fix(render): reject empty tag paths, sharpen the raw-form test

F1: test_no_raw_or_partial_form's first assertion only excluded one
literal shape (Ok [Var ["{name"]]), so it could not actually catch a
future raw/unescaped constructor under a different name. Replace it
with an assertion of the real parse result for {{{name}}}
(Ok [Var ["{name"]; Text "}"]), documented behaviour rather than a
guarantee this test cannot check -- the real guarantee is structural:
node has exactly four constructors and none of them is raw.

F2: {{.}}, {{#}}, {{^}} and {{/}} used to parse to a Var/Section/
Inverted with an empty path, reachable but never designed. This engine
has no "current context" for a bare dot to mean, so a bare-dot or
empty-sigil path is now a parse error at lex time, covering all four
sigil forms via one path helper. The existing "empty tag {{}}" branch
is unchanged and still reachable (a fully empty body is a distinct
case from a sigil with an empty path).

1 files changed, 14 insertions, 4 deletions

diff --git a/lib/render/template.ml b/lib/render/template.ml
index 8de91a6..3e35d53 100644
--- a/lib/render/template.ml
+++ b/lib/render/template.ml
@@ -38,13 +38,23 @@ let lex src =
             let b = trim body in
             if b = "" then Error "empty tag {{}}"
             else
+              (* Every sigil form (var, #, ^, /) resolves a dotted path; an
+                 empty path ("{{.}}", "{{#}}", "{{^}}", "{{/}}") names
+                 nothing -- there is no "current context" concept for a bare
+                 dot to mean, so it is a parse error rather than an invented
+                 behaviour Task 3 would have to define. *)
+              let with_path ctor rest =
+                match path rest with
+                | [] -> Error "empty tag path: {{.}} and {{#}} name nothing"
+                | p -> Ok (ctor p)
+              in
               match b.[0] with
-              | '#' -> Ok (TOpen (path (String.sub b 1 (String.length b - 1))))
-              | '^' -> Ok (TInv (path (String.sub b 1 (String.length b - 1))))
-              | '/' -> Ok (TClose (path (String.sub b 1 (String.length b - 1))))
+              | '#' -> with_path (fun p -> TOpen p) (String.sub b 1 (String.length b - 1))
+              | '^' -> with_path (fun p -> TInv p) (String.sub b 1 (String.length b - 1))
+              | '/' -> with_path (fun p -> TClose p) (String.sub b 1 (String.length b - 1))
               | '!' -> Ok TComment
               | '>' -> Error "partials are not supported: a template may not include another file"
-              | _ -> Ok (TVar (path b))
+              | _ -> with_path (fun p -> TVar p) b
           in
           (match tag with
            | Error e -> Error e