build/test: Add -Dtest-slow-targets and move mips module tests behind it.

The idea is that these tests are just too slow to include by default on a
contributor's local machine. If they want to run these, they need to opt in.

build.zig

@@ -380,6 +380,7 @@ pub fn build(b: *std.Build) !void {
 
     const test_filters = b.option([]const []const u8, "test-filter", "Skip tests that do not match any filter") orelse &[0][]const u8{};
     const test_target_filters = b.option([]const []const u8, "test-target-filter", "Skip tests whose target triple do not match any filter") orelse &[0][]const u8{};
+    const test_slow_targets = b.option(bool, "test-slow-targets", "Enable running module tests for targets that have a slow compiler backend") orelse false;
 
     const test_cases_options = b.addOptions();
 
@@ -459,6 +460,7 @@ pub fn build(b: *std.Build) !void {
     test_step.dependOn(tests.addModuleTests(b, .{
         .test_filters = test_filters,
         .test_target_filters = test_target_filters,
+        .test_slow_targets = test_slow_targets,
         .root_src = "test/behavior.zig",
         .name = "behavior",
         .desc = "Run the behavior tests",
@@ -473,6 +475,7 @@ pub fn build(b: *std.Build) !void {
     test_step.dependOn(tests.addModuleTests(b, .{
         .test_filters = test_filters,
         .test_target_filters = test_target_filters,
+        .test_slow_targets = test_slow_targets,
         .root_src = "test/c_import.zig",
         .name = "c-import",
         .desc = "Run the @cImport tests",
@@ -486,6 +489,7 @@ pub fn build(b: *std.Build) !void {
     test_step.dependOn(tests.addModuleTests(b, .{
         .test_filters = test_filters,
         .test_target_filters = test_target_filters,
+        .test_slow_targets = test_slow_targets,
         .root_src = "lib/compiler_rt.zig",
         .name = "compiler-rt",
         .desc = "Run the compiler_rt tests",
@@ -500,6 +504,7 @@ pub fn build(b: *std.Build) !void {
     test_step.dependOn(tests.addModuleTests(b, .{
         .test_filters = test_filters,
         .test_target_filters = test_target_filters,
+        .test_slow_targets = test_slow_targets,
         .root_src = "lib/c.zig",
         .name = "universal-libc",
         .desc = "Run the universal libc tests",
@@ -527,6 +532,7 @@ pub fn build(b: *std.Build) !void {
     test_step.dependOn(tests.addModuleTests(b, .{
         .test_filters = test_filters,
         .test_target_filters = test_target_filters,
+        .test_slow_targets = test_slow_targets,
         .root_src = "lib/std/std.zig",
         .name = "std",
         .desc = "Run the standard library tests",

test/tests.zig

@@ -27,6 +27,12 @@ const TestTarget = struct {
     use_lld: ?bool = null,
     pic: ?bool = null,
     strip: ?bool = null,
+
+    // This is intended for targets that are known to be slow to compile. These are acceptable to
+    // run in CI, but should not be run on developer machines by default. As an example, at the time
+    // of writing, this includes LLVM's MIPS backend which takes upwards of 20 minutes longer to
+    // compile tests than other backends.
+    slow_backend: bool = false,
 };
 
 const test_targets = blk: {
@@ -310,6 +316,7 @@ const test_targets = blk: {
                 .os_tag = .linux,
                 .abi = .none,
             },
+            .slow_backend = true,
         },
         .{
             .target = .{
@@ -318,6 +325,7 @@ const test_targets = blk: {
                 .abi = .musl,
             },
             .link_libc = true,
+            .slow_backend = true,
         },
         .{
             .target = .{
@@ -326,6 +334,7 @@ const test_targets = blk: {
                 .abi = .gnueabihf,
             },
             .link_libc = true,
+            .slow_backend = true,
         },
 
         .{
@@ -334,6 +343,7 @@ const test_targets = blk: {
                 .os_tag = .linux,
                 .abi = .none,
             },
+            .slow_backend = true,
         },
         .{
             .target = .{
@@ -342,6 +352,7 @@ const test_targets = blk: {
                 .abi = .musl,
             },
             .link_libc = true,
+            .slow_backend = true,
         },
         .{
             .target = .{
@@ -350,6 +361,7 @@ const test_targets = blk: {
                 .abi = .gnueabihf,
             },
             .link_libc = true,
+            .slow_backend = true,
         },
 
         .{
@@ -401,7 +413,8 @@ const test_targets = blk: {
             .link_libc = true,
         },
 
-        // Disabled until LLVM fixes their O(N^2) codegen.
+        // Disabled until LLVM fixes their O(N^2) codegen. Note that this is so bad that we don't
+        // even want to include this in CI with `slow_backend`.
         // https://github.com/ziglang/zig/issues/18872
         //.{
         //    .target = .{
@@ -412,7 +425,8 @@ const test_targets = blk: {
         //    .use_llvm = true,
         //},
 
-        // Disabled until LLVM fixes their O(N^2) codegen.
+        // Disabled until LLVM fixes their O(N^2) codegen. Note that this is so bad that we don't
+        // even want to include this in CI with `slow_backend`.
         // https://github.com/ziglang/zig/issues/18872
         //.{
         //    .target = .{
@@ -966,6 +980,7 @@ pub fn addRunTranslatedCTests(
 const ModuleTestOptions = struct {
     test_filters: []const []const u8,
     test_target_filters: []const []const u8,
+    test_slow_targets: bool,
     root_src: []const u8,
     name: []const u8,
     desc: []const u8,
@@ -982,6 +997,8 @@ pub fn addModuleTests(b: *std.Build, options: ModuleTestOptions) *Step {
     const step = b.step(b.fmt("test-{s}", .{options.name}), options.desc);
 
     for (test_targets) |test_target| {
+        if (!options.test_slow_targets and test_target.slow_backend) continue;
+
         if (options.skip_non_native and !test_target.target.isNative())
             continue;