std: Stabilize utf8_error_error_len feature

Stabilizes:

* `Utf8Error::error_len`

Closes #40494

src/liballoc/tests/lib.rs

@@ -28,7 +28,6 @@
 #![feature(test)]
 #![feature(unboxed_closures)]
 #![feature(unicode)]
-#![feature(utf8_error_error_len)]
 
 extern crate alloc;
 extern crate test;

src/libcore/str/mod.rs

@@ -207,7 +207,7 @@ impl Utf8Error {
     ///   that starts at the index given by `valid_up_to()`.
     ///   Decoding should resume after that sequence
     ///   (after inserting a U+FFFD REPLACEMENT CHARACTER) in case of lossy decoding.
-    #[unstable(feature = "utf8_error_error_len", reason ="new", issue = "40494")]
+    #[stable(feature = "utf8_error_error_len", since = "1.20.0")]
     pub fn error_len(&self) -> Option<usize> {
         self.error_len.map(|len| len as usize)
     }