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A tutorial on creating a drop-in replacement for rustc.

Many tools benefit from being a drop-in replacement for a compiler. By this, I mean that any user of the tool can use mytool in all the ways they would normally use rustc - whether manually compiling a single file or as part of a complex make project or Cargo build, etc. That could be a lot of work; rustc, like most compilers, takes a large number of command line arguments which can affect compilation in complex and interacting ways. Emulating all of this behaviour in your tool is annoying at best, especically if you are making many of the same calls into librustc that the compiler is.

The kind of things I have in mind are tools like rustdoc or a future rustfmt. These want to operate as closely as possible to real compilation, but have totally different outputs (documentation and formatted source code, respectively). Another use case is a customised compiler. Say you want to add a custom code generation phase after macro expansion, then creating a new tool should be easier than forking the compiler (and keeping it up to date as the compiler evolves).

I have gradually been trying to improve the API of librustc to make creating a drop-in tool easier to produce (many others have also helped improve these interfaces over the same time frame). It is now pretty simple to make a tool which is as close to rustc as you want it to be. In this tutorial I'll show how.

Note/warning, everything I talk about in this tutorial is internal API for rustc. It is all extremely unstable and likely to change often and in unpredictable ways. Maintaining a tool which uses these APIs will be non- trivial, although hopefully easier than maintaining one that does similar things without using them.

This tutorial starts with a very high level view of the rustc compilation process and of some of the code that drives compilation. Then I'll describe how that process can be customised. In the final section of the tutorial, I'll go through an example - stupid-stats - which shows how to build a drop-in tool.

Overview of the compilation process

Compilation using rustc happens in several phases. We start with parsing, this includes lexing. The output of this phase is an AST (abstract syntax tree). There is a single AST for each crate (indeed, the entire compilation process operates over a single crate). Parsing abstracts away details about individual files which will all have been read in to the AST in this phase. At this stage the AST includes all macro uses, attributes will still be present, and nothing will have been eliminated due to cfgs.

The next phase is configuration and macro expansion. This can be thought of as a function over the AST. The unexpanded AST goes in and an expanded AST comes out. Macros and syntax extensions are expanded, and cfg attributes will cause some code to disappear. The resulting AST won't have any macros or macro uses left in.

The code for these first two phases is in libsyntax.

After this phase, the compiler allocates ids to each node in the AST (technically not every node, but most of them). If we are writing out dependencies, that happens now.

The next big phase is analysis. This is the most complex phase and uses the bulk of the code in rustc. This includes name resolution, type checking, borrow checking, type and lifetime inference, trait selection, method selection, linting, and so forth. Most error detection is done in this phase (although parse errors are found during parsing). The 'output' of this phase is a bunch of side tables containing semantic information about the source program. The analysis code is in librustc and a bunch of other crates with the 'librustc_' prefix.

Next is translation, this translates the AST (and all those side tables) into LLVM IR (intermediate representation). We do this by calling into the LLVM libraries, rather than actually writing IR directly to a file. The code for this is in librustc_trans.

The next phase is running the LLVM backend. This runs LLVM's optimisation passes on the generated IR and then generates machine code. The result is object files. This phase is all done by LLVM, it is not really part of the rust compiler. The interface between LLVM and rustc is in librustc_llvm.

Finally, we link the object files into an executable. Again we outsource this to other programs and it's not really part of the rust compiler. The interface is in librustc_back (which also contains some things used primarily during translation).

All these phases are coordinated by the driver. To see the exact sequence, look at the compile_input function in librustc_driver/driver.rs. The driver (which is found in librust_driver) handles all the highest level coordination of compilation - handling command line arguments, maintaining compilation state (primarily in the Session), and calling the appropriate code to run each phase of compilation. It also handles high level coordination of pretty printing and testing. To create a drop-in compiler replacement or a compiler replacement, we leave most of compilation alone and customise the driver using its APIs.

The driver customisation APIs

There are two primary ways to customise compilation - high level control of the driver using CompilerCalls and controlling each phase of compilation using a CompileController. The former lets you customise handling of command line arguments etc., the latter lets you stop compilation early or execute code between phases.

CompilerCalls

CompilerCalls is a trait that you implement in your tool. It contains a fairly ad-hoc set of methods to hook in to the process of processing command line arguments and driving the compiler. For details, see the comments in librustc_driver/lib.rs. I'll summarise the methods here.

early_callback and late_callback let you call arbitrary code at different points - early is after command line arguments have been parsed, but before anything is done with them; late is pretty much the last thing before compilation starts, i.e., after all processing of command line arguments, etc. is done. Currently, you get to choose whether compilation stops or continues at each point, but you don't get to change anything the driver has done. You can record some info for later, or perform other actions of your own.

some_input and no_input give you an opportunity to modify the primary input to the compiler (usually the input is a file containing the top module for a crate, but it could also be a string). You could record the input or perform other actions of your own.

