Reserve identifiers starting with __ for compiler usage

crates/analyzer/src/analyzer_error.rs

@@ -565,6 +565,23 @@ pub enum AnalyzerError {
         #[label("Uncovered")]
         uncovered: SourceSpan,
     },
+
+    #[diagnostic(
+        severity(Error),
+        code(reserved_identifier),
+        help("prefix `__` can't be used"),
+        url(
+            "https://doc.veryl-lang.org/book/06_appendix/02_semantic_error.html#reserved_identifier"
+        )
+    )]
+    #[error("{identifier} is reverved for compiler usage")]
+    ReservedIdentifier {
+        identifier: String,
+        #[source_code]
+        input: NamedSource,
+        #[label("Error location")]
+        error_location: SourceSpan,
+    },
 }
 
 impl AnalyzerError {
@@ -906,4 +923,12 @@ impl AnalyzerError {
             uncovered: uncovered.into(),
         }
     }
+
+    pub fn reserved_identifier(identifier: &str, source: &str, token: &Token) -> Self {
+        AnalyzerError::ReservedIdentifier {
+            identifier: identifier.to_string(),
+            input: AnalyzerError::named_source(source, token),
+            error_location: token.into(),
+        }
+    }
 }

crates/analyzer/src/handlers/check_identifier.rs

@@ -130,6 +130,15 @@ impl<'a> CheckIdentifier<'a> {
         };
 
         let identifier = token.to_string();
+
+        if identifier.starts_with("__") {
+            self.errors.push(AnalyzerError::reserved_identifier(
+                &identifier,
+                self.text,
+                token,
+            ));
+        }
+
         if let Some(prefix) = prefix {
             if !identifier.starts_with(prefix) {
                 self.errors.push(AnalyzerError::invalid_identifier(

crates/analyzer/src/tests.rs

@@ -600,3 +600,17 @@ fn uncovered_branch() {
     let errors = analyze(code);
     assert!(matches!(errors[0], AnalyzerError::UncoveredBranch { .. }));
 }
+
+#[test]
+fn reserved_identifier() {
+    let code = r#"
+    module __ModuleA {
+    }
+    "#;
+
+    let errors = analyze(code);
+    assert!(matches!(
+        errors[0],
+        AnalyzerError::ReservedIdentifier { .. }
+    ));
+}