Auto merge of #58051 - SimonSapin:str_escape, r=alexcrichton

Stabilize str::escape_* methods with new return types…

… that implement `Display` and `Iterator<Item=char>`, as proposed in FCP: https://github.com/rust-lang/rust/issues/27791#issuecomment-376864727

14 files changed, 326 insertions, 199 deletions

diff --git a/src/liballoc/lib.rs b/src/liballoc/lib.rs
index 189ba84eeed..d4ee428a3b5 100644
--- a/src/liballoc/lib.rs
+++ b/src/liballoc/lib.rs
@@ -68,7 +68,6 @@
 #![warn(intra_doc_link_resolution_failure)]
 #![warn(missing_debug_implementations)]
 
-#![cfg_attr(not(test), feature(fn_traits))]
 #![cfg_attr(not(test), feature(generator_trait))]
 #![cfg_attr(test, feature(test))]
 
@@ -86,6 +85,7 @@
 #![feature(dropck_eyepatch)]
 #![feature(exact_size_is_empty)]
 #![feature(fmt_internals)]
+#![feature(fn_traits)]
 #![feature(fundamental)]
 #![feature(futures_api)]
 #![feature(lang_items)]
@@ -100,6 +100,7 @@
 #![feature(receiver_trait)]
 #![feature(specialization)]
 #![feature(staged_api)]
+#![feature(std_internals)]
 #![feature(str_internals)]
 #![feature(trusted_len)]
 #![feature(try_reserve)]
diff --git a/src/liballoc/str.rs b/src/liballoc/str.rs
index 1fd4c9978a6..a36804bddff 100644
--- a/src/liballoc/str.rs
+++ b/src/liballoc/str.rs
@@ -29,8 +29,6 @@
 #![allow(unused_imports)]
 
 use core::borrow::Borrow;
-use core::fmt;
-use core::str as core_str;
 use core::str::pattern::{Pattern, Searcher, ReverseSearcher, DoubleEndedSearcher};
 use core::mem;
 use core::ptr;
@@ -443,45 +441,6 @@ impl str {
         return s;
     }
 
-    /// Escapes each char in `s` with [`char::escape_debug`].
-    ///
-    /// Note: only extended grapheme codepoints that begin the string will be
-    /// escaped.
-    ///
-    /// [`char::escape_debug`]: primitive.char.html#method.escape_debug
-    #[unstable(feature = "str_escape",
-               reason = "return type may change to be an iterator",
-               issue = "27791")]
-    pub fn escape_debug(&self) -> String {
-        let mut string = String::with_capacity(self.len());
-        let mut chars = self.chars();
-        if let Some(first) = chars.next() {
-            string.extend(first.escape_debug_ext(true))
-        }
-        string.extend(chars.flat_map(|c| c.escape_debug_ext(false)));
-        string
-    }
-
-    /// Escapes each char in `s` with [`char::escape_default`].
-    ///
-    /// [`char::escape_default`]: primitive.char.html#method.escape_default
-    #[unstable(feature = "str_escape",
-               reason = "return type may change to be an iterator",
-               issue = "27791")]
-    pub fn escape_default(&self) -> String {
-        self.chars().flat_map(|c| c.escape_default()).collect()
-    }
-
-    /// Escapes each char in `s` with [`char::escape_unicode`].
-    ///
-    /// [`char::escape_unicode`]: primitive.char.html#method.escape_unicode
-    #[unstable(feature = "str_escape",
-               reason = "return type may change to be an iterator",
-               issue = "27791")]
-    pub fn escape_unicode(&self) -> String {
-        self.chars().flat_map(|c| c.escape_unicode()).collect()
-    }
-
     /// Converts a [`Box<str>`] into a [`String`] without copying or allocating.
     ///
     /// [`String`]: string/struct.String.html
@@ -612,3 +571,4 @@ impl str {
 pub unsafe fn from_boxed_utf8_unchecked(v: Box<[u8]>) -> Box<str> {
     Box::from_raw(Box::into_raw(v) as *mut str)
 }
+
diff --git a/src/liballoc/tests/lib.rs b/src/liballoc/tests/lib.rs
index a76fd87a1a9..2b63ac5c7d2 100644
--- a/src/liballoc/tests/lib.rs
+++ b/src/liballoc/tests/lib.rs
@@ -5,7 +5,6 @@
 #![feature(pattern)]
 #![feature(repeat_generic_slice)]
 #![feature(slice_sort_by_cached_key)]
-#![feature(str_escape)]
 #![feature(try_reserve)]
 #![feature(unboxed_closures)]
 #![feature(vecdeque_rotate)]
diff --git a/src/liballoc/tests/str.rs b/src/liballoc/tests/str.rs
index f4e6bc9ea31..a1dc763f6d8 100644
--- a/src/liballoc/tests/str.rs
+++ b/src/liballoc/tests/str.rs
@@ -990,15 +990,15 @@ fn test_split_at_boundscheck() {
 
