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).

Configuring the compiler

By default, rustc is built without most debug information. To enable debug info, set debug = true in your config.toml.

Setting debug = true turns on many different debug options (e.g., debug-assertions, debug-logging, etc.) which can be individually tweaked if you want to, but many people simply set debug = true. Check out the comments in config.toml.example for more info.

You will need to rebuild the compiler once you've changed any configuration options.

-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 MinGW, sorry. If you have trouble or the backtraces are full of unknown, you might want to find some way to use Linux, Mac, or MSVC on Windows.

In the default configuration (without debug set to true), 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 set debug = true, you will get line numbers for the stack trace. Then the backtrace will look like this:

stack backtrace:
   (~~~~ LINES REMOVED BY ME FOR BREVITY ~~~~)
             at /home/user/rust/compiler/rustc_typeck/src/check/cast.rs:110
   7: rustc_typeck::check::cast::CastCheck::check
             at /home/user/rust/compiler/rustc_typeck/src/check/cast.rs:572
             at /home/user/rust/compiler/rustc_typeck/src/check/cast.rs:460
             at /home/user/rust/compiler/rustc_typeck/src/check/cast.rs:370
   (~~~~ LINES REMOVED BY ME FOR BREVITY ~~~~)
  33: rustc_driver::driver::compile_input
             at /home/user/rust/compiler/rustc_driver/src/driver.rs:1010
             at /home/user/rust/compiler/rustc_driver/src/driver.rs:212
  34: rustc_driver::run_compiler
             at /home/user/rust/compiler/rustc_driver/src/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 panic on the nth error on delay_span_bug. If you leave off =n, the compiler will assume 1 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 + ();
}
$ rustc +stage1 error.rs
error[E0277]: cannot add `()` to `{integer}`
 --> error.rs:2:7
  |
2 |       1 + ();
  |         ^ no implementation for `{integer} + ()`
  |
  = help: the trait `Add<()>` is not implemented for `{integer}`

error: aborting due to previous error

Now, where does the error above come from?

$ RUST_BACKTRACE=1 rustc +stage1 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/compiler/rustc_errors/src/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/compiler/rustc_middle/src/traits/error_reporting.rs:823
   8: rustc::traits::error_reporting::<impl rustc::infer::InferCtxt<'a, 'tcx>>
             ::report_fulfillment_errors
             at /home/user/rust/compiler/rustc_middle/src/traits/error_reporting.rs:160
             at /home/user/rust/compiler/rustc_middle/src/traits/error_reporting.rs:112
   9: rustc_typeck::check::FnCtxt::select_obligations_where_possible
             at /home/user/rust/compiler/rustc_typeck/src/check/mod.rs:2192
  (~~~ IRRELEVANT PART OF BACKTRACE REMOVED BY ME ~~~)
  36: rustc_driver::run_compiler
             at /home/user/rust/compiler/rustc_driver/src/lib.rs:253

Cool, now I have a backtrace for the error!

Getting logging output

The compiler uses the tracing crate for logging.

For details see the guide section on tracing

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

Viewing Spanview output (.html files)

In addition to graphviz output, MIR debugging flags include an option to generate a MIR representation called Spanview that uses HTML to highlight code regions in the original source code and display compiler metadata associated with each region. -Z dump-mir-spanview, for example, highlights spans associated with each MIR Statement, Terminator, and/or BasicBlock.

These .html files use CSS features to dynamically expand spans obscured by overlapping spans, and native tooltips (based on the HTML title attribute) to reveal the actual MIR elements, as text.

To view these files, simply use a modern browser, or a CSS-capable HTML preview feature in a modern IDE. (The default HTML preview pane in VS Code is known to work, for instance.)

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.

Debugging type layouts

The (permanently) unstable #[rustc_layout] attribute can be used to dump the Layout of the type it is attached to. For example:


#![allow(unused)]
#![feature(rustc_attrs)]

fn main() {
#[rustc_layout(debug)]
type T<'a> = &'a u32;
}

Will emit the following:

error: layout_of(&'a u32) = Layout {
    fields: Primitive,
    variants: Single {
        index: 0,
    },
    abi: Scalar(
        Scalar {
            value: Pointer,
            valid_range: 1..=18446744073709551615,
        },
    ),
    largest_niche: Some(
        Niche {
            offset: Size {
                raw: 0,
            },
            scalar: Scalar {
                value: Pointer,
                valid_range: 1..=18446744073709551615,
            },
        },
    ),
    align: AbiAndPrefAlign {
        abi: Align {
            pow2: 3,
        },
        pref: Align {
            pow2: 3,
        },
    },
    size: Size {
        raw: 8,
    },
}
 --> src/lib.rs:4:1
  |
4 | type T<'a> = &'a u32;
  | ^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error