Add `BufRead::skip_until`

[WilliamVenner]

Proposal

Problem statement

Rust provides a way to "read everything up to x" (BufRead::read_until), but overlooks that you may want to discard that data, ergo, ignore it.

Motivation, use-cases

When reading a binary file format, you may encounter NUL-terminated strings. These can be easily read using BufRead::read_until to read up to a 0 byte, but what if I don't care about that information? What if I want to skip over it? It is tempting to use BufRead::read_until(..., b'\0', &mut Vec::new()) but the optimiser does not optimise away the unused Vec and resulting allocations in this case.

Solution sketches

Solution 1

Make the buf argument of BufRead::read_until an Option<&mut Vec<u8>>

• Introduces a breaking change
• Results in slower code for both Some and None cases
• Harshly non-self-documenting, needs explanation for its behaviour, isn't immediately obvious

Solution 2

Copy + paste the BufRead::read_until implementation and remove the buf argument and its resulting extend_from_slice calls.

• Specialized for this use case
• In my benchmarks is roughly x2 faster than BufRead::read_until(..., b'\0', &mut Vec::new())

[joshtriplett]

Discussed in today's libs meetup. 👍 for solution 2: add a skip_until function that skips over the data.

Please do submit a PR.

Sorry for the long latency.

[WilliamVenner]

PR rust-lang/rust#98943 🎉