nothing variable (#527)

* nothing variable

* corrected comments

* added color to nothing like bool

* compare nothing with values

* comparison tests

crates/nu-parser/src/flatten.rs

@@ -7,6 +7,7 @@ use std::fmt::{Display, Formatter, Result};
 #[derive(Debug, Eq, PartialEq, Ord, PartialOrd)]
 pub enum FlatShape {
     Garbage,
+    Nothing,
     Bool,
     Int,
     Float,
@@ -29,6 +30,7 @@ impl Display for FlatShape {
     fn fmt(&self, f: &mut Formatter) -> Result {
         match self {
             FlatShape::Garbage => write!(f, "flatshape_garbage"),
+            FlatShape::Nothing => write!(f, "flatshape_nothing"),
             FlatShape::Bool => write!(f, "flatshape_bool"),
             FlatShape::Int => write!(f, "flatshape_int"),
             FlatShape::Float => write!(f, "flatshape_float"),
@@ -127,6 +129,9 @@ pub fn flatten_expression(
         Expr::Garbage => {
             vec![(expr.span, FlatShape::Garbage)]
         }
+        Expr::Nothing => {
+            vec![(expr.span, FlatShape::Nothing)]
+        }
         Expr::Int(_) => {
             vec![(expr.span, FlatShape::Int)]
         }

crates/nu-parser/src/parser.rs

@@ -1186,6 +1186,16 @@ pub fn parse_variable_expr(
             },
             None,
         );
+    } else if contents == b"$nothing" {
+        return (
+            Expression {
+                expr: Expr::Nothing,
+                span,
+                ty: Type::Nothing,
+                custom_completion: None,
+            },
+            None,
+        );
     } else if contents == b"$nu" {
         return (
             Expression {
@@ -3483,6 +3493,7 @@ pub fn find_captures_in_expr(
         }
         Expr::ImportPattern(_) => {}
         Expr::Garbage => {}
+        Expr::Nothing => {}
         Expr::GlobPattern(_) => {}
         Expr::Int(_) => {}
         Expr::Keyword(_, _, expr) => {

crates/nu-parser/src/type_check.rs

@@ -252,6 +252,8 @@ pub fn math_result_type(
                 (Type::Filesize, Type::Filesize) => (Type::Bool, None),
 
                 (x, y) if x == y => (Type::Bool, None),
+                (Type::Nothing, _) => (Type::Bool, None),
+                (_, Type::Nothing) => (Type::Bool, None),
                 (Type::Unknown, _) => (Type::Bool, None),
                 (_, Type::Unknown) => (Type::Bool, None),
                 _ => {
@@ -276,6 +278,8 @@ pub fn math_result_type(
                 (Type::Duration, Type::Duration) => (Type::Bool, None),
                 (Type::Filesize, Type::Filesize) => (Type::Bool, None),
 
+                (Type::Nothing, _) => (Type::Bool, None),
+                (_, Type::Nothing) => (Type::Bool, None),
                 (Type::Unknown, _) => (Type::Bool, None),
                 (_, Type::Unknown) => (Type::Bool, None),
                 _ => {