Removes two entry points for memory leaks

Sources/GraphQL/Language/AST.swift

@@ -91,7 +91,7 @@ public final class Token: @unchecked Sendable {
      * including ignored tokens. <SOF> is always the first node and <EOF>
      * the last.
      */
-    public let prev: Token?
+    public private(set) weak var prev: Token?
     public internal(set) var next: Token?
 
     init(

Sources/GraphQL/Validation/ValidationContext.swift

@@ -176,9 +176,10 @@ public final class ValidationContext: ASTValidationContext {
         recursiveVariableUsages = [:]
 
         super.init(ast: ast) { _ in }
-        onError = { error in
-            self.errors.append(error)
-        }
+    }
+
+    public override func report(error: GraphQLError) {
+        errors.append(error)
     }
 
     func getSchema() -> GraphQLSchema? {

Tests/GraphQLTests/GraphQLMemoryRetentionTests.swift

@@ -0,0 +1,65 @@
+import Testing
+
+@testable import GraphQL
+
+// Holds a weak reference to a Token so we can observe when ARC frees it.
+private final class WeakTokenBox {
+    weak var token: Token?
+}
+
+@Suite struct GraphQLMemoryRetentionTests {
+    private func makeSchema() throws -> GraphQLSchema {
+        try GraphQLSchema(
+            query: GraphQLObjectType(
+                name: "Query",
+                fields: ["hello": GraphQLField(type: GraphQLString)]
+            )
+        )
+    }
+
+    // Regression test for Token.prev being a strong reference.
+    //
+    // Adjacent tokens in the linked list formed a mutual retain cycle:
+    // SOF.next → T1 and T1.prev → SOF. When the Document went out of scope,
+    // neither token could be freed. Making prev `weak` breaks the cycle.
+    @Test func tokenChainIsReleasedAfterDocumentGoesOutOfScope() throws {
+        let box = WeakTokenBox()
+
+        func parseAndCapture() throws {
+            let document = try parse(source: "{ hello }")
+            // startToken is SOF (no prev). Capture the next token, which
+            // has a .prev back to SOF — the exact edge the fix targets.
+            box.token = document.loc?.startToken.next
+            #expect(box.token != nil)
+        }
+
+        try parseAndCapture()
+        #expect(box.token == nil)
+    }
+
+    // Regression test for ValidationContext.init capturing `self` in a closure.
+    //
+    // The onError closure stored on ASTValidationContext captured ValidationContext
+    // strongly, creating a cycle that prevented the context (and the Document it
+    // holds) from ever being freed. Overriding report(error:) instead eliminates
+    // the closure and the cycle.
+    //
+    // SOF (startToken) has no .prev, so it has no token-chain cycle. If it is
+    // retained after this scope it must be because ValidationContext leaked and
+    // is still holding a copy of the Document.
+    @Test func validationContextReleasesDocumentAfterValidation() throws {
+        let schema = try makeSchema()
+        let box = WeakTokenBox()
+
+        func validateAndCapture() throws {
+            let document = try parse(source: "{ hello }")
+            box.token = document.loc?.startToken  // SOF — no prev, no token cycle
+            let errors = validate(schema: schema, ast: document)
+            #expect(errors.isEmpty)
+            #expect(box.token != nil)
+        }
+
+        try validateAndCapture()
+        #expect(box.token == nil)
+    }
+}