Specialize Vec::extend to Vec::extend_from_slice

[fhartwig]

I tried using the existing SpecExtend as a helper trait for this, but the instances would always conflict with the instances higher up in the file, so I created a new helper trait.

Benchmarking extend vs extend_from_slice with an slice of 1000 u64s gives the following results:

before:
running 2 tests
test tests::bench_extend_from_slice ... bench: 166 ns/iter (+/- 78)
test tests::bench_extend_trait ... bench: 1,187 ns/iter (+/- 697)
after:
running 2 tests
test tests::bench_extend_from_slice ... bench: 149 ns/iter (+/- 87)
test tests::bench_extend_trait ... bench: 138 ns/iter (+/- 70)

[rust-highfive]

r? @alexcrichton

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[fhartwig]

I'm not really sure what to do with the docstring on extend_from_slice. Saying that extend specialises to the same thing seems premature, considering that specialisation isn't stable yet. But leaving it probably also isn't ideal.

[apasel422]

Out of curiosity, what were the conflicts with SpecExtend?

[fhartwig]

@apasel422 As far as I remember, I tried to do
impl <'a, T: 'a + Copy, I: IntoIterator<Item=&'a T>> SpecExtend<I> for Vec<T>
and
impl<'a, T: Copy> SpecExtend<&'a[T]> for Vec<T>
and both of these would get a conflicting implementation error pointing to the impl<I: IntoIterator> SpecExtend<I> for Vec<I::Item> that comes before. I might be getting this wrong though, but I can look it up tonight.

EDIT:
Here are the errors that I got when I tried implementing SpecExtend. Looking at them now, they kind of make sense to me:

error[E0119]: conflicting implementations of trait `SpecExtend<_>` for type `vec::Vec<_>`:
--> src/libcollections/vec.rs:1597:1
|
1538 | impl<I, T> SpecExtend<I> for Vec<T> where I: IntoIterator<Item=T> {
| - first implementation here
...
1597 | impl <'a, T: 'a + Copy, I: IntoIterator<Item=&'a T>> SpecExtend<I> for Vec<T> {
| ^ conflicting implementation for `vec::Vec<_>`
error[E0119]: conflicting implementations of trait `SpecExtend<&[_]>` for type `vec::Vec<_>`:
--> src/libcollections/vec.rs:1603:1
|
1538 | impl<I, T> SpecExtend<I> for Vec<T> where I: IntoIterator<Item=T> {
| - first implementation here
...
1603 | impl<'a, T: Copy> SpecExtend<&'a[T]> for Vec<T> {
| ^ conflicting implementation for `vec::Vec<_>`

[alexcrichton]

@bors: r+

Thanks!

[bors]

📌 Commit 63ee8d0 has been approved by alexcrichton

[bors]

⌛ Testing commit 63ee8d0 with merge 5059dfe...

[bors]

💔 Test failed - auto-linux-cross-opt

[arielb1]

@bors retry

[bors]

⚡ Previous build results for auto-win-msvc-64-cargotest are reusable. Rebuilding only auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt, auto-win-msvc-64-opt-rustbuild...

[bors]

⛄ The build was interrupted to prioritize another pull request.

[bors]

⌛ Testing commit 63ee8d0 with merge 030bc49...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-rustbuild
Approved by: alexcrichton
Pushing 030bc49 to master...