compiler: Enable -Wdate-time for C/C++ code in most Zig commands.

In particular: `build-obj` (& friends), `test`, `run`, and `translate-c`.

Not `cc`/`c++`, however, as we don't advertise deterministic builds for those
commands. For those, it is expected that people set up the necessary flags
themselves if they want determinism. (See #20811 for one such flag.)

src/Compilation.zig

@@ -94,6 +94,8 @@ native_system_include_paths: []const []const u8,
 /// Corresponds to `-u <symbol>` for ELF/MachO and `/include:<symbol>` for COFF/PE.
 force_undefined_symbols: std.StringArrayHashMapUnmanaged(void),
 
+warn_nondeterminism: bool,
+
 c_object_table: std.AutoArrayHashMapUnmanaged(*CObject, void) = .{},
 win32_resource_table: if (dev.env.supports(.win32_resource)) std.AutoArrayHashMapUnmanaged(*Win32Resource, void) else struct {
     pub fn keys(_: @This()) [0]void {
@@ -1175,6 +1177,7 @@ pub const CreateOptions = struct {
     hash_style: link.File.Elf.HashStyle = .both,
     entry: Entry = .default,
     force_undefined_symbols: std.StringArrayHashMapUnmanaged(void) = .{},
+    warn_nondeterminism: bool = false,
     stack_size: ?u64 = null,
     image_base: ?u64 = null,
     version: ?std.SemanticVersion = null,
@@ -1572,6 +1575,7 @@ pub fn create(gpa: Allocator, arena: Allocator, options: CreateOptions) !*Compil
             .native_system_include_paths = options.native_system_include_paths,
             .wasi_emulated_libs = options.wasi_emulated_libs,
             .force_undefined_symbols = options.force_undefined_symbols,
+            .warn_nondeterminism = options.warn_nondeterminism,
             .link_eh_frame_hdr = link_eh_frame_hdr,
             .global_cc_argv = options.global_cc_argv,
             .file_system_inputs = options.file_system_inputs,
@@ -2625,6 +2629,7 @@ fn addNonIncrementalStuffToCacheManifest(
     man.hash.add(comp.include_compiler_rt);
     man.hash.add(comp.rc_includes);
     man.hash.addListOfBytes(comp.force_undefined_symbols.keys());
+    man.hash.add(comp.warn_nondeterminism);
     man.hash.addListOfBytes(comp.framework_dirs);
     try link.hashAddSystemLibs(man, comp.system_libs);
 
@@ -5731,6 +5736,10 @@ pub fn addCCArgs(
                 },
             }
 
+            if (comp.warn_nondeterminism) {
+                try argv.append("-Wdate-time");
+            }
+
             if (target_util.supports_fpic(target) and mod.pic) {
                 try argv.append("-fPIC");
             }

src/main.zig

@@ -895,6 +895,7 @@ fn buildOutputType(
     var test_no_exec = false;
     var entry: Compilation.CreateOptions.Entry = .default;
     var force_undefined_symbols: std.StringArrayHashMapUnmanaged(void) = .{};
+    var warn_nondeterminism: bool = false;
     var stack_size: ?u64 = null;
     var image_base: ?u64 = null;
     var link_eh_frame_hdr = false;
@@ -1150,8 +1151,6 @@ fn buildOutputType(
                         n_jobs = num;
                     } else if (mem.eql(u8, arg, "--subsystem")) {
                         subsystem = try parseSubSystem(args_iter.nextOrFatal());
-                    } else if (mem.eql(u8, arg, "-O")) {
-                        mod_opts.optimize_mode = parseOptimizeMode(args_iter.nextOrFatal());
                     } else if (mem.startsWith(u8, arg, "-fentry=")) {
                         entry = .{ .named = arg["-fentry=".len..] };
                     } else if (mem.eql(u8, arg, "--force_undefined")) {
@@ -1299,6 +1298,10 @@ fn buildOutputType(
                         target_mcpu = arg["-mcpu=".len..];
                     } else if (mem.startsWith(u8, arg, "-O")) {
                         mod_opts.optimize_mode = parseOptimizeMode(arg["-O".len..]);
+                        warn_nondeterminism = mod_opts.optimize_mode != .Debug;
+                    } else if (mem.eql(u8, arg, "-O")) {
+                        mod_opts.optimize_mode = parseOptimizeMode(args_iter.nextOrFatal());
+                        warn_nondeterminism = mod_opts.optimize_mode != .Debug;
                     } else if (mem.eql(u8, arg, "--dynamic-linker")) {
                         create_module.dynamic_linker = args_iter.nextOrFatal();
                     } else if (mem.eql(u8, arg, "--sysroot")) {
@@ -3348,6 +3351,7 @@ fn buildOutputType(
         .link_emit_relocs = link_emit_relocs,
         .entry = entry,
         .force_undefined_symbols = force_undefined_symbols,
+        .warn_nondeterminism = warn_nondeterminism,
         .stack_size = stack_size,
         .image_base = image_base,
         .formatted_panics = formatted_panics,