Dont allow variable names contain '__'

docs/syntax.md

@@ -372,7 +372,10 @@ some_var
 foo/bar
 ```
 
-A variable can be anything matching the regex `[A-Za-z0-9_.-/]+`.
+A variable can be anything matching the regex `[A-Za-z0-9_.-/]+` but with some restrictions:
+
+- It can not start with `//`
+- It can not contain `__`
 
 A variable is a name for some immutable expression. It is possible to rebind variables with the same name.
 
@@ -681,7 +684,10 @@ The constructors inherit the name of their types and become functions (`Tree/Nod
 
 ### Variables
 
-A variable can be anything matching the regex `[A-Za-z0-9_.-/]+`.
+A variable can be anything matching the regex `[A-Za-z0-9_.-/]+` but with some restrictions:
+
+- It can not start with `//`
+- It can not contain `__`
 
 A variable is a name for some immutable expression. It is possible to rebind variables with the same name.
 

src/fun/parser.rs

@@ -821,8 +821,17 @@ pub trait ParserCommons<'a>: Parser<'a> {
   }
 
   fn parse_bend_name(&mut self) -> ParseResult<Name> {
+    let ini_idx = *self.index();
     let name = self.take_while(|c| c.is_ascii_alphanumeric() || c == '_' || c == '.' || c == '-' || c == '/');
-    if name.is_empty() { self.expected("name") } else { Ok(Name::new(name.to_owned())) }
+    let end_idx = *self.index();
+    if name.contains("__") {
+      let msg = "Variable names are not allowed to contain \"__\".".to_string();
+      self.with_ctx(Err(msg), ini_idx, end_idx)
+    } else if name.is_empty() {
+      self.expected("name")
+    } else {
+      Ok(Name::new(name.to_owned()))
+    }
   }
 
   /// Consumes exactly the text without skipping.

tests/golden_tests/compile_file/variable_name_double_underscore.bend

@@ -0,0 +1,2 @@
+def main:
+  return __this_should_fail__(*)

tests/snapshots/compile_file__variable_name_double_underscore.bend.snap

@@ -0,0 +1,9 @@
+---
+source: tests/golden_tests.rs
+input_file: tests/golden_tests/compile_file/variable_name_double_underscore.bend
+---
+Errors:
+In tests/golden_tests/compile_file/variable_name_double_underscore.bend :
+- expected: expression
+- detected:
+  2 |   return __this_should_fail__(*)