Add starts with operator (#5061)

* add starts_with operator

* added a test

crates/nu-protocol/src/ast/expression.rs

@@ -34,6 +34,7 @@ impl Expression {
                     Operator::Plus | Operator::Minus => 90,
                     Operator::NotContains
                     | Operator::Contains
+                    | Operator::StartsWith
                     | Operator::LessThan
                     | Operator::LessThanOrEqual
                     | Operator::GreaterThan

crates/nu-protocol/src/ast/operator.rs

@@ -23,6 +23,7 @@ pub enum Operator {
     And,
     Or,
     Pow,
+    StartsWith,
 }
 
 impl Display for Operator {
@@ -46,6 +47,7 @@ impl Display for Operator {
             Operator::Pow => write!(f, "**"),
             Operator::LessThanOrEqual => write!(f, "<="),
             Operator::GreaterThanOrEqual => write!(f, ">="),
+            Operator::StartsWith => write!(f, "=^"),
         }
     }
 }

crates/nu-protocol/src/value/mod.rs

@@ -2031,6 +2031,27 @@ impl Value {
         }
     }
 
+    pub fn starts_with(&self, op: Span, rhs: &Value) -> Result<Value, ShellError> {
+        let span = span(&[self.span()?, rhs.span()?]);
+
+        match (self, rhs) {
+            (Value::String { val: lhs, .. }, Value::String { val: rhs, .. }) => Ok(Value::Bool {
+                val: lhs.starts_with(rhs),
+                span,
+            }),
+            (Value::CustomValue { val: lhs, span }, rhs) => {
+                lhs.operation(*span, Operator::StartsWith, op, rhs)
+            }
+            _ => Err(ShellError::OperatorMismatch {
+                op_span: op,
+                lhs_ty: self.get_type(),
+                lhs_span: self.span()?,
+                rhs_ty: rhs.get_type(),
+                rhs_span: rhs.span()?,
+            }),
+        }
+    }
+
     pub fn not_contains(&self, op: Span, rhs: &Value) -> Result<Value, ShellError> {
         let span = span(&[self.span()?, rhs.span()?]);