src/compiler-debugging.md - rust-lang/rustc-dev-guide - Git at Google

 # Debugging the compiler

 <!-- toc -->

 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](./incrcomp-debugging.html) or the [LLVM Debugging
 chapter](./backend/debugging.md)).

 ## Configuring the compiler

 By default, rustc is built without most debug information. To enable debug info,
 set `debug = true` in your bootstrap.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 `bootstrap.toml` with options:

 ```toml
 [rust]
 debug = true
 debuginfo-level = 2
 ```

 > NOTE:
 > This will use a lot of disk space
 > (upwards of <!-- date-check Aug 2022 --> 35GB),
 > and will take a lot more compile time.
 > With `debuginfo-level = 1` (the default when `debug = 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 `bootstrap.toml`.

 ```toml
 [rust]
 new-symbol-mangling = false
 ```

 > See the comments in `bootstrap.example.toml` for more info.

 You will need to rebuild the compiler after changing any configuration option.

 ## Suppressing the ICE file

 By default, if rustc encounters an Internal Compiler Error (ICE) it will dump the ICE contents to an
 ICE file within the current working directory named `rustc-ice-<timestamp>-<pid>.txt`. If this is
 not desirable, you can prevent the ICE file from being created with `RUSTC_ICE=0`.

 ## Getting a backtrace
 [getting-a-backtrace]: #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:

 ```text
 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:

 ```text
 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
 ```

 ## `-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-internals`, which generally enables printing more
 info that could be useful for debugging.

 Right below you can find elaborate explainers on a selected few.

 ### Getting a backtrace for errors
 [getting-a-backtrace-for-errors]: #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. If you leave off `=n`, the compiler will
 assume `1` for `n` and thus panic on the first error it encounters.

 For example:

 ```bash
 $ cat error.rs
 ```

 ```rust
 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!

 ### Debugging delayed bugs

 The `-Z eagerly-emit-delayed-bugs` option makes it easy to debug delayed bugs.
 It turns them into normal errors, i.e. makes them visible. This can be used in
 combination with `-Z treat-err-as-bug` to stop at a particular delayed bug and
 get a backtrace.

 ### Getting the error creation location

 `-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`:
 - it will print the locations for all errors emitted
 - it does not require a compiler built with debug symbols
 - you don't have to read through a big stack trace.

 ## Getting logging output

 The compiler uses the [`tracing`] crate for logging.

 [`tracing`]: https://docs.rs/tracing

 For details see [the guide section on tracing](./tracing.md)

 ## Narrowing (Bisecting) Regressions

 The [cargo-bisect-rustc][bisect] 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][bisect-tutorial] on how to use
 it.

 [bisect]: https://github.com/rust-lang/cargo-bisect-rustc
 [bisect-tutorial]: https://rust-lang.github.io/cargo-bisect-rustc/tutorial.html

 ## Downloading Artifacts from Rust's CI

 The [rustup-toolchain-install-master][rtim] 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.

 [rtim]: https://github.com/kennytm/rustup-toolchain-install-master

 ## `#[rustc_*]` TEST attributes

 The compiler defines a whole lot of internal (perma-unstable) attributes some of which are useful
 for debugging by dumping extra compiler-internal information. These are prefixed with `rustc_` and
 are gated behind the internal feature `rustc_attrs` (enabled via e.g. `#![feature(rustc_attrs)]`).

 For a complete and up to date list, see [`builtin_attrs`]. More specifically, the ones marked `TEST`.
 Here are some notable ones:

 | Attribute | Description |
 |----------------|-------------|
 | `rustc_def_path` | Dumps the [`def_path_str`] of an item. |
 | `rustc_dump_def_parents` | Dumps the chain of `DefId` parents of certain definitions. |
 | `rustc_dump_item_bounds` | Dumps the [`item_bounds`] of an item. |
 | `rustc_dump_predicates` | Dumps the [`predicates_of`] an item. |
 | `rustc_dump_vtable` | Dumps the vtable layout of an impl, or a type alias of a dyn type. |
 | `rustc_hidden_type_of_opaques` | Dumps the [hidden type of each opaque types][opaq] in the crate. |
 | `rustc_layout` | [See this section](#debugging-type-layouts). |
 | `rustc_object_lifetime_default` | Dumps the [object lifetime defaults] of an item. |
 | `rustc_outlives` | Dumps implied bounds of an item. More precisely, the [`inferred_outlives_of`] an item. |
 | `rustc_regions` | Dumps NLL closure region requirements. |
 | `rustc_symbol_name` | Dumps the mangled & demangled [`symbol_name`] of an item. |
 | `rustc_variances` | Dumps the [variances] of an item. |

 Right below you can find elaborate explainers on a selected few.

