Describes restrictions on generic parameters for php_class (#194)

Author: ju1ius

guide/src/macros/classes.md

@@ -32,6 +32,34 @@ You can rename the property with options:
 - `rename` - Allows you to rename the property, e.g.
   `#[prop(rename = "new_name")]`
 
+## Restrictions
+
+### No lifetime parameters
+
+Rust lifetimes are used by the Rust compiler to reason about a program's memory safety.
+They are a compile-time only concept;
+there is no way to access Rust lifetimes at runtime from a dynamic language like PHP.
+
+As soon as Rust data is exposed to PHP,
+there is no guarantee which the Rust compiler can make on how long the data will live.
+PHP is a reference-counted language and those references can be held
+for an arbitrarily long time, which is untraceable by the Rust compiler.
+The only possible way to express this correctly is to require that any `#[php_class]`
+does not borrow data for any lifetime shorter than the `'static` lifetime,
+i.e. the `#[php_class]` cannot have any lifetime parameters.
+
+When you need to share ownership of data between PHP and Rust,
+instead of using borrowed references with lifetimes, consider using
+reference-counted smart pointers such as [Arc](https://doc.rust-lang.org/std/sync/struct.Arc.html).
+
+### No generic parameters
+
+A Rust struct `Foo<T>` with a generic parameter `T` generates new compiled implementations
+each time it is used with a different concrete type for `T`.
+These new implementations are generated by the compiler at each usage site.
+This is incompatible with wrapping `Foo` in PHP,
+where there needs to be a single compiled implementation of `Foo` which is integrated with the PHP interpreter.
+
 ## Example
 
 This example creates a PHP class `Human`, adding a PHP property `address`.