src/tools/miri/CONTRIBUTING.md - toolchain/rustc - Git at Google

 # Contribution Guide

 If you want to hack on Miri yourself, great!  Here are some resources you might
 find useful.

 ## Getting started

 Check out the issues on this GitHub repository for some ideas. In particular,
 look for the green `E-*` labels which mark issues that should be rather
 well-suited for onboarding. For more ideas or help with hacking on Miri, you can
 contact us on the [Rust Zulip]. See the [Rust website](https://www.rust-lang.org/governance/teams/compiler#team-miri)
 for a list of Miri maintainers.

 [Rust Zulip]: https://rust-lang.zulipchat.com

 ### Larger-scale contributions

 If you are thinking about making a larger-scale contribution -- in particular anything that needs
 more than can reasonably fit in a single PR to be feature-complete -- then please talk to us before
 writing significant amounts of code. Generally, we will ask that you follow a three-step "project"
 process for such contributions:

 1. Clearly define the **goal** of the project. This defines the scope of the project, i.e. which
    part of which APIs should be supported. If this involves functions that expose a big API surface
    with lots of flags, the project may want to support only a tiny subset of flags; that should be
    documented. A good way to express the goal is with one or more test cases that Miri should be
    able to successfully execute when the project is completed. It is a good idea to get feedback
    from team members already at this stage to ensure that the project is reasonably scoped and
    aligns with our interests.
 2. Make a **design** for how to realize the goal. A larger project will likely have to do global
    changes to Miri, like adding new global state to the `Machine` type or new methods to the
    `FileDescription` trait. Often we have to iterate on those changes, which can quite substantially
    change how the final implementation looks like.

     The design should be reasonably concrete, i.e. for new global state or methods the corresponding
    Rust types and method signatures should be spelled out. We realize that it can be hard to make a
    design without doing implementation work, in particular if you are not yet familiar with the
    codebase. Doing draft implementations in phase 2 of this process is perfectly fine, just please
    be aware that we might request fundamental changes that can require significantly reworking what
    you already did. If you open a PR in this stage, please clearly indicate that this project is
    still in the design stage.

 3. Finish the **implementation** and have it reviewed.

 This process is largely informal, and its primary goal is to more clearly communicate expectations.
 Please get in touch with us if you have any questions!

 ## Preparing the build environment

 Miri heavily relies on internal and unstable rustc interfaces to execute MIR,
 which means it is important that you install a version of rustc that Miri
 actually works with.

 The `rust-version` file contains the commit hash of rustc that Miri is currently
 tested against. Other versions will likely not work. After installing
 [`rustup-toolchain-install-master`], you can run the following command to
 install that exact version of rustc as a toolchain:
 ```
 ./miri toolchain
 ```
 This will set up a rustup toolchain called `miri` and set it as an override for
 the current directory.

 You can also create a `.auto-everything` file (contents don't matter, can be empty), which
 will cause any `./miri` command to automatically call `./miri toolchain`, `clippy` and `rustfmt`
 for you. If you don't want all of these to happen, you can add individual `.auto-toolchain`,
 `.auto-clippy` and `.auto-fmt` files respectively.

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

 ## Building and testing Miri

 Invoking Miri requires getting a bunch of flags right and setting up a custom
 sysroot. The `miri` script takes care of that for you. With the
 build environment prepared, compiling Miri is just one command away:

 ```
 ./miri build
 ```

 Run `./miri` without arguments to see the other commands our build tool
 supports.

 ### Testing the Miri driver

 The Miri driver compiled from `src/bin/miri.rs` is the "heart" of Miri: it is
 basically a version of `rustc` that, instead of compiling your code, runs it.
 It accepts all the same flags as `rustc` (though the ones only affecting code
 generation and linking obviously will have no effect) [and more][miri-flags].

