Allow parsing left-unbounded range (..10)

It is implemented as a preliminary check when parsing a call and relies
on a fact that a token that successfully parses as a range is unlikely
to be a valid path or command name.

crates/nu-parser/src/parser.rs

@@ -469,6 +469,15 @@ pub fn parse_call(
     spans: &[Span],
     expand_aliases: bool,
 ) -> (Expression, Option<ParseError>) {
+    // We might be parsing left-unbounded range ("..10")
+    let bytes = working_set.get_span_contents(spans[0]);
+    if let (Some(b'.'), Some(b'.')) = (bytes.get(0), bytes.get(1)) {
+        let (range_expr, range_err) = parse_range(working_set, spans[0]);
+        if range_err.is_none() {
+            return (range_expr, range_err);
+        }
+    }
+
     // assume spans.len() > 0?
     let mut pos = 0;
     let mut shorthand = vec![];

crates/nu-parser/tests/test_parser.rs

@@ -371,6 +371,37 @@ mod range {
         }
     }
 
+    #[test]
+    fn parse_left_unbounded_range() {
+        let engine_state = EngineState::new();
+        let mut working_set = StateWorkingSet::new(&engine_state);
+
+        let (block, err) = parse(&mut working_set, None, b"..10", true);
+
+        assert!(err.is_none());
+        assert!(block.len() == 1);
+        match &block[0] {
+            Statement::Pipeline(Pipeline { expressions }) => {
+                assert!(expressions.len() == 1);
+                assert!(matches!(
+                    expressions[0],
+                    Expression {
+                        expr: Expr::Range(
+                            None,
+                            Some(_),
+                            RangeOperator {
+                                inclusion: RangeInclusion::Inclusive,
+                                ..
+                            }
+                        ),
+                        ..
+                    }
+                ))
+            }
+            _ => panic!("No match"),
+        }
+    }
+
     #[test]
     fn parse_negative_range() {
         let engine_state = EngineState::new();