Add quickstart for how to build and run the compiler (rust-lang#1951)

* Add quickstart for how to build and run the compiler

The chapter is quite long, and a lot of the information is, while
valuable, not very important for newcomers. I think it makes sense to
have a condensed version for anyone just wanting to get started with
only the most important information.

* A few improvements to quickstart

src/doc/rustc-dev-guide/src/SUMMARY.md

@@ -8,6 +8,7 @@
 # Building and debugging `rustc`
 
 - [How to build and run the compiler](./building/how-to-build-and-run.md)
+    - [Quickstart](./building/quickstart.md)
     - [Prerequisites](./building/prerequisites.md)
     - [Suggested Workflows](./building/suggested.md)
     - [Distribution artifacts](./building/build-install-distribution-artifacts.md)

src/doc/rustc-dev-guide/src/building/how-to-build-and-run.md

@@ -5,6 +5,11 @@
 The compiler is built using a tool called `x.py`. You will need to
 have Python installed to run it.
 
+## Quick Start
+
+For a less in-depth quick-start of getting the compiler running, see [quickstart](./quickstart.md).
+
+
 ## Get the source code
 
 The main repository is [`rust-lang/rust`][repo]. This contains the compiler,

src/doc/rustc-dev-guide/src/building/quickstart.md

@@ -0,0 +1,64 @@
+# Quickstart
+
+This is a quickstart guide about getting the compiler running. For more information in the individual steps,
+see the other pages in this chapter.
+
+First, clone the repository:
+
+```sh
+git clone https://github.com/rust-lang/rust.git
+cd rust
+```
+
+When building the compiler, we don't use `cargo` directly, instead we use a wrapper called "x".
+It is invoked with `./x`.
+
+We need to create a configuration for the build. Use `./x setup` to create a good default.
+
+```sh
+./x setup
+```
+
+Then, we can build the compiler. Use `./x build` to build the compiler, standard library and a few tools.
+You can also `./x check` to just check it.
+All these commands can take specific components/paths as arguments, for example `./x check compiler` to just check the compiler.
+
+```sh
+./x build
+```
+
+> When doing a change to the compiler that does not affect the way it compiles the standard library
+(so for example, a change to an error message), use `--keep-stage-std 1` to avoid recompiling it.
+
+After building the compiler and standard library, you now have a working compiler toolchain.
+You can use it with rustup by linking it.
+
+```sh
+rustup toolchain link stage1 build/host/stage1
+```
+
+Now you have a toolchain called `stage1` linked to your build. You can use it to test the compiler.
+
+```sh
+rustc +stage1 testfile.rs
+```
+
+After doing a change, you can run the compiler test suite with `./x test`.
+
+`./x test` runs the full test suite, which is slow and rarely what you want.
+Usually, `./x test tests/ui` is what you want after a comiler change,
+testing all [UI tests](../tests/ui.md) that invoke the compiler on a specific test file and check the output.
+
+```sh
+./x test tests/ui
+```
+
+Use `--bless` if you've made a change and want to update the `.stderr` files with the new output.
+
+> `./x suggest` can also be helpful for suggesting which tests to run after a change.
+
+Congrats, you are now ready to make a change to the compiler! If you have more questions,
+[the full chapter](./how-to-build-and-run.md) might contain the answers, and if it doesn't,
+feel free to ask for help on [Zulip](https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp).
+
+If you use VSCode, `./x setup` will ask you if you want to set up the config. For other editors, check out [suggested workflows](./suggested.md).