Add CompactCowStr

Author: SimonSapin

src/compact_cow_str.rs

@@ -0,0 +1,231 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use std::borrow::{Borrow, Cow};
+use std::cmp;
+use std::fmt;
+use std::hash;
+use std::marker::PhantomData;
+use std::mem;
+use std::ops::Deref;
+use std::slice;
+use std::str;
+
+// All bits set except the highest
+const MAX_LEN: usize = !0 >> 1;
+
+// Only the highest bit
+const OWNED_TAG: usize = MAX_LEN + 1;
+
+/// Like `Cow<'a, str>`, but with smaller `std::mem::size_of`. (Two words instead of four.)
+pub struct CompactCowStr<'a> {
+    // `tagged_len` is a tag in its highest bit, and the string length in the rest of the bits.
+    //
+    // * If the tag is 1, the memory pointed to by `ptr` is owned
+    //   and the lifetime parameter is irrelevant.
+    //   `ptr` and `len` are the components of a `Box<str>`.
+    //
+    // * If the tag is 0, the memory is borrowed.
+    //   `ptr` and `len` are the components of a `&'a str`.
+
+    // FIXME: https://github.com/rust-lang/rust/issues/27730 use NonZero or Shared
+    ptr: *const u8,
+    tagged_len: usize,
+    phantom: PhantomData<&'a str>,
+}
+
+impl<'a> From<&'a str> for CompactCowStr<'a> {
+    #[inline]
+    fn from(s: &'a str) -> Self {
+        let len = s.len();
+        assert!(len <= MAX_LEN);
+        CompactCowStr {
+            ptr: s.as_ptr(),
+            tagged_len: len,
+            phantom: PhantomData,
+        }
+    }
+}
+
+impl<'a> From<Box<str>> for CompactCowStr<'a> {
+    #[inline]
+    fn from(s: Box<str>) -> Self {
+        let ptr = s.as_ptr();
+        let len = s.len();
+        assert!(len <= MAX_LEN);
+        mem::forget(s);
+        CompactCowStr {
+            ptr: ptr,
+            tagged_len: len | OWNED_TAG,
+            phantom: PhantomData,
+        }
+    }
+}
+
+impl<'a> CompactCowStr<'a> {
+    /// Whether this string refers to borrowed memory
+    /// (as opposed to owned, which would be freed when `CompactCowStr` goes out of scope).
+    #[inline]
+    pub fn is_borrowed(&self) -> bool {
+        (self.tagged_len & OWNED_TAG) == 0
+    }
+
+    /// The length of this string
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.tagged_len & !OWNED_TAG
+    }
+
+    // Intentionally private since it is easy to use incorrectly.
+    #[inline]
+    fn as_raw_str(&self) -> *const str {
+        unsafe {
+            str::from_utf8_unchecked(slice::from_raw_parts(self.ptr, self.len()))
+        }
+    }
+
+    /// If this string is borrowed, return a slice with the original lifetime,
+    /// not borrowing `self`.
+    ///
+    /// (`Deref` is implemented unconditionally, but returns a slice with a shorter lifetime.)
+    #[inline]
+    pub fn as_str(&self) -> Option<&'a str> {
+        if self.is_borrowed() {
+            Some(unsafe { &*self.as_raw_str() })
+        } else {
+            None
+        }
+    }
+}
+
+impl<'a> Clone for CompactCowStr<'a> {
+    #[inline]
+    fn clone(&self) -> Self {
+        if self.is_borrowed() {
+            CompactCowStr { ..*self }
+        } else {
+            Self::from(Box::from(&**self))
+        }
+    }
+}
+
+impl<'a> Drop for CompactCowStr<'a> {
+    #[inline]
+    fn drop(&mut self) {
+        if !self.is_borrowed() {
+            unsafe {
+                Box::from_raw(self.as_raw_str() as *mut str);
+            }
+        }
+    }
+}
+
+impl<'a> Deref for CompactCowStr<'a> {
+    type Target = str;
+
+    #[inline]
+    fn deref(&self) -> &str {
+        unsafe {
+            &*self.as_raw_str()
+        }
+    }
+}
+
+impl<'a> From<CompactCowStr<'a>> for Cow<'a, str> {
+    #[inline]
+    fn from(cow: CompactCowStr<'a>) -> Self {
+        unsafe {
+            let raw = cow.as_raw_str();
+            let is_borrowed = cow.is_borrowed();
+            mem::forget(cow);
+            if is_borrowed {
+                Cow::Borrowed(&*raw)
+            } else {
+                Cow::Owned(String::from(Box::from_raw(raw as *mut str)))
+            }
+        }
+    }
+}
+
+impl<'a> From<String> for CompactCowStr<'a> {
+    #[inline]
+    fn from(s: String) -> Self {
+        Self::from(s.into_boxed_str())
+    }
+}
+
+impl<'a> From<Cow<'a, str>> for CompactCowStr<'a> {
+    #[inline]
+    fn from(s: Cow<'a, str>) -> Self {
+        match s {
+            Cow::Borrowed(s) => Self::from(s),
+            Cow::Owned(s) => Self::from(s),
+        }
+    }
+}
+
+impl<'a> AsRef<str> for CompactCowStr<'a> {
+    #[inline]
+    fn as_ref(&self) -> &str {
+        self
+    }
+}
+
+impl<'a> Borrow<str> for CompactCowStr<'a> {
+    #[inline]
+    fn borrow(&self) -> &str {
+        self
+    }
+}
+
+impl<'a> Default for CompactCowStr<'a> {
+    #[inline]
+    fn default() -> Self {
+        Self::from("")
+    }
+}
+
+impl<'a> hash::Hash for CompactCowStr<'a> {
+    #[inline]
+    fn hash<H: hash::Hasher>(&self, hasher: &mut H) {
+        str::hash(self, hasher)
+    }
+}
+
+impl<'a, T: AsRef<str>> PartialEq<T> for CompactCowStr<'a> {
+    #[inline]
+    fn eq(&self, other: &T) -> bool {
+        str::eq(self, other.as_ref())
+    }
+}
+
+impl<'a, T: AsRef<str>> PartialOrd<T> for CompactCowStr<'a> {
+    #[inline]
+    fn partial_cmp(&self, other: &T) -> Option<cmp::Ordering> {
+        str::partial_cmp(self, other.as_ref())
+    }
+}
+
+impl<'a> Eq for CompactCowStr<'a> {}
+
+impl<'a> Ord for CompactCowStr<'a> {
+    #[inline]
+    fn cmp(&self, other: &Self) -> cmp::Ordering {
+        str::cmp(self, other)
+    }
+}
+
+impl<'a> fmt::Display for CompactCowStr<'a> {
+    #[inline]
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        str::fmt(self, formatter)
+    }
+}
+
+impl<'a> fmt::Debug for CompactCowStr<'a> {
+    #[inline]
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        str::fmt(self, formatter)
+    }
+}

