Add AsciiExt::into_ascii_{upper,lower}case

The default implementations (with where Self: Sized) are so that methods
that take `self` by value can exist in a trait that’s implemented for
dynamically-sized types (`str` and `[u8]`).

CC #27809 (comment)

src/libstd/ascii.rs

@@ -139,6 +139,112 @@ pub trait AsciiExt {
     /// ```
     #[unstable(feature = "ascii", issue = "27809")]
     fn make_ascii_lowercase(&mut self);
+
+    /// Converts this type to its ASCII upper case,
+    /// consuming the value to avoid allocating memory where `to_ascii_uppercase` would.
+    ///
+    /// See `to_ascii_uppercase` for more information.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(ascii)]
+    ///
+    /// use std::ascii::AsciiExt;
+    ///
+    /// let ascii: String = "a".to_owned();
+    ///
+    /// let upper = ascii.into_ascii_uppercase();
+    ///
+    /// assert_eq!(upper, "A");
+    /// ```
+    #[unstable(feature = "ascii", issue = "27809")]
+    fn into_ascii_uppercase(self) -> Self::Owned where Self: Sized {
+        self.to_ascii_uppercase()
+    }
+
+    /// Converts this type to its ASCII lower case,
+    /// consuming the value to avoid allocating memory where `to_ascii_lowercase` would.
+    ///
+    /// See `to_ascii_lowercase` for more information.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(ascii)]
+    ///
+    /// use std::ascii::AsciiExt;
+    ///
+    /// let ascii: String = "A".to_owned();
+    ///
+    /// let lower = ascii.into_ascii_lowercase();
+    ///
+    /// assert_eq!(lower, "a");
+    /// ```
+    #[unstable(feature = "ascii", issue = "27809")]
+    fn into_ascii_lowercase(self) -> Self::Owned where Self: Sized {
+        self.to_ascii_lowercase()
+    }
+}
+
+/// Implement `into_ascii_lowercase` and `into_ascii_uppercase` without memory allocation,
+/// defer other methods to `str`.
+#[unstable(feature = "ascii", issue = "27809")]
+impl AsciiExt for String {
+    type Owned = Self;
+
+    #[inline] fn is_ascii(&self) -> bool { (**self).is_ascii() }
+    #[inline] fn to_ascii_uppercase(&self) -> Self { (**self).to_ascii_uppercase() }
+    #[inline] fn to_ascii_lowercase(&self) -> Self { (**self).to_ascii_lowercase() }
+    #[inline] fn eq_ignore_ascii_case(&self, o: &Self) -> bool { (**self).eq_ignore_ascii_case(o) }
+    #[inline] fn make_ascii_uppercase(&mut self) { (**self).make_ascii_uppercase() }
+    #[inline] fn make_ascii_lowercase(&mut self) { (**self).make_ascii_lowercase() }
+
+    fn into_ascii_lowercase(mut self) -> Self {
+        unsafe {
+            for byte in self.as_mut_vec() {
+                *byte = byte.to_ascii_lowercase()
+            }
+        }
+        self
+    }
+
+    fn into_ascii_uppercase(mut self) -> Self {
+        unsafe {
+            for byte in self.as_mut_vec() {
+                *byte = byte.to_ascii_uppercase()
+            }
+        }
+        self
+    }
+}
+
+/// Implement `into_ascii_lowercase` and `into_ascii_uppercase` without memory allocation,
+/// defer other methods to `[u8]`.
+#[unstable(feature = "ascii", issue = "27809")]
+impl AsciiExt for Vec<u8> {
+    type Owned = Self;
+
+    #[inline] fn is_ascii(&self) -> bool { (**self).is_ascii() }
+    #[inline] fn to_ascii_uppercase(&self) -> Self { (**self).to_ascii_uppercase() }
+    #[inline] fn to_ascii_lowercase(&self) -> Self { (**self).to_ascii_lowercase() }
+    #[inline] fn eq_ignore_ascii_case(&self, o: &Self) -> bool { (**self).eq_ignore_ascii_case(o) }
+    #[inline] fn make_ascii_uppercase(&mut self) { (**self).make_ascii_uppercase() }
+    #[inline] fn make_ascii_lowercase(&mut self) { (**self).make_ascii_lowercase() }
+
+    fn into_ascii_lowercase(mut self) -> Self {
+        for byte in &mut self {
+            *byte = byte.to_ascii_lowercase()
+        }
+        self
+    }
+
+    fn into_ascii_uppercase(mut self) -> Self {
+        for byte in &mut self {
+            *byte = byte.to_ascii_uppercase()
+        }
+        self
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]