Ignore parse_pretty, it is unfortunate and hopefully will get improved. There is a default implementation, so you can pretend it doesn't exist.

build_controller returns a CompileController object for more fine-grained control of compilation, it is described next.

We might add more options in the future.

CompilerController

CompilerController is a struct consisting of PhaseControllers and flags. Currently, there is only flag, make_glob_map which signals whether to produce a map of glob imports (used by save-analysis and potentially other tools). There are probably flags in the session that should be moved here.

There is a PhaseController for each of the phases described in the above summary of compilation (and we could add more in the future for finer-grained control). They are all after_ a phase because they are checked at the end of a phase (again, that might change), e.g., CompilerController::after_parse controls what happens immediately after parsing (and before macro expansion).

Each PhaseController contains a flag called stop which indicates whether compilation should stop or continue, and a callback to be executed at the point indicated by the phase. The callback is called whether or not compilation continues.

Information about the state of compilation is passed to these callbacks in a CompileState object. This contains all the information the compiler has. Note that this state information is immutable - your callback can only execute code using the compiler state, it can't modify the state. (If there is demand, we could change that). The state available to a callback depends on where during compilation the callback is called. For example, after parsing there is an AST but no semantic analysis (because the AST has not been analysed yet). After translation, there is translation info, but no AST or analysis info (since these have been consumed/forgotten).

An example - stupid-stats

Our example tool is very simple, it simply collects some simple and not very useful statistics about a program; it is called stupid-stats. You can find the (more heavily commented) complete source for the example on Github. To build, just do cargo build. To run on a file foo.rs, do cargo run foo.rs (assuming you have a Rust program called foo.rs. You can also pass any command line arguments that you would normally pass to rustc). When you run it you'll see output similar to

In crate: foo,

Found 12 uses of `println!`;
The most common number of arguments is 1 (67% of all functions);
25% of functions have four or more arguments.

To make things easier, when we talk about functions, we're excluding methods and closures.

You can also use the executable as a drop-in replacement for rustc, because after all, that is the whole point of this exercise. So, however you use rustc in your makefile setup, you can use target/stupid (or whatever executable you end up with) instead. That might mean setting an environment variable or it might mean renaming your executable to rustc and setting your PATH. Similarly, if you're using Cargo, you'll need to rename the executable to rustc and set the PATH. Alternatively, you should be able to use multirust to get around all the PATH stuff (although I haven't actually tried that).

(Note that this example prints to stdout. I'm not entirely sure what Cargo does with stdout from rustc under different circumstances. If you don't see any output, try inserting a panic! after the println!s to error out, then Cargo should dump stupid-stats' stdout to Cargo's stdout).

Let's start with the main function for our tool, it is pretty simple:

fn main() {
    let args: Vec<_> = std::env::args().collect();
    rustc_driver::run_compiler(&args, &mut StupidCalls::new());
    std::env::set_exit_status(0);
}

The first line grabs any command line arguments. The second line calls the compiler driver with those arguments. The final line sets the exit code for the program.

The only interesting thing is the StupidCalls object we pass to the driver. This is our implementation of the CompilerCalls trait and is what will make this tool different from rustc.

StupidCalls is a mostly empty struct:

struct StupidCalls {
    default_calls: RustcDefaultCalls,
}

This tool is so simple that it doesn't need to store any data here, but usually you would. We embed a RustcDefaultCalls object to delegate to in our impl when we want exactly the same behaviour as the Rust compiler. Mostly you don't want to do that (or at least don't need to) in a tool. However, Cargo calls rustc with the --print file-names, so we delegate in late_callback and no_input to keep Cargo happy.

Most of the rest of the impl of CompilerCalls is trivial:

impl<'a> CompilerCalls<'a> for StupidCalls {
    fn early_callback(&mut self,
                        _: &getopts::Matches,
                        _: &config::Options,
                        _: &diagnostics::registry::Registry,
                        _: ErrorOutputType)
                      -> Compilation {
        Compilation::Continue
    }

    fn late_callback(&mut self,
                     m: &getopts::Matches,
                     s: &Session,
                     i: &Input,
                     odir: &Option<Path>,
                     ofile: &Option<Path>)
                     -> Compilation {
        self.default_calls.late_callback(m, s, i, odir, ofile);
        Compilation::Continue
    }

    fn some_input(&mut self,
                  input: Input,
                  input_path: Option<Path>)
                  -> (Input, Option<Path>) {
        (input, input_path)
    }

    fn no_input(&mut self,
                m: &getopts::Matches,
                o: &config::Options,
                odir: &Option<Path>,
                ofile: &Option<Path>,
                r: &diagnostics::registry::Registry)
                -> Option<(Input, Option<Path>)> {
        self.default_calls.no_input(m, o, odir, ofile, r);

