BufReader/Writer extension methods: is_empty, buffer

[fintelia]

Update: this is the tracking issue for:

impl<R: Read> BufReader<R> {
    pub fn buffer(&self) -> &[u8] {…}

    #[rustc_deprecated(since = "1.26.0", reason = "use .buffer().is_empty() instead")]
    pub fn is_empty(&self) -> bool {…}
}

is_empty is both unstable and deprecated, it should be removed.

---

There is currently no way to tell whether there is any data buffered inside a BufReader. This is unfortunate because it means that an application has no way to know whether calls to read() and fill_buffer() will return instantly or trigger a potentially expensive call to the underlying Reader. I propose adding an is_empty() method to BufReader to fill this gap.

[jonhoo]

@kennytm now that #45369 has been merged, I'm guessing this should get the C-tracking-issue label?

[dbrgn]

What's left for this to stabilize?

I'm parsing bytes from a BufReader to MsgPack using read_value. The function will stop parsing once a valid MsgPack value has been found. But in my application the parsing should fail if the buffer hasn't been consumed completely. Checking buf.is_empty() would probably be the cleanest way to do so.

[jonhoo]

@dbrgn the BufReader::buf_bytes method proposed in #48022 may also be useful for that purpose, though that may take even longer to land.

[SimonSapin]

This is the tracking issue for:

impl<R: Read> BufReader<R> {
    pub fn is_empty(&self) -> bool { /*…*/ }
}

Looks good to me to stabilize.

@rfcbot fcp merge

[rfcbot]

Team member @SimonSapin has proposed to merge this. The next step is review by the rest of the tagged teams:

• @Kimundi
• @SimonSapin
• @alexcrichton
• @aturon
• @dtolnay
• @sfackler
• @withoutboats

No concerns currently listed.

Once a majority of reviewers approve (and none object), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[sfackler]

This seems fine to stabilize, but it seems like it'd be more useful to know how many bytes are buffered, rather than just that there are some number of bytes.

[SimonSapin]

Why not both? Slices have is_empty even though it’s somewhat redundant with len.

[sfackler]

For sure. Would it be reasonable to try to squeeze that into this stabilization?