Add support for the exists tag.

js/test_cases/exists/README.md

@@ -0,0 +1 @@
+Exists appears to follow the same truthiness rules that sections follow, rather than merely checking if a value exists.

js/test_cases/exists/input1.json

@@ -0,0 +1 @@
+{"things": ["Alice", "Bob", "Chris"]}

js/test_cases/exists/input10.json

@@ -0,0 +1 @@
+{"things": {}}

js/test_cases/exists/input11.json

@@ -0,0 +1 @@
+["cat", "dog"]

js/test_cases/exists/input2.json

@@ -0,0 +1 @@
+{"things": {"name": "Alice", "keyboard": "K-Type"}}

js/test_cases/exists/input3.json

@@ -0,0 +1 @@
+{"there_are_no_things": 4}

js/test_cases/exists/input4.json

@@ -0,0 +1 @@
+{"things": "just a string"}

js/test_cases/exists/input5.json

@@ -0,0 +1 @@
+{"things": false}

js/test_cases/exists/input6.json

@@ -0,0 +1 @@
+{"things": null}

js/test_cases/exists/input7.json

@@ -0,0 +1 @@
+{"things": 0}

js/test_cases/exists/input8.json

@@ -0,0 +1 @@
+{"things": ""}

js/test_cases/exists/input9.json

@@ -0,0 +1 @@
+{"things": []}

js/test_cases/exists/main.dust

@@ -0,0 +1,5 @@
+{?things}
+Thing: {things}
+{:else}
+No things {.}
+{/things}

src/renderer/renderer.rs

@@ -114,9 +114,6 @@ impl<'a> DustRenderer<'a> {
                     // an empty array or null), Dust uses the
                     // original context before walking the path as
                     // the context for rendering the else block
-                    //
-                    // TODO: do filters apply? I don't think so
-                    // but I should test
                     return match &container.else_contents {
                         Some(body) => self.render_body(&body, breadcrumbs),
                         None => Ok("".to_owned()),
@@ -139,6 +136,21 @@ impl<'a> DustRenderer<'a> {
                     }
                 }
             }
+            DustTag::DTExists(container) => {
+                let val = walk_path(breadcrumbs, &container.path.keys);
+                let loop_elements: Vec<&dyn ContextElement> = self.get_loop_elements(val)?;
+                if loop_elements.is_empty() {
+                    return match &container.else_contents {
+                        Some(body) => self.render_body(&body, breadcrumbs),
+                        None => Ok("".to_owned()),
+                    };
+                } else {
+                    return match &container.contents {
+                        None => Ok("".to_owned()),
+                        Some(body) => self.render_body(&body, breadcrumbs),
+                    };
+                }
+            }
             _ => (), // TODO: Implement the rest
         }
         Ok("".to_owned())