Tweak rendering of cfg'd out item

```
error[E0433]: failed to resolve: could not find `doesnt_exist` in `inner`
  --> $DIR/diagnostics-cross-crate.rs:18:23
   |
LL |     cfged_out::inner::doesnt_exist::hello();
   |                       ^^^^^^^^^^^^ could not find `doesnt_exist` in `inner`
   |
note: found an item that was configured out
  --> $DIR/auxiliary/cfged_out.rs:6:13
   |
LL |     #[cfg(false)]
   |           ----- the item is gated here
LL |     pub mod doesnt_exist {
   |             ^^^^^^^^^^^^
```

2 files changed, 38 insertions, 29 deletions

diff --git a/compiler/rustc_resolve/src/diagnostics.rs b/compiler/rustc_resolve/src/diagnostics.rs
index e09fbcc8245..11def7e4876 100644
--- a/compiler/rustc_resolve/src/diagnostics.rs
+++ b/compiler/rustc_resolve/src/diagnostics.rs
@@ -2845,16 +2845,13 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                 continue;
             }
 
-            let note = errors::FoundItemConfigureOut { span: ident.span };
-            err.subdiagnostic(note);
-
-            if let CfgEntry::NameValue { value: Some((feature, _)), .. } = cfg.0 {
-                let note = errors::ItemWasBehindFeature { feature, span: cfg.1 };
-                err.subdiagnostic(note);
+            let item_was = if let CfgEntry::NameValue { value: Some((feature, _)), .. } = cfg.0 {
+                errors::ItemWas::BehindFeature { feature, span: cfg.1 }
             } else {
-                let note = errors::ItemWasCfgOut { span: cfg.1 };
-                err.subdiagnostic(note);
-            }
+                errors::ItemWas::CfgOut { span: cfg.1 }
+            };
+            let note = errors::FoundItemConfigureOut { span: ident.span, item_was };
+            err.subdiagnostic(note);
         }
     }
 }
diff --git a/compiler/rustc_resolve/src/errors.rs b/compiler/rustc_resolve/src/errors.rs
index d6b1e4de6ea..2747ba135ed 100644
--- a/compiler/rustc_resolve/src/errors.rs
+++ b/compiler/rustc_resolve/src/errors.rs
@@ -1,10 +1,13 @@
 use rustc_errors::codes::*;
-use rustc_errors::{Applicability, ElidedLifetimeInPathSubdiag, MultiSpan};
+use rustc_errors::{
+    Applicability, Diag, ElidedLifetimeInPathSubdiag, EmissionGuarantee, IntoDiagArg, MultiSpan,
+    Subdiagnostic,
+};
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_span::{Ident, Span, Symbol};
 
-use crate::Res;
 use crate::late::PatternSource;
+use crate::{Res, fluent_generated as fluent};
 
 #[derive(Diagnostic)]
 #[diag(resolve_generic_params_from_outer_item, code = E0401)]
@@ -1201,26 +1204,35 @@ pub(crate) struct IdentInScopeButItIsDesc<'a> {
     pub(crate) imported_ident_desc: &'a str,
 }
 
-#[derive(Subdiagnostic)]
-#[note(resolve_found_an_item_configured_out)]
 pub(crate) struct FoundItemConfigureOut {
-    #[primary_span]
-    pub(crate) span: Span,
-}
-
-#[derive(Subdiagnostic)]
-#[note(resolve_item_was_behind_feature)]
-pub(crate) struct ItemWasBehindFeature {
-    pub(crate) feature: Symbol,
-    #[primary_span]
-    pub(crate) span: Span,
-}
-
-#[derive(Subdiagnostic)]
-#[note(resolve_item_was_cfg_out)]
-pub(crate) struct ItemWasCfgOut {
-    #[primary_span]
     pub(crate) span: Span,
+    pub(crate) item_was: ItemWas,
+}
+
+pub(crate) enum ItemWas {
+    BehindFeature { feature: Symbol, span: Span },
+    CfgOut { span: Span },
+}
+
+impl Subdiagnostic for FoundItemConfigureOut {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
+        let mut multispan: MultiSpan = self.span.into();
+        match self.item_was {
+            ItemWas::BehindFeature { feature, span } => {
+                let key = "feature".into();
+                let value = feature.into_diag_arg(&mut None);
+                let msg = diag.dcx.eagerly_translate_to_string(
+                    fluent::resolve_item_was_behind_feature,
+                    [(&key, &value)].into_iter(),
+                );
+                multispan.push_span_label(span, msg);
+            }
+            ItemWas::CfgOut { span } => {
+                multispan.push_span_label(span, fluent::resolve_item_was_cfg_out);
+            }
+        }
+        diag.span_note(multispan, fluent::resolve_found_an_item_configured_out);
+    }
 }
 
 #[derive(Diagnostic)]