src/rustc-driver/intro.md

# `rustc_driver` and `rustc_interface`

## `rustc_driver`

The [`rustc_driver`] is essentially `rustc`'s `main` function.
It acts as the glue for running the various phases of the compiler in the correct order,
using the interface defined in the [`rustc_interface`] crate. Where possible, using [`rustc_driver`] rather than [`rustc_interface`] is recommended.

The main entry point of [`rustc_driver`] is [`rustc_driver::run_compiler`][rd_rc].
This builder accepts the same command-line args as rustc as well as an implementation of [`Callbacks`] and a couple of other optional options.
[`Callbacks`] is a `trait` that allows for custom compiler configuration,
as well as allowing custom code to run after different phases of the compilation.

## `rustc_interface`

The [`rustc_interface`] crate provides a low level API to external users for manually driving the compilation process,
allowing third parties to effectively use `rustc`'s internals as a library for analyzing a crate or for ad hoc emulating of the compiler for cases where [`rustc_driver`] is not flexible enough (i.e. `rustdoc` compiling code and serving output).

The main entry point of [`rustc_interface`] ([`rustc_interface::run_compiler`][i_rc]) takes a configuration variable for the compiler
and a `closure` taking a yet unresolved [`Compiler`].
[`run_compiler`][i_rc] creates a `Compiler` from the configuration and passes it to the `closure`.
Inside the `closure` you can use the `Compiler` to call various functions to compile a crate and get the results.
You can see a minimal example of how to use [`rustc_interface`] [here][example].

You can see an example of how to use the various functions using [`rustc_interface`] needs by looking at the `rustc_driver` implementation,
specifically [`rustc_driver_impl::run_compiler`][rdi_rc]
(not to be confused with [`rustc_interface::run_compiler`][i_rc]).

> **Warning:** By its very nature, the internal compiler APIs are always going
> to be unstable. That said, we do try not to break things unnecessarily.


[`Compiler`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_interface/interface/struct.Compiler.html
[`rustc_driver`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_driver/
[`rustc_interface`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_interface/index.html
[`Callbacks`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_driver/trait.Callbacks.html
[example]: https://github.com/rust-lang/rustc-dev-guide/blob/main/examples/rustc-interface-example.rs
[i_rc]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_interface/interface/fn.run_compiler.html
[rd_rc]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_driver/fn.run_compiler.html
[rdi_rc]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_driver_impl/fn.run_compiler.html