Writing tools in Bootstrap

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 it 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::ToolRustcPrivate

  Use this for tools that use the rustc_private mechanism, and thus depend on the locally built rustc and its rlib artifacts. 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::ToolRustcPrivate, 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.