 #[test]
 fn test_escape_unicode() {
-    assert_eq!("abc".escape_unicode(), "\\u{61}\\u{62}\\u{63}");
-    assert_eq!("a c".escape_unicode(), "\\u{61}\\u{20}\\u{63}");
-    assert_eq!("\r\n\t".escape_unicode(), "\\u{d}\\u{a}\\u{9}");
-    assert_eq!("'\"\\".escape_unicode(), "\\u{27}\\u{22}\\u{5c}");
-    assert_eq!("\x00\x01\u{fe}\u{ff}".escape_unicode(), "\\u{0}\\u{1}\\u{fe}\\u{ff}");
-    assert_eq!("\u{100}\u{ffff}".escape_unicode(), "\\u{100}\\u{ffff}");
-    assert_eq!("\u{10000}\u{10ffff}".escape_unicode(), "\\u{10000}\\u{10ffff}");
-    assert_eq!("ab\u{fb00}".escape_unicode(), "\\u{61}\\u{62}\\u{fb00}");
-    assert_eq!("\u{1d4ea}\r".escape_unicode(), "\\u{1d4ea}\\u{d}");
+    assert_eq!("abc".escape_unicode().to_string(), "\\u{61}\\u{62}\\u{63}");
+    assert_eq!("a c".escape_unicode().to_string(), "\\u{61}\\u{20}\\u{63}");
+    assert_eq!("\r\n\t".escape_unicode().to_string(), "\\u{d}\\u{a}\\u{9}");
+    assert_eq!("'\"\\".escape_unicode().to_string(), "\\u{27}\\u{22}\\u{5c}");
+    assert_eq!("\x00\x01\u{fe}\u{ff}".escape_unicode().to_string(), "\\u{0}\\u{1}\\u{fe}\\u{ff}");
+    assert_eq!("\u{100}\u{ffff}".escape_unicode().to_string(), "\\u{100}\\u{ffff}");
+    assert_eq!("\u{10000}\u{10ffff}".escape_unicode().to_string(), "\\u{10000}\\u{10ffff}");
+    assert_eq!("ab\u{fb00}".escape_unicode().to_string(), "\\u{61}\\u{62}\\u{fb00}");
+    assert_eq!("\u{1d4ea}\r".escape_unicode().to_string(), "\\u{1d4ea}\\u{d}");
 }
 
