Hide Repr details from io::Error, and rework io::Error::new_const.

library/std/src/ffi/c_str.rs

@@ -1077,7 +1077,7 @@ 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_const(io::ErrorKind::InvalidInput, &"data provided contains a nul byte")
+        io::const_io_error!(io::ErrorKind::InvalidInput, "data provided contains a nul byte")
     }
 }
 

library/std/src/fs.rs

@@ -2263,9 +2263,9 @@ impl DirBuilder {
         match path.parent() {
             Some(p) => self.create_dir_all(p)?,
             None => {
-                return Err(io::Error::new_const(
+                return Err(io::const_io_error!(
                     io::ErrorKind::Uncategorized,
-                    &"failed to create whole tree",
+                    "failed to create whole tree",
                 ));
             }
         }

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

@@ -357,9 +357,9 @@ impl<R: Read> Read for BufReader<R> {
             let mut bytes = Vec::new();
             self.read_to_end(&mut bytes)?;
             let string = crate::str::from_utf8(&bytes).map_err(|_| {
-                io::Error::new_const(
+                io::const_io_error!(
                     io::ErrorKind::InvalidData,
-                    &"stream did not contain valid UTF-8",
+                    "stream did not contain valid UTF-8",
                 )
             })?;
             *buf += string;

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

@@ -1,7 +1,7 @@
 use crate::error;
 use crate::fmt;
 use crate::io::{
-    self, Error, ErrorKind, IntoInnerError, IoSlice, Seek, SeekFrom, Write, DEFAULT_BUF_SIZE,
+    self, ErrorKind, IntoInnerError, IoSlice, Seek, SeekFrom, Write, DEFAULT_BUF_SIZE,
 };
 use crate::mem;
 use crate::ptr;
@@ -168,9 +168,9 @@ impl<W: Write> BufWriter<W> {
 
             match r {
                 Ok(0) => {
-                    return Err(Error::new_const(
+                    return Err(io::const_io_error!(
                         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

@@ -4,7 +4,7 @@ mod tests;
 use crate::io::prelude::*;
 
 use crate::cmp;
-use crate::io::{self, Error, ErrorKind, IoSlice, IoSliceMut, ReadBuf, SeekFrom};
+use crate::io::{self, ErrorKind, IoSlice, IoSliceMut, ReadBuf, SeekFrom};
 
 use core::convert::TryInto;
 
@@ -297,9 +297,9 @@ where
                 self.pos = n;
                 Ok(self.pos)
             }
-            None => Err(Error::new_const(
+            None => Err(io::const_io_error!(
                 ErrorKind::InvalidInput,
-                &"invalid seek to a negative or overflowing position",
+                "invalid seek to a negative or overflowing position",
             )),
         }
     }
@@ -400,9 +400,9 @@ 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_const(
+        io::const_io_error!(
             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

@@ -1,6 +1,9 @@
 #[cfg(test)]
 mod tests;
 
+mod repr_unpacked;
+use repr_unpacked::Repr;
+
 use crate::convert::From;
 use crate::error;
 use crate::fmt;
@@ -66,15 +69,38 @@ impl fmt::Debug for Error {
     }
 }
 
-enum Repr {
+enum ErrorData<C> {
     Os(i32),
     Simple(ErrorKind),
-    // &str is a fat pointer, but &&str is a thin pointer.
-    SimpleMessage(ErrorKind, &'static &'static str),
-    Custom(Box<Custom>),
+    SimpleMessage(&'static SimpleMessage),
+    Custom(C),
+}
+
+#[repr(align(4))]
+#[doc(hidden)]
+pub(crate) struct SimpleMessage {
+    kind: ErrorKind,
+    message: &'static str,
+}
+
+impl SimpleMessage {
+    pub(crate) const fn new(kind: ErrorKind, message: &'static str) -> Self {
+        Self { kind, message }
+    }
+}
+
+/// Create and return an `io::Error` for a given `ErrorKind` and constant
+/// message. This doesn't allocate.
+pub(crate) macro const_io_error($kind:expr, $message:expr $(,)?) {
+    $crate::io::error::Error::from_static_message({
+        const MESSAGE_DATA: $crate::io::error::SimpleMessage =
+            $crate::io::error::SimpleMessage::new($kind, $message);
+        &MESSAGE_DATA
+    })
 }
 
