Store backtrace for must_produce_diag

compiler/rustc_errors/src/lib.rs

@@ -442,8 +442,8 @@ struct DiagCtxtInner {
     emitter: Box<DynEmitter>,
 
     /// Must we produce a diagnostic to justify the use of the expensive
-    /// `trimmed_def_paths` function?
-    must_produce_diag: bool,
+    /// `trimmed_def_paths` function? Backtrace is the location of the call.
+    must_produce_diag: Option<Backtrace>,
 
     /// Has this diagnostic context printed any diagnostics? (I.e. has
     /// `self.emitter.emit_diagnostic()` been called?
@@ -572,10 +572,11 @@ impl Drop for DiagCtxtInner {
         }
 
         if !self.has_printed && !self.suppressed_expected_diag && !std::thread::panicking() {
-            if self.must_produce_diag {
+            if let Some(backtrace) = &self.must_produce_diag {
                 panic!(
-                    "must_produce_diag: trimmed_def_paths called but no diagnostics emitted; \
-                       use `DelayDm` for lints or `with_no_trimmed_paths` for debugging"
+                    "must_produce_diag: `trimmed_def_paths` called but no diagnostics emitted; \
+                       use `DelayDm` for lints or `with_no_trimmed_paths` for debugging. \
+                       called at: {backtrace}"
                 );
             }
         }
@@ -721,7 +722,7 @@ impl DiagCtxt {
         *delayed_bugs = Default::default();
         *deduplicated_err_count = 0;
         *deduplicated_warn_count = 0;
-        *must_produce_diag = false;
+        *must_produce_diag = None;
         *has_printed = false;
         *suppressed_expected_diag = false;
         *taught_diagnostics = Default::default();
@@ -1091,8 +1092,13 @@ impl DiagCtxt {
 
     /// Used when trimmed_def_paths is called and we must produce a diagnostic
     /// to justify its cost.
+    #[track_caller]
     pub fn set_must_produce_diag(&self) {
-        self.inner.borrow_mut().must_produce_diag = true;
+        assert!(
+            self.inner.borrow().must_produce_diag.is_none(),
+            "should only need to collect a backtrace once"
+        );
+        self.inner.borrow_mut().must_produce_diag = Some(Backtrace::capture());
     }
 }
 
@@ -1387,7 +1393,7 @@ impl DiagCtxtInner {
             deduplicated_err_count: 0,
             deduplicated_warn_count: 0,
             emitter,
-            must_produce_diag: false,
+            must_produce_diag: None,
             has_printed: false,
             suppressed_expected_diag: false,
             taught_diagnostics: Default::default(),