Debugging the compiler

This chapter contains a few tips to debug the compiler. These tips aim to be useful no matter what you are working on. Some of the other chapters have advice about specific parts of the compiler (e.g. the Queries Debugging and Testing chapter or the LLVM Debugging chapter).

-Z flags

The compiler has a bunch of -Z flags. These are unstable flags that are only enabled on nightly. Many of them are useful for debugging. To get a full listing of -Z flags, use -Z help.

One useful flag is -Z verbose, which generally enables printing more info that could be useful for debugging.

Getting a backtrace

When you have an ICE (panic in the compiler), you can set RUST_BACKTRACE=1 to get the stack trace of the panic! like in normal Rust programs. IIRC backtraces don't work on Mac and on MinGW, sorry. If you have trouble or the backtraces are full of unknown, you might want to find some way to use Linux or MSVC on Windows.

In the default configuration, you don't have line numbers enabled, so the backtrace looks like this:

stack backtrace:
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
   1: std::sys_common::backtrace::_print
   2: std::panicking::default_hook::{{closure}}
   3: std::panicking::default_hook
   4: std::panicking::rust_panic_with_hook
   5: std::panicking::begin_panic
   (~~~~ LINES REMOVED BY ME FOR BREVITY ~~~~)
  32: rustc_typeck::check_crate
  33: <std::thread::local::LocalKey<T>>::with
  34: <std::thread::local::LocalKey<T>>::with
  35: rustc::ty::context::TyCtxt::create_and_enter
  36: rustc_driver::driver::compile_input
  37: rustc_driver::run_compiler

If you want line numbers for the stack trace, you can enable debug = true in your config.toml and rebuild the compiler (debuginfo-level = 1 will also add line numbers, but debug = true gives full debuginfo). Then the backtrace will look like this:

stack backtrace:
   (~~~~ LINES REMOVED BY ME FOR BREVITY ~~~~)
             at /home/user/rust/src/librustc_typeck/check/cast.rs:110
   7: rustc_typeck::check::cast::CastCheck::check
             at /home/user/rust/src/librustc_typeck/check/cast.rs:572
             at /home/user/rust/src/librustc_typeck/check/cast.rs:460
             at /home/user/rust/src/librustc_typeck/check/cast.rs:370
   (~~~~ LINES REMOVED BY ME FOR BREVITY ~~~~)
  33: rustc_driver::driver::compile_input
             at /home/user/rust/src/librustc_driver/driver.rs:1010
             at /home/user/rust/src/librustc_driver/driver.rs:212
  34: rustc_driver::run_compiler
             at /home/user/rust/src/librustc_driver/lib.rs:253

Getting a backtrace for errors

If you want to get a backtrace to the point where the compiler emits an error message, you can pass the -Z treat-err-as-bug=n, which will make the compiler skip n errors or delay_span_bug calls and then panic on the next one. If you leave off =n, the compiler will assume 0 for n and thus panic on the first error it encounters.

This can also help when debugging delay_span_bug calls - it will make the first delay_span_bug call panic, which will give you a useful backtrace.

For example:

$ cat error.rs
fn main() {
    1 + ();
}
$ ./build/x86_64-unknown-linux-gnu/stage1/bin/rustc error.rs
error[E0277]: the trait bound `{integer}: std::ops::Add<()>` is not satisfied
 --> error.rs:2:7
  |
2 |     1 + ();
  |       ^ no implementation for `{integer} + ()`
  |
  = help: the trait `std::ops::Add<()>` is not implemented for `{integer}`

error: aborting due to previous error

$ # Now, where does the error above come from?
$ RUST_BACKTRACE=1 \
    ./build/x86_64-unknown-linux-gnu/stage1/bin/rustc \
    error.rs \
    -Z treat-err-as-bug
error[E0277]: the trait bound `{integer}: std::ops::Add<()>` is not satisfied
 --> error.rs:2:7
  |
2 |     1 + ();
  |       ^ no implementation for `{integer} + ()`
  |
  = help: the trait `std::ops::Add<()>` is not implemented for `{integer}`

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.24.0-dev running on x86_64-unknown-linux-gnu

note: run with `RUST_BACKTRACE=1` for a backtrace

thread 'rustc' panicked at 'encountered error with `-Z treat_err_as_bug',
/home/user/rust/src/librustc_errors/lib.rs:411:12
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose
backtrace.
stack backtrace:
  (~~~ IRRELEVANT PART OF BACKTRACE REMOVED BY ME ~~~)
   7: rustc::traits::error_reporting::<impl rustc::infer::InferCtxt<'a, 'tcx>>
             ::report_selection_error
             at /home/user/rust/src/librustc/traits/error_reporting.rs:823
   8: rustc::traits::error_reporting::<impl rustc::infer::InferCtxt<'a, 'tcx>>
             ::report_fulfillment_errors
             at /home/user/rust/src/librustc/traits/error_reporting.rs:160
             at /home/user/rust/src/librustc/traits/error_reporting.rs:112
   9: rustc_typeck::check::FnCtxt::select_obligations_where_possible
             at /home/user/rust/src/librustc_typeck/check/mod.rs:2192
  (~~~ IRRELEVANT PART OF BACKTRACE REMOVED BY ME ~~~)
  36: rustc_driver::run_compiler
             at /home/user/rust/src/librustc_driver/lib.rs:253
$ # Cool, now I have a backtrace for the error

