Improve OpenOptions append+truncate error message

library/std/src/fs/tests.rs

@@ -1506,6 +1506,7 @@ fn open_flavors() {
 
     // This error string is set by std itself so we are not at the whim of the OS here.
     let invalid_options = "creating or truncating a file requires write or append access";
+    let append_truncate_error = "append and truncate cannot both be enabled";
 
     // Test various combinations of creation modes and access modes.
     //
@@ -1543,15 +1544,21 @@ fn open_flavors() {
 
     // append
     check!(c(&a).create_new(true).open(&tmpdir.join("d")));
-    error_contains!(c(&a).create(true).truncate(true).open(&tmpdir.join("d")), invalid_options);
-    error_contains!(c(&a).truncate(true).open(&tmpdir.join("d")), invalid_options);
+    error_contains!(
+        c(&a).create(true).truncate(true).open(&tmpdir.join("d")),
+        append_truncate_error
+    );
+    error_contains!(c(&a).truncate(true).open(&tmpdir.join("d")), append_truncate_error);
     check!(c(&a).create(true).open(&tmpdir.join("d")));
     check!(c(&a).open(&tmpdir.join("d")));
 
     // read-append
     check!(c(&ra).create_new(true).open(&tmpdir.join("e")));
-    error_contains!(c(&ra).create(true).truncate(true).open(&tmpdir.join("e")), invalid_options);
-    error_contains!(c(&ra).truncate(true).open(&tmpdir.join("e")), invalid_options);
+    error_contains!(
+        c(&ra).create(true).truncate(true).open(&tmpdir.join("e")),
+        append_truncate_error
+    );
+    error_contains!(c(&ra).truncate(true).open(&tmpdir.join("e")), append_truncate_error);
     check!(c(&ra).create(true).open(&tmpdir.join("e")));
     check!(c(&ra).open(&tmpdir.join("e")));
 
@@ -2362,7 +2369,6 @@ fn test_open_options_invalid_combinations() {
         (|| OO::new().create(true).read(true).clone(), "create without write"),
         (|| OO::new().create_new(true).read(true).clone(), "create_new without write"),
         (|| OO::new().truncate(true).read(true).clone(), "truncate without write"),
-        (|| OO::new().truncate(true).append(true).clone(), "truncate with append"),
     ];
 
     for (make_opts, desc) in test_cases {
@@ -2377,7 +2383,13 @@ fn test_open_options_invalid_combinations() {
             "{desc} - wrong error message"
         );
     }
+    let result = OO::new().truncate(true).append(true).open("nonexistent.txt");
 
+    assert!(result.is_err(), "truncate with append should fail");
+
+    let err = result.unwrap_err();
+    assert_eq!(err.kind(), ErrorKind::InvalidInput);
+    assert_eq!(err.to_string(), "append and truncate cannot both be enabled");
     let result = OO::new().open("nonexistent.txt");
     assert!(result.is_err(), "no access mode should fail");
     let err = result.unwrap_err();

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

@@ -1186,7 +1186,7 @@ impl OpenOptions {
                 if self.truncate && !self.create_new {
                     return Err(io::Error::new(
                         io::ErrorKind::InvalidInput,
-                        "creating or truncating a file requires write or append access",
+                        "append and truncate cannot both be enabled",
                     ));
                 }
             }

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

@@ -301,7 +301,7 @@ impl OpenOptions {
                 if self.truncate && !self.create_new {
                     return Err(io::Error::new(
                         io::ErrorKind::InvalidInput,
-                        "creating or truncating a file requires write or append access",
+                        "append and truncate cannot both be enabled",
                     ));
                 }
             }