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

library/std/src/ffi/c_str.rs

@@ -1036,7 +1036,7 @@ impl fmt::Display for NulError {
 impl From<NulError> for io::Error {
     /// Converts a [`NulError`] into a [`io::Error`].
     fn from(_: NulError) -> io::Error {
-        io::Error::new(io::ErrorKind::InvalidInput, "data provided contains a nul byte")
+        io::Error::new_const(io::ErrorKind::InvalidInput, &"data provided contains a nul byte")
     }
 }
 

library/std/src/fs.rs

@@ -2188,7 +2188,10 @@ impl DirBuilder {
         match path.parent() {
             Some(p) => self.create_dir_all(p)?,
             None => {
-                return Err(io::Error::new(io::ErrorKind::Other, "failed to create whole tree"));
+                return Err(io::Error::new_const(
+                    io::ErrorKind::Other,
+                    &"failed to create whole tree",
+                ));
             }
         }
         match self.inner.mkdir(path) {

library/std/src/io/buffered/bufwriter.rs

@@ -164,9 +164,9 @@ impl<W: Write> BufWriter<W> {
 
             match r {
                 Ok(0) => {
-                    return Err(Error::new(
+                    return Err(Error::new_const(
                         ErrorKind::WriteZero,
-                        "failed to write the buffered data",
+                        &"failed to write the buffered data",
                     ));
                 }
                 Ok(n) => guard.consume(n),

library/std/src/io/cursor.rs

@@ -229,9 +229,9 @@ where
                 self.pos = n;
                 Ok(self.pos)
             }
-            None => Err(Error::new(
+            None => Err(Error::new_const(
                 ErrorKind::InvalidInput,
-                "invalid seek to a negative or overflowing position",
+                &"invalid seek to a negative or overflowing position",
             )),
         }
     }
@@ -328,9 +328,9 @@ fn slice_write_vectored(
 // Resizing write implementation
 fn vec_write(pos_mut: &mut u64, vec: &mut Vec<u8>, buf: &[u8]) -> io::Result<usize> {
     let pos: usize = (*pos_mut).try_into().map_err(|_| {
-        Error::new(
+        Error::new_const(
             ErrorKind::InvalidInput,
-            "cursor position exceeds maximum possible vector length",
+            &"cursor position exceeds maximum possible vector length",
         )
     })?;
     // Make sure the internal buffer is as least as big as where we

library/std/src/io/error.rs

@@ -69,6 +69,8 @@ impl fmt::Debug for Error {
 enum Repr {
     Os(i32),
     Simple(ErrorKind),
+    // &str is a fat pointer, but &&str is a thin pointer.
+    SimpleMessage(ErrorKind, &'static &'static str),
     Custom(Box<Custom>),
 }
 
@@ -259,6 +261,18 @@ impl Error {
         Error { repr: Repr::Custom(Box::new(Custom { kind, error })) }
     }
 
+    /// Creates a new I/O error from a known kind of error as well as a
+    /// constant message.
+    ///
+    /// This function does not allocate.
+    ///
+    /// This function should maybe change to
+    /// `new_const<const MSG: &'static str>(kind: ErrorKind)`
+    /// in the future, when const generics allow that.
+    pub(crate) const fn new_const(kind: ErrorKind, message: &'static &'static str) -> Error {
+        Self { repr: Repr::SimpleMessage(kind, message) }
+    }
+
     /// Returns an error representing the last OS error which occurred.
     ///
     /// This function reads the value of `errno` for the target platform (e.g.
@@ -342,6 +356,7 @@ impl Error {
             Repr::Os(i) => Some(i),
             Repr::Custom(..) => None,
             Repr::Simple(..) => None,
+            Repr::SimpleMessage(..) => None,
         }
     }
 
