Rollup merge of rust-lang#121403 - kornelski:io-oom, r=dtolnay

impl From<TryReserveError> for io::Error

There's an obvious mapping between these two errors, and it makes I/O code less noisy.

I've chosen to use simple `ErrorKind::OutOfMemory` `io::Error`, without keeping `TryReserveError` for the `source()`, because:

* It matches current uses in libstd,
* `ErrorData::Custom` allocates, which is a risky proposition for handling OOM errors specifically.
* Currently `TryReserveError` has no public fields/methods, so it's usefulness is limited. How allocators should report errors, especially custom and verbose ones is still an open question.

Just in case I've added note in the doccomment that this may change.

The compiler forced me to declare stability of this impl. I think this implementation is simple enough that it doesn't need full-blown stabilization period, and I've marked it for the next release, but of course I can adjust the attribute if needed.

Author: Nadrieril

library/std/src/fs.rs

@@ -261,7 +261,7 @@ pub fn read<P: AsRef<Path>>(path: P) -> io::Result<Vec<u8>> {
         let mut file = File::open(path)?;
         let size = file.metadata().map(|m| m.len() as usize).ok();
         let mut bytes = Vec::new();
-        bytes.try_reserve_exact(size.unwrap_or(0)).map_err(|_| io::ErrorKind::OutOfMemory)?;
+        bytes.try_reserve_exact(size.unwrap_or(0))?;
         io::default_read_to_end(&mut file, &mut bytes, size)?;
         Ok(bytes)
     }
@@ -304,7 +304,7 @@ pub fn read_to_string<P: AsRef<Path>>(path: P) -> io::Result<String> {
         let mut file = File::open(path)?;
         let size = file.metadata().map(|m| m.len() as usize).ok();
         let mut string = String::new();
-        string.try_reserve_exact(size.unwrap_or(0)).map_err(|_| io::ErrorKind::OutOfMemory)?;
+        string.try_reserve_exact(size.unwrap_or(0))?;
         io::default_read_to_string(&mut file, &mut string, size)?;
         Ok(string)
     }
@@ -777,14 +777,14 @@ impl Read for &File {
     // Reserves space in the buffer based on the file size when available.
     fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
         let size = buffer_capacity_required(self);
-        buf.try_reserve(size.unwrap_or(0)).map_err(|_| io::ErrorKind::OutOfMemory)?;
+        buf.try_reserve(size.unwrap_or(0))?;
         io::default_read_to_end(self, buf, size)
     }
 
     // Reserves space in the buffer based on the file size when available.
     fn read_to_string(&mut self, buf: &mut String) -> io::Result<usize> {
         let size = buffer_capacity_required(self);
-        buf.try_reserve(size.unwrap_or(0)).map_err(|_| io::ErrorKind::OutOfMemory)?;
+        buf.try_reserve(size.unwrap_or(0))?;
         io::default_read_to_string(self, buf, size)
     }
 }

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

@@ -344,7 +344,7 @@ impl<R: ?Sized + Read> Read for BufReader<R> {
     // delegate to the inner implementation.
     fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
         let inner_buf = self.buffer();
-        buf.try_reserve(inner_buf.len()).map_err(|_| io::ErrorKind::OutOfMemory)?;
+        buf.try_reserve(inner_buf.len())?;
         buf.extend_from_slice(inner_buf);
         let nread = inner_buf.len();
         self.discard_buffer();

library/std/src/io/error.rs

@@ -83,6 +83,18 @@ impl From<alloc::ffi::NulError> for Error {
     }
 }
 
+#[stable(feature = "io_error_from_try_reserve", since = "CURRENT_RUSTC_VERSION")]
+impl From<alloc::collections::TryReserveError> for Error {
+    /// Converts `TryReserveError` to an error with [`ErrorKind::OutOfMemory`].
+    ///
+    /// `TryReserveError` won't be available as the error `source()`,
+    /// but this may change in the future.
+    fn from(_: alloc::collections::TryReserveError) -> Error {
+        // ErrorData::Custom allocates, which isn't great for handling OOM errors.
+        ErrorKind::OutOfMemory.into()
+    }
+}
+
 // Only derive debug in tests, to make sure it
 // doesn't accidentally get printed.
 #[cfg_attr(test, derive(Debug))]

library/std/src/io/impls.rs

@@ -304,7 +304,7 @@ impl Read for &[u8] {
     #[inline]
     fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
         let len = self.len();
-        buf.try_reserve(len).map_err(|_| ErrorKind::OutOfMemory)?;
+        buf.try_reserve(len)?;
         buf.extend_from_slice(*self);
         *self = &self[len..];
         Ok(len)
@@ -452,7 +452,7 @@ impl<A: Allocator> Read for VecDeque<u8, A> {
     fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
         // The total len is known upfront so we can reserve it in a single call.
         let len = self.len();
-        buf.try_reserve(len).map_err(|_| ErrorKind::OutOfMemory)?;
+        buf.try_reserve(len)?;
 
         let (front, back) = self.as_slices();
         buf.extend_from_slice(front);

library/std/src/io/mod.rs

@@ -465,7 +465,7 @@ pub(crate) fn default_read_to_end<R: Read + ?Sized>(
 
         if buf.len() == buf.capacity() {
             // buf is full, need more space
-            buf.try_reserve(PROBE_SIZE).map_err(|_| ErrorKind::OutOfMemory)?;
+            buf.try_reserve(PROBE_SIZE)?;
         }
 
         let mut spare = buf.spare_capacity_mut();
@@ -834,7 +834,7 @@ pub trait Read {
     ///         if src_buf.is_empty() {
     ///             break;
     ///         }
-    ///         dest_vec.try_reserve(src_buf.len()).map_err(|_| io::ErrorKind::OutOfMemory)?;
+    ///         dest_vec.try_reserve(src_buf.len())?;
     ///         dest_vec.extend_from_slice(src_buf);
     ///
     ///         // Any irreversible side effects should happen after `try_reserve` succeeds,

library/std/src/io/tests.rs

@@ -692,3 +692,13 @@ fn read_buf_full_read() {
 
     assert_eq!(BufReader::new(FullRead).fill_buf().unwrap().len(), DEFAULT_BUF_SIZE);
 }
+
+#[test]
+// 64-bit only to be sure the allocator will fail fast on an impossible to satsify size
+#[cfg(target_pointer_width = "64")]
+fn try_oom_error() {
+    let mut v = Vec::<u8>::new();
+    let reserve_err = v.try_reserve(isize::MAX as usize - 1).unwrap_err();
+    let io_err = io::Error::from(reserve_err);
+    assert_eq!(io::ErrorKind::OutOfMemory, io_err.kind());
+}