Implement the first and last helpers.

js/test_cases/helpers_first/input1.json

@@ -0,0 +1,23 @@
+{
+  "people": [
+    {
+      "name": "Alice",
+      "pet": "cat"
+    },
+    {
+      "name": "Bob",
+      "pet": "dog"
+    },
+    {
+      "name": "Chris",
+      "pet": "lizard"
+    }
+  ],
+  "toys": [
+    "ball",
+    "bone"
+  ],
+  "scalar": 7,
+  "name": "global name",
+  "pet": "global pet"
+}

js/test_cases/helpers_first/main.dust

@@ -0,0 +1,29 @@
+Tags inside a first{~n}
+==================={~n}
+{#people}
+    {name}{@first petname="fluffy"},{pet}{petname}{/first}
+{/people}{~n}
+
+first inside a scalar{~n}
+====================={~n}
+{#scalar}
+    {name}{@first petname="fluffy"},{pet}{petname}{/first}
+{/scalar}{~n}
+
+Nested first inside another non-array section{~n}
+============================================={~n}
+{#people}
+    {#toys}
+        {name}'s pet {pet} plays with a {.}{@first}, {/first}
+    {/toys}
+{/people}{~n}
+
+Else block inside a first{~n}
+========================={~n}
+{#people}
+    {name}{@first},{pet}{petname}{:else}elseblock{/first}
+{/people}{~n}
+
+Sep outside any section{~n}
+======================={~n}
+{@first}first is printed outside any section{/first}

js/test_cases/helpers_last/input1.json

@@ -0,0 +1,23 @@
+{
+  "people": [
+    {
+      "name": "Alice",
+      "pet": "cat"
+    },
+    {
+      "name": "Bob",
+      "pet": "dog"
+    },
+    {
+      "name": "Chris",
+      "pet": "lizard"
+    }
+  ],
+  "toys": [
+    "ball",
+    "bone"
+  ],
+  "scalar": 7,
+  "name": "global name",
+  "pet": "global pet"
+}

js/test_cases/helpers_last/main.dust

@@ -0,0 +1,29 @@
+Tags inside a last{~n}
+=================={~n}
+{#people}
+    {name}{@last petname="fluffy"},{pet}{petname}{/last}
+{/people}{~n}
+
+last inside a scalar{~n}
+===================={~n}
+{#scalar}
+    {name}{@last petname="fluffy"},{pet}{petname}{/last}
+{/scalar}{~n}
+
+Nested last inside another non-array section{~n}
+============================================{~n}
+{#people}
+    {#toys}
+        {name}'s pet {pet} plays with a {.}{@last}, {/last}
+    {/toys}
+{/people}{~n}
+
+Else block inside a last{~n}
+========================{~n}
+{#people}
+    {name}{@last},{pet}{petname}{:else}elseblock{/last}
+{/people}{~n}
+
+Sep outside any section{~n}
+======================={~n}
+{@last}last is printed outside any section{/last}

src/renderer/renderer.rs

@@ -694,8 +694,90 @@ impl<'a> DustRenderer<'a> {
                     }
                 }
             }
-            DustTag::DTHelperFirst(parameterized_block) => {}
-            DustTag::DTHelperLast(parameterized_block) => {}
+            DustTag::DTHelperFirst(parameterized_block) => {
+                let new_breadcrumbs = self.new_breadcrumbs_partial(
+                    breadcrumbs,
+                    breadcrumbs,
+                    None,
+                    &parameterized_block.explicit_context,
+                );
+
+                let index = self.get(breadcrumbs, &vec!["$idx"]);
+                let len = self.get(breadcrumbs, &vec!["$len"]);
+                match (index, len) {
+                    (Err(_), _) | (_, Err(_)) => return Ok("".to_owned()),
+                    (Ok(index_resolved), Ok(len_resolved)) => {
+                        // Iteration contexts use OwnedLiteral::LPositiveinteger
+                        let index_cast = index_resolved
+                            .get_context_element_reference()
+                            .to_any()
+                            .downcast_ref::<OwnedLiteral>();
+                        let len_cast = len_resolved
+                            .get_context_element_reference()
+                            .to_any()
+                            .downcast_ref::<OwnedLiteral>();
+                        match (index_cast, len_cast) {
+                            (
+                                Some(OwnedLiteral::LPositiveInteger(index_number)),
+                                Some(OwnedLiteral::LPositiveInteger(len_number)),
+                            ) => {
+                                if *index_number != 0 {
+                                    return Ok("".to_owned());
+                                } else {
+                                    return self.render_maybe_body(
+                                        &parameterized_block.contents,
+                                        new_breadcrumbs.as_ref().unwrap_or(breadcrumbs),
+                                        blocks,
+                                    );
+                                }
+                            }
+                            _ => return Ok("".to_owned()),
+                        }
+                    }
+                }
+            }
+            DustTag::DTHelperLast(parameterized_block) => {
+                let new_breadcrumbs = self.new_breadcrumbs_partial(
+                    breadcrumbs,
+                    breadcrumbs,
+                    None,
+                    &parameterized_block.explicit_context,
+                );
+
+                let index = self.get(breadcrumbs, &vec!["$idx"]);
+                let len = self.get(breadcrumbs, &vec!["$len"]);
+                match (index, len) {
+                    (Err(_), _) | (_, Err(_)) => return Ok("".to_owned()),
+                    (Ok(index_resolved), Ok(len_resolved)) => {
+                        // Iteration contexts use OwnedLiteral::LPositiveinteger
+                        let index_cast = index_resolved
+                            .get_context_element_reference()
+                            .to_any()
+                            .downcast_ref::<OwnedLiteral>();
+                        let len_cast = len_resolved
+                            .get_context_element_reference()
+                            .to_any()
+                            .downcast_ref::<OwnedLiteral>();
+                        match (index_cast, len_cast) {
+                            (
+                                Some(OwnedLiteral::LPositiveInteger(index_number)),
+                                Some(OwnedLiteral::LPositiveInteger(len_number)),
+                            ) => {
+                                if *index_number != len_number - 1 {
+                                    return Ok("".to_owned());
+                                } else {
+                                    return self.render_maybe_body(
+                                        &parameterized_block.contents,
+                                        new_breadcrumbs.as_ref().unwrap_or(breadcrumbs),
+                                        blocks,
+                                    );
+                                }
+                            }
+                            _ => return Ok("".to_owned()),
+                        }
+                    }
+                }
+            }
         }
 
         Ok("".to_owned())