Bootstrap Tool Writing Modes

src/building/bootstrapping/writing-tools-in-bootstrap.md

80b734148767ac02567b23d35c32aeef9fbdea356b3d655b0000000300000578

# 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 "stage0-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.

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Title: Bootstrap Tool Writing Modes

Summary

Bootstrap provides three modes for writing tools: `Mode::ToolBootstrap` for tools using the stage0 compiler, `Mode::ToolStd` for tools relying on the locally built std, and `Mode::ToolRustc` for tools dependent on both the locally built `rustc` and target `std`. Regardless of the mode, the tool's `Step` implementation must return `ToolBuildResult` and use `ToolBuild`.