From cada71e3f4a0bcd2886b278523dd07eeff2e3e8e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Beno=C3=AEt=20du=20Garreau?= <bdgdlm@outlook.com>
Date: Mon, 19 Dec 2022 20:16:49 +0100
Subject: [PATCH 1/3] `io::Chain`: specialize some `Read` methods

---
 library/std/src/io/mod.rs | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/library/std/src/io/mod.rs b/library/std/src/io/mod.rs
index 23a13523fc275..23159459a70e0 100644
--- a/library/std/src/io/mod.rs
+++ b/library/std/src/io/mod.rs
@@ -2389,6 +2389,39 @@ impl<T: Read, U: Read> Read for Chain<T, U> {
         }
         self.second.read_vectored(bufs)
     }
+
+    #[inline]
+    fn is_read_vectored(&self) -> bool {
+        self.first.is_read_vectored() || self.second.is_read_vectored()
+    }
+
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
+        let mut read = 0;
+        if !self.done_first {
+            read += self.first.read_to_end(buf)?;
+            self.done_first = true;
+        }
+        read += self.second.read_to_end(buf)?;
+        Ok(read)
+    }
+
+    fn read_buf(&mut self, mut buf: BorrowedCursor<'_>) -> Result<()> {
+        if buf.capacity() == 0 {
+            return Ok(());
+        }
+
+        if !self.done_first {
+            let old_len = buf.written();
+            self.first.read_buf(buf.reborrow())?;
+
+            if buf.written() != old_len {
+                return Ok(());
+            } else {
+                self.done_first = true;
+            }
+        }
+        self.second.read_buf(buf)
+    }
 }
 
 #[stable(feature = "chain_bufread", since = "1.9.0")]

From cba6e102ec49d81086a780c12ef3a51f1c2bd340 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Beno=C3=AEt=20du=20Garreau?= <bdgdlm@outlook.com>
Date: Mon, 19 Dec 2022 20:36:21 +0100
Subject: [PATCH 2/3] `io::Chain`: specialize some `BufRead` methods

---
 library/std/src/io/mod.rs | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/library/std/src/io/mod.rs b/library/std/src/io/mod.rs
index 23159459a70e0..3617b956e8069 100644
--- a/library/std/src/io/mod.rs
+++ b/library/std/src/io/mod.rs
@@ -2429,9 +2429,7 @@ impl<T: BufRead, U: BufRead> BufRead for Chain<T, U> {
     fn fill_buf(&mut self) -> Result<&[u8]> {
         if !self.done_first {
             match self.first.fill_buf()? {
-                buf if buf.is_empty() => {
-                    self.done_first = true;
-                }
+                buf if buf.is_empty() => self.done_first = true,
                 buf => return Ok(buf),
             }
         }
@@ -2441,6 +2439,21 @@ impl<T: BufRead, U: BufRead> BufRead for Chain<T, U> {
     fn consume(&mut self, amt: usize) {
         if !self.done_first { self.first.consume(amt) } else { self.second.consume(amt) }
     }
+
+    fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize> {
+        let mut read = 0;
+        if !self.done_first {
+            let n = self.first.read_until(byte, buf)?;
+            read += n;
+
+            match buf.last() {
+                Some(b) if *b == byte && n != 0 => return Ok(read),
+                _ => self.done_first = true,
+            }
+        }
+        read += self.second.read_until(byte, buf)?;
+        Ok(read)
+    }
 }
 
 impl<T, U> SizeHint for Chain<T, U> {

From ebc59703292fd216d7ec1e027e0f3b78387a8ddf Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Beno=C3=AEt=20du=20Garreau?= <bdgdlm@outlook.com>
Date: Mon, 12 Jun 2023 18:17:48 +0200
Subject: [PATCH 3/3] Add tests and comments about `read_to_string` and
 `read_line` specializations

---
 library/std/src/io/mod.rs   |  6 ++++++
 library/std/src/io/tests.rs | 11 +++++++++++
 2 files changed, 17 insertions(+)

diff --git a/library/std/src/io/mod.rs b/library/std/src/io/mod.rs
index 3617b956e8069..244512e2780c5 100644
--- a/library/std/src/io/mod.rs
+++ b/library/std/src/io/mod.rs
@@ -2405,6 +2405,9 @@ impl<T: Read, U: Read> Read for Chain<T, U> {
         Ok(read)
     }
 
+    // We don't override `read_to_string` here because an UTF-8 sequence could
+    // be split between the two parts of the chain
+
     fn read_buf(&mut self, mut buf: BorrowedCursor<'_>) -> Result<()> {
         if buf.capacity() == 0 {
             return Ok(());
@@ -2454,6 +2457,9 @@ impl<T: BufRead, U: BufRead> BufRead for Chain<T, U> {
         read += self.second.read_until(byte, buf)?;
         Ok(read)
     }
+
+    // We don't override `read_line` here because an UTF-8 sequence could be
+    // split between the two parts of the chain
 }
 
 impl<T, U> SizeHint for Chain<T, U> {
diff --git a/library/std/src/io/tests.rs b/library/std/src/io/tests.rs
index f4a886d889a99..4a205f30d44ba 100644
--- a/library/std/src/io/tests.rs
+++ b/library/std/src/io/tests.rs
@@ -231,6 +231,17 @@ fn chain_bufread() {
     cmp_bufread(chain1, chain2, &testdata[..]);
 }
 
+#[test]
+fn chain_splitted_char() {
+    let chain = b"\xc3".chain(b"\xa9".as_slice());
+    assert_eq!(crate::io::read_to_string(chain).unwrap(), "é");
+
+    let mut chain = b"\xc3".chain(b"\xa9\n".as_slice());
+    let mut buf = String::new();
+    assert_eq!(chain.read_line(&mut buf).unwrap(), 3);
+    assert_eq!(buf, "é\n");
+}
+
 #[test]
 fn bufreader_size_hint() {
     let testdata = b"ABCDEFGHIJKL";