- Don't allow to chain let bindings like: let a = let b = 2.

- Give a proper warning when creating an incorrect let binding.

src/bin/expect_tests/error_messages_tests.ml

@@ -0,0 +1,22 @@
+open Cli_expect
+
+let%expect_test _ =
+  run_ligo_bad [ "compile-contract" ; "../../test/contracts/negative/gitlab_111.religo" ; "main" ] ;
+  [%expect {|
+    ligo: : Parse error in file "gitlab_111.religo", line 28, characters 0-3, after "=" and before "let":
+          This is an incorrect let binding. 
+          -
+          Examples of correct let bindings: 
+          let a: int = 4;
+          let (a: int, b: int) = (1, 2);
+          let func = (a: int, b: int) => a + b;
+           {}
+
+     If you're not sure how to fix this error, you can
+     do one of the following:
+
+    * Visit our documentation: https://ligolang.org/docs/intro/what-and-why/
+    * Ask a question on our Discord: https://discord.gg/9rhYaEt
+    * Open a gitlab issue: https://gitlab.com/ligolang/ligo/issues/new
+    * Check the changelog by running 'ligo changelog' |} ] ;
+

src/passes/1-parser/reasonligo/Parser.mly

@@ -418,13 +418,13 @@ unit:
 (* Expressions *)
 
 interactive_expr:
-  expr EOF { $1 }
+  expr_with_let_expr EOF { $1 }
 
 expr:
   base_cond__open(expr) | switch_expr(base_cond) { $1 }
 
 base_cond__open(x):
-  base_expr(x) | conditional(x) { $1 }
+  base_expr(x) | conditional(expr_with_let_expr) { $1 }
 
 base_cond:
   base_cond__open(base_cond) { $1 }
@@ -567,7 +567,7 @@ fun_expr:
     in EFun {region; value=f} }
 
 base_expr(right_expr):
-  let_expr(right_expr) | disj_expr_level | fun_expr { $1 }
+  disj_expr_level | fun_expr { $1 }
 
 conditional(right_expr):
   if_then_else(right_expr) | if_then(right_expr) { $1 }
@@ -609,6 +609,7 @@ base_if_then_else:
 closed_if:
   base_if_then_else__open(closed_if)
 | switch_expr(base_if_then_else) { $1 }
+| let_expr(expr_with_let_expr) { $1 }
 
 switch_expr(right_expr):
   "switch" switch_expr_ "{" cases(right_expr) "}" {
@@ -896,8 +897,12 @@ update_record:
       rbrace = $6}
     in {region; value} }
 
+expr_with_let_expr:
+  expr { $1 }
+| let_expr(expr_with_let_expr) { $1 }
+
 sequence_or_record_in:
-  expr ";" sep_or_term_list(expr,";") {
+  expr_with_let_expr ";" sep_or_term_list(expr_with_let_expr,";") {
     let elts, _region = $3 in
     let s_elts = Utils.nsepseq_cons $1 $2 elts
     in PaSequence {s_elts; s_terminator=None}
@@ -907,7 +912,7 @@ sequence_or_record_in:
     let r_elts = Utils.nsepseq_cons $1 $2 elts
     in PaRecord {r_elts; r_terminator = None}
   }
-| expr ";"? { PaSingleExpr $1 }
+| expr_with_let_expr ";"? { PaSingleExpr $1 }
 
 sequence_or_record:
   "{" sequence_or_record_in "}" {