 [`builtin_attrs`]: https://github.com/rust-lang/rust/blob/master/compiler/rustc_feature/src/builtin_attrs.rs
 [`def_path_str`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.def_path_str
 [`inferred_outlives_of`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.inferred_outlives_of
 [`item_bounds`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.item_bounds
 [`predicates_of`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.predicates_of
 [`symbol_name`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.symbol_name
 [object lifetime defaults]: https://doc.rust-lang.org/reference/lifetime-elision.html#default-trait-object-lifetimes
 [opaq]: ./opaque-types-impl-trait-inference.md
 [variances]: ./variance.md

 ### Formatting Graphviz output (.dot files)
 [formatting-graphviz-output]: #formatting-graphviz-output

 Some compiler options for debugging specific features yield graphviz graphs -
 e.g. the `#[rustc_mir(borrowck_graphviz_postflow="suffix.dot")]` attribute
 on a function dumps various borrow-checker dataflow graphs in conjunction with
 `-Zdump-mir-dataflow`.

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

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

 ### Debugging type layouts

 The internal attribute `#[rustc_layout]` can be used to dump the [`Layout`] of
 the type it is attached to. For example:

 ```rust
 #![feature(rustc_attrs)]

 #[rustc_layout(debug)]
 type T<'a> = &'a u32;
 ```

 Will emit the following:

 ```text
 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
 ```

 [`Layout`]: https://doc.rust-lang.org/nightly/nightly-rustc/stable_mir/abi/struct.Layout.html


 ## Configuring CodeLLDB for debugging `rustc`

 If you are using VSCode, and have edited your `bootstrap.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"):

 ```javascript
 // .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"]
       }
     ]
   }
 ```

 [CodeLLDB]: https://marketplace.visualstudio.com/items?itemName=vadimcn.vscode-lldb
	# Debugging the compiler

	<!-- toc -->

	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](./incrcomp-debugging.html) or the [LLVM Debugging
	chapter](./backend/debugging.md)).

	## Configuring the compiler

	By default, rustc is built without most debug information. To enable debug info,
	set `debug = true` in your bootstrap.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 `bootstrap.toml` with options:

	```toml
	[rust]
	debug = true
	debuginfo-level = 2
	```

	> NOTE:
	> This will use a lot of disk space
	> (upwards of <!-- date-check Aug 2022 --> 35GB),
	> and will take a lot more compile time.
	> With `debuginfo-level = 1` (the default when `debug = 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 `bootstrap.toml`.

	```toml
	[rust]
	new-symbol-mangling = false
	```

	> See the comments in `bootstrap.example.toml` for more info.

	You will need to rebuild the compiler after changing any configuration option.

	## Suppressing the ICE file

	By default, if rustc encounters an Internal Compiler Error (ICE) it will dump the ICE contents to an
	ICE file within the current working directory named `rustc-ice-<timestamp>-<pid>.txt`. If this is
	not desirable, you can prevent the ICE file from being created with `RUSTC_ICE=0`.

	## Getting a backtrace
	[getting-a-backtrace]: #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:

	```text
	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:

	```text
	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
	```

	## `-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-internals`, which generally enables printing more
	info that could be useful for debugging.

	Right below you can find elaborate explainers on a selected few.

	### Getting a backtrace for errors
	[getting-a-backtrace-for-errors]: #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. If you leave off `=n`, the compiler will
	assume `1` for `n` and thus panic on the first error it encounters.

	For example:

	```bash
	$ cat error.rs
	```

	```rust
	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!

	### Debugging delayed bugs

	The `-Z eagerly-emit-delayed-bugs` option makes it easy to debug delayed bugs.
	It turns them into normal errors, i.e. makes them visible. This can be used in
	combination with `-Z treat-err-as-bug` to stop at a particular delayed bug and
	get a backtrace.

	### Getting the error creation location

	`-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`:
	- it will print the locations for all errors emitted
	- it does not require a compiler built with debug symbols
	- you don't have to read through a big stack trace.