 #[test]
@@ -1009,31 +1009,32 @@ fn test_escape_debug() {
     // they are escaped. However, when the character is unescaped (e.g., for
     // printable characters), only a single backslash appears (as the character
     // itself appears in the debug string).
-    assert_eq!("abc".escape_debug(), "abc");
-    assert_eq!("a c".escape_debug(), "a c");
-    assert_eq!("éèê".escape_debug(), "éèê");
-    assert_eq!("\r\n\t".escape_debug(), "\\r\\n\\t");
-    assert_eq!("'\"\\".escape_debug(), "\\'\\\"\\\\");
-    assert_eq!("\u{7f}\u{ff}".escape_debug(), "\\u{7f}\u{ff}");
-    assert_eq!("\u{100}\u{ffff}".escape_debug(), "\u{100}\\u{ffff}");
-    assert_eq!("\u{10000}\u{10ffff}".escape_debug(), "\u{10000}\\u{10ffff}");
-    assert_eq!("ab\u{200b}".escape_debug(), "ab\\u{200b}");
-    assert_eq!("\u{10d4ea}\r".escape_debug(), "\\u{10d4ea}\\r");
-    assert_eq!("\u{301}a\u{301}bé\u{e000}".escape_debug(), "\\u{301}a\u{301}bé\\u{e000}");
+    assert_eq!("abc".escape_debug().to_string(), "abc");
+    assert_eq!("a c".escape_debug().to_string(), "a c");
+    assert_eq!("éèê".escape_debug().to_string(), "éèê");
+    assert_eq!("\r\n\t".escape_debug().to_string(), "\\r\\n\\t");
+    assert_eq!("'\"\\".escape_debug().to_string(), "\\'\\\"\\\\");
+    assert_eq!("\u{7f}\u{ff}".escape_debug().to_string(), "\\u{7f}\u{ff}");
+    assert_eq!("\u{100}\u{ffff}".escape_debug().to_string(), "\u{100}\\u{ffff}");
+    assert_eq!("\u{10000}\u{10ffff}".escape_debug().to_string(), "\u{10000}\\u{10ffff}");
+    assert_eq!("ab\u{200b}".escape_debug().to_string(), "ab\\u{200b}");
+    assert_eq!("\u{10d4ea}\r".escape_debug().to_string(), "\\u{10d4ea}\\r");
+    assert_eq!("\u{301}a\u{301}bé\u{e000}".escape_debug().to_string(),
+               "\\u{301}a\u{301}bé\\u{e000}");
 }
 
 #[test]
 fn test_escape_default() {
-    assert_eq!("abc".escape_default(), "abc");
-    assert_eq!("a c".escape_default(), "a c");
-    assert_eq!("éèê".escape_default(), "\\u{e9}\\u{e8}\\u{ea}");
-    assert_eq!("\r\n\t".escape_default(), "\\r\\n\\t");
-    assert_eq!("'\"\\".escape_default(), "\\'\\\"\\\\");
-    assert_eq!("\u{7f}\u{ff}".escape_default(), "\\u{7f}\\u{ff}");
-    assert_eq!("\u{100}\u{ffff}".escape_default(), "\\u{100}\\u{ffff}");
-    assert_eq!("\u{10000}\u{10ffff}".escape_default(), "\\u{10000}\\u{10ffff}");
-    assert_eq!("ab\u{200b}".escape_default(), "ab\\u{200b}");
-    assert_eq!("\u{10d4ea}\r".escape_default(), "\\u{10d4ea}\\r");
+    assert_eq!("abc".escape_default().to_string(), "abc");
+    assert_eq!("a c".escape_default().to_string(), "a c");
+    assert_eq!("éèê".escape_default().to_string(), "\\u{e9}\\u{e8}\\u{ea}");
+    assert_eq!("\r\n\t".escape_default().to_string(), "\\r\\n\\t");
+    assert_eq!("'\"\\".escape_default().to_string(), "\\'\\\"\\\\");
+    assert_eq!("\u{7f}\u{ff}".escape_default().to_string(), "\\u{7f}\\u{ff}");
+    assert_eq!("\u{100}\u{ffff}".escape_default().to_string(), "\\u{100}\\u{ffff}");
+    assert_eq!("\u{10000}\u{10ffff}".escape_default().to_string(), "\\u{10000}\\u{10ffff}");
+    assert_eq!("ab\u{200b}".escape_default().to_string(), "ab\\u{200b}");
+    assert_eq!("\u{10d4ea}\r".escape_default().to_string(), "\\u{10d4ea}\\r");
 }
 
