How Bootstrap does it

The core concept in Bootstrap is a build Step, which are chained together by Builder::ensure. Builder::ensure takes a Step as input, and runs the Step if and only if it has not already been run. Let's take a closer look at Step.

Synopsis of Step

A Step represents a granular collection of actions involved in the process of producing some artifact. It can be thought of like a rule in Makefiles. The Step trait is defined as:

pub trait Step: 'static + Clone + Debug + PartialEq + Eq + Hash {
    type Output: Clone;

    const DEFAULT: bool = false;
    const ONLY_HOSTS: bool = false;

    // Required methods
    fn run(self, builder: &Builder<'_>) -> Self::Output;
    fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_>;

    // Provided method
    fn make_run(_run: RunConfig<'_>) { ... }
}

• run is the function that is responsible for doing the work. Builder::ensure invokes run.
• should_run is the command-line interface, which determines if an invocation such as x build foo should run a given Step. In a “default” context where no paths are provided, then make_run is called directly.
• make_run is invoked only for things directly asked via the CLI and not for steps which are dependencies of other steps.

The entry points

There's a couple of preliminary steps before core Bootstrap code is reached:

1. Shell script or make: ./x or ./x.ps1 or make
2. Convenience wrapper script: x.py
3. src/bootstrap/bootstrap.py
4. src/bootstrap/src/bin/main.rs

See src/bootstrap/README.md for a more specific description of the implementation details.