Add callable parameter attributes

Author: Kirill Nesmeyanov

Writerside/labels.list

@@ -22,4 +22,5 @@
     <secondary-label id="wip" name="WIP" color="red">Work in progress</secondary-label>
     <secondary-label id="beta" name="β" color="tangerine">Beta</secondary-label>
     <secondary-label id="1.1" name="TypeLang 1.1" color="strawberry">Since type-lang/parser v1.1</secondary-label>
+    <secondary-label id="1.2" name="TypeLang 1.2" color="strawberry">Since type-lang/parser v1.2</secondary-label>
 </labels>

Writerside/topics/language/callable-types.md

@@ -231,3 +231,49 @@ _before the type_ or _before the parameter name._
 
 </tab>
 </tabs>
+
+
+## Attributes
+
+<secondary-label ref="1.2"/>
+
+Each callable parameter allows you to define list of additional attributes.
+An attribute is additional metadata for a parameter.
+
+<tabs>
+<tab title="Examples">
+
+> Simple attribute with one argument for each callable parameter.
+> ```typescript
+> Example\Functor(#[type<int8>] int $a): void
+> ```
+
+> Multiple attributes in one group.
+> ```typescript
+> Example\Functor(#[type<int8>, const] int&): void
+> ```
+
+> Multiple attribute groups.
+> ```typescript
+> Example\OnCreate(
+>     #[deprecated]
+>     #[inline]
+>     (callback(T): void) $callback=,
+> ): void
+> ```
+
+</tab>
+<tab title="Counterexamples">
+
+> Only valid identifiers are allowed.
+> ```typescript
+> Example\Functor(#[42] int $a): void
+> ```
+> ```
+> Syntax error, unexpected "42"
+> ```
+> {collapsible="true" collapsed-title="TypeLang\Parser\Exception\ParseException"}
+> {style="warning"}
+
+</tab>
+</tabs>