There are three types of tools you can write in bootstrap:
Mode::ToolBootstrap Use this for tools that don’t need anything from the in-tree compiler and can run with the stage0 rustc. The output is placed in the “bootstrap-tools” directory. This mode is for general-purpose tools built entirely with the stage0 compiler, including target libraries and only works for stage 0.
Mode::ToolStd Use this for tools that rely on the locally built std. The output goes into the “stageN-tools” directory. This mode is rarely used, mainly for compiletest which requires libtest.
Mode::ToolRustc Use this for tools that depend on both the locally built rustc and the target std. This is more complex than the other modes because the tool must be built with the same compiler used for rustc and placed in the “stageN-tools” directory. When you choose Mode::ToolRustc, ToolBuild implementation takes care of this automatically. If you need to use the builder’s compiler for something specific, you can get it from ToolBuildResult, which is returned by the tool's Step.
Regardless of the tool type you must return ToolBuildResult from the tool’s Step implementation and use ToolBuild inside it.