Compilation: fix reference trace behavior without -freference-trace

When `-freference-trace` is not passed, we want to show exactly one
reference trace. Previously, we set the reference trace root in `Sema`
iff there were no other failed analyses. However, this results in an
arbitrary error being the one with the reference trace after error
sorting. It is also incompatible with incremental compilation, where
some errors might be unreferenced. Instead, set the field on all
analysis errors, and decide in `Compilation.getAllErrorsAlloc` which
reference trace[s] to actually show.

src/Compilation.zig

@@ -3328,7 +3328,7 @@ pub fn getAllErrorsAlloc(comp: *Compilation) !ErrorBundle {
     if (comp.zcu) |zcu| zcu_errors: {
         for (zcu.failed_files.keys(), zcu.failed_files.values()) |file, error_msg| {
             if (error_msg) |msg| {
-                try addModuleErrorMsg(zcu, &bundle, msg.*);
+                try addModuleErrorMsg(zcu, &bundle, msg.*, false);
             } else {
                 // Must be ZIR or Zoir errors. Note that this may include AST errors.
                 _ = try file.getTree(gpa); // Tree must be loaded.
@@ -3378,6 +3378,7 @@ pub fn getAllErrorsAlloc(comp: *Compilation) !ErrorBundle {
             break :s entries.slice();
         };
         defer sorted_failed_analysis.deinit(gpa);
+        var added_any_analysis_error = false;
         for (sorted_failed_analysis.items(.key), sorted_failed_analysis.items(.value)) |anal_unit, error_msg| {
             if (comp.incremental) {
                 const refs = try zcu.resolveReferences();
@@ -3389,7 +3390,9 @@ pub fn getAllErrorsAlloc(comp: *Compilation) !ErrorBundle {
                 zcu.fmtAnalUnit(anal_unit),
             });
 
-            try addModuleErrorMsg(zcu, &bundle, error_msg.*);
+            try addModuleErrorMsg(zcu, &bundle, error_msg.*, added_any_analysis_error);
+            added_any_analysis_error = true;
+
             if (zcu.cimport_errors.get(anal_unit)) |errors| {
                 for (errors.getMessages()) |err_msg_index| {
                     const err_msg = errors.getErrorMessage(err_msg_index);
@@ -3412,13 +3415,13 @@ pub fn getAllErrorsAlloc(comp: *Compilation) !ErrorBundle {
             }
         }
         for (zcu.failed_codegen.values()) |error_msg| {
-            try addModuleErrorMsg(zcu, &bundle, error_msg.*);
+            try addModuleErrorMsg(zcu, &bundle, error_msg.*, false);
         }
         for (zcu.failed_types.values()) |error_msg| {
-            try addModuleErrorMsg(zcu, &bundle, error_msg.*);
+            try addModuleErrorMsg(zcu, &bundle, error_msg.*, false);
         }
         for (zcu.failed_exports.values()) |value| {
-            try addModuleErrorMsg(zcu, &bundle, value.*);
+            try addModuleErrorMsg(zcu, &bundle, value.*, false);
         }
 
         const actual_error_count = zcu.intern_pool.global_error_set.getNamesFromMainThread().len;
@@ -3527,7 +3530,7 @@ pub fn getAllErrorsAlloc(comp: *Compilation) !ErrorBundle {
         // We don't actually include the error here if `!include_compile_log_sources`.
         // The sorting above was still necessary, though, to get `log_text` in the right order.
         if (include_compile_log_sources) {
-            try addModuleErrorMsg(zcu, &bundle, messages.items[0]);
+            try addModuleErrorMsg(zcu, &bundle, messages.items[0], false);
         }
 
         break :compile_log_text try log_text.toOwnedSlice(gpa);
@@ -3631,10 +3634,14 @@ pub const ErrorNoteHashContext = struct {
     }
 };
 
+const default_reference_trace_len = 2;
 pub fn addModuleErrorMsg(
     zcu: *Zcu,
     eb: *ErrorBundle.Wip,
     module_err_msg: Zcu.ErrorMsg,
+    /// If `-freference-trace` is not specified, we only want to show the one reference trace.
+    /// So, this is whether we have already emitted an error with a reference trace.
+    already_added_error: bool,
 ) !void {
     const gpa = eb.gpa;
     const ip = &zcu.intern_pool;
@@ -3657,14 +3664,18 @@ pub fn addModuleErrorMsg(
     var ref_traces: std.ArrayListUnmanaged(ErrorBundle.ReferenceTrace) = .empty;
     defer ref_traces.deinit(gpa);
 
-    if (module_err_msg.reference_trace_root.unwrap()) |rt_root| {
+    rt: {
+        const rt_root = module_err_msg.reference_trace_root.unwrap() orelse break :rt;
+        const max_references = zcu.comp.reference_trace orelse refs: {
+            if (already_added_error) break :rt;
+            break :refs default_reference_trace_len;
+        };
+
         const all_references = try zcu.resolveReferences();
 
         var seen: std.AutoHashMapUnmanaged(InternPool.AnalUnit, void) = .empty;
         defer seen.deinit(gpa);
 
-        const max_references = zcu.comp.reference_trace orelse Sema.default_reference_trace_len;
-
         var referenced_by = rt_root;
         while (all_references.get(referenced_by)) |maybe_ref| {
             const ref = maybe_ref orelse break;

src/Sema.zig

@@ -191,7 +191,6 @@ const LowerZon = @import("Sema/LowerZon.zig");
 const arith = @import("Sema/arith.zig");
 
 pub const default_branch_quota = 1000;
-pub const default_reference_trace_len = 2;
 
 pub const InferredErrorSet = struct {
     /// The function body from which this error set originates.
@@ -2580,7 +2579,7 @@ pub fn failWithOwnedErrorMsg(sema: *Sema, block: ?*Block, err_msg: *Zcu.ErrorMsg
     if (build_options.enable_debug_extensions and zcu.comp.debug_compile_errors) {
         var wip_errors: std.zig.ErrorBundle.Wip = undefined;
         wip_errors.init(gpa) catch @panic("out of memory");
-        Compilation.addModuleErrorMsg(zcu, &wip_errors, err_msg.*) catch @panic("out of memory");
+        Compilation.addModuleErrorMsg(zcu, &wip_errors, err_msg.*, false) catch @panic("out of memory");
         std.debug.print("compile error during Sema:\n", .{});
         var error_bundle = wip_errors.toOwnedBundle("") catch @panic("out of memory");
         error_bundle.renderToStdErr(.{ .ttyconf = .no_color });
@@ -2600,10 +2599,7 @@ pub fn failWithOwnedErrorMsg(sema: *Sema, block: ?*Block, err_msg: *Zcu.ErrorMsg
         }
     }
 
-    const use_ref_trace = if (zcu.comp.reference_trace) |n| n > 0 else zcu.failed_analysis.count() == 0;
+    err_msg.reference_trace_root = sema.owner.toOptional();
-    if (use_ref_trace) {
-        err_msg.reference_trace_root = sema.owner.toOptional();
-    }
 
     const gop = try zcu.failed_analysis.getOrPut(gpa, sema.owner);
     if (gop.found_existing) {