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The Rustc Driver and Interface
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.
The rustc_interface crate provides external users with an (unstable) API
for running code at particular times during the compilation process, allowing
third parties to effectively use rustc's internals as a library for
analysing a crate or emulating the compiler in-process (e.g. the RLS or rustdoc).
For those using rustc as a library, the rustc_interface::interface::run_compiler()
function is the main entrypoint to the compiler. It takes a configuration for the compiler
and a closure that takes a Compiler. run_compiler creates a Compiler from the
configuration and passes it to the closure. Inside the closure, you can use the Compiler
to drive queries to compile a crate and get the results. This is what the rustc_driver does too.
You can see what queries are currently available through the rustdocs for Compiler.
You can see an example of how to use them by looking at the rustc_driver implementation,
specifically the rustc_driver::run_compiler function (not to be confused with
rustc_interface::interface::run_compiler). The rustc_driver::run_compiler function
takes a bunch of command-line args and some other configurations and
drives the compilation to completion.
rustc_driver::run_compiler also takes a Callbacks. In the past, when
the rustc_driver::run_compiler was the primary way to use the compiler as a
library, these callbacks were used to have some custom code run after different
phases of the compilation. If you read Appendix A, you may notice the use of the
types CompilerCalls and CompileController, which no longer exist. Callbacks
replaces this functionality.
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.