parser: improve error message for missing semicolon in let/{}

In the same vein as be18b7dc2¹.

Before:
  error: syntax error, expecting ';'

After:
  error: syntax error, expecting ';' to end binding

[1]: be18b7dc25

Change-Id: Iffbcb113d2b892a50c646c9875e970376a6a6964

lix/libexpr/parser/grammar.hh

@@ -208,6 +208,7 @@ namespace d {
     // Terminators.
     namespace t {
         struct inherit : one<';'> { };
+        struct binding : one<';'> { };
     }
 }
 
@@ -438,7 +439,7 @@ struct bindings : opt<
     list<
         sor<
             inherit,
-            seq<binding, seps, must<one<';'>>>
+            seq<binding, seps, must<d::t::binding>>
         >,
         seps
     >
@@ -447,9 +448,9 @@ struct bindings : opt<
 struct repl_binding : binding {};
 
 struct repl_bindings : seq<
-    list<repl_binding, one<';'>, t::sep>,
+    list<repl_binding, d::t::binding, t::sep>,
     /* Optional semicolon at the end for convenience */
-    opt<seps, one<';'>>
+    opt<seps, d::t::binding>
 > {};
 
 struct op {

lix/libexpr/parser/parser-impl1.inc.cc

@@ -56,6 +56,7 @@ error_message_for(grammar::v1::expr::select) = "expecting selection expression";
 error_message_for(grammar::v1::t::kw_then) = "expecting 'then'";
 error_message_for(grammar::v1::t::kw_else) = "expecting 'else'";
 error_message_for(grammar::v1::t::kw_in) = "expecting 'in'";
+error_message_for(grammar::v1::d::t::binding) = "expecting ';' to end binding";
 error_message_for(grammar::v1::d::t::inherit) = "expecting ';' to end 'inherit' bindings";
 
 struct SyntaxErrors

tests/functional2/lang/let/in-unterminated.nix

@@ -0,0 +1,3 @@
+let
+  a = ""
+in a

tests/functional2/lang/let/parse-fail-unterminated.err.exp

@@ -0,0 +1,6 @@
+error: syntax error, expecting ';' to end binding
+       at /pwd/in.nix:3:1:
+            2|   a = ""
+            3| in a
+             | ^
+            4|

tests/functional2/lang/let/test.toml

@@ -5,3 +5,7 @@ runner = "eval-okay"
 runner = "eval-okay"
 in = "in-ancient-let.nix"
 flags = [ "--extra-deprecated-features", "ancient-let" ]
+
+[[test]]
+runner = "parse-fail"
+in = "in-unterminated.nix"