Improved trace output selection using program phase filtering (#2851)

By implementing these improvements, users will have the ability to choose specific parts of the trace output.

Currently, when executing a file using the explorer with the --trace_file=- or --trace_file=filename.txt flag, the resulting output is an extensive and verbose log containing all the information.

In this PR, I have introduced the `ProgramPhase` enum class, which have distinct phases encountered during the compilation of a program in the explorer. Each member of this enum class corresponds to a specific phase, signifying the relevant information to be included in the trace output.

The phases covered by the `ProgramPhase` enum class are as follows:

1. Printing the source program
2. Name resolution
3. Control flow resolution
4. Type checking
5. Unformed variable resolution
6. Printing declarations
7. Printing the timings
8. Printing whole output.

These phases can be selected by passing the following compiler flags along with `--trace_file=-`.
`-trace_source_program`, `-trace_name_resolution`, `-trace_control_flow_resolution`, `-trace_type_checking`, `-trace_unformed_variables_resolution`, `-trace_declarations`, `-trace_execution`, `-trace_timing` and `-trace_all`. If none of these flags is passed only execution trace will be added to the output.

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

explorer/main.cpp

@@ -32,6 +32,7 @@ namespace path = llvm::sys::path;
 auto ExplorerMain(int argc, char** argv, void* static_for_main_addr,
                   llvm::StringRef relative_prelude_path) -> int {
   llvm::setBugReportMsg(
+
       "Please report issues to "
       "https://github.com/carbon-language/carbon-lang/issues and include the "
       "crash backtrace.\n");
@@ -49,6 +50,36 @@ auto ExplorerMain(int argc, char** argv, void* static_for_main_addr,
       "trace_file",
       cl::desc("Output file for tracing; set to `-` to output to stdout."));
 
+  cl::list<ProgramPhase> allowed_program_phases(
+      cl::desc("Select the program phases to include in the output. By "
+               "default, only the execution trace will be added to the trace "
+               "output. Use a combination of the following flags to include "
+               "outputs for multiple phases:"),
+      cl::values(
+          clEnumValN(ProgramPhase::SourceProgram, "trace_source_program",
+                     "Include trace output for the Source Program phase."),
+          clEnumValN(ProgramPhase::NameResolution, "trace_name_resolution",
+                     "Include trace output for the Name Resolution phase."),
+          clEnumValN(
+              ProgramPhase::ControlFlowResolution,
+              "trace_control_flow_resolution",
+              "Include trace output for the Control Flow Resolution phase."),
+          clEnumValN(ProgramPhase::TypeChecking, "trace_type_checking",
+                     "Include trace output for the Type Checking phase."),
+          clEnumValN(ProgramPhase::UnformedVariableResolution,
+                     "trace_unformed_variables_resolution",
+                     "Include trace output for the Unformed Variables "
+                     "Resolution phase."),
+          clEnumValN(ProgramPhase::Declarations, "trace_declarations",
+                     "Include trace output for printing Declarations."),
+          clEnumValN(ProgramPhase::Execution, "trace_execution",
+                     "Include trace output for Program Execution."),
+          clEnumValN(
+              ProgramPhase::Timing, "trace_timing",
+              "Include timing logs for each phase, indicating the time taken."),
+          clEnumValN(ProgramPhase::All, "trace_all",
+                     "Include trace output for all phases.")));
+
   // Use the executable path as a base for the relative prelude path.
   std::string exe =
       llvm::sys::fs::getMainExecutable(argv[0], static_for_main_addr);
@@ -66,7 +97,10 @@ auto ExplorerMain(int argc, char** argv, void* static_for_main_addr,
   // Set up a stream for trace output.
   std::unique_ptr<llvm::raw_ostream> scoped_trace_stream;
   TraceStream trace_stream;
+
   if (!trace_file_name.empty()) {
+    // Adding allowed phases in the trace_stream
+    trace_stream.set_allowed_phases(allowed_program_phases);
     if (trace_file_name == "-") {
       trace_stream.set_stream(&llvm::outs());
     } else {
@@ -87,11 +121,6 @@ auto ExplorerMain(int argc, char** argv, void* static_for_main_addr,
   if (result.ok()) {
     // Print the return code to stdout.
     llvm::outs() << "result: " << *result << "\n";
-
-    // When there's a dedicated trace file, print the return code to it too.
-    if (scoped_trace_stream) {
-      trace_stream << "result: " << *result << "\n";
-    }
     return EXIT_SUCCESS;
   } else {
     llvm::errs() << result.error() << "\n";