- `--test-args` will pass arguments through to the test runner. For `tests/ui`,
  this is `compiletest`. For unit tests and doc tests this is the `libtest`
  runner.

Most test runners accept `--help`,
which you can use to find out the options accepted by the runner.

## Environment Variables

During bootstrapping, there are a bunch of compiler-internal environment
variables that are used. If you are trying to run an intermediate version of
`rustc`, sometimes you may need to set some of these environment variables
manually. Otherwise, you get an error like the following:

```text
thread 'main' panicked at 'RUSTC_STAGE was not set: NotPresent', library/core/src/result.rs:1165:5
```

If `./stageN/bin/rustc` gives an error about environment variables, that usually
means something is quite wrong -- such as you're trying to compile `rustc` or
`std` or something which depends on environment variables. In the unlikely case
that you actually need to invoke `rustc` in such a situation, you can tell the
bootstrap shim to print all `env` variables by adding `-vvv` to your `x`
command.

Finally, bootstrap makes use of the [cc-rs crate] which has [its own
method][env-vars] of configuring `C` compilers and `C` flags via environment
variables.


## Clarification of build command's `stdout`

In this part, we will investigate the build command's `stdout` in an action
(similar, but more detailed and complete documentation compare to topic above).
When you execute `x build --dry-run` command, the build output will be something
like the following:

```text
Building stage0 library artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
Copying stage0 library from stage0 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Building stage0 compiler artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
Copying stage0 rustc from stage0 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Assembling stage1 compiler (x86_64-unknown-linux-gnu)
Building stage1 library artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
Copying stage1 library from stage1 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Building stage1 tool rust-analyzer-proc-macro-srv (x86_64-unknown-linux-gnu)
Building rustdoc for stage1 (x86_64-unknown-linux-gnu)
```

### Building stage0 {std,compiler} artifacts

These steps use the provided (downloaded, usually) compiler to compile the local
Rust source into libraries we can use.

### Copying stage0 \{std,rustc\}

This copies the library and compiler artifacts from `cargo` into
`stage0-sysroot/lib/rustlib/{target-triple}/lib`

### Assembling stage1 compiler

This copies the libraries we built in "building `stage0` ... artifacts" into the
`stage1` compiler's `lib/` directory. These are the host libraries that the
compiler itself uses to run. These aren't actually used by artifacts the new
compiler generates. This step also copies the `rustc` and `rustdoc` binaries we
generated into `build/$HOST/stage/bin`.

The `stage1/bin/rustc` is a fully functional compiler built with stage0 (precompiled) compiler and std.
To use a compiler built entirely from source with the in-tree compiler and std, you need to build the
stage2 compiler, which is compiled using the stage1 (in-tree) compiler and std.