 [miri-flags]: README.md#miri--z-flags-and-environment-variables

 For example, you can (cross-)run the driver on a particular file by doing

 ```sh
 ./miri run tests/pass/format.rs
 ./miri run tests/pass/hello.rs --target i686-unknown-linux-gnu
 ```

 Tests in ``pass-dep`` need to be run using ``./miri run --dep <filename>``.
 For example:
 ```sh
 ./miri run --dep tests/pass-dep/shims/libc-fs.rs
 ```

 You can (cross-)run the entire test suite using:

 ```sh
 ./miri test
 ./miri test --target i686-unknown-linux-gnu
 ```

 `./miri test FILTER` only runs those tests that contain `FILTER` in their filename (including the
 base directory, e.g. `./miri test fail` will run all compile-fail tests). Multiple filters are
 supported: `./miri test FILTER1 FILTER2` runs all tests that contain either string.

 #### Fine grained logging

 You can get a trace of which MIR statements are being executed by setting the
 `MIRI_LOG` environment variable.  For example:

 ```sh
 MIRI_LOG=info ./miri run tests/pass/vec.rs
 ```

 Setting `MIRI_LOG` like this will configure logging for Miri itself as well as
 the `rustc_middle::mir::interpret` and `rustc_mir::interpret` modules in rustc. You
 can also do more targeted configuration, e.g. the following helps debug the
 stacked borrows implementation:

 ```sh
 MIRI_LOG=rustc_mir::interpret=info,miri::stacked_borrows ./miri run tests/pass/vec.rs
 ```

 Note that you will only get `info`, `warn` or `error` messages if you use a prebuilt compiler.
 In order to get `debug` and `trace` level messages, you need to build miri with a locally built
 compiler that has `debug=true` set in `config.toml`.

 #### Debugging error messages

 You can set `MIRI_BACKTRACE=1` to get a backtrace of where an
 evaluation error was originally raised.


 ### UI testing

 We use ui-testing in Miri, meaning we generate `.stderr` and `.stdout` files for the output
 produced by Miri. You can use `./miri test --bless` to automatically (re)generate these files when
 you add new tests or change how Miri presents certain output.

 Note that when you also use `MIRIFLAGS` to change optimizations and similar, the ui output
 will change in unexpected ways. In order to still be able
 to run the other checks while ignoring the ui output, use `MIRI_SKIP_UI_CHECKS=1 ./miri test`.

 For more info on how to configure ui tests see [the documentation on the ui test crate][ui_test]

 [ui_test]: https://github.com/oli-obk/ui_test/blob/main/README.md

 ### Testing `cargo miri`

 Working with the driver directly gives you full control, but you also lose all
 the convenience provided by cargo. Once your test case depends on a crate, it
 is probably easier to test it with the cargo wrapper. You can install your
 development version of Miri using

 ```
 ./miri install
 ```

 and then you can use it as if it was installed by `rustup` as a component of the
 `miri` toolchain. Note that the `miri` and `cargo-miri` executables are placed
 in the `miri` toolchain's sysroot to prevent conflicts with other toolchains.
 The Miri binaries in the `cargo` bin directory (usually `~/.cargo/bin`) are managed by rustup.

 There's a test for the cargo wrapper in the `test-cargo-miri` directory; run `./run-test.py` in
 there to execute it. You can pass `--target` to execute the test for another target.

 ### Using a modified standard library

 Miri re-builds the standard library into a custom sysroot, so it is fairly easy
 to test Miri against a modified standard library -- you do not even have to
 build Miri yourself, the Miri shipped by `rustup` will work. All you have to do
 is set the `MIRI_LIB_SRC` environment variable to the `library` folder of a
 `rust-lang/rust` repository checkout. Note that changing files in that directory
 does not automatically trigger a re-build of the standard library; you have to
 clear the Miri build cache manually (on Linux, `rm -rf ~/.cache/miri`;
 on Windows, `rmdir /S "%LOCALAPPDATA%\rust-lang\miri\cache"`;
 and on macOS, `rm -rf ~/Library/Caches/org.rust-lang.miri`).

 ### Benchmarking

 Miri comes with a few benchmarks; you can run `./miri bench` to run them with the locally built
 Miri. Note: this will run `./miri install` as a side-effect. Also requires `hyperfine` to be
 installed (`cargo install hyperfine`).