        // This is not optimal error handling.
        panic!("No input supplied to stupid-stats");
    }

    fn build_controller(&mut self, _: &Session) -> driver::CompileController<'a> {
        ...
    }
}

We don't do anything for either of the callbacks, nor do we change the input if the user supplies it. If they don't, we just panic!, this is the simplest way to handle the error, but not very user-friendly, a real tool would give a constructive message or perform a default action.

In build_controller we construct our CompileController. We only want to parse, and we want to inspect macros before expansion, so we make compilation stop after the first phase (parsing). The callback after that phase is where the tool does it's actual work by walking the AST. We do that by creating an AST visitor and making it walk the AST from the top (the crate root). Once we've walked the crate, we print the stats we've collected:

fn build_controller(&mut self, _: &Session) -> driver::CompileController<'a> {
    // We mostly want to do what rustc does, which is what basic() will return.
    let mut control = driver::CompileController::basic();
    // But we only need the AST, so we can stop compilation after parsing.
    control.after_parse.stop = Compilation::Stop;

    // And when we stop after parsing we'll call this closure.
    // Note that this will give us an AST before macro expansions, which is
    // not usually what you want.
    control.after_parse.callback = box |state| {
        // Which extracts information about the compiled crate...
        let krate = state.krate.unwrap();

        // ...and walks the AST, collecting stats.
        let mut visitor = StupidVisitor::new();
        visit::walk_crate(&mut visitor, krate);

        // And finally prints out the stupid stats that we collected.
        let cratename = match attr::find_crate_name(&krate.attrs[]) {
            Some(name) => name.to_string(),
            None => String::from_str("unknown_crate"),
        };
        println!("In crate: {},\n", cratename);
        println!("Found {} uses of `println!`;", visitor.println_count);

        let (common, common_percent, four_percent) = visitor.compute_arg_stats();
        println!("The most common number of arguments is {} ({:.0}% of all functions);",
                 common, common_percent);
        println!("{:.0}% of functions have four or more arguments.", four_percent);
    };

    control
}

That is all it takes to create your own drop-in compiler replacement or custom compiler! For the sake of completeness I'll go over the rest of the stupid-stats tool.

struct StupidVisitor {
    println_count: usize,
    arg_counts: Vec<usize>,
}

The StupidVisitor struct just keeps track of the number of println!s it has seen and the count for each number of arguments. It implements syntax::visit::Visitor to walk the AST. Mostly we just use the default methods, these walk the AST taking no action. We override visit_item and visit_mac to implement custom behaviour when we walk into items (items include functions, modules, traits, structs, and so forth, we're only interested in functions) and macros:

impl<'v> visit::Visitor<'v> for StupidVisitor {
    fn visit_item(&mut self, i: &'v ast::Item) {
        match i.node {
            ast::Item_::ItemFn(ref decl, _, _, _, _) => {
                // Record the number of args.
                self.increment_args(decl.inputs.len());
            }
            _ => {}
        }

        // Keep walking.
        visit::walk_item(self, i)
    }

    fn visit_mac(&mut self, mac: &'v ast::Mac) {
        // Find its name and check if it is "println".
        let ast::Mac_::MacInvocTT(ref path, _, _) = mac.node;
        if path_to_string(path) == "println" {
            self.println_count += 1;
        }

        // Keep walking.
        visit::walk_mac(self, mac)
    }
}

The increment_args method increments the correct count in StupidVisitor::arg_counts. After we're done walking, compute_arg_stats does some pretty basic maths to come up with the stats we want about arguments.

What next?

These APIs are pretty new and have a long way to go until they're really good. If there are improvements you'd like to see or things you'd like to be able to do, let me know in a comment or GitHub issue. In particular, it's not clear to me exactly what extra flexibility is required. If you have an existing tool that would be suited to this setup, please try it out and let me know if you have problems.

It'd be great to see Rustdoc converted to using these APIs, if that is possible (although long term, I'd prefer to see Rustdoc run on the output from save- analysis, rather than doing its own analysis). Other parts of the compiler (e.g., pretty printing, testing) could be refactored to use these APIs internally (I already changed save-analysis to use CompilerController). I've been experimenting with a prototype rustfmt which also uses these APIs.