 #[derive(Debug)]
+#[repr(align(4))]
 struct Custom {
     kind: ErrorKind,
     error: Box<dyn error::Error + Send + Sync>,
@@ -396,7 +422,7 @@ impl From<ErrorKind> for Error {
     /// ```
     #[inline]
     fn from(kind: ErrorKind) -> Error {
-        Error { repr: Repr::Simple(kind) }
+        Error { repr: Repr::new_simple(kind) }
     }
 }
 
@@ -461,20 +487,22 @@ impl Error {
     }
 
     fn _new(kind: ErrorKind, error: Box<dyn error::Error + Send + Sync>) -> Error {
-        Error { repr: Repr::Custom(Box::new(Custom { kind, error })) }
+        Error { repr: Repr::new_custom(Box::new(Custom { kind, error })) }
     }
 
-    /// Creates a new I/O error from a known kind of error as well as a
-    /// constant message.
+    /// 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.
+    /// You should not use this directly, and instead use the `const_io_error!`
+    /// macro: `io::const_io_error!(ErrorKind::Something, "some_message")`.
+    ///
+    /// This function should maybe change to `from_static_message<const MSG: &'static
+    /// str>(kind: ErrorKind)` in the future, when const generics allow that.
     #[inline]
-    pub(crate) const fn new_const(kind: ErrorKind, message: &'static &'static str) -> Error {
-        Self { repr: Repr::SimpleMessage(kind, message) }
+    pub(crate) const fn from_static_message(msg: &'static SimpleMessage) -> Error {
+        Self { repr: Repr::new_simple_message(msg) }
     }
 
     /// Returns an error representing the last OS error which occurred.
@@ -532,7 +560,7 @@ impl Error {
     #[must_use]
     #[inline]
     pub fn from_raw_os_error(code: i32) -> Error {
-        Error { repr: Repr::Os(code) }
+        Error { repr: Repr::new_os(code) }
     }
 
     /// Returns the OS error that this error represents (if any).
@@ -568,11 +596,11 @@ impl Error {
     #[must_use]
     #[inline]
     pub fn raw_os_error(&self) -> Option<i32> {
-        match self.repr {
-            Repr::Os(i) => Some(i),
-            Repr::Custom(..) => None,
-            Repr::Simple(..) => None,
-            Repr::SimpleMessage(..) => None,
+        match self.repr.data() {
+            ErrorData::Os(i) => Some(i),
+            ErrorData::Custom(..) => None,
+            ErrorData::Simple(..) => None,
+            ErrorData::SimpleMessage(..) => None,
         }
     }
 
@@ -607,11 +635,11 @@ impl Error {
     #[must_use]
     #[inline]
     pub fn get_ref(&self) -> Option<&(dyn error::Error + Send + Sync + 'static)> {
-        match self.repr {
-            Repr::Os(..) => None,
-            Repr::Simple(..) => None,
-            Repr::SimpleMessage(..) => None,
-            Repr::Custom(ref c) => Some(&*c.error),
+        match self.repr.data() {
+            ErrorData::Os(..) => None,
+            ErrorData::Simple(..) => None,
+            ErrorData::SimpleMessage(..) => None,
+            ErrorData::Custom(c) => Some(&*c.error),
         }
     }
 
@@ -681,11 +709,11 @@ impl Error {
     #[must_use]
     #[inline]
     pub fn get_mut(&mut self) -> Option<&mut (dyn error::Error + Send + Sync + 'static)> {
-        match self.repr {
-            Repr::Os(..) => None,
-            Repr::Simple(..) => None,
-            Repr::SimpleMessage(..) => None,
-            Repr::Custom(ref mut c) => Some(&mut *c.error),
+        match self.repr.data_mut() {
+            ErrorData::Os(..) => None,
+            ErrorData::Simple(..) => None,
+            ErrorData::SimpleMessage(..) => None,
+            ErrorData::Custom(c) => Some(&mut *c.error),
         }
     }
 
