impl Add<{str, Cow<str>}> for Cow<str> #36430

llogiq · 2016-09-12T21:57:21Z

llogiq · 2016-09-13T22:06:52Z

r? @alexcrichton – or do we need to discuss this first?

alexcrichton · 2016-09-14T18:07:39Z

src/libcollections/borrow.rs

+        if self == "" {
+            Cow::Borrowed(rhs)
+        } else if rhs == "" {
+            self.clone()


This can just be self, right?

alexcrichton · 2016-09-14T18:08:23Z

Thanks for the PR! Could you expand on the rationale for this as well? E.g. it seems specifically for the optimization where one side is empty?

llogiq · 2016-09-14T21:34:48Z

Exactly. I have found this optimization while looking at Raph Levien's xi-editor, and thought it'd be a good idea to have it in libcollections.

alilleybrinker · 2016-09-16T14:16:46Z

There's a relevant discussion happening in the internals forum right now: https://internals.rust-lang.org/t/implement-add-for-string-string/4088/7

Personally, I am opposed to adding further impls where Add is used for string concatenation. There is obviously already one impl for this use case, but I think this was a design mistake, and shouldn't be made worse. It would be preferable, if possible, to have some other operator for concatenation in general, and then implement said operator for all relevant types (I do not know how feasible such a change is at this point, it may be that it is not feasible at all). If nothing else, Add as implemented here is non-commutative, which goes against what I imagine is many people's intuition (certainly mine) for the addition operator.

As an addendum to that, it may be a good idea to lay out what basic mathematical operator properties we like for the operator trait implementations to respect (commutativity, associativity, etc.). Obviously, as they are safe traits, such properties couldn't be relied on in safe code, but including an explanation of them in the docs would (hopefully) encourage the creation of implementations which respect users' intuitions for those operators.

llogiq · 2016-09-16T15:14:17Z

I know where you come from, but Add for String is pretty much entrenched by now (I even had to allow the clippy lints for it due to complaints). And I disagree that we'd be making things worse – if you're opposed to String addition, you can always extend. OTOH, improving Cow ergonomics is a win in my book.

alilleybrinker · 2016-09-16T19:52:13Z

Yeah, you may be right given that string concatenation via Add already exists in the language. There's no good reason to leave performance improvements on the table out of an ideological opposition to something that's already been stabilized. Removing the Add impls for string concatenation and replacing them with some alternative operator may be a thing to consider for Rust 2.0 (if/when that arrives).

alexcrichton · 2016-09-25T20:30:12Z

I've tagged for discussion in libs triage, but I agree that it's probably too late to remove Add on String and those types. It's already stable today so it'd be a bit odd to hold off on this purely because of that.

Also @llogiq can you add AddAssign implementation as well?

llogiq · 2016-09-27T19:26:49Z

That took way longer than usual due to my notebook (which was broken by my kids) failing when it gets hot – for example during a Rust build.

@alexcrichton should I add anything else?

alexcrichton · 2016-09-29T00:48:05Z

Ah nope, just waiting on the libs team to discuss. Our conclusion was that this should be good to go. Want to squash the commits as well?

llogiq · 2016-09-29T05:15:11Z

~~My son Gregor was born yesterday, so since then I'm in childrearing mode. Feel free to squash while merging. 😄~~

scratch that, the kid's sleeping, found the time to squash. 😀

This does not actually add anything that wasn't there, but is merely an optimization for the given cases, which would have incurred additional heap allocation for adding empty strings, and improving the ergonomics of `Cow` with strings.

alexcrichton · 2016-09-29T22:45:21Z

@bors: r+

Oh wow, congratulations! I can definitely take over from here :)

bors · 2016-09-29T22:45:23Z

📌 Commit dd13a80 has been approved by alexcrichton

bors · 2016-09-29T22:50:32Z

⌛ Testing commit dd13a80 with merge c717cfa...

impl Add<{str, Cow<str>}> for Cow<str> cc #35837

ollie27

It might be worth adding tests for Cow + &str as well.