 ## Configuring `rust-analyzer`

 To configure `rust-analyzer` and VS Code for working on Miri, save the following
 to `.vscode/settings.json` in your local Miri clone:

 ```json
 {
     "rust-analyzer.rustc.source": "discover",
     "rust-analyzer.linkedProjects": [
         "Cargo.toml",
         "cargo-miri/Cargo.toml",
         "miri-script/Cargo.toml",
     ],
     "rust-analyzer.check.invocationLocation": "root",
     "rust-analyzer.check.invocationStrategy": "once",
     "rust-analyzer.check.overrideCommand": [
         "env",
         "MIRI_AUTO_OPS=no",
         "./miri",
         "clippy", // make this `check` when working with a locally built rustc
         "--message-format=json",
     ],
     // Contrary to what the name suggests, this also affects proc macros.
     "rust-analyzer.cargo.buildScripts.invocationLocation": "root",
     "rust-analyzer.cargo.buildScripts.invocationStrategy": "once",
     "rust-analyzer.cargo.buildScripts.overrideCommand": [
         "env",
         "MIRI_AUTO_OPS=no",
         "./miri",
         "check",
         "--message-format=json",
     ],
 }
 ```

 > #### Note
 >
 > If you are [building Miri with a locally built rustc][], set
 > `rust-analyzer.rustcSource` to the relative path from your Miri clone to the
 > root `Cargo.toml` of the locally built rustc. For example, the path might look
 > like `../rust/Cargo.toml`.

 See the rustc-dev-guide's docs on ["Configuring `rust-analyzer` for `rustc`"][rdg-r-a]
 for more information about configuring VS Code and `rust-analyzer`.

 [rdg-r-a]: https://rustc-dev-guide.rust-lang.org/building/suggested.html#configuring-rust-analyzer-for-rustc

 ## Advanced topic: Working on Miri in the rustc tree

 We described above the simplest way to get a working build environment for Miri,
 which is to use the version of rustc indicated by `rustc-version`. But
 sometimes, that is not enough.

 A big part of the Miri driver is shared with rustc, so working on Miri will
 sometimes require also working on rustc itself. In this case, you should *not*
 work in a clone of the Miri repository, but in a clone of the
 [main Rust repository](https://github.com/rust-lang/rust/). There is a copy of
 Miri located at `src/tools/miri` that you can work on directly. A maintainer
 will eventually sync those changes back into this repository.

 When working on Miri in the rustc tree, here's how you can run tests:

 ```
 ./x.py test miri
 ```

 `--bless` will work, too.

 You can also directly run Miri on a Rust source file:

 ```
 ./x.py run miri --stage 1 --args src/tools/miri/tests/pass/hello.rs
 ```

 ## Advanced topic: Syncing with the rustc repo

 We use the [`josh` proxy](https://github.com/josh-project/josh) to transmit changes between the
 rustc and Miri repositories. You can install it as follows:

 ```sh
 RUSTFLAGS="--cap-lints=warn" cargo +stable install josh-proxy --git https://github.com/josh-project/josh --tag r23.12.04
 ```

 Josh will automatically be started and stopped by `./miri`.

 ### Importing changes from the rustc repo

 *Note: this usually happens automatically, so these steps rarely have to be done by hand.*

 We assume we start on an up-to-date master branch in the Miri repo.

 ```sh
 # Fetch and merge rustc side of the history. Takes ca 5 min the first time.
 # This will also update the `rustc-version` file.
 ./miri rustc-pull
 # Update local toolchain and apply formatting.
 ./miri toolchain && ./miri fmt
 git commit -am "rustup"
 ```

 Now push this to a new branch in your Miri fork, and create a PR. It is worth
 running `./miri test` locally in parallel, since the test suite in the Miri repo
 is stricter than the one on the rustc side, so some small tweaks might be
 needed.