@@ -720,11 +748,11 @@ impl Error {
     #[must_use = "`self` will be dropped if the result is not used"]
     #[inline]
     pub fn into_inner(self) -> Option<Box<dyn error::Error + Send + Sync>> {
-        match self.repr {
-            Repr::Os(..) => None,
-            Repr::Simple(..) => None,
-            Repr::SimpleMessage(..) => None,
-            Repr::Custom(c) => Some(c.error),
+        match self.repr.into_data() {
+            ErrorData::Os(..) => None,
+            ErrorData::Simple(..) => None,
+            ErrorData::SimpleMessage(..) => None,
+            ErrorData::Custom(c) => Some(c.error),
         }
     }
 
@@ -750,44 +778,46 @@ impl Error {
     #[must_use]
     #[inline]
     pub fn kind(&self) -> ErrorKind {
-        match self.repr {
-            Repr::Os(code) => sys::decode_error_kind(code),
-            Repr::Custom(ref c) => c.kind,
-            Repr::Simple(kind) => kind,
-            Repr::SimpleMessage(kind, _) => kind,
+        match self.repr.data() {
+            ErrorData::Os(code) => sys::decode_error_kind(code),
+            ErrorData::Custom(ref c) => c.kind,
+            ErrorData::Simple(kind) => kind,
+            ErrorData::SimpleMessage(m) => m.kind,
         }
     }
 }
 
 impl fmt::Debug for Repr {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match *self {
-            Repr::Os(code) => fmt
+        match self.data() {
+            ErrorData::Os(code) => fmt
                 .debug_struct("Os")
                 .field("code", &code)
                 .field("kind", &sys::decode_error_kind(code))
                 .field("message", &sys::os::error_string(code))
                 .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()
-            }
+            ErrorData::Custom(c) => fmt::Debug::fmt(&c, fmt),
+            ErrorData::Simple(kind) => fmt.debug_tuple("Kind").field(&kind).finish(),
+            ErrorData::SimpleMessage(msg) => fmt
+                .debug_struct("Error")
+                .field("kind", &msg.kind)
+                .field("message", &msg.message)
+                .finish(),
         }
     }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl fmt::Display for Error {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match self.repr {
-            Repr::Os(code) => {
+        match self.repr.data() {
+            ErrorData::Os(code) => {
                 let detail = sys::os::error_string(code);
                 write!(fmt, "{} (os error {})", detail, code)
             }
-            Repr::Custom(ref c) => c.error.fmt(fmt),
-            Repr::Simple(kind) => write!(fmt, "{}", kind.as_str()),
-            Repr::SimpleMessage(_, &msg) => msg.fmt(fmt),
+            ErrorData::Custom(ref c) => c.error.fmt(fmt),
+            ErrorData::Simple(kind) => write!(fmt, "{}", kind.as_str()),
+            ErrorData::SimpleMessage(msg) => msg.message.fmt(fmt),
         }
     }
 }
@@ -796,29 +826,29 @@ impl fmt::Display for Error {
 impl error::Error for Error {
     #[allow(deprecated, deprecated_in_future)]
     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(),
+        match self.repr.data() {
+            ErrorData::Os(..) | ErrorData::Simple(..) => self.kind().as_str(),
+            ErrorData::SimpleMessage(msg) => msg.message,
+            ErrorData::Custom(ref c) => c.error.description(),
         }
     }
 
     #[allow(deprecated)]
     fn cause(&self) -> Option<&dyn error::Error> {
-        match self.repr {
-            Repr::Os(..) => None,
-            Repr::Simple(..) => None,
-            Repr::SimpleMessage(..) => None,
-            Repr::Custom(ref c) => c.error.cause(),
+        match self.repr.data() {
+            ErrorData::Os(..) => None,
+            ErrorData::Simple(..) => None,
+            ErrorData::SimpleMessage(..) => None,
+            ErrorData::Custom(ref c) => c.error.cause(),
         }
     }
 
     fn source(&self) -> Option<&(dyn error::Error + 'static)> {
-        match self.repr {
-            Repr::Os(..) => None,
-            Repr::Simple(..) => None,
-            Repr::SimpleMessage(..) => None,
-            Repr::Custom(ref c) => c.error.source(),
+        match self.repr.data() {
+            ErrorData::Os(..) => None,
+            ErrorData::Simple(..) => None,
+            ErrorData::SimpleMessage(..) => None,
+            ErrorData::Custom(ref c) => c.error.source(),
         }
     }
 }