Preserve spans when hiding do_not_recommend impls

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

@@ -816,13 +816,21 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
     pub(super) fn apply_do_not_recommend(
         &self,
         obligation: &mut PredicateObligation<'tcx>,
+        root_obligation: &PredicateObligation<'tcx>,
     ) -> bool {
         let mut base_cause = obligation.cause.code().clone();
         let mut applied_do_not_recommend = false;
         loop {
             if let ObligationCauseCode::ImplDerived(ref c) = base_cause {
                 if self.tcx.do_not_recommend_impl(c.impl_or_alias_def_id) {
                     let code = (*c.derived.parent_code).clone();
+                    // Keep more precise spans that still point within the parent obligation,
+                    // but do not let hidden impl details move the span outside of it.
+                    if code == *root_obligation.cause.code()
+                        && !root_obligation.cause.span.contains(obligation.cause.span)
+                    {
+                        obligation.cause.span = root_obligation.cause.span;
+                    }
                     obligation.cause.map_code(|_| code);
                     obligation.predicate = c.derived.parent_trait_pred.upcast(self.tcx);
                     applied_do_not_recommend = true;

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

@@ -298,7 +298,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             error.code,
             FulfillmentErrorCode::Select(crate::traits::SelectionError::Unimplemented)
                 | FulfillmentErrorCode::Project(_)
-        ) && self.apply_do_not_recommend(&mut error.obligation)
+        ) && self.apply_do_not_recommend(&mut error.obligation, &error.root_obligation)
         {
             error.code = FulfillmentErrorCode::Select(SelectionError::Unimplemented);
         }

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

@@ -12,6 +12,26 @@ LL | impl AsExpression<Text> for &'_ str {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = help: for that trait implementation, expected `Text`, found `Integer`
 
-error: aborting due to 1 previous error
+error[E0277]: the trait bound `X: A` is not satisfied
+  --> $DIR/as_expression.rs:60:15
+   |
+LL |     X.start().foo().finish();
+   |               ^^^ unsatisfied trait bound
+   |
+help: the trait `A` is not implemented for `X`
+  --> $DIR/as_expression.rs:70:1
+   |
+LL | struct X;
+   | ^^^^^^^^
+note: required by a bound in `Ext::foo`
+  --> $DIR/as_expression.rs:79:23
+   |
+LL |     fn foo(self) -> Self
+   |        --- required by a bound in this associated function
+LL |     where
+LL |         Self: Sized + A;
+   |                       ^ required by this bound in `Ext::foo`
+
+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.next.stderr

@@ -22,6 +22,26 @@ LL |     where
 LL |         T: AsExpression<Self::SqlType>,
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Foo::check`
 
-error: aborting due to 1 previous error
+error[E0277]: the trait bound `X: A` is not satisfied
+  --> $DIR/as_expression.rs:60:15
+   |
+LL |     X.start().foo().finish();
+   |               ^^^ unsatisfied trait bound
+   |
+help: the trait `A` is not implemented for `X`
+  --> $DIR/as_expression.rs:70:1
+   |
+LL | struct X;
+   | ^^^^^^^^
+note: required by a bound in `Ext::foo`
+  --> $DIR/as_expression.rs:79:23
+   |
+LL |     fn foo(self) -> Self
+   |        --- required by a bound in this associated function
+LL |     where
+LL |         Self: Sized + A;
+   |                       ^ required by this bound in `Ext::foo`
+
+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

@@ -55,4 +55,49 @@ impl<T> Foo for T where T: Expression {}
 fn main() {
     SelectInt.check("bar");
     //~^ ERROR the trait bound `&str: AsExpression<Integer>` is not satisfied
+
+    // Regression test for https://github.com/rust-lang/rust/issues/156475.
+    X.start().foo().finish();
+    //~^ ERROR the trait bound `X: A` is not satisfied
+}
+
+trait A {}
+trait B {}
+
+#[diagnostic::do_not_recommend]
+impl<T: B> A for T {}
+
+struct X;
+
+trait Start {
+    fn start(self) -> Self;
+}
+
+trait Ext {
+    fn foo(self) -> Self
+    where
+        Self: Sized + A;
+}
+
+trait Finish {
+    fn finish(self);
+}
+
+impl<T> Start for T {
+    fn start(self) -> Self {
+        self
+    }
+}
+
+impl<T> Ext for T {
+    fn foo(self) -> Self
+    where
+        Self: Sized + A,
+    {
+        self
+    }
+}
+
+impl<T> Finish for T {
+    fn finish(self) {}
 }