 ### Exporting changes to the rustc repo

 We will use the josh proxy to push to your fork of rustc. Run the following in the Miri repo,
 assuming we are on an up-to-date master branch:

 ```sh
 # Push the Miri changes to your rustc fork (substitute your github handle for YOUR_NAME).
 ./miri rustc-push YOUR_NAME miri
 ```

 This will create a new branch called `miri` in your fork, and the output should include a link that
 creates a rustc PR to integrate those changes into the main repository. If that PR has conflicts,
 you need to pull rustc changes into Miri first, and then re-do the rustc push.

 If this fails due to authentication problems, it can help to make josh push via ssh instead of
 https. Add the following to your `.gitconfig`:

 ```toml
 [url "[email protected]:"]
     pushInsteadOf = https://github.com/
 ```

 ## Further environment variables

 The following environment variables are relevant to `./miri`:

 * `MIRI_AUTO_OPS` indicates whether the automatic execution of rustfmt, clippy and toolchain setup
   (as controlled by the `./auto-*` files) should be skipped. If it is set to `no`, they are skipped.
   This is used to allow automated IDE actions to avoid the auto ops.
 * `MIRI_LOG`, `MIRI_BACKTRACE` control logging and backtrace printing during Miri executions.
 * `MIRI_TEST_THREADS` (recognized by `./miri test`) sets the number of threads to use for running
   tests. By default, the number of cores is used.
 * `MIRI_SKIP_UI_CHECKS` (recognized by `./miri test`) disables checking that the `stderr` or
   `stdout` files match the actual output.

 Furthermore, the usual environment variables recognized by `cargo miri` also work for `./miri`, e.g.
 `MIRI_LIB_SRC`. Note that `MIRIFLAGS` is ignored by `./miri test` as each test controls the flags it
 is run with.

 The following environment variables are *internal* and must not be used by
 anyone but Miri itself. They are used to communicate between different Miri
 binaries, and as such worth documenting:

 * `CARGO_EXTRA_FLAGS` is understood by `./miri` and passed to all host cargo invocations.
   It is reserved for CI usage; setting the wrong flags this way can easily confuse the script.
 * `MIRI_BE_RUSTC` can be set to `host` or `target`. It tells the Miri driver to
   actually not interpret the code but compile it like rustc would. With `target`, Miri sets
   some compiler flags to prepare the code for interpretation; with `host`, this is not done.
   This environment variable is useful to be sure that the compiled `rlib`s are compatible
   with Miri.
 * `MIRI_CALLED_FROM_SETUP` is set during the Miri sysroot build,
   which will re-invoke `cargo-miri` as the `rustc` to use for this build.
 * `MIRI_CALLED_FROM_RUSTDOC` when set to any value tells `cargo-miri` that it is
   running as a child process of `rustdoc`, which invokes it twice for each doc-test
   and requires special treatment, most notably a check-only build before interpretation.
   This is set by `cargo-miri` itself when running as a `rustdoc`-wrapper.
 * `MIRI_CWD` when set to any value tells the Miri driver to change to the given
   directory after loading all the source files, but before commencing
   interpretation. This is useful if the interpreted program wants a different
   working directory at run-time than at build-time.
 * `MIRI_LOCAL_CRATES` is set by `cargo-miri` to tell the Miri driver which
   crates should be given special treatment in diagnostics, in addition to the
   crate currently being compiled.
 * `MIRI_ORIG_RUSTDOC` is set and read by different phases of `cargo-miri` to remember the
   value of `RUSTDOC` from before it was overwritten.
 * `MIRI_REPLACE_LIBRS_IF_NOT_TEST` when set to any value enables a hack that helps bootstrap
   run the standard library tests in Miri.
 * `MIRI_TEST_TARGET` is set by `./miri test` (and `./x.py test miri`) to tell the test harness about
   the chosen target.
 * `MIRI_VERBOSE` when set to any value tells the various `cargo-miri` phases to
   perform verbose logging.
 * `MIRI_HOST_SYSROOT` is set by bootstrap to tell `cargo-miri` which sysroot to use for *host*
   operations.
 * `RUSTC_BLESS` is set by `./miri test` (and `./x.py test miri`) to indicate bless-mode to the test
   harness.
	# Contribution Guide

	If you want to hack on Miri yourself, great! Here are some resources you might
	find useful.

