Add a developer documentation to our book

Author: celinval

.gitignore

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 /book
+**/target
+
+.idea
+*.swp
+*.swo
+.vscode

SUMMARY.md

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 - [Charter](./CHARTER.md)
 - [Meetings](./meetings/README.md)
 - [Draft RFCs](./draft-rfcs/README.md)
+- [Developer Documentation](./dev/README.md)

dev/README.md

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+# Development documentation
+
+This folder contains some documentation useful for the development of Stable MIR
+
+## Toolchain versions
+
+Our CI currently run a nightly job that build and run our test suites against the latest nightly  toolchain.
+This repository also contains a .toolchain file that specifies to use `nightly` channel.
+
+However, if you already have the nightly toolchain installed, this will not update the toolchain to
+the latest nightly.
+You need to explicitly do that.
+
+If you see some errors while compiling our tools, please ensure you have the latest nightly.
+You might also want to check [our nightly runs](https://github.com/rust-lang/project-stable-mir/actions/workflows/nightly.yml?query=branch%3Amain) to ensure they are not currently broken.
+If so, you can check what was the last successful nightly run, and use its nightly version.
+
+## Tools
+
+We currently have two different tools that is used to help with testing StableMIR
+
+### TestDrive
+
+This is a small driver that we build on the top of the rust compiler.
+By default, this driver behaves exactly like `rustc`, and it can be used to build a crate
+or multiple packages using cargo.
+
+This driver also contains multiple checks that will inspect the stable MIR of the crate that was compiled.
+In order to run those tests, you need to pass an extra argument `--smir-check`.
+
+Let's say we have a crate `input.rs`, in order to run all checks, one should invoke:
+
+```shell
+cargo run -p test-drive -- --smir-check test.rs
+# or run the test-drive directly where ${BIN_PATH} is the binary location
+${BIN_PATH}/test-drive --smir-check test.rs
+```
+
+In order to run this as part of a cargo build, you can run from a workspace:
+
+```shell
+# Only check SMIR for the target crate
+RUSTC=${BIN_PATH} cargo rustc -p ${TARGET_PACKAGE} -- --smir-check
+
+# Check SMIR for all crates being compiled, including dependencies
+RUSTC=${BIN_PATH} RUSTFLAGS=--smir-check cargo build
+```
+
+This driver accepts a few other options that are all preceded with `--smir` prefix[^outdated]. For example
+ - `--smir-fixme`: Will run checks that currently trigger a known bug in StableMIR.
+ - `--smir-verbose`: Print status and failure messages.
+
+[^outdated]: Since these are test tools, this documentation may be outdated.
+
+### Compiletest
+
+This is a little utility built on the top of `ui_test` crate.
+It scans our test folders and run tests according to the selected mode.
+For more details on how to run this utility, check its help option:
+```shell
+cargo run -p compiletest -- --help
+```
+
+## Test Suites
+
+We have a few different test suites for Stable MIR:
+  - **Rust compiler [`ui-fulldeps/stable-mir`](https://github. com/rust-lang/rust/tree/master/tests/ui-fulldeps/stable-mir):**
+    These are very simple sanity checks that live inside the main rust repository.
+    These tests should cover very basic functionality related to the translation of internal structures to stable ones.
+  - **Rust compiler suites:** We are enabling the execution of rust compiler test suites.
+  These suites are run with the rust respository compiletest. 
+  To run them, I recommend using our script `scripts/run_rustc_tests.sh` since there are a lot of arguments.
+  - **Local suites:** These are suites hosted in this repository inside `tests/`.
+  These are run using our local `compilest` and
+  `scripts/run_own_tests.sh` is a script to run all local suites
+    - `fixme`: Single file crates that are currently failing.
+      These tests are run using `--smir-fixme`.
+    - `sanity-checks`: Small examples that exercise some specific construct.
+      These tests succeed if compilation and all smir checks succeed.