Add Generic Associated Types support (#124)

* tests: put tests for GAT

* feat: support GAT

* tests: add normal type parameter to generic associated type

* tests: add checks for type binding

* feat: support type binding to have type arguments

* chore: regenerate parser

* chore: fix formatting

Author: kawaemon

corpus/declarations.txt

@@ -1128,6 +1128,89 @@ pub trait Graph {
       (associated_type (type_identifier) (trait_bounds (scoped_type_identifier (identifier) (type_identifier))))
       (associated_type (type_identifier)))))
 
+============================================
+Generic Associated Types
+============================================
+
+pub trait Database {
+    type F<'a, D>: Future<Output = D> + 'a;
+}
+
+impl Database for Foo {
+    type F<'a, D> = DatabaseFuture<'a, D>;
+}
+
+fn use_database1<D: Database<F<'a, TD> = F>>() {}
+
+fn use_database2<D>()
+where
+    D: Database<F<'a, TD> = F>,
+{}
+
+---
+
+(source_file
+  (trait_item
+    (visibility_modifier)
+    (type_identifier)
+    (declaration_list
+      (associated_type
+        (type_identifier)
+        (type_parameters (lifetime (identifier)) (type_identifier))
+        (trait_bounds
+          (generic_type
+            (type_identifier)
+            (type_arguments (type_binding (type_identifier) (type_identifier))))
+          (lifetime (identifier))))))
+
+  (impl_item
+    (type_identifier)
+    (type_identifier)
+    (declaration_list
+      (type_item
+        (type_identifier)
+        (type_parameters (lifetime (identifier)) (type_identifier))
+        (generic_type
+          (type_identifier)
+          (type_arguments (lifetime (identifier)) (type_identifier))))))
+
+  (function_item
+    (identifier)
+    (type_parameters
+      (constrained_type_parameter
+        (type_identifier)
+        (trait_bounds
+          (generic_type
+            (type_identifier)
+            (type_arguments
+              (type_binding
+                (type_identifier)
+                (type_arguments
+                  (lifetime (identifier))
+                  (type_identifier))
+                (type_identifier)))))))
+    (parameters)
+    (block))
+
+  (function_item
+    (identifier)
+      (type_parameters (type_identifier))
+      (parameters)
+      (where_clause
+        (where_predicate
+          (type_identifier)
+          (trait_bounds
+            (generic_type
+              (type_identifier)
+              (type_arguments
+                 (type_binding
+                   (type_identifier)
+                   (type_arguments
+                     (lifetime (identifier))
+                     (type_identifier))
+                   (type_identifier)))))))
+    (block)))
+
 =====================
 Higher-ranked types
 =====================

grammar.js

@@ -481,6 +481,7 @@ module.exports = grammar({
     associated_type: $ => seq(
       'type',
       field('name', $._type_identifier),
+      field('type_parameters', optional($.type_parameters)),
       field('bounds', optional($.trait_bounds)),
       ';'
     ),
@@ -786,6 +787,7 @@ module.exports = grammar({
 
     type_binding: $ => seq(
       field('name', $._type_identifier),
+      field('type_arguments', optional($.type_arguments)),
       '=',
       field('type', $._type)
     ),
@@ -1450,7 +1452,7 @@ module.exports = grammar({
     self: $ => 'self',
     super: $ => 'super',
     crate: $ => 'crate',
-    
+
     metavariable: $ => /\$[a-zA-Z_]\w*/
   }
 })

src/grammar.json

@@ -2728,6 +2728,22 @@
             "name": "_type_identifier"
           }
         },
+        {
+          "type": "FIELD",
+          "name": "type_parameters",
+          "content": {
+            "type": "CHOICE",
+            "members": [
+              {
+                "type": "SYMBOL",
+                "name": "type_parameters"
+              },
+              {
+                "type": "BLANK"
+              }
+            ]
+          }
+        },
         {
           "type": "FIELD",
           "name": "bounds",
@@ -4429,6 +4445,22 @@
             "name": "_type_identifier"
           }
         },
+        {
+          "type": "FIELD",
+          "name": "type_arguments",
+          "content": {
+            "type": "CHOICE",
+            "members": [
+              {
+                "type": "SYMBOL",
+                "name": "type_arguments"
+              },
+              {
+                "type": "BLANK"
+              }
+            ]
+          }
+        },
         {
           "type": "STRING",
           "value": "="

src/node-types.json

@@ -607,6 +607,16 @@
             "named": true
           }
         ]
+      },
+      "type_parameters": {
+        "multiple": false,
+        "required": false,
+        "types": [
+          {
+            "type": "type_parameters",
+            "named": true
+          }
+        ]
       }
     }
   },
@@ -3923,6 +3933,16 @@
             "named": true
           }
         ]
+      },
+      "type_arguments": {
+        "multiple": false,
+        "required": false,
+        "types": [
+          {
+            "type": "type_arguments",
+            "named": true
+          }
+        ]
       }
     }
   },