diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index 363a2667174..8d776642249 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -232,6 +232,7 @@
     all(target_vendor = "fortanix", target_env = "sgx"),
     feature(slice_index_methods, coerce_unsized, sgx_platform)
 )]
+#![cfg_attr(windows, feature(round_char_boundary))]
 //
 // Language features:
 #![feature(alloc_error_handler)]
diff --git a/library/std/src/sys/windows/c.rs b/library/std/src/sys/windows/c.rs
index f58dcf1287b..d1e6594b3f2 100644
--- a/library/std/src/sys/windows/c.rs
+++ b/library/std/src/sys/windows/c.rs
@@ -6,13 +6,15 @@
 
 use crate::ffi::CStr;
 use crate::mem;
-use crate::os::raw::{c_char, c_int, c_long, c_longlong, c_uint, c_ulong, c_ushort};
+use crate::os::raw::{c_char, c_long, c_longlong, c_uint, c_ulong, c_ushort};
 use crate::os::windows::io::{BorrowedHandle, HandleOrInvalid, HandleOrNull};
 use crate::ptr;
 use core::ffi::NonZero_c_ulong;
 
 use libc::{c_void, size_t, wchar_t};
 
+pub use crate::os::raw::c_int;
+
 #[path = "c/errors.rs"] // c.rs is included from two places so we need to specify this
 mod errors;
 pub use errors::*;
@@ -47,16 +49,19 @@ pub type ACCESS_MASK = DWORD;
 
 pub type LPBOOL = *mut BOOL;
 pub type LPBYTE = *mut BYTE;
+pub type LPCCH = *const CHAR;
 pub type LPCSTR = *const CHAR;
+pub type LPCWCH = *const WCHAR;
 pub type LPCWSTR = *const WCHAR;
+pub type LPCVOID = *const c_void;
 pub type LPDWORD = *mut DWORD;
 pub type LPHANDLE = *mut HANDLE;
 pub type LPOVERLAPPED = *mut OVERLAPPED;
 pub type LPPROCESS_INFORMATION = *mut PROCESS_INFORMATION;
 pub type LPSECURITY_ATTRIBUTES = *mut SECURITY_ATTRIBUTES;
 pub type LPSTARTUPINFO = *mut STARTUPINFO;
+pub type LPSTR = *mut CHAR;
 pub type LPVOID = *mut c_void;
-pub type LPCVOID = *const c_void;
 pub type LPWCH = *mut WCHAR;
 pub type LPWIN32_FIND_DATAW = *mut WIN32_FIND_DATAW;
 pub type LPWSADATA = *mut WSADATA;
@@ -132,6 +137,10 @@ pub const MAX_PATH: usize = 260;
 
 pub const FILE_TYPE_PIPE: u32 = 3;
 
+pub const CP_UTF8: DWORD = 65001;
+pub const MB_ERR_INVALID_CHARS: DWORD = 0x08;
+pub const WC_ERR_INVALID_CHARS: DWORD = 0x80;
+
 #[repr(C)]
 #[derive(Copy)]
 pub struct WIN32_FIND_DATAW {
@@ -1155,6 +1164,25 @@ extern "system" {
         lpFilePart: *mut LPWSTR,
     ) -> DWORD;
     pub fn GetFileAttributesW(lpFileName: LPCWSTR) -> DWORD;
+
+    pub fn MultiByteToWideChar(
+        CodePage: UINT,
+        dwFlags: DWORD,
+        lpMultiByteStr: LPCCH,
+        cbMultiByte: c_int,
+        lpWideCharStr: LPWSTR,
+        cchWideChar: c_int,
+    ) -> c_int;
+    pub fn WideCharToMultiByte(
+        CodePage: UINT,
+        dwFlags: DWORD,
+        lpWideCharStr: LPCWCH,
+        cchWideChar: c_int,
+        lpMultiByteStr: LPSTR,
+        cbMultiByte: c_int,
+        lpDefaultChar: LPCCH,
+        lpUsedDefaultChar: LPBOOL,
+    ) -> c_int;
 }
 
