Parse and infer tuple indices

crates/ra_syntax/src/validation/field_expr.rs

@@ -0,0 +1,12 @@
+use crate::{ast::{self, FieldKind},
+            SyntaxError,
+            SyntaxErrorKind::*,
+};
+
+pub(crate) fn validate_field_expr_node(node: &ast::FieldExpr, errors: &mut Vec<SyntaxError>) {
+    if let Some(FieldKind::Index(idx)) = node.field_access() {
+        if idx.text().chars().any(|c| c < '0' || c > '9') {
+            errors.push(SyntaxError::new(InvalidTupleIndexFormat, idx.range()));
+        }
+    }
+}