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

llogiq · Sep 12, 2016

cc #35837

llogiq · Sep 13, 2016

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

alexcrichton · Sep 14, 2016

alexcrichton reviewed Sep 14, 2016

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alexcrichton · Sep 14, 2016

src/libcollections/borrow.rs

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


This can just be self, right?

alexcrichton · Sep 14, 2016

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 · Sep 14, 2016

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 · Sep 16, 2016

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 · Sep 16, 2016

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 · Sep 16, 2016

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 · Sep 25, 2016

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 · Sep 27, 2016

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 · Sep 29, 2016

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 · Sep 29, 2016

~~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. 😀

alexcrichton · Sep 29, 2016

@bors: r+

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

bors · Sep 29, 2016

📌 Commit dd13a80 has been approved by alexcrichton

bors · Sep 29, 2016

⌛️ Testing commit dd13a80 with merge c717cfa...

ollie27 · Sep 29, 2016

ollie27 suggested changes Sep 29, 2016

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

ollie27 · Sep 29, 2016

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 · Sep 29, 2016

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 · Sep 29, 2016

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 · Sep 29, 2016

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 · Sep 29, 2016

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 · Sep 29, 2016

src/libcollectionstest/cow_str.rs

+    }
+}
+
+fn check_cow_add_assign() {


This will need #[test].

ollie27 · Sep 29, 2016

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 · Sep 29, 2016

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 · Sep 30, 2016

alexcrichton added the T-libs label Sep 13, 2016

rust-highfive assigned alexcrichton Sep 13, 2016

llogiq force-pushed the llogiq:cow_add branch from b9b4810 to dd13a80 Sep 29, 2016

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

Auto merge of #36430 - llogiq:cow_add, r=alexcrichton
impl Add<{str, Cow<str>}> for Cow<str> cc #35837

c717cfa

bors merged commit dd13a80 into rust-lang:master Sep 30, 2016
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impl Add<{str, Cow<str>}> for Cow<str> #36430

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llogiq commented Sep 12, 2016

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bors commented Sep 30, 2016

Hide details View details bors merged commit dd13a80 into rust-lang:master Sep 30, 2016 2 checks passed

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llogiq commented Sep 13, 2016 •

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llogiq commented Sep 29, 2016 •

edited

llogiq force-pushed the llogiq:cow_add branch from `b9b4810` to `dd13a80` Sep 29, 2016

ollie27 suggested changes Sep 29, 2016

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