 #[test]
diff --git a/src/libcore/char/methods.rs b/src/libcore/char/methods.rs
index 72967b9adf7..122e5f3affd 100644
--- a/src/libcore/char/methods.rs
+++ b/src/libcore/char/methods.rs
@@ -189,10 +189,8 @@ impl char {
     /// An extended version of `escape_debug` that optionally permits escaping
     /// Extended Grapheme codepoints. This allows us to format characters like
     /// nonspacing marks better when they're at the start of a string.
-    #[doc(hidden)]
-    #[unstable(feature = "str_internals", issue = "0")]
     #[inline]
-    pub fn escape_debug_ext(self, escape_grapheme_extended: bool) -> EscapeDebug {
+    pub(crate) fn escape_debug_ext(self, escape_grapheme_extended: bool) -> EscapeDebug {
         let init_state = match self {
             '\t' => EscapeDefaultState::Backslash('t'),
             '\r' => EscapeDefaultState::Backslash('r'),
diff --git a/src/libcore/internal_macros.rs b/src/libcore/internal_macros.rs
index d12800f7124..9f0f6fd49fb 100644
--- a/src/libcore/internal_macros.rs
+++ b/src/libcore/internal_macros.rs
@@ -75,3 +75,47 @@ macro_rules! forward_ref_op_assign {
         }
     }
 }
+
+/// Create a zero-size type similar to a closure type, but named.
+#[unstable(feature = "std_internals", issue = "0")]
+macro_rules! impl_fn_for_zst {
+    ($(
+        $( #[$attr: meta] )*
+        // FIXME: when libcore is in the 2018 edition, use `?` repetition in
+        // $( <$( $li : lifetime ),+> )?
+        struct $Name: ident impl$( <$( $lifetime : lifetime ),+> )* Fn =
+            |$( $arg: ident: $ArgTy: ty ),*| -> $ReturnTy: ty
+            $body: block;
+    )+) => {
+        $(
+            $( #[$attr] )*
+            struct $Name;
+
+            impl $( <$( $lifetime ),+> )* Fn<($( $ArgTy, )*)> for $Name {
+                #[inline]
+                extern "rust-call" fn call(&self, ($( $arg, )*): ($( $ArgTy, )*)) -> $ReturnTy {
+                    $body
+                }
+            }
+
+            impl $( <$( $lifetime ),+> )* FnMut<($( $ArgTy, )*)> for $Name {
+                #[inline]
+                extern "rust-call" fn call_mut(
+                    &mut self,
+                    ($( $arg, )*): ($( $ArgTy, )*)
+                ) -> $ReturnTy {
+                    Fn::call(&*self, ($( $arg, )*))
+                }
+            }
+
+            impl $( <$( $lifetime ),+> )* FnOnce<($( $ArgTy, )*)> for $Name {
+                type Output = $ReturnTy;
+
+                #[inline]
+                extern "rust-call" fn call_once(self, ($( $arg, )*): ($( $ArgTy, )*)) -> $ReturnTy {
+                    Fn::call(&self, ($( $arg, )*))
+                }
+            }
+        )+
+    }
+}
diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs
index 8b728c9414d..419980225c3 100644
--- a/src/libcore/lib.rs
+++ b/src/libcore/lib.rs
@@ -95,6 +95,7 @@
 #![feature(simd_ffi)]
 #![feature(specialization)]
 #![feature(staged_api)]
+#![feature(std_internals)]
 #![feature(stmt_expr_attributes)]
 #![feature(unboxed_closures)]
 #![feature(unsized_locals)]
diff --git a/src/libcore/str/mod.rs b/src/libcore/str/mod.rs
index 6c08e545c5c..363eae04f03 100644
--- a/src/libcore/str/mod.rs
+++ b/src/libcore/str/mod.rs
@@ -8,10 +8,13 @@ use self::pattern::Pattern;
 use self::pattern::{Searcher, ReverseSearcher, DoubleEndedSearcher};
 
 use char;
-use fmt;
+use fmt::{self, Write};
 use iter::{Map, Cloned, FusedIterator, TrustedLen, TrustedRandomAccess, Filter};
+use iter::{Flatten, FlatMap, Chain};
 use slice::{self, SliceIndex, Split as SliceSplit};
 use mem;
+use ops::Try;
+use option;
 
 pub mod pattern;
 
@@ -1345,33 +1348,14 @@ impl FusedIterator for Lines<'_> {}
 #[allow(deprecated)]
 pub struct LinesAny<'a>(Lines<'a>);
 
