x86_64: fix @errorName data

The final offset was clobbering the first error name, which is revealed
by an out of bounds when the global error set is empty.

Closes #22362

src/codegen.zig

@@ -148,19 +148,23 @@ pub fn generateLazySymbol(
     if (lazy_sym.ty == .anyerror_type) {
         alignment.* = .@"4";
         const err_names = ip.global_error_set.getNamesFromMainThread();
-        mem.writeInt(u32, try code.addManyAsArray(4), @intCast(err_names.len), endian);
-        var offset = code.items.len;
-        try code.resize((err_names.len + 1) * 4);
+        var offset_index: u32 = @intCast(code.items.len);
+        var string_index: u32 = @intCast(4 * (1 + err_names.len + @intFromBool(err_names.len > 0)));
+        try code.resize(offset_index + string_index);
+        mem.writeInt(u32, code.items[offset_index..][0..4], @intCast(err_names.len), endian);
+        if (err_names.len == 0) return .ok;
+        offset_index += 4;
         for (err_names) |err_name_nts| {
             const err_name = err_name_nts.toSlice(ip);
-            mem.writeInt(u32, code.items[offset..][0..4], @intCast(code.items.len), endian);
-            offset += 4;
+            mem.writeInt(u32, code.items[offset_index..][0..4], string_index, endian);
+            offset_index += 4;
             try code.ensureUnusedCapacity(err_name.len + 1);
             code.appendSliceAssumeCapacity(err_name);
             code.appendAssumeCapacity(0);
+            string_index += @intCast(err_name.len + 1);
         }
-        mem.writeInt(u32, code.items[offset..][0..4], @intCast(code.items.len), endian);
-        return Result.ok;
+        mem.writeInt(u32, code.items[offset_index..][0..4], string_index, endian);
+        return .ok;
     } else if (Type.fromInterned(lazy_sym.ty).zigTypeTag(pt.zcu) == .@"enum") {
         alignment.* = .@"1";
         const enum_ty = Type.fromInterned(lazy_sym.ty);
@@ -171,8 +175,8 @@ pub fn generateLazySymbol(
             code.appendSliceAssumeCapacity(tag_name);
             code.appendAssumeCapacity(0);
         }
-        return Result.ok;
-    } else return .{ .fail = try ErrorMsg.create(
+        return .ok;
+    } else return .{ .fail = try .create(
         gpa,
         src_loc,
         "TODO implement generateLazySymbol for {s} {}",

test/standalone/build.zig.zon

@@ -180,6 +180,9 @@
         .run_output_caching = .{
             .path = "run_output_caching",
         },
+        .empty_global_error_set = .{
+            .path = "empty_global_error_set",
+        },
     },
     .paths = .{
         "build.zig",

test/standalone/empty_global_error_set/build.zig

@@ -0,0 +1,24 @@
+const std = @import("std");
+const builtin = @import("builtin");
+
+pub fn build(b: *std.Build) void {
+    const test_step = b.step("test", "Test it");
+    b.default_step = test_step;
+
+    for ([_]bool{ false, true }) |use_llvm| {
+        const main = b.addObject(.{
+            .name = "main",
+            .root_module = b.createModule(.{
+                .root_source_file = b.path("main.zig"),
+                .target = b.resolveTargetQuery(.{
+                    .cpu_arch = .x86_64,
+                    .os_tag = .linux,
+                }),
+            }),
+            .use_llvm = use_llvm,
+            .use_lld = use_llvm,
+        });
+        _ = main.getEmittedBin();
+        test_step.dependOn(&main.step);
+    }
+}

test/standalone/empty_global_error_set/main.zig

@@ -0,0 +1,4 @@
+fn errorName(err: anyerror) [:0]const u8 {
+    return @errorName(err);
+}
+export const error_name: *const anyopaque = &errorName;