ollie27 · 2016-09-29T18:08:21Z

src/libcollections/borrow.rs

@@ -270,3 +270,49 @@ impl<'a, T: ?Sized + ToOwned> AsRef<T> for Cow<'a, T> {
        self
    }
 }
+
+#[stable(feature = "cow_add", since = "1.13.0")]


These will need to be 1.14.0 now.

ollie27 · 2016-09-29T22:54:35Z

src/libcollections/borrow.rs

+        } else if rhs == "" {
+            self
+        } else {
+            Cow::Owned(self.into_owned() + rhs)


If self is a Cow::Borrowed it would be better to use String::with_capacity to create the new string so we avoid an extra reallocation.

ollie27 · 2016-09-29T22:57:35Z

src/libcollections/borrow.rs

+impl<'a> AddAssign<&'a str> for Cow<'a, str> {
+    fn add_assign(&mut self, rhs: &'a str) {
+        if rhs == "" { return; }
+        self.to_mut().push_str(rhs);


This doesn't behave the same as the Add implementations. If self is empty this should result in a Cow::Borrowed like with Add not Cow::Owned.

ollie27 · 2016-09-29T23:00:35Z

src/libcollectionstest/cow_str.rs

@@ -0,0 +1,65 @@
+// Copyright 2012-2013-2014 The Rust Project Developers. See the COPYRIGHT


You'll need to add mod cow_str; to src/libcollectionstest/lib.rs for this file to be used.

Also I'm pretty sure "2012-2013-2014" isn't the correct year 😉.

ollie27 · 2016-09-29T23:02:12Z

src/libcollectionstest/cow_str.rs

+    assert_eq!("Hello, Rustaceans!", borrowed1 + owned2);
+
+    assert_eq!("Hello, World!", owned1 + borrowed2);
+    assert_eq!("Hello, Rustaceans!", owned1 + owned2);


These two should presumably start with "Hi, ".

Also owned1 and owned2 have been consumed by this point so will need cloning or something earlier.

ollie27 · 2016-09-29T23:02:33Z

src/libcollectionstest/cow_str.rs

+    }
+}
+
+fn check_cow_add_assign() {


This will need #[test].

ollie27 · 2016-09-29T23:05:31Z

src/libcollections/borrow.rs

+impl<'a> Add<&'a str> for Cow<'a, str> {
+    type Output = Cow<'a, str>;
+
+    fn add(self, rhs: &'a str) -> Self {


I think it would be worth adding documentation to these implementations as the semantics aren't really obvious. Especially explaining why the lifetimes have been restricted like they have.

ollie27 · 2016-09-29T23:11:35Z

src/libcollections/borrow.rs

+    type Output = Cow<'a, str>;
+
+    fn add(self, rhs: &'a str) -> Self {
+        if self == "" {


nit: might be better to use .is_empty().

bors · 2016-09-30T03:25:43Z

alexcrichton added the T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. label Sep 13, 2016

rust-highfive assigned alexcrichton Sep 13, 2016

alexcrichton reviewed Sep 14, 2016
View reviewed changes

llogiq force-pushed the cow_add branch from b9b4810 to dd13a80 Compare September 29, 2016 12:59

bors added a commit that referenced this pull request Sep 29, 2016

Auto merge of #36430 - llogiq:cow_add, r=alexcrichton

c717cfa

impl Add<{str, Cow<str>}> for Cow<str> cc #35837

ollie27 suggested changes Sep 29, 2016

View reviewed changes

bors merged commit dd13a80 into rust-lang:master Sep 30, 2016

llogiq deleted the cow_add branch December 28, 2016 23:45

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impl Add<{str, Cow<str>}> for Cow<str> #36430

impl Add<{str, Cow<str>}> for Cow<str> #36430

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bors commented Sep 30, 2016

		@@ -0,0 +1,65 @@
		// Copyright 2012-2013-2014 The Rust Project Developers. See the COPYRIGHT

impl Add<{str, Cow<str>}> for Cow<str> #36430

impl Add<{str, Cow<str>}> for Cow<str> #36430

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bors commented Sep 30, 2016

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