Rollup merge of #122299 - compiler-errors:bt-for-must-diag, r=nnether…

…cote

Store backtrace for `must_produce_diag`

This makes it significantly easier to debug a `must_produce_diag` ICE, since we have no other way to know where the heck the bug originates from.

Backtrace rendering kinda sucks right now since we're just printing it in the panic message; happy to apply some suggestions to make it prettier or reuse other bug printing machinery, but also don't want to iterate too much on the rendering since this really is just for debug purposes.

r? nnethercote

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());
     }
 }
 
@@ -1384,7 +1390,7 @@ 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(),