-/// A nameable, cloneable fn type
-#[derive(Clone)]
-struct LinesAnyMap;
-
-impl<'a> Fn<(&'a str,)> for LinesAnyMap {
-    #[inline]
-    extern "rust-call" fn call(&self, (line,): (&'a str,)) -> &'a str {
+impl_fn_for_zst! {
+    /// A nameable, cloneable fn type
+    #[derive(Clone)]
+    struct LinesAnyMap impl<'a> Fn = |line: &'a str| -> &'a str {
         let l = line.len();
         if l > 0 && line.as_bytes()[l - 1] == b'\r' { &line[0 .. l - 1] }
         else { line }
-    }
-}
-
-impl<'a> FnMut<(&'a str,)> for LinesAnyMap {
-    #[inline]
-    extern "rust-call" fn call_mut(&mut self, (line,): (&'a str,)) -> &'a str {
-        Fn::call(&*self, (line,))
-    }
-}
-
-impl<'a> FnOnce<(&'a str,)> for LinesAnyMap {
-    type Output = &'a str;
-
-    #[inline]
-    extern "rust-call" fn call_once(self, (line,): (&'a str,)) -> &'a str {
-        Fn::call(&self, (line,))
-    }
+    };
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -2727,7 +2711,7 @@ impl str {
         let inner = self
             .as_bytes()
             .split(IsAsciiWhitespace)
-            .filter(IsNotEmpty)
+            .filter(BytesIsNotEmpty)
             .map(UnsafeBytesToStr);
         SplitAsciiWhitespace { inner }
     }
@@ -3964,6 +3948,146 @@ impl str {
         let me = unsafe { self.as_bytes_mut() };
         me.make_ascii_lowercase()
     }
+
+    /// Return an iterator that escapes each char in `s` with [`char::escape_debug`].
+    ///
+    /// Note: only extended grapheme codepoints that begin the string will be
+    /// escaped.
+    ///
+    /// [`char::escape_debug`]: ../std/primitive.char.html#method.escape_debug
+    ///
+    /// # Examples
+    ///
+    /// As an iterator:
+    ///
+    /// ```
+    /// for c in "❤\n!".escape_debug() {
+    ///     print!("{}", c);
+    /// }
+    /// println!();
+    /// ```
+    ///
+    /// Using `println!` directly:
+    ///
+    /// ```
+    /// println!("{}", "❤\n!".escape_debug());
+    /// ```
+    ///
+    ///
+    /// Both are equivalent to:
+    ///
+    /// ```
+    /// println!("❤\\n!");
+    /// ```
+    ///
+    /// Using `to_string`:
+    ///
+    /// ```
+    /// assert_eq!("❤\n!".escape_debug().to_string(), "❤\\n!");
+    /// ```
+    #[stable(feature = "str_escape", since = "1.34.0")]
+    pub fn escape_debug(&self) -> EscapeDebug {
+        let mut chars = self.chars();
+        EscapeDebug {
+            inner: chars.next()
+                .map(|first| first.escape_debug_ext(true))
+                .into_iter()
+                .flatten()
+                .chain(chars.flat_map(CharEscapeDebugContinue))
+        }
+    }
+
+    /// Return an iterator that escapes each char in `s` with [`char::escape_default`].
+    ///
+    /// [`char::escape_default`]: ../std/primitive.char.html#method.escape_default
+    ///
+    /// # Examples
+    ///
+    /// As an iterator:
+    ///
+    /// ```
+    /// for c in "❤\n!".escape_default() {
+    ///     print!("{}", c);
+    /// }
+    /// println!();
+    /// ```
+    ///
+    /// Using `println!` directly:
+    ///
+    /// ```
+    /// println!("{}", "❤\n!".escape_default());
+    /// ```
+    ///
+    ///
+    /// Both are equivalent to:
+    ///
+    /// ```
+    /// println!("\\u{{2764}}\n!");
+    /// ```
+    ///
+    /// Using `to_string`:
+    ///
+    /// ```
+    /// assert_eq!("❤\n!".escape_default().to_string(), "\\u{2764}\\n!");
