how to build and run compiler, first shot

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 # How to build the compiler and run what you built
+
+The compiler is built using a tool called `x.py`. You will need to
+have Python installed to run it. But before we get to that, if you're going to 
+be hacking on rustc, you'll want to tweak the configuration of the compiler. The default
+configuration is oriented towards running the compiler as a user, not a developer.
+
+### Create a config.toml
+
+To start, copy [`config.toml.example`] to `config.toml`:
+
+[`config.toml.example`]: https://github.com/rust-lang/rust/blob/master/config.toml.example
+
+```bash
+> cd $RUST_CHECKOUT
+> cp config.toml.example config.toml
+```
+
+Then you will want to open up the file and change the following
+settings (and possibly others, such as `llvm.ccache`):
+
+```
+[llvm]
+# Enables LLVM assertions, which will check that the LLVM bitcode generated
+# by the compiler is internally consistent. These are particularly helpful
+# if you edit `trans`.
+assertions = true
+
+[rust]
+# This enables some assertions, but more importantly it enables the `debug!` logging
+# macros that are essential for debugging rustc.
+debug-assertions = true
+
+# This will make your build more parallel; it costs a bit of runtime
+# performance perhaps (less inlining) but it's worth it.
+codegen-units = 0
+
+# I always enable full debuginfo, though debuginfo-lines is more important.
+debuginfo = true
+
+# Gives you line numbers for backtraces.
+debuginfo-lines = true
+
+# Using the system allocator (instead of jemalloc) means that tools
+# like valgrind and memcache work better.
+use-jemalloc = false
+```
+
+### Running x.py and building a stage1 compiler
+
+Once you've created a config.toml, you are now ready to run
+`x.py`. There are a lot of options here, but let's start with what is
+probably the best "go to" command for building a local rust:
+
+```
+./x.py build --incremental --stage 1 src/libstd
+```
+
+What this command will do is the following:
+
+- Using the beta compiler (also called stage 0), it will build the
+  standard library and rustc from the `src` directory. The resulting
+  compiler is called the "stage 1" compiler.
+  - During this build, the `--incremental` switch enables incremental
+    compilation, so that if you later rebuild after editing things in
+    `src`, you can save a bit of time.
+- Using this stage 1 compiler, it will build the standard library.
+  (this is what the `src/libstd`) means.
+
+This is just a subset of the full rustc build. The **full** rustc build (what you
+get if you just say `./x.py build`) has quite a few more steps:
+
+- Build stage1 rustc with stage0 compiler
+- Build libstd with stage1 compiler (up to here is the same)
+- Build stage2 rustc with stage1 compiler (this part is new)
+- Build libstd with stage2 compiler
+- Build librustdoc and a bunch of other things
+
+### Creating a rustup toolchain
+
+Once you have successfully built rustc, you will have created a bunch
+of files in your `build` directory. In order to actually run the
+resulting rustc, we recommend creating rustup toolchains. The first
+one will run the stage1 compiler (which we built above). The second
+will execute the stage2 compiler (which we did not build, but which
+you will likely build at some point).
+
+```
+> rustup toolchain link stage1 build/<host-triple>/stage1
+> rustup toolchain link stage2 build/<host-triple>/stage2
+```
+
+Now you can run the rustc you built with. If you run with `-vV`, you
+should see a version number ending in `-dev`, indicating a build from
+your local environment:
+
+```
+> rustc +stage1 -vV
+rustc 1.25.0-dev
+binary: rustc
+commit-hash: unknown
+commit-date: unknown
+host: x86_64-unknown-linux-gnu
+release: 1.25.0-dev
+LLVM version: 4.0
+```
+
+### Other x.py commands
+
+Here are a few other useful x.py commands. We'll cover some of them in detail in other sections:
+
+- Building things:
+  - `./x.py clean` -- clean up the build directory (`rm -rf build` works too, but then you have to rebuild LLVM)
+  - `./x.py build --stage 1` -- builds everything using the stage 1 compiler, not just up to libstd
+  - `./x.py build` -- builds the stage2 compiler
+- Running tests (see the section [running tests](./running-tests.html) for more details):
+  - `./x.py test --stage 1 src/libstd` -- runs the `#[test]` tests from libstd
+  - `./x.py test --stage 1 src/test/run-pass` -- runs the `run-pass` test suite