Improved IO Bytes Size Hint

[Xavientois]

After trying to implement better size_hint() return values for File in this PR and changing to implementing it for BufReader in this PR, I have arrived at this implementation that provides tighter bounds for the Bytes iterator of various readers including BufReader, Empty, and Chain.

Unfortunately, for BufReader, the size_hint only improves after calling fill_buffer due to it using the contents of the buffer for the hint. Nevertheless, the the tighter bounds should result in better pre-allocation of space to handle the contents of the Bytes iterator.

Closes #81052

[rust-highfive]

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[bors]

☔ The latest upstream changes (presumably #81578) made this pull request unmergeable. Please resolve the merge conflicts.

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

configure: rust.channel         := nightly
configure: rust.debug-assertions := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
configure: 
---
skip untracked path cpu-usage.csv during rustfmt invocations
skip untracked path src/doc/book/ during rustfmt invocations
skip untracked path src/doc/rust-by-example/ during rustfmt invocations
skip untracked path src/llvm-project/ during rustfmt invocations
Diff in /checkout/library/std/src/io/util.rs at line 4:
Running `"/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/rustfmt" "--config-path" "/checkout" "--edition" "2018" "--unstable-features" "--skip-children" "--check" "/checkout/library/std/src/io/util.rs"` failed.
If you're running `tidy`, try again with `--bless`. Or, if you just want to format code, run `./x.py fmt` instead.
 mod tests;
 use crate::fmt;
 use crate::fmt;
-use crate::io::{self, BufRead, Initializer, IoSlice, IoSliceMut, Read, Seek, SeekFrom, SizeHint, Write};
+use crate::io::{
+    self, BufRead, Initializer, IoSlice, IoSliceMut, Read, Seek, SeekFrom, SizeHint, Write,
 
 
 /// A reader which is always at EOF.
failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test --stage 2 src/tools/tidy
Build completed unsuccessfully in 0:00:16

[Dylan-DPC-zz]

@cramertj this is ready for review

[chadbrewbaker]

Sorry if I am blind, but I didn't see an implementation. A benchmark would be nice. BufReader is a pain point for me right now in several ways.

Chiefly, read_until doesn't support stream processing. You should be able to pass in a function that does a left fold over fixed buffer reads until it hits the end of line or whatnot. I'm agnostic if the buffer stays as Vector that doesn't grow or is a u8 array.

fn read_until_streaming(&mut self, byte: u8, buf: &mut Vec<u8>, left_fold_func, init_state ) ->
 (Result<usize>, final_state) {
    }

A "hello world" might preform an XOR over the bytes of each line.

struct XorState {
    sum: u8 =0;
}

fun xor_fold( &mut state: XorState, buf: &mut Vec<u8>, byte: u8){
   // while (buf.peek() != byte)
   // state.sum = xor(state.sum, buf.get() );
}

[Xavientois]

@chadbrewbaker I'm not sure I understand your question. If you're wondering where the updated size hint is implemented for BufReader it is on line 441 of library/std/src/io/buffered/bufreader.rs.

Are you asking benchmarks to show the performance boost for this improved size hint? I am not sure what I would be checking since the goal of this PR is to provide tighter bounds from the size hint when possible, which is shown by the unit tests I included.

[chadbrewbaker]

Sorry, missed the unit test. Ok.

[cramertj]

@bors r+

[bors]

📌 Commit 7674ae1 has been approved by cramertj

[bors]

⌛ Testing commit 7674ae1 with merge b347d73d836ef2d36e2e8160b567c5b3edeee259...

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

Removing intermediate container 93ce468f24a5
 ---> 7ae884a133af
Step 5/10 : RUN npm install es-check -g
 ---> Running in 440ac6ccb27e
/node-v14.4.0-linux-x64/bin/es-check -> /node-v14.4.0-linux-x64/lib/node_modules/es-check/index.js

> spawn-sync@1.0.15 postinstall /node-v14.4.0-linux-x64/lib/node_modules/es-check/node_modules/spawn-sync
> node postinstall
+ es-check@5.2.1
added 95 packages from 44 contributors in 3.766s
Removing intermediate container 440ac6ccb27e
 ---> b4d6f0a1ef82
---
Cloning into 'rust-toolstate'...
<Nothing changed>
+ es-check es5 ../src/librustdoc/html/static/main.js ../src/librustdoc/html/static/settings.js ../src/librustdoc/html/static/source-script.js ../src/librustdoc/html/static/storage.js

Cannot read property 'includes' of undefined

[bors]

💔 Test failed - checks-actions

[JohnTitor]

Unrelated CI failure, @bors retry