Remove a [derive(Clone)] for Rust 1.20

This should be the relevant issue: rust-lang/rust#43690.

Cargo.toml

@@ -61,3 +61,7 @@ name = "chars"
 
 [[example]]
 name = "is_zalgo"
+
+
+[package.metadata.docs.rs]
+features = ["nightly"]

build.rs

@@ -8,12 +8,17 @@ use rustc_version::{Version, version};
 fn run_rustc_version() {
     let version = version().unwrap();
 
+    if version >= Version::parse("1.21.0").unwrap() {
+        // This should be the relevant issue: https://github.com/rust-lang/rust/pull/43690
+        println!("cargo:rustc-cfg=fn_clone");
+    }
+
     if version >= Version::parse("1.26.0").unwrap() {
         println!("cargo:rustc-cfg=stable_fused_iterator");
     }
 
     if version >= Version::parse("1.27.0").unwrap() {
-        // We won't be able to define `Iterator::try_fold` and `DoubleEndedIterator::try_fold`
+        // We won't be able to define `Iterator::try_fold` and `DoubleEndedIterator::try_rfold`
         // in stable anyway because both require a type parameter bounded by `ops::Try` which is
         // not going to stabilize yet.
         //println!("cargo:rustc-cfg=stable_iterator_try_fold");

src/unapply_iter.rs

@@ -10,7 +10,9 @@ use core::ops;
 ///
 /// [`unapply_iter`]: fn.unapply_iter.html
 #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
-#[derive(Clone, Debug)]
+// This should be the relevant issue: https://github.com/rust-lang/rust/pull/43690
+#[cfg_attr(fn_clone, derive(Clone))]
+#[derive(Debug)]
 pub struct UnapplyIter<I> {
     pub(crate) inner: iter::Filter<I, fn(&char) -> bool>,
 }