	## Getting started

	Check out the issues on this GitHub repository for some ideas. In particular,
	look for the green `E-*` labels which mark issues that should be rather
	well-suited for onboarding. For more ideas or help with hacking on Miri, you can
	contact us on the [Rust Zulip]. See the [Rust website](https://www.rust-lang.org/governance/teams/compiler#team-miri)
	for a list of Miri maintainers.

	[Rust Zulip]: https://rust-lang.zulipchat.com

	### Larger-scale contributions

	If you are thinking about making a larger-scale contribution -- in particular anything that needs
	more than can reasonably fit in a single PR to be feature-complete -- then please talk to us before
	writing significant amounts of code. Generally, we will ask that you follow a three-step "project"
	process for such contributions:

	1. Clearly define the goal of the project. This defines the scope of the project, i.e. which
	part of which APIs should be supported. If this involves functions that expose a big API surface
	with lots of flags, the project may want to support only a tiny subset of flags; that should be
	documented. A good way to express the goal is with one or more test cases that Miri should be
	able to successfully execute when the project is completed. It is a good idea to get feedback
	from team members already at this stage to ensure that the project is reasonably scoped and
	aligns with our interests.
	2. Make a design for how to realize the goal. A larger project will likely have to do global
	changes to Miri, like adding new global state to the `Machine` type or new methods to the
	`FileDescription` trait. Often we have to iterate on those changes, which can quite substantially
	change how the final implementation looks like.

	The design should be reasonably concrete, i.e. for new global state or methods the corresponding
	Rust types and method signatures should be spelled out. We realize that it can be hard to make a
	design without doing implementation work, in particular if you are not yet familiar with the
	codebase. Doing draft implementations in phase 2 of this process is perfectly fine, just please
	be aware that we might request fundamental changes that can require significantly reworking what
	you already did. If you open a PR in this stage, please clearly indicate that this project is
	still in the design stage.

	3. Finish the implementation and have it reviewed.

	This process is largely informal, and its primary goal is to more clearly communicate expectations.
	Please get in touch with us if you have any questions!

	## Preparing the build environment

	Miri heavily relies on internal and unstable rustc interfaces to execute MIR,
	which means it is important that you install a version of rustc that Miri
	actually works with.

	The `rust-version` file contains the commit hash of rustc that Miri is currently
	tested against. Other versions will likely not work. After installing
	[`rustup-toolchain-install-master`], you can run the following command to
	install that exact version of rustc as a toolchain:
	```
	./miri toolchain
	```
	This will set up a rustup toolchain called `miri` and set it as an override for
	the current directory.

	You can also create a `.auto-everything` file (contents don't matter, can be empty), which
	will cause any `./miri` command to automatically call `./miri toolchain`, `clippy` and `rustfmt`
	for you. If you don't want all of these to happen, you can add individual `.auto-toolchain`,
	`.auto-clippy` and `.auto-fmt` files respectively.

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

	## Building and testing Miri

	Invoking Miri requires getting a bunch of flags right and setting up a custom
	sysroot. The `miri` script takes care of that for you. With the
	build environment prepared, compiling Miri is just one command away:

	```
	./miri build
	```

	Run `./miri` without arguments to see the other commands our build tool
	supports.

	### Testing the Miri driver

	The Miri driver compiled from `src/bin/miri.rs` is the "heart" of Miri: it is
	basically a version of `rustc` that, instead of compiling your code, runs it.
	It accepts all the same flags as `rustc` (though the ones only affecting code
	generation and linking obviously will have no effect) [and more][miri-flags].

