Improve fs error open_from unix

Consistency for #79399
Suggested by JohnTitor

Improve fs error invaild input for sys_common

The text was duplicated from unix.

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

@@ -2,7 +2,7 @@ use crate::os::unix::prelude::*;
 
 use crate::ffi::{CStr, CString, OsStr, OsString};
 use crate::fmt;
-use crate::io::{self, Error, ErrorKind, IoSlice, IoSliceMut, SeekFrom};
+use crate::io::{self, Error, IoSlice, IoSliceMut, SeekFrom};
 use crate::mem;
 use crate::path::{Path, PathBuf};
 use crate::ptr;
@@ -1152,14 +1152,12 @@ pub fn canonicalize(p: &Path) -> io::Result<PathBuf> {
 
 fn open_from(from: &Path) -> io::Result<(crate::fs::File, crate::fs::Metadata)> {
     use crate::fs::File;
+    use crate::sys_common::fs::NOT_FILE_ERROR;
 
     let reader = File::open(from)?;
     let metadata = reader.metadata()?;
     if !metadata.is_file() {
-        return Err(Error::new_const(
-            ErrorKind::InvalidInput,
-            &"the source path is not an existing regular file",
-        ));
+        return Err(NOT_FILE_ERROR);
     }
     Ok((reader, metadata))
 }

library/std/src/sys_common/fs.rs

@@ -4,15 +4,17 @@ use crate::fs;
 use crate::io::{self, Error, ErrorKind};
 use crate::path::Path;
 
+pub(crate) const NOT_FILE_ERROR: Error = Error::new_const(
+    ErrorKind::InvalidInput,
+    &"the source path is neither a regular file nor a symlink to a regular file",
+);
+
 pub fn copy(from: &Path, to: &Path) -> io::Result<u64> {
     let mut reader = fs::File::open(from)?;
     let metadata = reader.metadata()?;
 
     if !metadata.is_file() {
-        return Err(Error::new_const(
-            ErrorKind::InvalidInput,
-            &"the source path is not an existing regular file",
-        ));
+        return Err(NOT_FILE_ERROR);
     }
 
     let mut writer = fs::File::create(to)?;