Improve implementation of rstrip and lstrip

NEWS.md

@@ -47,6 +47,8 @@ This section lists changes that do not have deprecation warnings.
 Library improvements
 --------------------
 
+  * The functions `strip`, `lstrip` and `rstrip` now return `SubString` ([#22496]).
+
   * The functions `base` and `digits` digits now accept a negative
     base (like `ndigits` did) ([#21692]).
 

base/strings/util.jl

@@ -137,15 +137,16 @@ julia> lstrip(a)
 ```
 """
 function lstrip(s::AbstractString, chars::Chars=_default_delims)
+    e = endof(s)
     i = start(s)
     while !done(s,i)
         c, j = next(s,i)
         if !(c in chars)
-            return s[i:end]
+            return SubString(s, i, e)
         end
         i = j
     end
-    s[end+1:end]
+    SubString(s, e+1, e)
 end
 
 """
@@ -171,11 +172,11 @@ function rstrip(s::AbstractString, chars::Chars=_default_delims)
     while !done(r,i)
         c, j = next(r,i)
         if !(c in chars)
-            return s[1:end-i+1]
+            return SubString(s, 1, endof(s)-i+1)
         end
         i = j
     end
-    s[1:0]
+    SubString(s, 1, 0)
 end
 
 """

test/strings/util.jl

@@ -26,7 +26,7 @@ for s in ("", " ", " abc", "abc ", "  abc  "), f in (lstrip, rstrip, strip)
         ft = f(t)
         @test s == t
         @test fs == ft
-        @test typeof(ft) == typeof(t[1:end])
+        @test typeof(ft) == SubString{T}
 
         b = convert(SubString{T}, t)
         fb = f(b)