	[miri-flags]: README.md#miri--z-flags-and-environment-variables

	For example, you can (cross-)run the driver on a particular file by doing

	```sh
	./miri run tests/pass/format.rs
	./miri run tests/pass/hello.rs --target i686-unknown-linux-gnu
	```

	Tests in ``pass-dep`` need to be run using ``./miri run --dep <filename>``.
	For example:
	```sh
	./miri run --dep tests/pass-dep/shims/libc-fs.rs
	```

	You can (cross-)run the entire test suite using:

	```sh
	./miri test
	./miri test --target i686-unknown-linux-gnu
	```

	`./miri test FILTER` only runs those tests that contain `FILTER` in their filename (including the
	base directory, e.g. `./miri test fail` will run all compile-fail tests). Multiple filters are
	supported: `./miri test FILTER1 FILTER2` runs all tests that contain either string.

	#### Fine grained logging

	You can get a trace of which MIR statements are being executed by setting the
	`MIRI_LOG` environment variable. For example:

	```sh
	MIRI_LOG=info ./miri run tests/pass/vec.rs
	```

	Setting `MIRI_LOG` like this will configure logging for Miri itself as well as
	the `rustc_middle::mir::interpret` and `rustc_mir::interpret` modules in rustc. You
	can also do more targeted configuration, e.g. the following helps debug the
	stacked borrows implementation:

	```sh
	MIRI_LOG=rustc_mir::interpret=info,miri::stacked_borrows ./miri run tests/pass/vec.rs
	```

	Note that you will only get `info`, `warn` or `error` messages if you use a prebuilt compiler.
	In order to get `debug` and `trace` level messages, you need to build miri with a locally built
	compiler that has `debug=true` set in `config.toml`.

	#### Debugging error messages

	You can set `MIRI_BACKTRACE=1` to get a backtrace of where an
	evaluation error was originally raised.


	### UI testing

	We use ui-testing in Miri, meaning we generate `.stderr` and `.stdout` files for the output
	produced by Miri. You can use `./miri test --bless` to automatically (re)generate these files when
	you add new tests or change how Miri presents certain output.

	Note that when you also use `MIRIFLAGS` to change optimizations and similar, the ui output
	will change in unexpected ways. In order to still be able
	to run the other checks while ignoring the ui output, use `MIRI_SKIP_UI_CHECKS=1 ./miri test`.

	For more info on how to configure ui tests see [the documentation on the ui test crate][ui_test]

	[ui_test]: https://github.com/oli-obk/ui_test/blob/main/README.md

	### Testing `cargo miri`

	Working with the driver directly gives you full control, but you also lose all
	the convenience provided by cargo. Once your test case depends on a crate, it
	is probably easier to test it with the cargo wrapper. You can install your
	development version of Miri using

	```
	./miri install
	```

	and then you can use it as if it was installed by `rustup` as a component of the
	`miri` toolchain. Note that the `miri` and `cargo-miri` executables are placed
	in the `miri` toolchain's sysroot to prevent conflicts with other toolchains.
	The Miri binaries in the `cargo` bin directory (usually `~/.cargo/bin`) are managed by rustup.

	There's a test for the cargo wrapper in the `test-cargo-miri` directory; run `./run-test.py` in
	there to execute it. You can pass `--target` to execute the test for another target.

	### Using a modified standard library

	Miri re-builds the standard library into a custom sysroot, so it is fairly easy
	to test Miri against a modified standard library -- you do not even have to
	build Miri yourself, the Miri shipped by `rustup` will work. All you have to do
	is set the `MIRI_LIB_SRC` environment variable to the `library` folder of a
	`rust-lang/rust` repository checkout. Note that changing files in that directory
	does not automatically trigger a re-build of the standard library; you have to
	clear the Miri build cache manually (on Linux, `rm -rf ~/.cache/miri`;
	on Windows, `rmdir /S "%LOCALAPPDATA%\rust-lang\miri\cache"`;
	and on macOS, `rm -rf ~/Library/Caches/org.rust-lang.miri`).

	### Benchmarking

	Miri comes with a few benchmarks; you can run `./miri bench` to run them with the locally built
	Miri. Note: this will run `./miri install` as a side-effect. Also requires `hyperfine` to be
	installed (`cargo install hyperfine`).

