add to_string method for StringView

Author: Yu-zh

string/string.mbti

@@ -23,6 +23,7 @@ impl StringView {
   op_as_view(Self, start~ : Int = .., end? : Int) -> Self
   op_get(Self, Int) -> Char
   rev_get(Self, Int) -> Char
+  to_string(Self) -> String
 }
 impl Show for StringView
 

string/view.mbt

@@ -426,6 +426,20 @@ pub impl Show for StringView with output(self, logger) {
   String::output(substr, logger)
 }
 
+///|
+/// Returns a new String containing a copy of the characters in this view.
+/// 
+/// # Examples
+/// 
+/// ```
+/// let str = "Hello World"
+/// let view = str[0:5]  // "Hello"
+/// inspect!(view.to_string(), content="Hello")
+/// ```
+pub fn to_string(self : StringView) -> String {
+  self.str.substring(start=self.start, end=self.end)
+}
+
 ///|
 /// Returns an iterator over the Unicode characters in the string view.
 pub fn StringView::iter(self : StringView) -> Iter[Char] {

string/view_test.mbt

@@ -514,3 +514,10 @@ test "panic stringview negative index3" {
   let _ = str[-1:-2]
 
 }
+
+test "to_string" {
+  let s = "Hello World"
+  let view = s[0:5]
+  let ss = "\{view} Moonbit"
+  inspect!(ss, content="Hello Moonbit")
+}