+    /// ```
+    #[stable(feature = "str_escape", since = "1.34.0")]
+    pub fn escape_default(&self) -> EscapeDefault {
+        EscapeDefault { inner: self.chars().flat_map(CharEscapeDefault) }
+    }
+
+    /// Return an iterator that escapes each char in `s` with [`char::escape_unicode`].
+    ///
+    /// [`char::escape_unicode`]: ../std/primitive.char.html#method.escape_unicode
+    ///
+    /// # Examples
+    ///
+    /// As an iterator:
+    ///
+    /// ```
+    /// for c in "❤\n!".escape_unicode() {
+    ///     print!("{}", c);
+    /// }
+    /// println!();
+    /// ```
+    ///
+    /// Using `println!` directly:
+    ///
+    /// ```
+    /// println!("{}", "❤\n!".escape_unicode());
+    /// ```
+    ///
+    ///
+    /// Both are equivalent to:
+    ///
+    /// ```
+    /// println!("\\u{{2764}}\\u{{a}}\\u{{21}}");
+    /// ```
+    ///
+    /// Using `to_string`:
+    ///
+    /// ```
+    /// assert_eq!("❤\n!".escape_unicode().to_string(), "\\u{2764}\\u{a}\\u{21}");
+    /// ```
+    #[stable(feature = "str_escape", since = "1.34.0")]
+    pub fn escape_unicode(&self) -> EscapeUnicode {
+        EscapeUnicode { inner: self.chars().flat_map(CharEscapeUnicode) }
+    }
+}
+
+impl_fn_for_zst! {
+    #[derive(Clone)]
+    struct CharEscapeDebugContinue impl Fn = |c: char| -> char::EscapeDebug {
+        c.escape_debug_ext(false)
+    };
+
+    #[derive(Clone)]
+    struct CharEscapeUnicode impl Fn = |c: char| -> char::EscapeUnicode {
+        c.escape_unicode()
+    };
+    #[derive(Clone)]
+    struct CharEscapeDefault impl Fn = |c: char| -> char::EscapeDefault {
+        c.escape_default()
+    };
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -4011,102 +4135,36 @@ pub struct SplitWhitespace<'a> {
 #[stable(feature = "split_ascii_whitespace", since = "1.34.0")]
 #[derive(Clone, Debug)]
 pub struct SplitAsciiWhitespace<'a> {
-    inner: Map<Filter<SliceSplit<'a, u8, IsAsciiWhitespace>, IsNotEmpty>, UnsafeBytesToStr>,
-}
-
-#[derive(Clone)]
-struct IsWhitespace;
-
-impl FnOnce<(char, )> for IsWhitespace {
-    type Output = bool;
-
-    #[inline]
-    extern "rust-call" fn call_once(mut self, arg: (char, )) -> bool {
-        self.call_mut(arg)
-    }
-}
-
-impl FnMut<(char, )> for IsWhitespace {
-    #[inline]
-    extern "rust-call" fn call_mut(&mut self, arg: (char, )) -> bool {
-        arg.0.is_whitespace()
-    }
-}
-
-#[derive(Clone)]
-struct IsAsciiWhitespace;
-
-impl<'a> FnOnce<(&'a u8, )> for IsAsciiWhitespace {
-    type Output = bool;
-
-    #[inline]
-    extern "rust-call" fn call_once(mut self, arg: (&u8, )) -> bool {
-        self.call_mut(arg)
-    }
-}
-
-impl<'a> FnMut<(&'a u8, )> for IsAsciiWhitespace {
-    #[inline]
-    extern "rust-call" fn call_mut(&mut self, arg: (&u8, )) -> bool {
-        arg.0.is_ascii_whitespace()
-    }
-}
-
-#[derive(Clone)]
-struct IsNotEmpty;
-
-impl<'a, 'b> FnOnce<(&'a &'b str, )> for IsNotEmpty {
-    type Output = bool;
-
-    #[inline]
-    extern "rust-call" fn call_once(mut self, arg: (&'a &'b str, )) -> bool {
-        self.call_mut(arg)
-    }
-}
-
-impl<'a, 'b> FnMut<(&'a &'b str, )> for IsNotEmpty {
-    #[inline]
-    extern "rust-call" fn call_mut(&mut self, arg: (&'a &'b str, )) -> bool {
-        !arg.0.is_empty()
-    }
+    inner: Map<Filter<SliceSplit<'a, u8, IsAsciiWhitespace>, BytesIsNotEmpty>, UnsafeBytesToStr>,
 }
 
