:label supports Civet reserved words (but not JS reserved words)

civet.dev/reference.md

@@ -2102,7 +2102,9 @@ while item?
 
 ::: info
 Labels have the colon on the left to avoid conflict with implicit object
-literals.  The colons are optional in `break` and `continue`.
+literals.  The colons are optional in `break` and `continue`,
+except for Civet reserved words (e.g. `and`) where a colon is required.
+JavaScript reserved words are invalid as labels.
 :::
 
 ### Controlling Loop Value

source/parser.hera

@@ -4460,7 +4460,10 @@ LabelledStatement
 
 Label
   # NOTE: `:label` instead of `label:` to not clash with implicit object literal
-  Colon:colon Identifier:id Whitespace:w ->
+  Colon:colon IdentifierName:id Whitespace:w ->
+    // If `id` is a JavaScript reserved word, JS or TS will output an error.
+    // `:void` is sometimes useful as a return-type annotation, so leave that.
+    if (id.name === "void") return $skip
     return {
       type: "Label",
       name: id.name,
@@ -4469,8 +4472,13 @@ Label
 
 # Argument to break/continue, which can include colon or not in input,
 # but should not have colon in output
+# Colon is required if the identifier is a Civet keyword,
+# except for 'loop' which we special-case
 LabelIdentifier
-  Colon? Identifier:id -> id
+  Colon IdentifierName:id -> id
+  # Infinite loop of break/continue isn't helpful, so treat as label
+  &Loop IdentifierName:id -> id
+  Identifier
 
 LabelledItem
   Statement

test/label.civet

@@ -1,4 +1,4 @@
-{testCase, wrapper} from ./helper.civet
+{testCase, throws, wrapper} from ./helper.civet
 
 describe "labels", ->
   testCase """
@@ -107,3 +107,34 @@ describe "labels", ->
       }
     }
   """
+
+  testCase """
+    reserved word label
+    ---
+    :and while x
+      break :and
+    ---
+    and: while (x) {
+      break and
+    }
+  """
+
+  throws """
+    reserved word label without colon
+    ---
+    :and while x
+      break and
+    ---
+    ParseError
+  """
+
+  testCase """
+    loop label
+    ---
+    :loop while x
+      break loop
+    ---
+    loop: while (x) {
+      break loop
+    }
+  """