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Rollup merge of #132085 - celinval:update-smir-doc, r=compiler-errors

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Update StableMIR doc to reflect current status

Update stable-mir documentation, since we no longer use git subtree, and we have 2 different crates.
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compiler/stable_mir/README.md
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@ -1,93 +1,33 @@
This crate is regularly synced with its mirror in the rustc repo at `compiler/rustc_smir`.
This crate is currently developed in-tree together with the compiler.
We use `git subtree` for this to preserve commits and allow the rustc repo to
edit these crates without having to touch this repo. This keeps the crates compiling
while allowing us to independently work on them here. The effort of keeping them in
sync is pushed entirely onto us, without affecting rustc workflows negatively.
This may change in the future, but changes to policy should only be done via a
compiler team MCP.
Our goal is to start publishing `stable_mir` into crates.io.
Until then, users will use this as any other rustc crate, by installing
the rustup component `rustc-dev`, and declaring `stable-mir` as an external crate.
## Instructions for working on this crate locally
Since the crate is the same in the rustc repo and here, the dependencies on rustc_* crates
will only either work here or there, but never in both places at the same time. Thus we use
optional dependencies on the rustc_* crates, requiring local development to use
```
cargo build --no-default-features -Zavoid-dev-deps
```
in order to compile successfully.
## Instructions for syncing
### Updating this repository
In the rustc repo, execute
```
git subtree push --prefix=compiler/rustc_smir url_to_your_fork_of_project_stable_mir some_feature_branch
```
and then open a PR of your `some_feature_branch` against https://github.com/rust-lang/project-stable-mir
### Updating the rustc library
First we need to bump our stack limit, as the rustc repo otherwise quickly hits that:
```
ulimit -s 60000
```
#### Maximum function recursion depth (1000) reached
Then we need to disable `dash` as the default shell for sh scripts, as otherwise we run into a
hard limit of a recursion depth of 1000:
```
sudo dpkg-reconfigure dash
```
and then select `No` to disable dash.
#### Patching your `git worktree`
The regular git worktree does not scale to repos of the size of the rustc repo.
So download the `git-subtree.sh` from https://github.com/gitgitgadget/git/pull/493/files and run
```
sudo cp --backup /path/to/patched/git-subtree.sh /usr/lib/git-core/git-subtree
sudo chmod --reference=/usr/lib/git-core/git-subtree~ /usr/lib/git-core/git-subtree
sudo chown --reference=/usr/lib/git-core/git-subtree~ /usr/lib/git-core/git-subtree
```
#### Actually doing a sync
In the rustc repo, execute
```
git subtree pull --prefix=compiler/rustc_smir https://github.com/rust-lang/project-stable-mir smir
```
Note: only ever sync to rustc from the project-stable-mir's `smir` branch. Do not sync with your own forks.
Then open a PR against rustc just like a regular PR.
See the StableMIR ["Getting Started"](https://rust-lang.github.io/project-stable-mir/getting-started.html)
guide for more information.
## Stable MIR Design
The stable-mir will follow a similar approach to proc-macro2. It’s
implementation will eventually be broken down into two main crates:
The stable-mir will follow a similar approach to proc-macro2. Its
implementation is split between two main crates:
- `stable_mir`: Public crate, to be published on crates.io, which will contain
the stable data structure as well as proxy APIs to make calls to the
compiler.
- `rustc_smir`: The compiler crate that will translate from internal MIR to
SMIR. This crate will also implement APIs that will be invoked by
stable-mir to query the compiler for more information.
the stable data structure as well as calls to `rustc_smir` APIs. The
translation between stable and internal constructs will also be done in this crate,
however, this is currently implemented in the `rustc_smir` crate.[^translation].
- `rustc_smir`: This crate implements the public APIs to the compiler.
It is responsible for gathering all the information requested, and providing
the data in its unstable form.
This will help tools to communicate with the rust compiler via stable APIs. Tools will depend on
`stable_mir` crate, which will invoke the compiler using APIs defined in `rustc_smir`. I.e.:
[^translation]: This is currently implemented in the `rustc_smir` crate,
but we are working to change that.
I.e.,
tools will depend on `stable_mir` crate,
which will invoke the compiler using APIs defined in `rustc_smir`.
I.e.:
```
┌──────────────────────────────────┐ ┌──────────────────────────────────┐
@ -104,9 +44,3 @@ This will help tools to communicate with the rust compiler via stable APIs. Tool
More details can be found here:
https://hackmd.io/XhnYHKKuR6-LChhobvlT-g?view
For now, the code for these two crates are in separate modules of this crate.
The modules have the same name for simplicity. We also have a third module,
`rustc_internal` which will expose APIs and definitions that allow users to
gather information from internal MIR constructs that haven't been exposed in
the `stable_mir` module.