Rollup merge of #83353 - m-ou-se:io-error-avoid-alloc, r=nagisa

Add internal io::Error::new_const to avoid allocations.

This makes it possible to have a io::Error containing a message with zero allocations, and uses that everywhere to avoid the *three* allocations involved in `io::Error::new(kind, "message")`.

The function signature isn't perfect, because it needs a reference to the `&str`. So for now, this is just a `pub(crate)` function. Later, we'll be able to use `fn new_const<MSG: &'static str>(kind: ErrorKind)` to make that a bit better. (Then we'll also be able to use some ZST trickery if that would result in more efficient code.)

See https://github.com/rust-lang/rust/issues/83352

1 files changed, 2 insertions, 2 deletions

diff --git a/library/std/src/io/impls.rs b/library/std/src/io/impls.rs
index 9870cfc4c95..6891bd8a664 100644
--- a/library/std/src/io/impls.rs
+++ b/library/std/src/io/impls.rs
@@ -263,7 +263,7 @@ impl Read for &[u8] {
     #[inline]
     fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
         if buf.len() > self.len() {
-            return Err(Error::new(ErrorKind::UnexpectedEof, "failed to fill whole buffer"));
+            return Err(Error::new_const(ErrorKind::UnexpectedEof, &"failed to fill whole buffer"));
         }
         let (a, b) = self.split_at(buf.len());
 
@@ -345,7 +345,7 @@ impl Write for &mut [u8] {
         if self.write(data)? == data.len() {
             Ok(())
         } else {
-            Err(Error::new(ErrorKind::WriteZero, "failed to write whole buffer"))
+            Err(Error::new_const(ErrorKind::WriteZero, &"failed to write whole buffer"))
         }
     }