@@ -377,6 +392,7 @@ impl Error {
         match self.repr {
             Repr::Os(..) => None,
             Repr::Simple(..) => None,
+            Repr::SimpleMessage(..) => None,
             Repr::Custom(ref c) => Some(&*c.error),
         }
     }
@@ -448,6 +464,7 @@ impl Error {
         match self.repr {
             Repr::Os(..) => None,
             Repr::Simple(..) => None,
+            Repr::SimpleMessage(..) => None,
             Repr::Custom(ref mut c) => Some(&mut *c.error),
         }
     }
@@ -484,6 +501,7 @@ impl Error {
         match self.repr {
             Repr::Os(..) => None,
             Repr::Simple(..) => None,
+            Repr::SimpleMessage(..) => None,
             Repr::Custom(c) => Some(c.error),
         }
     }
@@ -512,6 +530,7 @@ impl Error {
             Repr::Os(code) => sys::decode_error_kind(code),
             Repr::Custom(ref c) => c.kind,
             Repr::Simple(kind) => kind,
+            Repr::SimpleMessage(kind, _) => kind,
         }
     }
 }
@@ -527,6 +546,9 @@ impl fmt::Debug for Repr {
                 .finish(),
             Repr::Custom(ref c) => fmt::Debug::fmt(&c, fmt),
             Repr::Simple(kind) => fmt.debug_tuple("Kind").field(&kind).finish(),
+            Repr::SimpleMessage(kind, &message) => {
+                fmt.debug_struct("Error").field("kind", &kind).field("message", &message).finish()
+            }
         }
     }
 }
@@ -541,6 +563,7 @@ impl fmt::Display for Error {
             }
             Repr::Custom(ref c) => c.error.fmt(fmt),
             Repr::Simple(kind) => write!(fmt, "{}", kind.as_str()),
+            Repr::SimpleMessage(_, &msg) => msg.fmt(fmt),
         }
     }
 }
@@ -551,6 +574,7 @@ impl error::Error for Error {
     fn description(&self) -> &str {
         match self.repr {
             Repr::Os(..) | Repr::Simple(..) => self.kind().as_str(),
+            Repr::SimpleMessage(_, &msg) => msg,
             Repr::Custom(ref c) => c.error.description(),
         }
     }
@@ -560,6 +584,7 @@ impl error::Error for Error {
         match self.repr {
             Repr::Os(..) => None,
             Repr::Simple(..) => None,
+            Repr::SimpleMessage(..) => None,
             Repr::Custom(ref c) => c.error.cause(),
         }
     }
@@ -568,6 +593,7 @@ impl error::Error for Error {
         match self.repr {
             Repr::Os(..) => None,
             Repr::Simple(..) => None,
+            Repr::SimpleMessage(..) => None,
             Repr::Custom(ref c) => c.error.source(),
         }
     }

library/std/src/io/error/tests.rs

@@ -1,9 +1,15 @@
 use super::{Custom, Error, ErrorKind, Repr};
 use crate::error;
 use crate::fmt;
+use crate::mem::size_of;
 use crate::sys::decode_error_kind;
 use crate::sys::os::error_string;
 
+#[test]
+fn test_size() {
+    assert!(size_of::<Error>() <= size_of::<[usize; 2]>());
+}
+
 #[test]
 fn test_debug_error() {
     let code = 6;
@@ -51,3 +57,13 @@ fn test_downcasting() {
     let extracted = err.into_inner().unwrap();
     extracted.downcast::<TestError>().unwrap();
 }
+
+#[test]
+fn test_const() {
+    const E: Error = Error::new_const(ErrorKind::NotFound, &"hello");
+
+    assert_eq!(E.kind(), ErrorKind::NotFound);
+    assert_eq!(E.to_string(), "hello");
+    assert!(format!("{:?}", E).contains("\"hello\""));
+    assert!(format!("{:?}", E).contains("NotFound"));
+}

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"))
         }
     }
 

library/std/src/io/mod.rs

@@ -333,7 +333,7 @@ where
         let ret = f(g.buf);
         if str::from_utf8(&g.buf[g.len..]).is_err() {
             ret.and_then(|_| {
-                Err(Error::new(ErrorKind::InvalidData, "stream did not contain valid UTF-8"))
+                Err(Error::new_const(ErrorKind::InvalidData, &"stream did not contain valid UTF-8"))
             })
         } else {
             g.len = g.buf.len();
@@ -429,7 +429,7 @@ pub(crate) fn default_read_exact<R: Read + ?Sized>(this: &mut R, mut buf: &mut [
         }
     }
     if !buf.is_empty() {
-        Err(Error::new(ErrorKind::UnexpectedEof, "failed to fill whole buffer"))
+        Err(Error::new_const(ErrorKind::UnexpectedEof, &"failed to fill whole buffer"))
     } else {
         Ok(())
     }
@@ -1432,7 +1432,10 @@ pub trait Write {
         while !buf.is_empty() {
             match self.write(buf) {
                 Ok(0) => {
-                    return Err(Error::new(ErrorKind::WriteZero, "failed to write whole buffer"));
+                    return Err(Error::new_const(
+                        ErrorKind::WriteZero,
+                        &"failed to write whole buffer",
+                    ));
                 }
                 Ok(n) => buf = &buf[n..],
                 Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
@@ -1497,7 +1500,10 @@ pub trait Write {
         while !bufs.is_empty() {
             match self.write_vectored(bufs) {
                 Ok(0) => {
-                    return Err(Error::new(ErrorKind::WriteZero, "failed to write whole buffer"));
+                    return Err(Error::new_const(
+                        ErrorKind::WriteZero,
+                        &"failed to write whole buffer",
+                    ));
                 }
                 Ok(n) => bufs = IoSlice::advance(bufs, n),
                 Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
@@ -1571,7 +1577,7 @@ pub trait Write {
                 if output.error.is_err() {
                     output.error
                 } else {
-                    Err(Error::new(ErrorKind::Other, "formatter error"))
+                    Err(Error::new_const(ErrorKind::Other, &"formatter error"))
                 }
             }
         }