src/lib.rs

@@ -91,6 +91,7 @@ pub use nth::parse_nth;
 pub use serializer::{ToCss, CssStringWriter, serialize_identifier, serialize_string, TokenSerializationType};
 pub use parser::{Parser, Delimiter, Delimiters, SourcePosition, ParseError, BasicParseError, ParserInput};
 pub use unicode_range::UnicodeRange;
+pub use compact_cow_str::CompactCowStr;
 
 // For macros
 #[doc(hidden)] pub use macros::_internal__to_lowercase;
@@ -116,6 +117,7 @@ mod color;
 mod nth;
 mod serializer;
 mod unicode_range;
+mod compact_cow_str;
 
 #[cfg(test)] mod tests;
 #[cfg(test)] mod size_of_tests;

src/size_of_tests.rs

@@ -2,6 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+use compact_cow_str::CompactCowStr;
 use std::borrow::Cow;
 use tokenizer::{Token, NumericValue, PercentageValue};
 
@@ -35,3 +36,4 @@ size_of_test!(token, Token, 56);
 size_of_test!(numeric_value, NumericValue, 16);
 size_of_test!(percentage_value, PercentageValue, 16);
 size_of_test!(std_cow_str, Cow<'static, str>, 32);
+size_of_test!(compact_cow_str, CompactCowStr, 16);

src/tokenizer.rs

@@ -17,7 +17,7 @@ use self::Token::*;
 
 /// One of the pieces the CSS input is broken into.
 ///
-/// Some components use `CowString` in order to borrow from the original input string
+/// Some components use `Cow` in order to borrow from the original input string
 /// and avoid allocating/copying when possible.
 #[derive(PartialEq, Debug, Clone)]
 pub enum Token<'a> {