borrowck/errors: fix i18n error in delayed bug

During borrowck, the `MultiSpan` from a buffered diagnostic is cloned and
used to emit a delayed bug indicating a diagnostic was buffered - when
the buffered diagnostic is translated, then the cloned `MultiSpan` may
contain labels which can only render with the diagnostic's arguments, but
the delayed bug being emitted won't have those arguments. Adds a function
which clones `MultiSpan` without also cloning the contained labels, and
use this function when creating the buffered diagnostic delayed bug.

Signed-off-by: David Wood <david@davidtw.co>

compiler/rustc_borrowck/src/lib.rs

@@ -2306,11 +2306,10 @@ mod error {
 
         pub fn buffer_error(&mut self, t: DiagnosticBuilder<'_, ErrorGuaranteed>) {
             if let None = self.tainted_by_errors {
-                self.tainted_by_errors = Some(
-                    self.tcx
-                        .sess
-                        .delay_span_bug(t.span.clone(), "diagnostic buffered but not emitted"),
-                )
+                self.tainted_by_errors = Some(self.tcx.sess.delay_span_bug(
+                    t.span.clone_ignoring_labels(),
+                    "diagnostic buffered but not emitted",
+                ))
             }
             t.buffer(&mut self.buffered);
         }

compiler/rustc_error_messages/src/lib.rs

@@ -533,6 +533,14 @@ impl MultiSpan {
     pub fn has_span_labels(&self) -> bool {
         self.span_labels.iter().any(|(sp, _)| !sp.is_dummy())
     }
+
+    /// Clone this `MultiSpan` without keeping any of the span labels - sometimes a `MultiSpan` is
+    /// to be re-used in another diagnostic, but includes `span_labels` which have translated
+    /// messages. These translated messages would fail to translate without their diagnostic
+    /// arguments which are unlikely to be cloned alongside the `Span`.
+    pub fn clone_ignoring_labels(&self) -> Self {
+        Self { primary_spans: self.primary_spans.clone(), ..MultiSpan::new() }
+    }
 }
 
 impl From<Span> for MultiSpan {