library/std/src/io/tests.rs

@@ -152,12 +152,12 @@ fn take_eof() {
 
     impl Read for R {
         fn read(&mut self, _: &mut [u8]) -> io::Result<usize> {
-            Err(io::Error::new(io::ErrorKind::Other, ""))
+            Err(io::Error::new_const(io::ErrorKind::Other, &""))
         }
     }
     impl BufRead for R {
         fn fill_buf(&mut self) -> io::Result<&[u8]> {
-            Err(io::Error::new(io::ErrorKind::Other, ""))
+            Err(io::Error::new_const(io::ErrorKind::Other, &""))
         }
         fn consume(&mut self, _amt: usize) {}
     }

library/std/src/net/mod.rs

@@ -88,6 +88,6 @@ where
         }
     }
     Err(last_err.unwrap_or_else(|| {
-        Error::new(ErrorKind::InvalidInput, "could not resolve to any addresses")
+        Error::new_const(ErrorKind::InvalidInput, &"could not resolve to any addresses")
     }))
 }

library/std/src/net/udp.rs

@@ -173,7 +173,7 @@ impl UdpSocket {
     pub fn send_to<A: ToSocketAddrs>(&self, buf: &[u8], addr: A) -> io::Result<usize> {
         match addr.to_socket_addrs()?.next() {
             Some(addr) => self.0.send_to(buf, &addr),
-            None => Err(Error::new(ErrorKind::InvalidInput, "no addresses to send data to")),
+            None => Err(Error::new_const(ErrorKind::InvalidInput, &"no addresses to send data to")),
         }
     }
 

library/std/src/sys/hermit/fd.rs

@@ -46,19 +46,19 @@ impl FileDesc {
         self.duplicate_path(&[])
     }
     pub fn duplicate_path(&self, _path: &[u8]) -> io::Result<FileDesc> {
-        Err(io::Error::new(ErrorKind::Other, "duplicate isn't supported"))
+        Err(io::Error::new_const(ErrorKind::Other, &"duplicate isn't supported"))
     }
 
     pub fn nonblocking(&self) -> io::Result<bool> {
         Ok(false)
     }
 
     pub fn set_cloexec(&self) -> io::Result<()> {
-        Err(io::Error::new(ErrorKind::Other, "cloexec isn't supported"))
+        Err(io::Error::new_const(ErrorKind::Other, &"cloexec isn't supported"))
     }
 
     pub fn set_nonblocking(&self, _nonblocking: bool) -> io::Result<()> {
-        Err(io::Error::new(ErrorKind::Other, "nonblocking isn't supported"))
+        Err(io::Error::new_const(ErrorKind::Other, &"nonblocking isn't supported"))
     }
 }
 

library/std/src/sys/hermit/fs.rs

@@ -226,7 +226,7 @@ impl OpenOptions {
             (false, _, true) => Ok(O_WRONLY | O_APPEND),
             (true, _, true) => Ok(O_RDWR | O_APPEND),
             (false, false, false) => {
-                Err(io::Error::new(ErrorKind::InvalidInput, "invalid access mode"))
+                Err(io::Error::new_const(ErrorKind::InvalidInput, &"invalid access mode"))
             }
         }
     }
@@ -236,12 +236,18 @@ impl OpenOptions {
             (true, false) => {}
             (false, false) => {
                 if self.truncate || self.create || self.create_new {
-                    return Err(io::Error::new(ErrorKind::InvalidInput, "invalid creation mode"));
+                    return Err(io::Error::new_const(
+                        ErrorKind::InvalidInput,
+                        &"invalid creation mode",
+                    ));
                 }
             }
             (_, true) => {
                 if self.truncate && !self.create_new {
-                    return Err(io::Error::new(ErrorKind::InvalidInput, "invalid creation mode"));
+                    return Err(io::Error::new_const(
+                        ErrorKind::InvalidInput,
+                        &"invalid creation mode",
+                    ));
                 }
             }
         }

library/std/src/sys/hermit/mod.rs

@@ -54,7 +54,10 @@ pub fn unsupported<T>() -> crate::io::Result<T> {
 }
 
 pub fn unsupported_err() -> crate::io::Error {
-    crate::io::Error::new(crate::io::ErrorKind::Other, "operation not supported on HermitCore yet")
+    crate::io::Error::new_const(
+        crate::io::ErrorKind::Other,
+        &"operation not supported on HermitCore yet",
+    )
 }
 
 // This enum is used as the storage for a bunch of types which can't actually