Add flag to dump the raw SemIR in the event of a crash. (#6558)

toolchain/check/check.cpp

@@ -9,6 +9,7 @@
 
 #include "common/check.h"
 #include "common/map.h"
+#include "common/pretty_stack_trace_function.h"
 #include "toolchain/check/check_unit.h"
 #include "toolchain/check/context.h"
 #include "toolchain/check/cpp/import.h"
@@ -417,6 +418,27 @@ auto CheckParseTrees(
             &unit, tree_and_subtrees_getters.Get(unit.sem_ir->check_ir_id()));
       }));
 
+  // Dump the raw SemIR in the event of a crash. We dump it to a separate file
+  // to keep the stack trace manageable.
+  PrettyStackTraceFunction sem_ir_dumper([&](llvm::raw_ostream& output) {
+    if (!options.sem_ir_crash_dump.empty()) {
+      output << "Dumping raw SemIR to " << options.sem_ir_crash_dump << "\n";
+      std::error_code error_code;
+      llvm::raw_fd_ostream sem_ir_dump(options.sem_ir_crash_dump, error_code);
+      if (error_code) {
+        output << "Raw SemIR dump failed: " << error_code.category().name()
+               << ":" << error_code.value() << ": " << error_code.message()
+               << "\n";
+      } else {
+        for (const auto& unit_info : unit_infos) {
+          if (unit_info.is_checked && unit_info.unit->sem_ir != nullptr) {
+            unit_info.unit->sem_ir->Print(sem_ir_dump);
+          }
+        }
+      }
+    }
+  });
+
   Map<ImportKey, UnitAndImports*> api_map =
       BuildApiMapAndDiagnosePackaging(unit_infos);