Document current limitations

README.md

@@ -64,6 +64,7 @@ It supports all versions of PHP from 5.3 and major open-source projects:
 
 - [Explore 660+ Rector Rules](/docs/rector_rules_overview.md)
 - [How Does Rector Work?](/docs/how_it_works.md)
+- [Current Limitations](/docs/limitations.md)
 - [PHP Parser Nodes](https://github.com/rectorphp/php-parser-nodes-docs/blob/master/docs/nodes_overview.md)
 
 ### Advanced
@@ -81,14 +82,19 @@ It supports all versions of PHP from 5.3 and major open-source projects:
 
 ## Install
 
+The currently recommended way of installing Rector is as a Composer `dev` dependency:
+
 ```bash
 composer require rector/rector --dev
 ```
 
-- Having conflicts during `composer require`? → Use the [Rector Prefixed](https://github.com/rectorphp/rector-prefixed)
-- Using a different PHP version than Rector supports? → Use the [Docker image](/docs/how_to_run_rector_in_docker.md)
+If that does not work for you
+* due to version conflicts,
+* minimum PHP requirements,
+* does not give you a suitably recent version of Rector or
+* you do not want to install tools like Rector as Composer dependencies,
 
-<br>
+please make sure to review the [current limitations and installation alternatives](/docs/limitations.md).
 
 ## Running Rector
 

docs/how_to_run_rector_in_docker.md

@@ -3,6 +3,8 @@
 You can run Rector on your project using Docker.
 To make sure you are running latest version, use `docker pull rector/rector`.
 
+In case of problems, please make sure to review the [current limitations and installation alternatives](/docs/limitations.md).
+
 *Note that Rector inside Docker expects your application in `/project` directory - it is mounted via volume from the current directory (`$pwd`).*
 
 ```bash

docs/limitations.md

@@ -0,0 +1,69 @@
+# Current Limitations and Installation Alternatives
+
+At least until [#3490](https://github.com/rectorphp/rector/issues/3490) is resolved, Rector relies on _runtime PHP reflection_ to analyze your source code. That means the code Rector works on cannot merely be treated as _data_, but has to be loaded (through the autoloader) into memory to be analyzed by the PHP interpreter.
+
+> Read the [PHPStan 0.12.26 release announcement](https://phpstan.org/blog/zero-config-analysis-with-static-reflection) to learn more about runtime and static reflection. PHPStan is the static analysis tool used by Rector.
+
+This brings a few limitations and affects your choices of how you can run Rector.
+
+## PHP Language Level
+
+The PHP process that runs Rector code also needs to load and parse your source code. So, you need to run a version of PHP that is both compatible with your project code as well as Rector.
+
+This means you cannot run Rector on PHP 8 to work on a project that uses language features limited to previous PHP versions.
+
+> One example for this is [array and string offset access using curly braces](https://www.php.net/manual/de/migration74.deprecated.php#migration74.deprecated.core.array-string-access-curly-brace).
+
+But also the other way round, you cannot run Rector on PHP 7 to analyze a project with PHP 8 language features, like [constructor property promotion](https://stitcher.io/blog/constructor-promotion-in-php-8).
+
+Together with Rector's minimum PHP 7.3 version requirement, this might limit your options of how to run Rector.
+
+Note that using an older version of Rector might allow you to run a version of PHP <7.3.
+
+## Code Collisions
+
+Since Rector needs to load the code it works on in addition to the code it needs to run itself, conflicts may occur.
+
+One practical example of this is when you're running Rector on a project that uses an older version of Symfony. Since Rector uses a few Symfony Components itself, you may either run into problems when classes are re-declared, or when Rector happens to load and later (by accident) run parts of Symfony from your project installation. In the later case, the mix-up of different versions may lead to generally unpredictable results.
+
+To remedy this situation, [Rector Prefixed](https://github.com/rectorphp/rector-prefixed) has been created. Basically, it takes Rector and all of its dependencies and moves _all_ that code under a unique namespace prefix.
+
+(Add a note here: Drawbacks? Why isn't Rector-prefixed _the_ recommended approach?)
+
+## Installation Options
+
+With these limitations in mind, here are your installation options.
+
+### 1. Direct Composer install
+
+```bash
+composer require rector/rector --dev
+```
+
+Composer will try to find a version of Rector compatible with your current version of PHP. Also, since it will resolve a _single_ version for every dependency your project and Rector have in common, code collisions will not occur.
+
+The downside is that having Rector installed this way might affect the dependency versions your project will be able to use.
+
+### 2. Use Composer to install Rector Prefixed
+
+```bash
+composer require rector/rector-prefixed --dev
+```
+
+Since Rector Prefixed brings along most of the code it needs in a separate namespace, the impact and resulting limitations for your project's requirements are minimal.
+
+As of writing, the only additional dependency requirement is on `phpstan/phpstan`. But apart from that, Composer should be able to install it with whatever packages your project requires.
+
+If you need Rector `^0.9.0`, however, you need to run at least PHP 7.3.
+
+### 3. Use Docker to run Rector
+
+When you're using a different version of PHP than the desired version of Rector supports, you can run one of the Docker images we provide.
+
+See the [documentation on running in Docker](/docs/how_to_run_rector_in_docker.md) on how to do this.
+
+There is one caveat, though: As of writing, the [official Rector Docker images](https://hub.docker.com/r/rector/rector/tags?page=1&ordering=last_updated) are built with PHP 8.0 and use the non-prefixed version of Rector.
+
+So, you might run into problems with these images when your code uses language features not compatible with PHP 8, or when there is a collision between your project's and Rector's dependencies.
+
+[Work is underway](https://github.com/rectorphp/rector/pull/5667) to improve this situation and to provide Docker images with alternative PHP versions and/or a prefixed version of Rector.