rust-lang/rust — pull request #15915 by erickt (2014-07-23 14:07:27+00:00)
Title: std: rename MemWriter to SeekableMemWriter, add seekless MemWriter

std: rename MemWriter to SeekableMemWriter, add seekless MemWriter

Not all users of MemWriter need to seek, but having MemWriter seekable adds between 3-29% in overhead in certain circumstances. This fixes that performance gap by making a non-seekable MemWriter, and creating a new SeekableMemWriter for those circumstances when that functionality is actually needed.

```
test io::mem::test::bench_buf_reader ... bench: 682 ns/iter (+/- 85)
test io::mem::test::bench_buf_writer ... bench: 580 ns/iter (+/- 57)
test io::mem::test::bench_mem_reader ... bench: 793 ns/iter (+/- 99)
test io::mem::test::bench_mem_writer_001_0000 ... bench: 48 ns/iter (+/- 27)
test io::mem::test::bench_mem_writer_001_0010 ... bench: 65 ns/iter (+/- 27) = 153 MB/s
test io::mem::test::bench_mem_writer_001_0100 ... bench: 132 ns/iter (+/- 12) = 757 MB/s
test io::mem::test::bench_mem_writer_001_1000 ... bench: 802 ns/iter (+/- 151) = 1246 MB/s
test io::mem::test::bench_mem_writer_100_0000 ... bench: 481 ns/iter (+/- 28)
test io::mem::test::bench_mem_writer_100_0010 ... bench: 1957 ns/iter (+/- 126) = 510 MB/s
test io::mem::test::bench_mem_writer_100_0100 ... bench: 8222 ns/iter (+/- 434) = 1216 MB/s
test io::mem::test::bench_mem_writer_100_1000 ... bench: 82496 ns/iter (+/- 11191) = 1212 MB/s
test io::mem::test::bench_seekable_mem_writer_001_0000 ... bench: 48 ns/iter (+/- 2)
test io::mem::test::bench_seekable_mem_writer_001_0010 ... bench: 64 ns/iter (+/- 2) = 156 MB/s
test io::mem::test::bench_seekable_mem_writer_001_0100 ... bench: 129 ns/iter (+/- 7) = 775 MB/s
test io::mem::test::bench_seekable_mem_writer_001_1000 ... bench: 801 ns/iter (+/- 159) = 1248 MB/s
test io::mem::test::bench_seekable_mem_writer_100_0000 ... bench: 711 ns/iter (+/- 51)
test io::mem::test::bench_seekable_mem_writer_100_0010 ... bench: 2532 ns/iter (+/- 227) = 394 MB/s
test io::mem::test::bench_seekable_mem_writer_100_0100 ... bench: 8962 ns/iter (+/- 947) = 1115 MB/s
test io::mem::test::bench_seekable_mem_writer_100_1000 ... bench: 85086 ns/iter (+/- 11555) = 1175 MB/s
```