	## Getting logging output

	The compiler uses the [`tracing`] crate for logging.

	[`tracing`]: https://docs.rs/tracing

	For details see [the guide section on tracing](./tracing.md)

	## Narrowing (Bisecting) Regressions

	The [cargo-bisect-rustc][bisect] 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][bisect-tutorial] on how to use
	it.

	[bisect]: https://github.com/rust-lang/cargo-bisect-rustc
	[bisect-tutorial]: https://rust-lang.github.io/cargo-bisect-rustc/tutorial.html

	## Downloading Artifacts from Rust's CI

	The [rustup-toolchain-install-master][rtim] 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.

	[rtim]: https://github.com/kennytm/rustup-toolchain-install-master

	## `#[rustc_*]` TEST attributes

	The compiler defines a whole lot of internal (perma-unstable) attributes some of which are useful
	for debugging by dumping extra compiler-internal information. These are prefixed with `rustc_` and
	are gated behind the internal feature `rustc_attrs` (enabled via e.g. `#![feature(rustc_attrs)]`).

	For a complete and up to date list, see [`builtin_attrs`]. More specifically, the ones marked `TEST`.
	Here are some notable ones:

	\| Attribute \| Description \|
	\|----------------\|-------------\|
	\| `rustc_def_path` \| Dumps the [`def_path_str`] of an item. \|
	\| `rustc_dump_def_parents` \| Dumps the chain of `DefId` parents of certain definitions. \|
	\| `rustc_dump_item_bounds` \| Dumps the [`item_bounds`] of an item. \|
	\| `rustc_dump_predicates` \| Dumps the [`predicates_of`] an item. \|
	\| `rustc_dump_vtable` \| Dumps the vtable layout of an impl, or a type alias of a dyn type. \|
	\| `rustc_hidden_type_of_opaques` \| Dumps the [hidden type of each opaque types][opaq] in the crate. \|
	\| `rustc_layout` \| [See this section](#debugging-type-layouts). \|
	\| `rustc_object_lifetime_default` \| Dumps the [object lifetime defaults] of an item. \|
	\| `rustc_outlives` \| Dumps implied bounds of an item. More precisely, the [`inferred_outlives_of`] an item. \|
	\| `rustc_regions` \| Dumps NLL closure region requirements. \|
	\| `rustc_symbol_name` \| Dumps the mangled & demangled [`symbol_name`] of an item. \|
	\| `rustc_variances` \| Dumps the [variances] of an item. \|

	Right below you can find elaborate explainers on a selected few.

	[`builtin_attrs`]: https://github.com/rust-lang/rust/blob/master/compiler/rustc_feature/src/builtin_attrs.rs
	[`def_path_str`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.def_path_str
	[`inferred_outlives_of`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.inferred_outlives_of
	[`item_bounds`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.item_bounds
	[`predicates_of`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.predicates_of
	[`symbol_name`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.symbol_name
	[object lifetime defaults]: https://doc.rust-lang.org/reference/lifetime-elision.html#default-trait-object-lifetimes
	[opaq]: ./opaque-types-impl-trait-inference.md
	[variances]: ./variance.md

	### Formatting Graphviz output (.dot files)
	[formatting-graphviz-output]: #formatting-graphviz-output

	Some compiler options for debugging specific features yield graphviz graphs -
	e.g. the `#[rustc_mir(borrowck_graphviz_postflow="suffix.dot")]` attribute
	on a function dumps various borrow-checker dataflow graphs in conjunction with
	`-Zdump-mir-dataflow`.

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

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

	### Debugging type layouts

	The internal attribute `#[rustc_layout]` can be used to dump the [`Layout`] of
	the type it is attached to. For example:

	```rust
	#![feature(rustc_attrs)]

	#[rustc_layout(debug)]
	type T<'a> = &'a u32;
	```

	Will emit the following:

	```text
	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
	```

	[`Layout`]: https://doc.rust-lang.org/nightly/nightly-rustc/stable_mir/abi/struct.Layout.html


	## Configuring CodeLLDB for debugging `rustc`

	If you are using VSCode, and have edited your `bootstrap.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"):

	```javascript
	// .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"]
	}
	]
	}
	```

	[CodeLLDB]: https://marketplace.visualstudio.com/items?itemName=vadimcn.vscode-lldb