	## Configuring `rust-analyzer`

	To configure `rust-analyzer` and VS Code for working on Miri, save the following
	to `.vscode/settings.json` in your local Miri clone:

	```json
	{
	"rust-analyzer.rustc.source": "discover",
	"rust-analyzer.linkedProjects": [
	"Cargo.toml",
	"cargo-miri/Cargo.toml",
	"miri-script/Cargo.toml",
	],
	"rust-analyzer.check.invocationLocation": "root",
	"rust-analyzer.check.invocationStrategy": "once",
	"rust-analyzer.check.overrideCommand": [
	"env",
	"MIRI_AUTO_OPS=no",
	"./miri",
	"clippy", // make this `check` when working with a locally built rustc
	"--message-format=json",
	],
	// Contrary to what the name suggests, this also affects proc macros.
	"rust-analyzer.cargo.buildScripts.invocationLocation": "root",
	"rust-analyzer.cargo.buildScripts.invocationStrategy": "once",
	"rust-analyzer.cargo.buildScripts.overrideCommand": [
	"env",
	"MIRI_AUTO_OPS=no",
	"./miri",
	"check",
	"--message-format=json",
	],
	}
	```

	> #### Note
	>
	> If you are [building Miri with a locally built rustc][], set
	> `rust-analyzer.rustcSource` to the relative path from your Miri clone to the
	> root `Cargo.toml` of the locally built rustc. For example, the path might look
	> like `../rust/Cargo.toml`.

	See the rustc-dev-guide's docs on ["Configuring `rust-analyzer` for `rustc`"][rdg-r-a]
	for more information about configuring VS Code and `rust-analyzer`.

	[rdg-r-a]: https://rustc-dev-guide.rust-lang.org/building/suggested.html#configuring-rust-analyzer-for-rustc

	## Advanced topic: Working on Miri in the rustc tree

	We described above the simplest way to get a working build environment for Miri,
	which is to use the version of rustc indicated by `rustc-version`. But
	sometimes, that is not enough.

	A big part of the Miri driver is shared with rustc, so working on Miri will
	sometimes require also working on rustc itself. In this case, you should not
	work in a clone of the Miri repository, but in a clone of the
	[main Rust repository](https://github.com/rust-lang/rust/). There is a copy of
	Miri located at `src/tools/miri` that you can work on directly. A maintainer
	will eventually sync those changes back into this repository.

	When working on Miri in the rustc tree, here's how you can run tests:

	```
	./x.py test miri
	```

	`--bless` will work, too.

	You can also directly run Miri on a Rust source file:

	```
	./x.py run miri --stage 1 --args src/tools/miri/tests/pass/hello.rs
	```

	## Advanced topic: Syncing with the rustc repo

	We use the [`josh` proxy](https://github.com/josh-project/josh) to transmit changes between the
	rustc and Miri repositories. You can install it as follows:

	```sh
	RUSTFLAGS="--cap-lints=warn" cargo +stable install josh-proxy --git https://github.com/josh-project/josh --tag r23.12.04
	```

	Josh will automatically be started and stopped by `./miri`.

	### Importing changes from the rustc repo

	Note: this usually happens automatically, so these steps rarely have to be done by hand.

	We assume we start on an up-to-date master branch in the Miri repo.

	```sh
	# Fetch and merge rustc side of the history. Takes ca 5 min the first time.
	# This will also update the `rustc-version` file.
	./miri rustc-pull
	# Update local toolchain and apply formatting.
	./miri toolchain && ./miri fmt
	git commit -am "rustup"
	```

	Now push this to a new branch in your Miri fork, and create a PR. It is worth
	running `./miri test` locally in parallel, since the test suite in the Miri repo
	is stricter than the one on the rustc side, so some small tweaks might be
	needed.

