add builder.addFmt API and use it to test stage1 zig fmt

closes #1968

CMakeLists.txt

@@ -448,6 +448,7 @@ set(ZIG_STD_FILES
     "buf_set.zig"
     "buffer.zig"
     "build.zig"
+    "build/fmt.zig"
     "c/darwin.zig"
     "c/freebsd.zig"
     "c/index.zig"

build.zig

@@ -55,6 +55,8 @@ pub fn build(b: *Builder) !void {
     var test_stage2 = b.addTest("src-self-hosted/test.zig");
     test_stage2.setBuildMode(builtin.Mode.Debug);
 
+    const fmt_build_zig = b.addFmt([][]const u8{"build.zig"});
+
     var exe = b.addExecutable("zig", "src-self-hosted/main.zig");
     exe.setBuildMode(mode);
 
@@ -106,6 +108,11 @@ pub fn build(b: *Builder) !void {
     }
     const modes = chosen_modes[0..chosen_mode_index];
 
+    // run stage1 `zig fmt` on this build.zig file just to make sure it works
+    test_step.dependOn(&fmt_build_zig.step);
+    const fmt_step = b.step("test-fmt", "Run zig fmt against build.zig to make sure it works");
+    fmt_step.dependOn(&fmt_build_zig.step);
+
     test_step.dependOn(tests.addPkgTests(b, test_filter, "test/stage1/behavior.zig", "behavior", "Run the behavior tests", modes));
 
     test_step.dependOn(tests.addPkgTests(b, test_filter, "std/index.zig", "std", "Run the standard library tests", modes));

std/build.zig

@@ -15,6 +15,8 @@ const BufSet = std.BufSet;
 const BufMap = std.BufMap;
 const fmt_lib = std.fmt;
 
+pub const FmtStep = @import("build/fmt.zig").FmtStep;
+
 pub const Builder = struct {
     uninstall_tls: TopLevelStep,
     install_tls: TopLevelStep,
@@ -205,6 +207,10 @@ pub const Builder = struct {
         return remove_dir_step;
     }
 
+    pub fn addFmt(self: *Builder, paths: []const []const u8) *FmtStep {
+        return FmtStep.create(self, paths);
+    }
+
     pub fn version(self: *const Builder, major: u32, minor: u32, patch: u32) Version {
         return Version{
             .major = major,