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).
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
.
If you want to use GDB to debug rustc, please set config.toml
with options:
[rust] debug = true debuginfo-level = 2
NOTE: This will use a lot of disk space (upwards of 35GB), and will take a lot more compile time. With
debuginfo-level = 1
(the default whendebug = true
), you will be able to track the execution path, but will lose the symbol information for debugging.
The default configuration will enable symbol-mangling-version
v0. This requires at least GDB v10.2, otherwise you need to disable new symbol-mangling-version in config.toml
.
[rust] new-symbol-mangling = false
See the comments in
config.example.toml
for more info.
You will need to rebuild the compiler after changing any configuration option.
-Z
flagsThe 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.
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
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!
-Z track-diagnostics
can help figure out where errors are emitted. It uses #[track_caller]
for this and prints its location alongside the error:
$ RUST_BACKTRACE=1 rustc +stage1 error.rs -Z track-diagnostics error[E0277]: cannot add `()` to `{integer}` --> src\error.rs:2:7 | 2 | 1 + (); | ^ no implementation for `{integer} + ()` -Ztrack-diagnostics: created at compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs:638:39 | = help: the trait `Add<()>` is not implemented for `{integer}` = help: the following other types implement trait `Add<Rhs>`: <&'a f32 as Add<f32>> <&'a f64 as Add<f64>> <&'a i128 as Add<i128>> <&'a i16 as Add<i16>> <&'a i32 as Add<i32>> <&'a i64 as Add<i64>> <&'a i8 as Add<i8>> <&'a isize as Add<isize>> and 48 others For more information about this error, try `rustc --explain E0277`.
This is similar but different to -Z treat-err-as-bug
:
The compiler uses the tracing
crate for logging.
For details see the guide section on tracing
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
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.
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.
The (permanently) unstable #[rustc_layout]
attribute can be used to dump the Layout
of the type it is attached to. For example:
#![feature(rustc_attrs)] #[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
rustc
If you are using VSCode, and have edited your config.toml
to request debugging level 1 or 2 for the parts of the code you're interested in, then you should be able to use the CodeLLDB extension in VSCode to debug it.
Here is a sample launch.json
file, being used to run a stage 1 compiler direct from the directory where it is built (does not have to be “installed”):
// .vscode/launch.json { "version": "0.2.0", "configurations": [ { "type": "lldb", "request": "launch", "name": "Launch", "args": [], // array of string command-line arguments to pass to compiler "program": "${workspaceFolder}/build/host/stage1/bin/rustc", "windows": { // applicable if using windows "program": "${workspaceFolder}/build/host/stage1/bin/rustc.exe" }, "cwd": "${workspaceFolder}", // current working directory at program start "stopOnEntry": false, "sourceLanguages": ["rust"] } ] }