	### Exporting changes to the rustc repo

	We will use the josh proxy to push to your fork of rustc. Run the following in the Miri repo,
	assuming we are on an up-to-date master branch:

	```sh
	# Push the Miri changes to your rustc fork (substitute your github handle for YOUR_NAME).
	./miri rustc-push YOUR_NAME miri
	```

	This will create a new branch called `miri` in your fork, and the output should include a link that
	creates a rustc PR to integrate those changes into the main repository. If that PR has conflicts,
	you need to pull rustc changes into Miri first, and then re-do the rustc push.

	If this fails due to authentication problems, it can help to make josh push via ssh instead of
	https. Add the following to your `.gitconfig`:

	```toml
	[url "[email protected]:"]
	pushInsteadOf = https://github.com/
	```

	## Further environment variables

	The following environment variables are relevant to `./miri`:

	* `MIRI_AUTO_OPS` indicates whether the automatic execution of rustfmt, clippy and toolchain setup
	(as controlled by the `./auto-*` files) should be skipped. If it is set to `no`, they are skipped.
	This is used to allow automated IDE actions to avoid the auto ops.
	* `MIRI_LOG`, `MIRI_BACKTRACE` control logging and backtrace printing during Miri executions.
	* `MIRI_TEST_THREADS` (recognized by `./miri test`) sets the number of threads to use for running
	tests. By default, the number of cores is used.
	* `MIRI_SKIP_UI_CHECKS` (recognized by `./miri test`) disables checking that the `stderr` or
	`stdout` files match the actual output.

	Furthermore, the usual environment variables recognized by `cargo miri` also work for `./miri`, e.g.
	`MIRI_LIB_SRC`. Note that `MIRIFLAGS` is ignored by `./miri test` as each test controls the flags it
	is run with.

	The following environment variables are internal and must not be used by
	anyone but Miri itself. They are used to communicate between different Miri
	binaries, and as such worth documenting:

	* `CARGO_EXTRA_FLAGS` is understood by `./miri` and passed to all host cargo invocations.
	It is reserved for CI usage; setting the wrong flags this way can easily confuse the script.
	* `MIRI_BE_RUSTC` can be set to `host` or `target`. It tells the Miri driver to
	actually not interpret the code but compile it like rustc would. With `target`, Miri sets
	some compiler flags to prepare the code for interpretation; with `host`, this is not done.
	This environment variable is useful to be sure that the compiled `rlib`s are compatible
	with Miri.
	* `MIRI_CALLED_FROM_SETUP` is set during the Miri sysroot build,
	which will re-invoke `cargo-miri` as the `rustc` to use for this build.
	* `MIRI_CALLED_FROM_RUSTDOC` when set to any value tells `cargo-miri` that it is
	running as a child process of `rustdoc`, which invokes it twice for each doc-test
	and requires special treatment, most notably a check-only build before interpretation.
	This is set by `cargo-miri` itself when running as a `rustdoc`-wrapper.
	* `MIRI_CWD` when set to any value tells the Miri driver to change to the given
	directory after loading all the source files, but before commencing
	interpretation. This is useful if the interpreted program wants a different
	working directory at run-time than at build-time.
	* `MIRI_LOCAL_CRATES` is set by `cargo-miri` to tell the Miri driver which
	crates should be given special treatment in diagnostics, in addition to the
	crate currently being compiled.
	* `MIRI_ORIG_RUSTDOC` is set and read by different phases of `cargo-miri` to remember the
	value of `RUSTDOC` from before it was overwritten.
	* `MIRI_REPLACE_LIBRS_IF_NOT_TEST` when set to any value enables a hack that helps bootstrap
	run the standard library tests in Miri.
	* `MIRI_TEST_TARGET` is set by `./miri test` (and `./x.py test miri`) to tell the test harness about
	the chosen target.
	* `MIRI_VERBOSE` when set to any value tells the various `cargo-miri` phases to
	perform verbose logging.
	* `MIRI_HOST_SYSROOT` is set by bootstrap to tell `cargo-miri` which sysroot to use for host
	operations.
	* `RUSTC_BLESS` is set by `./miri test` (and `./x.py test miri`) to indicate bless-mode to the test
	harness.