Unrevert #18259 (#18280)

* Revert "Revert #182599 "add faster done for strings" (#18275)"

This reverts commit 6d179b3.

* Test that next(s, endof(s)) > endof(s.data)

(cherry picked from commit 62c3bfa)

Author: TotalVerb

base/strings/string.jl

@@ -80,6 +80,10 @@ end
     return Char(c), i
 end
 
+# This implementation relies on `next` returning a value past the end of the
+# String's underlying data, which is true for valid Strings
+done(s::String, state) = state > endof(s.data)
+
 @inline function next(s::String, i::Int)
     # function is split into this critical fast-path
     # for pure ascii data, such as parsing numbers,

test/strings/basic.jl

@@ -427,3 +427,9 @@ foobaz(ch) = reinterpret(Char, typemax(UInt32))
 
 # issue #17624, missing getindex method for String
 @test "abc"[:] == "abc"
+
+# issue #18280: next/nextind must return past String's underlying data
+for s in ("Hello", "Σ", "こんにちは", "😊😁")
+    @test next(s, endof(s))[2] > endof(s.data)
+    @test nextind(s, endof(s)) > endof(s.data)
+end