Add support for `let assert ... as` (#107)

grammar.js

@@ -357,23 +357,27 @@ module.exports = grammar({
         optional(field("arguments", $.arguments))
       ),
     todo: ($) =>
-      seq(
-        "todo",
-        optional(
-          choice(
-            // DEPRECATED: The 'as' syntax was introduced in v0.30.
-            seq("(", field("message", $.string), ")"),
-            seq("as", field("message", $._expression))
+      prec.left(
+        seq(
+          "todo",
+          optional(
+            choice(
+              // DEPRECATED: The 'as' syntax was introduced in v0.30.
+              seq("(", field("message", $.string), ")"),
+              seq("as", field("message", $._expression))
+            )
           )
         )
       ),
     panic: ($) =>
-      seq(
-        "panic",
-        optional(
-          choice(
-            seq("(", field("message", $.string), ")"),
-            seq("as", field("message", $._expression))
+      prec.left(
+        seq(
+          "panic",
+          optional(
+            choice(
+              seq("(", field("message", $.string), ")"),
+              seq("as", field("message", $._expression))
+            )
           )
         )
       ),
@@ -488,7 +492,13 @@ module.exports = grammar({
       ),
     _case_clause_tuple_access: ($) =>
       seq(field("tuple", $.identifier), ".", field("index", $.integer)),
-    let_assert: ($) => seq("let", "assert", $._assignment),
+    let_assert: ($) =>
+      seq(
+        "let",
+        "assert",
+        $._assignment,
+        optional(seq("as", field("message", $._expression)))
+      ),
     let: ($) => seq("let", $._assignment),
     use: ($) =>
       seq(

test/corpus/functions.txt

@@ -277,6 +277,8 @@ fn assert_assignment() {
   }
   let assert y = x
   let assert #(x, _) = #(1, 2)
+  let assert Ok(_) = Ok(10) as "This never fails"
+  let assert 10 = 11 as { "message1" <> "message2" }
 }
 fn assignment() {
   let x = {
@@ -504,7 +506,29 @@ fn field_access(x) {
           (discard))
         value: (tuple
           (integer)
-          (integer)))))
+              (integer)))
+          (let_assert
+            pattern: (record_pattern
+              name: (constructor_name)
+              arguments: (record_pattern_arguments
+                (record_pattern_argument
+                  pattern: (discard))))
+            value: (record
+              name: (constructor_name)
+              arguments: (arguments
+                (argument
+                  value: (integer))))
+            message: (string
+              (quoted_content)))
+          (let_assert
+            pattern: (integer)
+            value: (integer)
+            message: (block
+              (binary_expression
+                left: (string
+                  (quoted_content))
+                right: (string
+                  (quoted_content)))))))
   (function
     name: (identifier)
     parameters: (function_parameters)