-impl<'a, 'b> FnOnce<(&'a &'b [u8], )> for IsNotEmpty {
-    type Output = bool;
-
-    #[inline]
-    extern "rust-call" fn call_once(mut self, arg: (&'a &'b [u8], )) -> bool {
-        self.call_mut(arg)
-    }
-}
-
-impl<'a, 'b> FnMut<(&'a &'b [u8], )> for IsNotEmpty {
-    #[inline]
-    extern "rust-call" fn call_mut(&mut self, arg: (&'a &'b [u8], )) -> bool {
-        !arg.0.is_empty()
-    }
-}
+impl_fn_for_zst! {
+    #[derive(Clone)]
+    struct IsWhitespace impl Fn = |c: char| -> bool {
+        c.is_whitespace()
+    };
 
-#[derive(Clone)]
-struct UnsafeBytesToStr;
+    #[derive(Clone)]
+    struct IsAsciiWhitespace impl Fn = |byte: &u8| -> bool {
+        byte.is_ascii_whitespace()
+    };
 
-impl<'a> FnOnce<(&'a [u8], )> for UnsafeBytesToStr {
-    type Output = &'a str;
+    #[derive(Clone)]
+    struct IsNotEmpty impl<'a, 'b> Fn = |s: &'a &'b str| -> bool {
+        !s.is_empty()
+    };
 
-    #[inline]
-    extern "rust-call" fn call_once(mut self, arg: (&'a [u8], )) -> &'a str {
-        self.call_mut(arg)
-    }
-}
+    #[derive(Clone)]
+    struct BytesIsNotEmpty impl<'a, 'b> Fn = |s: &'a &'b [u8]| -> bool {
+        !s.is_empty()
+    };
 
-impl<'a> FnMut<(&'a [u8], )> for UnsafeBytesToStr {
-    #[inline]
-    extern "rust-call" fn call_mut(&mut self, arg: (&'a [u8], )) -> &'a str {
-        unsafe { from_utf8_unchecked(arg.0) }
-    }
+    #[derive(Clone)]
+    struct UnsafeBytesToStr impl<'a> Fn = |bytes: &'a [u8]| -> &'a str {
+        unsafe { from_utf8_unchecked(bytes) }
+    };
 }
 
-
 #[stable(feature = "split_whitespace", since = "1.1.0")]
 impl<'a> Iterator for SplitWhitespace<'a> {
     type Item = &'a str;
@@ -4216,3 +4274,74 @@ impl<'a> Iterator for EncodeUtf16<'a> {
 
 #[stable(feature = "fused", since = "1.26.0")]
 impl FusedIterator for EncodeUtf16<'_> {}
+
+/// The return type of [`str::escape_debug`].
+///
+/// [`str::escape_debug`]: ../../std/primitive.str.html#method.escape_debug
+#[stable(feature = "str_escape", since = "1.34.0")]
+#[derive(Clone, Debug)]
+pub struct EscapeDebug<'a> {
+    inner: Chain<
+        Flatten<option::IntoIter<char::EscapeDebug>>,
+        FlatMap<Chars<'a>, char::EscapeDebug, CharEscapeDebugContinue>
+    >,
+}
+
+/// The return type of [`str::escape_default`].
+///
+/// [`str::escape_default`]: ../../std/primitive.str.html#method.escape_default
+#[stable(feature = "str_escape", since = "1.34.0")]
+#[derive(Clone, Debug)]
+pub struct EscapeDefault<'a> {
+    inner: FlatMap<Chars<'a>, char::EscapeDefault, CharEscapeDefault>,
+}
+
+/// The return type of [`str::escape_unicode`].
+///
+/// [`str::escape_unicode`]: ../../std/primitive.str.html#method.escape_unicode
+#[stable(feature = "str_escape", since = "1.34.0")]
+#[derive(Clone, Debug)]
+pub struct EscapeUnicode<'a> {
+    inner: FlatMap<Chars<'a>, char::EscapeUnicode, CharEscapeUnicode>,
+}
+
+macro_rules! escape_types_impls {
+    ($( $Name: ident ),+) => {$(
+        #[stable(feature = "str_escape", since = "1.34.0")]
+        impl<'a> fmt::Display for $Name<'a> {
+            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+                self.clone().try_for_each(|c| f.write_char(c))
+            }
+        }
+
+        #[stable(feature = "str_escape", since = "1.34.0")]
+        impl<'a> Iterator for $Name<'a> {
+            type Item = char;
+
+            #[inline]
+            fn next(&mut self) -> Option<char> { self.inner.next() }
+
+            #[inline]
+            fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
+
+            #[inline]
+            fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R where
+                Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
+            {
+                self.inner.try_fold(init, fold)
+            }
+
+            #[inline]
+            fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
+                where Fold: FnMut(Acc, Self::Item) -> Acc,
+            {
+                self.inner.fold(init, fold)
+            }
+        }
+
+        #[stable(feature = "str_escape", since = "1.34.0")]
+        impl<'a> FusedIterator for $Name<'a> {}
+    )+}
+}
+
+escape_types_impls!(EscapeDebug, EscapeDefault, EscapeUnicode);
diff --git a/src/libgraphviz/lib.rs b/src/libgraphviz/lib.rs
index 7e7a9df93cd..489020d4ee7 100644
--- a/src/libgraphviz/lib.rs
+++ b/src/libgraphviz/lib.rs
@@ -277,7 +277,6 @@
 #![deny(rust_2018_idioms)]
 
