Cypress: allow enabling Rust Crypto via env var (#10951)

* Cypress: allow enabling Rust Crypto via env var

* prettify

cypress/support/config.json.ts

@@ -37,5 +37,13 @@ const CONFIG_JSON = {
 };
 
 beforeEach(() => {
-    cy.intercept({ method: "GET", pathname: "/config.json" }, { body: CONFIG_JSON });
+    const configJson = CONFIG_JSON;
+
+    // configure element to use rust crypto if the env var tells us so
+    if (Cypress.env("RUST_CRYPTO")) {
+        configJson["features"] = {
+            feature_rust_crypto: true,
+        };
+    }
+    cy.intercept({ method: "GET", pathname: "/config.json" }, { body: configJson });
 });

docs/cypress.md

@@ -45,6 +45,16 @@ To launch it:
 yarn run test:cypress:open
 ```
 
+### Running with Rust cryptography
+
+`matrix-js-sdk` is currently in the
+[process](https://github.com/vector-im/element-web/issues/21972) of being
+updated to replace its end-to-end encryption implementation to use the [Matrix
+Rust SDK](https://github.com/matrix-org/matrix-rust-sdk). This is not currently
+enabled by default, but it is possible to have Cypress configure Element to use
+the Rust crypto implementation by setting the environment variable
+`CYPRESS_RUST_CRYPTO=1`.
+
 ## How the Tests Work
 
 Everything Cypress-related lives in the `cypress/` subdirectory of react-sdk