List prepend implementation with tests, and a few small code clarity changes to list_push implementation

compiler/builtins/src/std.rs

@@ -510,7 +510,7 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         ),
     );
 
-    // push : List elem -> elem -> List elem
+    // push : List elem, elem -> List elem
     add_type(
         Symbol::LIST_PUSH,
         SolvedType::Func(
@@ -519,6 +519,15 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         ),
     );
 
+    // prepend : List elem, elem -> List elem
+    add_type(
+        Symbol::LIST_PREPEND,
+        SolvedType::Func(
+            vec![list_type(flex(TVAR1)), flex(TVAR1)],
+            Box::new(list_type(flex(TVAR1))),
+        ),
+    );
+
     // single : a -> List a
     add_type(
         Symbol::LIST_SINGLE,

compiler/builtins/src/unique.rs

@@ -699,6 +699,36 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         )
     });
 
+    // prepend : Attr * (List a)
+    //      , a
+    //     -> Attr * (List a)
+    //
+    // NOTE: we demand the new item to have the same uniqueness as the other list items.
+    // It could be allowed to add unique items to shared lists, but that requires special code gen
+    add_type(Symbol::LIST_PREPEND, {
+        let_tvars! { a, star1, star2 };
+
+        unique_function(
+            vec![
+                SolvedType::Apply(
+                    Symbol::ATTR_ATTR,
+                    vec![
+                        flex(star1),
+                        SolvedType::Apply(Symbol::LIST_LIST, vec![flex(a)]),
+                    ],
+                ),
+                flex(a),
+            ],
+            SolvedType::Apply(
+                Symbol::ATTR_ATTR,
+                vec![
+                    boolean(star2),
+                    SolvedType::Apply(Symbol::LIST_LIST, vec![flex(a)]),
+                ],
+            ),
+        )
+    });
+
     // List.map does not need to check the container rule on the input list.
     // There is no way in which this signature can cause unique values to be duplicated
     //