Getting logging output

These crates are used in compiler for logging:

  • log
  • env-logger: check the link to see the full RUSTC_LOG syntax

The compiler has a lot of debug! calls, which print out logging information at many points. These are very useful to at least narrow down the location of a bug if not to find it entirely, or just to orient yourself as to why the compiler is doing a particular thing.

To see the logs, you need to set the RUSTC_LOG environment variable to your log filter, e.g. to get the logs for a specific module, you can run the compiler as RUSTC_LOG=module::path rustc my-file.rs. All debug! output will then appear in standard error.

Note that unless you use a very strict filter, the logger will emit a lot of output, so use the most specific module(s) you can (comma-separated if multiple). It's typically a good idea to pipe standard error to a file and look at the log output with a text editor.

So to put it together.

# This puts the output of all debug calls in `librustc/traits` into
# standard error, which might fill your console backscroll.
$ RUSTC_LOG=rustc::traits rustc +local my-file.rs

# This puts the output of all debug calls in `librustc/traits` in
# `traits-log`, so you can then see it with a text editor.
$ RUSTC_LOG=rustc::traits rustc +local my-file.rs 2>traits-log

# Not recommended. This will show the output of all `debug!` calls
# in the Rust compiler, and there are a *lot* of them, so it will be
# hard to find anything.
$ RUSTC_LOG=debug rustc +local my-file.rs 2>all-log

# This will show the output of all `info!` calls in `rustc_trans`.
#
# There's an `info!` statement in `trans_instance` that outputs
# every function that is translated. This is useful to find out
# which function triggers an LLVM assertion, and this is an `info!`
# log rather than a `debug!` log so it will work on the official
# compilers.
$ RUSTC_LOG=rustc_trans=info rustc +local my-file.rs

How to keep or remove debug! and trace! calls from the resulting binary

While calls to error!, warn! and info! are included in every build of the compiler, calls to debug! and trace! are only included in the program if debug-assertions=yes is turned on in config.toml (it is turned off by default), so if you don't see DEBUG logs, especially if you run the compiler with RUSTC_LOG=rustc rustc some.rs and only see INFO logs, make sure that debug-assertions=yes is turned on in your config.toml.

I also think that in some cases just setting it will not trigger a rebuild, so if you changed it and you already have a compiler built, you might want to call x.py clean to force one.

Logging etiquette and conventions

Because calls to debug! are removed by default, in most cases, don't worry about adding "unnecessary" calls to debug! and leaving them in code you commit - they won't slow down the performance of what we ship, and if they helped you pinning down a bug, they will probably help someone else with a different one.

A loosely followed convention is to use debug!("foo(...)") at the start of a function foo and debug!("foo: ...") within the function. Another loosely followed convention is to use the {:?} format specifier for debug logs.

One thing to be careful of is expensive operations in logs.

If in the module rustc::foo you have a statement

debug!("{:?}", random_operation(tcx));

Then if someone runs a debug rustc with RUSTC_LOG=rustc::bar, then random_operation() will run.

This means that you should not put anything too expensive or likely to crash there - that would annoy anyone who wants to use logging for their own module. No-one will know it until someone tries to use logging to find another bug.

Formatting Graphviz output (.dot files)

Some compiler options for debugging specific features yield graphviz graphs - e.g. the #[rustc_mir(borrowck_graphviz_postflow="suffix.dot")] attribute dumps various borrow-checker dataflow graphs.

These all produce .dot files. To view these files, install graphviz (e.g. apt-get install graphviz) and then run the following commands:

$ dot -T pdf maybe_init_suffix.dot > maybe_init_suffix.pdf
$ firefox maybe_init_suffix.pdf # Or your favorite pdf viewer

Narrowing (Bisecting) Regressions

The cargo-bisect-rustc tool can be used as a quick and easy way to find exactly which PR caused a change in rustc behavior. It automatically downloads rustc PR artifacts and tests them against a project you provide until it finds the regression. You can then look at the PR to get more context on why it was changed. See this tutorial on how to use it.

Downloading Artifacts from Rust's CI

The rustup-toolchain-install-master tool by kennytm can be used to download the artifacts produced by Rust's CI for a specific SHA1 -- this basically corresponds to the successful landing of some PR -- and then sets them up for your local use. This also works for artifacts produced by @bors try. This is helpful when you want to examine the resulting build of a PR without doing the build yourself.