Avoid zeroing large stack buffers in stdio on Windows

library/std/src/sys/windows/stdio.rs

@@ -3,6 +3,7 @@
 use crate::char::decode_utf16;
 use crate::cmp;
 use crate::io;
+use crate::mem::MaybeUninit;
 use crate::os::windows::io::{FromRawHandle, IntoRawHandle};
 use crate::ptr;
 use crate::str;
@@ -169,13 +170,14 @@ fn write(
 }
 
 fn write_valid_utf8_to_console(handle: c::HANDLE, utf8: &str) -> io::Result<usize> {
-    let mut utf16 = [0u16; MAX_BUFFER_SIZE / 2];
+    let mut utf16: [MaybeUninit<u16>; MAX_BUFFER_SIZE / 2] = MaybeUninit::uninit_array();
     let mut len_utf16 = 0;
     for (chr, dest) in utf8.encode_utf16().zip(utf16.iter_mut()) {
-        *dest = chr;
+        *dest = MaybeUninit::new(chr);
         len_utf16 += 1;
     }
-    let utf16 = &utf16[..len_utf16];
+    // Safety: We've initialized `len_utf16` values.
+    let utf16: &[u16] = unsafe { MaybeUninit::slice_assume_init_ref(&utf16[..len_utf16]) };
 
     let mut written = write_u16s(handle, &utf16)?;
 
@@ -250,27 +252,33 @@ impl io::Read for Stdin {
             return Ok(bytes_copied);
         } else if buf.len() - bytes_copied < 4 {
             // Not enough space to get a UTF-8 byte. We will use the incomplete UTF8.
-            let mut utf16_buf = [0u16; 1];
+            let mut utf16_buf = [MaybeUninit::new(1); 1];
             // Read one u16 character.
             let read = read_u16s_fixup_surrogates(handle, &mut utf16_buf, 1, &mut self.surrogate)?;
             // Read bytes, using the (now-empty) self.incomplete_utf8 as extra space.
-            let read_bytes = utf16_to_utf8(&utf16_buf[..read], &mut self.incomplete_utf8.bytes)?;
+            let read_bytes = utf16_to_utf8(
+                unsafe { MaybeUninit::slice_assume_init_ref(&utf16_buf[..read]) },
+                &mut self.incomplete_utf8.bytes,
+            )?;
 
             // Read in the bytes from incomplete_utf8 until the buffer is full.
             self.incomplete_utf8.len = read_bytes as u8;
             // No-op if no bytes.
             bytes_copied += self.incomplete_utf8.read(&mut buf[bytes_copied..]);
             Ok(bytes_copied)
         } else {
-            let mut utf16_buf = [0u16; MAX_BUFFER_SIZE / 2];
+            let mut utf16_buf: [MaybeUninit<u16>; MAX_BUFFER_SIZE / 2] =
+                MaybeUninit::uninit_array();
             // In the worst case, a UTF-8 string can take 3 bytes for every `u16` of a UTF-16. So
             // we can read at most a third of `buf.len()` chars and uphold the guarantee no data gets
             // lost.
             let amount = cmp::min(buf.len() / 3, utf16_buf.len());
             let read =
                 read_u16s_fixup_surrogates(handle, &mut utf16_buf, amount, &mut self.surrogate)?;
-
-            match utf16_to_utf8(&utf16_buf[..read], buf) {
+            // Safety `read_u16s_fixup_surrogates` returns the number of items
+            // initialized.
+            let utf16s = unsafe { MaybeUninit::slice_assume_init_ref(&utf16_buf[..read]) };
+            match utf16_to_utf8(utf16s, buf) {
                 Ok(value) => return Ok(bytes_copied + value),
                 Err(e) => return Err(e),
             }
@@ -283,14 +291,14 @@ impl io::Read for Stdin {
 // This is a best effort, and might not work if we are not the only reader on Stdin.
 fn read_u16s_fixup_surrogates(
     handle: c::HANDLE,
-    buf: &mut [u16],
+    buf: &mut [MaybeUninit<u16>],
     mut amount: usize,
     surrogate: &mut u16,
 ) -> io::Result<usize> {
     // Insert possibly remaining unpaired surrogate from last read.
     let mut start = 0;
     if *surrogate != 0 {
-        buf[0] = *surrogate;
+        buf[0] = MaybeUninit::new(*surrogate);
         *surrogate = 0;
         start = 1;
         if amount == 1 {
@@ -303,7 +311,10 @@ fn read_u16s_fixup_surrogates(
     let mut amount = read_u16s(handle, &mut buf[start..amount])? + start;
 
     if amount > 0 {
-        let last_char = buf[amount - 1];
+        // Safety: The returned `amount` is the number of values initialized,
+        // and it is not 0, so we know that `buf[amount - 1]` have been
+        // initialized.
+        let last_char = unsafe { buf[amount - 1].assume_init() };
         if last_char >= 0xD800 && last_char <= 0xDBFF {
             // high surrogate
             *surrogate = last_char;
@@ -313,7 +324,8 @@ fn read_u16s_fixup_surrogates(
     Ok(amount)
 }
 
-fn read_u16s(handle: c::HANDLE, buf: &mut [u16]) -> io::Result<usize> {
+// Returns `Ok(n)` if it initialized `n` values in `buf`.
+fn read_u16s(handle: c::HANDLE, buf: &mut [MaybeUninit<u16>]) -> io::Result<usize> {
     // Configure the `pInputControl` parameter to not only return on `\r\n` but also Ctrl-Z, the
     // traditional DOS method to indicate end of character stream / user input (SUB).
     // See #38274 and https://stackoverflow.com/questions/43836040/win-api-readconsole.
@@ -346,8 +358,9 @@ fn read_u16s(handle: c::HANDLE, buf: &mut [u16]) -> io::Result<usize> {
         }
         break;
     }
-
-    if amount > 0 && buf[amount as usize - 1] == CTRL_Z {
+    // Safety: if `amount > 0`, then that many bytes were written, so
+    // `buf[amount as usize - 1]` has been initialized.
+    if amount > 0 && unsafe { buf[amount as usize - 1].assume_init() } == CTRL_Z {
         amount -= 1;
     }
     Ok(amount as usize)