {}
+
+fn needs_root<T: Root>() {}
+
+fn main() {
+    needs_root::<()>();
+    //~^ ERROR the trait bound `(): Root` is not satisfied
+}