 #![feature(nll)]
-#![feature(str_escape)]
 
 use LabelText::*;
 
@@ -538,7 +537,7 @@ impl<'a> LabelText<'a> {
             EscStr(s) => s,
             LabelStr(s) => {
                 if s.contains('\\') {
-                    (&*s).escape_default().into()
+                    (&*s).escape_default().to_string().into()
                 } else {
                     s
                 }
diff --git a/src/libsyntax/attr/mod.rs b/src/libsyntax/attr/mod.rs
index a4f5449ec54..29e86e0ecee 100644
--- a/src/libsyntax/attr/mod.rs
+++ b/src/libsyntax/attr/mod.rs
@@ -634,7 +634,7 @@ impl LitKind {
 
         match *self {
             LitKind::Str(string, ast::StrStyle::Cooked) => {
-                let escaped = string.as_str().escape_default();
+                let escaped = string.as_str().escape_default().to_string();
                 Token::Literal(token::Lit::Str_(Symbol::intern(&escaped)), None)
             }
             LitKind::Str(string, ast::StrStyle::Raw(n)) => {
diff --git a/src/libsyntax/lib.rs b/src/libsyntax/lib.rs
index c844f9e2a91..a56cdf623bf 100644
--- a/src/libsyntax/lib.rs
+++ b/src/libsyntax/lib.rs
@@ -15,7 +15,6 @@
 #![feature(rustc_attrs)]
 #![feature(rustc_diagnostic_macros)]
 #![feature(slice_sort_by_cached_key)]
-#![feature(str_escape)]
 #![feature(step_trait)]
 #![feature(try_trait)]
 #![feature(unicode_internals)]
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index c670f47b597..cdf805176a2 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -606,7 +606,7 @@ pub trait PrintState<'a> {
         match lit.node {
             ast::LitKind::Str(st, style) => self.print_string(&st.as_str(), style),
             ast::LitKind::Err(st) => {
-                let st = st.as_str().escape_debug();
+                let st = st.as_str().escape_debug().to_string();
                 let mut res = String::with_capacity(st.len() + 2);
                 res.push('\'');
                 res.push_str(&st);
diff --git a/src/libsyntax_ext/lib.rs b/src/libsyntax_ext/lib.rs
index 05c8084a51e..77e60c13637 100644
--- a/src/libsyntax_ext/lib.rs
+++ b/src/libsyntax_ext/lib.rs
@@ -10,7 +10,6 @@
 #![feature(proc_macro_span)]
 #![feature(decl_macro)]
 #![feature(nll)]
-#![feature(str_escape)]
 #![feature(rustc_diagnostic_macros)]
 
 #![recursion_limit="256"]
diff --git a/src/test/run-pass/threads-sendsync/sync-send-iterators-in-libcore.rs b/src/test/run-pass/threads-sendsync/sync-send-iterators-in-libcore.rs
index 903bbf51634..44beb9dc1e5 100644
--- a/src/test/run-pass/threads-sendsync/sync-send-iterators-in-libcore.rs
+++ b/src/test/run-pass/threads-sendsync/sync-send-iterators-in-libcore.rs
@@ -2,9 +2,6 @@
 // pretty-expanded FIXME #23616
 
 #![allow(warnings)]
-#![feature(iter_empty)]
-#![feature(iter_once)]
-#![feature(str_escape)]
 
 use std::iter::{empty, once, repeat};