Add tidy to the target of ./x check

src/bootstrap/src/core/build_steps/check.rs

@@ -841,3 +841,8 @@ tool_check_step!(BumpStage0 {
     mode: Mode::ToolBootstrap,
     default: false
 });
+
+// Tidy is implicitly checked when `./x test tidy` is executed
+// (if you set a pre-push hook, the command is called).
+// So this is mainly for people working on tidy.
+tool_check_step!(Tidy { path: "src/tools/tidy", mode: Mode::ToolBootstrap, default: false });

src/bootstrap/src/core/builder/mod.rs

@@ -1076,6 +1076,7 @@ impl<'a> Builder<'a> {
                 check::CoverageDump,
                 check::Linkchecker,
                 check::BumpStage0,
+                check::Tidy,
                 // This has special staging logic, it may run on stage 1 while others run on stage 0.
                 // It takes quite some time to build stage 1, so put this at the end.
                 //