 #[link(name = "ws2_32")]
diff --git a/library/std/src/sys/windows/stdio.rs b/library/std/src/sys/windows/stdio.rs
index c2cd48470bd..32c6ccffb7a 100644
--- a/library/std/src/sys/windows/stdio.rs
+++ b/library/std/src/sys/windows/stdio.rs
@@ -169,14 +169,27 @@ fn write(
 }
 
 fn write_valid_utf8_to_console(handle: c::HANDLE, utf8: &str) -> io::Result<usize> {
+    debug_assert!(!utf8.is_empty());
+
     let mut utf16 = [MaybeUninit::<u16>::uninit(); MAX_BUFFER_SIZE / 2];
-    let mut len_utf16 = 0;
-    for (chr, dest) in utf8.encode_utf16().zip(utf16.iter_mut()) {
-        *dest = MaybeUninit::new(chr);
-        len_utf16 += 1;
-    }
-    // Safety: We've initialized `len_utf16` values.
-    let utf16: &[u16] = unsafe { MaybeUninit::slice_assume_init_ref(&utf16[..len_utf16]) };
+    let utf8 = &utf8[..utf8.floor_char_boundary(utf16.len())];
+
+    let utf16: &[u16] = unsafe {
+        // Note that this theoretically checks validity twice in the (most common) case
+        // where the underlying byte sequence is valid utf-8 (given the check in `write()`).
+        let result = c::MultiByteToWideChar(
+            c::CP_UTF8,                      // CodePage
+            c::MB_ERR_INVALID_CHARS,         // dwFlags
+            utf8.as_ptr() as c::LPCCH,       // lpMultiByteStr
+            utf8.len() as c::c_int,          // cbMultiByte
+            utf16.as_mut_ptr() as c::LPWSTR, // lpWideCharStr
+            utf16.len() as c::c_int,         // cchWideChar
+        );
+        assert!(result != 0, "Unexpected error in MultiByteToWideChar");
+
+        // Safety: MultiByteToWideChar initializes `result` values.
+        MaybeUninit::slice_assume_init_ref(&utf16[..result as usize])
+    };
 
     let mut written = write_u16s(handle, &utf16)?;
 
@@ -189,8 +202,8 @@ fn write_valid_utf8_to_console(handle: c::HANDLE, utf8: &str) -> io::Result<usiz
         // a missing surrogate can be produced (and also because of the UTF-8 validation above),
         // write the missing surrogate out now.
         // Buffering it would mean we have to lie about the number of bytes written.
-        let first_char_remaining = utf16[written];
-        if first_char_remaining >= 0xDCEE && first_char_remaining <= 0xDFFF {
+        let first_code_unit_remaining = utf16[written];
+        if first_code_unit_remaining >= 0xDCEE && first_code_unit_remaining <= 0xDFFF {
             // low surrogate
             // We just hope this works, and give up otherwise
             let _ = write_u16s(handle, &utf16[written..written + 1]);
@@ -212,6 +225,7 @@ fn write_valid_utf8_to_console(handle: c::HANDLE, utf8: &str) -> io::Result<usiz
 }
 
 fn write_u16s(handle: c::HANDLE, data: &[u16]) -> io::Result<usize> {
+    debug_assert!(data.len() < u32::MAX as usize);
     let mut written = 0;
     cvt(unsafe {
         c::WriteConsoleW(
@@ -365,26 +379,32 @@ fn read_u16s(handle: c::HANDLE, buf: &mut [MaybeUninit<u16>]) -> io::Result<usiz
     Ok(amount as usize)
 }
 
-#[allow(unused)]
 fn utf16_to_utf8(utf16: &[u16], utf8: &mut [u8]) -> io::Result<usize> {
-    let mut written = 0;
-    for chr in char::decode_utf16(utf16.iter().cloned()) {
-        match chr {
-            Ok(chr) => {
-                chr.encode_utf8(&mut utf8[written..]);
-                written += chr.len_utf8();
-            }
-            Err(_) => {
-                // We can't really do any better than forget all data and return an error.
-                return Err(io::const_io_error!(
-                    io::ErrorKind::InvalidData,
-                    "Windows stdin in console mode does not support non-UTF-16 input; \
-                     encountered unpaired surrogate",
-                ));
-            }
-        }
+    debug_assert!(utf16.len() <= c::c_int::MAX as usize);
+    debug_assert!(utf8.len() <= c::c_int::MAX as usize);
+
+    let result = unsafe {
+        c::WideCharToMultiByte(
+            c::CP_UTF8,                    // CodePage
+            c::WC_ERR_INVALID_CHARS,       // dwFlags
+            utf16.as_ptr(),                // lpWideCharStr
+            utf16.len() as c::c_int,       // cchWideChar
+            utf8.as_mut_ptr() as c::LPSTR, // lpMultiByteStr
+            utf8.len() as c::c_int,        // cbMultiByte
+            ptr::null(),                   // lpDefaultChar
+            ptr::null_mut(),               // lpUsedDefaultChar
+        )
+    };
+    if result == 0 {
+        // We can't really do any better than forget all data and return an error.
+        Err(io::const_io_error!(
+            io::ErrorKind::InvalidData,
+            "Windows stdin in console mode does not support non-UTF-16 input; \
+            encountered unpaired surrogate",
+        ))
+    } else {
+        Ok(result as usize)
     }
-    Ok(written)
 }
 
 impl IncompleteUtf8 {