Use Vec::drain in BufWriter

[bluss]

Use Vec::drain in BufWriter

I happened past a comment that asked for functionality that we now have.

[rust-highfive]

r? @aturon

(rust_highfive has picked a reviewer for you, use r? to override)

[sfackler]

Does calling drain actually remove the items or is it lazy?

[bluss]

It removes all the drained items regardless if you iterate it or not. I checked the compilation of this use case back when I added drain, it should compile to the same code. (I.e. just the ptr::copy).

[bluss]

Regarding laziness, during iteration it only reads out values (ptr::read), the actual removal doesn't happen until the Drain value is dropped (ptr::copy and Vec::set_len).

[alexcrichton]

@bors: r+ b9543ae360e27804747759cd4cc085ca4aaee80d

Yay!

[bors]

⌛ Testing commit b9543ae with merge 181a8e5...

[bors]

💔 Test failed - auto-mac-64-opt

[bluss]

Sigh, forgot to remove an import and didn't see that warning

[bluss]

@bors r=alexcrichton

[bors]

📌 Commit 7b51c1c has been approved by alexcrichton

[bors]

⌛ Testing commit 7b51c1c with merge e4e9319...

[bluss]

Bors come back, stop being weird 😭

[alexcrichton]

This passed all tests except for the android builder which apparently disappeared, so I'm going to merge this manually.