Convert SafeMarkup::checkPlain() in render arrays to #plain_text

+++ b/core/modules/rest/src/Plugin/views/display/RestExport.php
@@ -318,10 +318,10 @@ public function render() {
-      $build['#markup'] = SafeMarkup::checkPlain($build['#markup']);
+      $build['#markup'] = Html::escape($build['#markup']);

This use #plain_text and unset #markup.

This implements #9, also looked for any further possible conversions and found another 2

@stefan.r I think the DiffFormatter changes will cause double escaping in the the config diff. The parent issue contains a test for this. I'd rather do the very simple and easy to follow changes here than make the changes to DiffFormatter without testing.

Needs for #11

Let's see if this fixes the test fail

I'm wondering whether we should here think about whether using checkPlain or the corresponding #plain_text is the right thing to do for all those places ...
but I guess at least for now it is the most practical thing to just convert and rethink the usecases later ...

@dawehner yeah - this patch is functionally equivalent to HEAD - I think we should just do the conversions are be done.

#18 was also meant to RTBC this.

+1 for the RTBC

@Cottser the problem with that test is that is it testing the render array rather than the output - we have tests that confirm #plain_text will always be escaped and we're testing here that #plain_text is set. Afaics there is no regression here.

+++ b/core/modules/rest/src/Tests/Views/StyleSerializerTest.php
@@ -480,16 +480,16 @@ public function testLivePreview() {
-    $rendered_json = $build['#markup'];
-    $this->assertEqual($rendered_json, $expected, 'Ensure the previewed json is escaped.');
+    $rendered_json = $build['#plain_text'];
+    $this->assertTrue(!isset($build['#markup']) && $rendered_json == $expected, 'Ensure the previewed json is escaped.');

@@ -505,7 +505,7 @@ public function testLivePreview() {
-    $rendered_xml = $build['#markup'];
+    $rendered_xml = $build['#plain_text'];
     $this->assertEqual($rendered_xml, $expected, 'Ensure we preview xml when we change the request format.');

These are the lines that prove we don't have a regression.

I had that same thought, technically we could render it if we're unsure #plain_text works as expected, I think we did something similar in the checkPlain() issue? @cottser feel free to update if you feel it makes the test stronger.

Cool

I also think the straight conversion is OK here.

Committed/pushed to 8.0.x, thanks!